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Issue
A&A
Volume 569, September 2014
Article Number A119
Number of page(s) 10
Section Astrophysical processes
DOI https://doi.org/10.1051/0004-6361/201323118
Published online 02 October 2014

© ESO, 2014

1. Introduction

Several carbonaceous solids are observed in the interstellar medium (ISM): nanodiamonds, fullerenes, polyaromatic hydrocarbons, amorphous carbons, hydrogenated amorphous carbons (HACs), and ice mantles that, when processed, can lead to the formation of organic residues. The hydrogenated solids comprise nanodiamonds, observed through vibration bands at 3.43 and 3.53 μm of hydrogen-terminated nanodiamonds (Chang et al. 1995; Guillois et al. 1999; Pirali et al. 2007) in equilibrium with the stellar radiation field. They are observed close to the stars around very few objects (Habart et al. 2004; Acke et al. 2006; Goto et al. 2009). The aromatic infrared bands (AIB), with emission bands observed around 3.3, 6.2, 7.7, 8.6, and 11.3 μm, characteristic of a (hydrogenated) polyaromatic material, are observed ubiquitously in the ISM. They have led to the polycyclic aromatic hydrocarbons (PAHs) hypothesis, which attributes the AIB bands to the infrared fluorescence of large PAH molecules upon absorption of energetic UV photons (Leger et al. 1984; Allamandola et al. 1985). The AIB astronomical spectra display a variability in their band profiles, and the observed sources have been classified into three classes A–C, based on a phenomenological decomposition of the band positions. By number, class A spectra are the most often observed, followed by class B, and a few class C. The evolution among the profiles is primarily linked to differences in the chemical composition of the emitting molecules or particles. Class A are probably the most aromatic ones, and class C display additional features that are attributed to a more aliphatic character, observationnally (Sloan et al. 2007; Boersma et al. 2008; Keller et al. 2008; Acke et al. 2010), and experimentally (Pino et al. 2008; Carpentier et al. 2012; Gadallah et al. 2013).

Hydrogenated amorphous carbons, also called a-C:H by physicists, for amorphous material made of C and H, are another component of interstellar dust. In this article we focus on this major phase, which probably hosts the largest carbon portion of these hydrogenated carbon materials. Allen & Wickramasenghe (1981) observed it for the first time at 3.4 μm against a Galactic center source. The features contributing to this absorption band were early associated to sp3 CH3 and CH2 stretching modes, as stated for example in Duley & Williams (1983). Since then, numerous experiments, observations and models have been employed to constrain its origin (e.g., Jones et al. 1983; 2013; Butchart et al. 1986; Mc Fadzean et al. 1989; Ehrenfreund et al. 1991; Sandford et al. 1991; 1995; Pendleton et al. 1994; Tielens et al. 1996; Geballe et al. 1998; Muñoz Caro et al. 2001; Chiar et al. 2002; Mennella et al. 2002; Pendleton & Allamandola 2002; Dartois et al. 2005; Godard et al. 2012). The a-C:H Galactic abundance has been estimated from the observed CH stretching modes.

Table 1

Characterization of different irradiated a-C:H, a-C:D and polyethylene films.

Depending on the intrinsic strength for CH modes and the degree of hydrogenation of the assumed material carriers, the implied cosmic carbon fraction varies from 2.6% to 35% (Sandford et al. 1991), above 2.5 to 4% based on the spectra of alkanes (Pendleton et al. 1994) and up to 20–30% for laboratory analogs of a-C:H (Duley 1994; Duley et al. 1998). With the increase in sensitivity studies, a-C:H absorptions have been observed in several extragalactic obscured active galactic nucleus (AGN) sources, mainly via the stretching modes (Pendleton et al. 1994; Mason et al. 2004; Dartois et al. 2004; Imanishi 2006; Risaliti et al. 2006; Imanishi et al. 2008).

Several laboratory analogs have been proposed to account for this carbonaceous component and provide a fit to the observed infrared features. These include analogs produced using a heated carbon rod (Schnaiter et al. 1998), laser-desorbed carbon (Mennella et al. 1999), and subsequent exposure to hydrogen plasma, carbon vapor obtained by striking arc discharges between carbon rods in a hydrogen atmosphere (Mennella et al. 2003), plasma deposition (Lee & Wdowiak 1993; Furton et al. 1999; Godard & Dartois 2010), or photoproduced a-C:H at low temperature (Dartois et al. 2005).

These analogs possess commonalities that allow us to define them as a family of carbonaceous materials. In addition to this common infrared active bands, the H/C ratio of the above mentioned analogs vary by more than an order of magnitude and also possess intrinsic structural differences that are sometimes much less evident because they imply less active IR modes of the carbonaceous backbone.

H2, the most abundant interstellar molecule, is involved in many of the dominant processes that govern the ISM. However, because of the lack of permanent dipole, coupled to a large rotational spacing (equivalent to about 1000 K in T), H2 is often difficult to observe. Astronomical observations of H2 toward relatively energetic media such as photon dominated regions (PDRs), shocks, proto-planetary nebulae (PPNe), or the inner parts, close to stars of young stellar objects (YSOs), suggest that formation of H2 on PAHs or small grains is very frequent in PDRs (e.g., Table 1 of Habart et al. 2005). In parallel, many hydrocarbons species are detected in PDRs, coincident with intense emission in the H2 ro-vibrational lines (Pety et al. 2005), following energetic UV excitation. In addition, pure gas-phase chemistry models are unsuccesful in reproducing the measured abundances in the ISM small hydrocarbons. Information on potential sources of these species are thus required to explain the observations. In particular, PAHs are among the targeted precursors to explain the occurrence of small hydrocarbons in PDRs (Pety et al. 2005). Small carbonaceous particles, such as a-C:H, may also play a role in the observed sequence of radicals and neutral species.

thumbnail Fig. 1

Schematic view of the SICAL experiment for VUV irradiation, mainly composed of a vacuum chamber containing several windows for FTIR measurement of the solid sample and VUV irradiation beam and quadrupole mass-spectrometer (QMS) for the detection of volatile species in the gas.

In this article we investigate, with experiments that combine infrared and mass spectrometry, the role of energetic UV photons of astrophysical interest that interact with a plasma-produced HAC, an analog to the a-C:H diffuse ISM dust. We also investigate its deuterated form to disambiguate involved species in mass-spectrometry analyses. The photolysis of this a-C:H (a-C:D) analog releases H2 (D2) as the main product, but also some small carbonaceous species that will replenish the gas phase in the diffuse ISM and more energetic environments such as PDRs. In this article we focus on the H2 and CH4 production efficiency. We describe in Sect. 2 the experimental setup and measurements. The results are given in Sect. 3, and the astrophysical implications are discussed in relation to other analogs in Sect. 4, followed by the conclusion in in Sect. 5.

2. Experimental details

thumbnail Fig. 2

Emission spectrum of the H2 discharge VUV lamp between 110 and 200 nm at different pressures, through one MgF2 window, which has an cutoff at 115 nm. The spectra were recorded by Cruz-Diaz et al. 2013 under a hydrogen pressure of 0.6 mbar (blue dashed line) and 1.0 mbar (black continuous line). The expected spectrum of our lamp for 0.75 mbar (red continuous line) is the interpolation of these two curves.

2.1. Experimental setup

The SICAL-X experimental setup used to study the vacuum ultraviolet (VUV) irradiation of deuterated and HAC (a-C:H, a-C:D) is shown in Fig. 1. It is mainly composed of a vacuum chamber, with a typical working pressure of ≤2 × 10-8 mbar, obtained with a Pfeiffer 2000l/s hybrid turbomolecular pump. Two ports equipped with CsI windows are connected to the chamber, allowing the entry of the IR probe beam. A VUV window (MgF2) interfaces the ultraviolet H2 discharge lamp. The HACs, produced in a different setup (SICAL-P) aside from the main experiment, is introduced as a thin film deposited on a ZnSe substrate, transparent in the mid-IR wavelength range. The substrate is cooled by a closed-cycle helium refrigerator (SRDK series CRYOCOOLER F-50 series Compressor Unit). The temperature of the substrate is adjustable by a resistive-type heater element and monitored with a thermocouple with an accuracy of ±0.1 K. The temperature accessed is about 10 K. The evolution of the film during irradiation is monitored with a Bruker Vertex 80v infrared Fourier transform spectrometer, at a resolution of 1 cm-1, in a spectral range covering between 7500 and 400 cm-1. During the irradiation many species are liberated from the film surface. We use a QMS quadrupole mass spectrometer (Quadera QMS 200) to follow the evolution of those masses.

2.2. HAC production

The hydrogenated and deuterated amorphous carbon films are prepared by a plasma-enhanced chemical vapor deposition method (PECVD), where radicals and ions resulting from a low pressure radio-frequency (RF) plasma (at 2.45 GHZ), of CH4 or CD4 gases, are deposited on a substrate (ZnSe, MgF2) under vacuum. This method has been described in previous works (Godard et al. 2011; Godard & Dartois 2010). The typical deposition time required to produce a film several micrometers thick is a few minutes. After the film is produced, the chamber is filled with nitrogen at a pressure slightly higher than the atmosphere and immediately transferred to the high vacuum chamber.

2.3. VUV irradiation

VUV photons are generated by a hydrogen flow-discharge lamp, using an Evenson cavity coupled to a 2.45 GHz RF microwave generator. The hydrogen pressure in the lamp was set to 0.75 mbar, which maximises the total number of photons whose energy lies between 6.8 and 10.5 eV (between 120 and 180 nm). The VUV and visible photons enter to the vacuum chamber through a MgF2 window with a cutoff at about 115 nm. The 110 to 200 nm emission spectra of the VUV lamp used in a similar setup and recorded by Cruz-Diaz et al. (2013) at 0.6 and 1 mbar are presented in Fig. 2, as well as the expected interpolated spectrum with our working H2 lamp pressure (0.75 mbar). Using the normalized spectrum and performing an integration over the energy range, the average energy per emitted photon is ⟨ Ephoton ⟩ ≈ 8.6 eV/photon.

In the chamber, a 20 mm diameter metallic diaphragm was placed 5 cm away in front of the MgF2 window to stop the photons that would otherwise irradiate other parts of the substrate holder or the chamber.

The VUV flux, defined as the number of photons per second per unit area, was obtained by using an actinometry measurement method. We used a polyethylene film, with a thickness of 15μm (Goodfellows), and replaced the HAC films on top of the substrate. This hydrocarbon polymer was chosen because it is well studied under irradiation with similar VUV sources (e.g., Truica-Marasescu & Wertheimer 2005, and references therein). VUV irradiation gives rise to a new absorption band at 965 cm-1 attributed to the formation of double bonds from a trans-vinylene group (−HC=CH−). The evolution of this band (Fig. 3b), well calibrated, gives access to the time-dependent dose at the exact same position where our interstellar analogs are subsequently processed. By scaling the time-dependent formation of trans-vinylene absorption to the calibrated measurement (Fig. 3a), we deduce a photon flux of Φ = 2.7 ± 0.6 × 1014 photon/(cm2 s) in our experiment at the substrate position. This measurement was carried out regularly to monitor the potential degradation of the MgF2 lamp interface.

thumbnail Fig. 3

a) Integrated absorption of the double bond in trans-vinylene group (–HC=CH–) arising at 965 cm-1, as a function of dose, for irradiated polyethylene films placed in the substrate holder. The curve is the mean of two independent measurements with the corresponding error bars. b) Corresponding spectra showing the band increasing with fluence over the irradiation dose. These measurements ensure a regular actinometric calibration of the VUV lamp flux (see text for details).

2.4. QMS mass measurement

The QMS was located at ~10 cm from the film and aligned to make a 45° angle with the film surface. A stable current of energetic electrons (70 eV) was produced by a heated filament. These electrons ionize molecules and atoms with which they collide, and polarized plates accelerate the ions through a quadrupole mass filter. Ions with a certain mass-to-charge ratio propagate until they reach a deviation zone and are detected by a secondary-emission multiplier detector (SEM). In this particular configuration, VUV photons eventually reflected from the film holder, which behave as a source of noise, cannot reach the detector. To properly monitor the background signal when the UV lamp is on (i.e., the true zero of the signal, including possible contributions by photons that are absorbed or reflected outside the path to the sample), we placed a shutter in front of the film to allow periodic on-off measurements by stopping the incident photons on the film. This background was subtracted to the signal coming from the film. Two hundred masses were monitored simultaneously, with a mass resolution of . Without external calibration, the relative quantity of the corresponding species can, to first order, be related to the signal by taking into account the difference in ionization cross-sections for different species (at 70 eV a factor of 3.5 is expected between CH4 and H2), and molecule fragments caused by electron impact-ionization (e.g., CH4 detected via CH, CH, CH, CH+, C+ and 13C contribution), ionization cross-sections ratio are given at the NIST1.

2.5. Temperature-programed desorption for a-C:D and a-C:H films

We began by cooling the shield down to 40 K, while the temperature of the window was maintained at 200 K before the irradiation; this minimizes the condensation of residual gas onto the film. After that, the substrate window temperature was rapidly cooled down until it reached about 10.2 K and the film irradiation was started. After a period of irradiation at a fixed temperature T, a TPD (temperature-programed desorption) is performed with a heating ramp of 2.5 to 5 K/min, in the two cases of a-C:H and a-C:D analogs. During the 2.5 K/min TPD, the VUV irradiation was kept on to avoid changing the experimental conditions. During the very first experiments, the ramps were stopped at 200 K and at up to 280 K in the following ones. Simultaneously, QMS measurements were carried out to follow the evolution of the species released from the film after irradiation.

3. Results

We present below the results obtained after the irradiation of several deuterated and HAC films prepared under the same conditions, and with variable thicknesses. They were irradiated at 10 K during several hours to simulate the conditions of an astrophysical dense cloud and produce the species in higher abundance than shorter irradiation times, so that we could perform quantitative measurements by comparing IR and mass-spectra measurements associated with a TPD program. Shorter irradiations were performed at higher temperatures (50 K,75 K,100 K,300 K) to allow the diffusion out of the film to proceed efficiently, within the experiment duration, for the main produced species, which may otherwise seem to be trapped at the lowest temperatures because of the much shorter time scale than in space.

3.1. IR measurement

3.1.1. Main features

The absorption spectra in the infrared region between 4000 and 1000 cm-1 of the prepared a-C:H and a-C:D films are presented in Fig. 4. In a-C:H films we distinguish the bending modes of C-H bonds between 1500 and 1300 cm-1 and stretching modes between 3100 and 2800 cm-1. There are four stretching modes; the asymmetric stretching modes in the methyl group CH3 and methylene group CH2 at about 2955 cm-1 and 2925 cm-1, and two symmetric modes for CH3 and CH2 groups at 2873 cm-1 and 2857 cm-1 (these two modes are blended, see Dartois et al. 2005).

thumbnail Fig. 4

a) Transmittance spectra of a-C:H films deposited on a ZnSe window at 10 K. b) Transmittance spectra of a-C:D. c) Fits performed on the asymmetric CH3 and CH2 stretching modes. d) Fits performed on the asymmetric CD3 and CD2 stretching modes.

For deuterium substitution the similar stretching modes are active, but they are shifted to lower frequencies, between 2015 and 2285 cm-1, and the relative intensity between the modes is also changed. The asymmetric features from CD3 and CD2 are located at about 2220 and 2200cm-1, while the symmetric modes are located at 2073 cm-1 and 2100 cm-1 (e.g., Tyrode & Hedberg 2011).

The film thicknesses (Table 1) were estimated from the interference pattern. The fringe spacing Δσ was measured and the sample thicknesses d were calculated using the formula with n the refractive index of the sample, and αIR the angle of the IR beam incidence with the sample normal (αIR = 45°). Godard et al. (2010) measured the refractive index for several a-C:H samples. They found that n varies between 1.25 and 1.85. Our films have an estimated refractive index of 1.7 ± 0.2.

3.1.2. a-C:H, a-C:D irradiation

The total irradiation time for a-C:H and a-C:D films is typically five hours for short-duration and 18 h for long-irradiation times. The different irradiations by VUV photons at 10.2 K show the destruction of C-H and C-D bonds. To follow the evolution of the intensity of stretching modes and estimate the number of destroyed C-H bonds during the irradiation, an IR spectrum was recorded every 20 min with 15 min of coadded scans. The evolution of the optical depths integrated over the aliphatic C-H stretch band between 2760 and 3140 cm-1, as a function of the irradiation dose for different experiments, is represented in Fig. 6 (right axis).

thumbnail Fig. 5

Optical-depth decrease corresponding to different times of irradiation for a) a-C:H film in the 3100–2700 cm-1 range. b) a-C:D film in the 2300–2000 cm-1 range. Each curve corresponds to an additional irradiation time of 20 min. The asterisk * denotes solid CO contamination around 2133 cm-1.

The corresponding number of destroyed C-H bonds, NC − H, is given by where Δτ is the variation in the optical depth after irradiation and , are the integrated absorption coefficients of the antisymmetric stretching modes of CH3 and CH2 groups, with values equal to 1.25 × 10-17 and 8.4 × 10-18 cm/group (Dartois et al. 2004).

To evaluate the corresponding C-D bonds destruction, we evaluated to first order the corresponding band strengths for deuterated amorphous carbons by comparing the relative integrated absorptions in a-C:H and a-C:D films, normalized to the same film thickness. The fit was made only for the more intense asymmetric stretching modes, which are better separated than the symmetric ones for a-C:H. The asymmetric stretching features of a-C:H were fitted by three Gaussian profiles corresponding to the CH3, CH2, and CH2 Fermi resonance contributions, then grouped into two profiles related to their CH3 or CH2 character, as in Dartois et al. (2004). We proceeded in the same way for a-C:D films with the assigned band positions for the CD3 and CD2 asymmetric stretching modes, plus the contributions of CD2 Fermi resonances (e.g., Tyrode & Hedberg 2011, band position assigned also by Raman spectroscopy). We show the corresponding profiles in Fig. 4. The a-C:D asymmetric stretching band strengths calculated in this work lead to A(a-CD3) = 9 × 10-18 cm/group and A(a-CD2) = 4.8 − 5.5 × 10-18 cm/group. In a way similar to NC − H, the number of destroyed C-D bonds C-D was estimated.

thumbnail Fig. 6

Number of destroyed C-H bonds NC − H and C-D bonds NC − D as a function of the photon dose.

Figure 6 (left axis) shows the total number of destroyed C-H and C-D bonds as function of the number of photons. We can deduce that a C-H bond is lost for about 70 photons (almost equal for C-D bond lost). This shows that this analog is particularly resistant to VUV irradiation. Part of this resistance with respect to other analogs is, in addition to the resilience of this material, related to its photoluminescence properties (Godard et al. 2010).

3.2. Mass measurement

In the following, we show that most of the C-H bonds destruction leads to the production of H2 molecules, but also to small carbonaceous molecules. With our experimental conditions, P ≲ 2 × 10-8 mbar, the quantity of measured H2 is dominated by the residual H2 that remains in the chamber after pumping. For this reason we also analyzed deuterated amorphous carbon films because the signal of D2 does not overlap with any other background signal. Photochemical properties of a-C:H and a-C:D are expected to behave similarly. Deuteration does not significantly change the electronic orbitals and electronic bonds in the material. During VUV photons irradiation, optically allowed electronic singlet-singlet transitions are predominant at wavelengths of about 140 nm and above (Skurat 2003). Chemical transformations are mainly caused by reactions of these excited singlet states, which are very similar in both a-C:H and a-C:D materials. As was previously shown by IR spectroscopy (Fig. 6), the number of destroyed C-H and C-D bonds for the same irradiation dose are similar in both materials, within measurements uncertainties. Photoionization becomes competitive at higher energies, and the results are expected to be closer to radiolysis.

3.2.1. D2 production

The mass spectra of D2 (m/z = 4 u), released from two a-C:D films and irradiated at 10 K, during the TPD at a ramp rate of 2.5 K/min, are shown in panel a and b of Fig. 7. The mass m/z = 5 u is presented as an adjacent background noise channel. In panel c, we present the TPD spectra of the same masses for an a-C:H film irradiated under the same conditions. D2 sublimation starts at a temperature of ≈17 K for the molecules produced closer to the surface and reaches maximum for the bulk at about 90–120 K, well above the expected temperature of a surface D2 ice layer TPD behavior (e.g., Fillion et al. 2009). These D2 molecules were produced within the film during the VUV irradiation and diffuse from the bulk. For a-C:H, which served as a blank reference experiment, no signal was observed for these two masses.

thumbnail Fig. 7

a) TPD spectra of a-C:D film after 18 h of irradiation. D2 released from a-C:D film and the mass m/z = 5 u as background mass channel. b) TPD spectra of a-C:D film after 5 h of irradiation. c) TPD spectra of a-C:H film after 5 h of irradiation. At mass m/z = 4 u the signal is quite negligible and overlaps the background signal.

The TPD desorption peak lies at a temperature above the expected physisorption barrier to move from site to site (inside, in the bulk, and at the surface), with a typical adsorption energy on surfaces for D2 of a few 10 meV. The TPD behavior is dominated by the permeation time needed to diffuse through the film, because the D2 molecules are produced inside the bulk of the film for VUV irradiation. Additional experiments performed at 50, 75 and 100 K are shown in Fig. 8. The irradiation time windows are shown in the upper part of each panel. The intensities were normalized to the D2 signal when the film is irradiated at 300 K (last peak), a temperature at which the diffusion is almost instantaneous. When the irradiation stops, the signal decays as a function of the diffusion of D2 molecules out of the film. The diffusion can be estimated for the 100 K and 75 K experiments from the observed mean decay-time constant. Below about 50−60 K, the diffusion decay- time constant becomes too low to be measured directly because of our pumping speed and QMS sensitivity limits. We define a typical decay-time constant τ as the time interval between the value of the D2 signal when the irradiation is stopped and the time when the signal has decayed to 10% of this value. At 100 K, this corresponds to about 1500 s, at 75 K it is about 4000 s. The diffusion time constants generally follow an exponential behavior as a function of the inverse of the temperature. At 50 K, the extrapolated decay-time constant would be of about 3 × 104 s, at 25 K about 107 s (three to four months), at 20 K about 1.8 × 108 s (about six years). These time constants are out of reach in the laboratory, but short in space.

thumbnail Fig. 8

Irradiation and TPD spectra of a-C:D film at 100 K (upper panel), 75 K (middle panel), and 50 K (lower panel). The irradiation sequences are shown with the upper bar and the blue curve gives the film temperature. See text for details.

3.2.2. CD4 production

The mass spectrum of CD4 (m/z = 20 u) released from an irradiated a-C:D film during the TPD at a ramp rate of 2.5 K/min is displayed in panel a of Fig. 9. The mass m/z = 21 u is presented as the background mass channel, as well as the mass m/z = 18 u (divided by 500) to check for possible contribution by water isotopologs. As for D2, a blank reference experiment with a-C:H was performed, monitoring the same channels, and is shown in panel b.

Two peaks are observed in the TPD of a-C:D film at m/z = 20 u, a first peak located at about 153 K, and a second one at 200 K. In the a-C:H film case, only the first peak around 150 K is observed, slightly shifted compared with the corresponding a-C:D film one, and the second peak at 200 K is absent.

After several hours of irradiation, the peak at m/z = 18 u arises around 150 K during the a-C:H TPD caused by the residual gas-phase water frozen onto cold surfaces during the experiment, under our high vacuum conditions. We clearly see the coincidence of the temporal evolution of this peak and the first peak that appears in the m/z = 20 u of a-C:D TPD spectrum. Based on the a-C:H experiment, where the relative intensities of the peaks ((m/z = 18 u)/(m/z = 20 u)≈ 500) approximately correspond to the natural O18 to O16 isotope ratio, we attribute it to a contribution by . The second large peak that appears only in the a-C:D TPD spectrum at mass m/z = 20 u, with a maximum around 200 K, is attributed to the CD4 produced by VUV photons within the film and diffusing from the bulk.

thumbnail Fig. 9

a) TPD spectra of CD4 released from a-C:D film and the mass m/z = 21 u as background mass channel. b) TPD spectra of a-C:D film after 5 h of irradiation. c) TPD spectra of a-C:H film, the two masses m/z = 20 u, (m/z = 18)/500 and the mass m/z = 21 u as background mass channel.

3.2.3. Mass distribution

To estimate the percentual abundance of each molecule, first we normalize each curve by the ionisation cross-section for each molecule, then we integrated each TPD curve signal. Systematics in the uncertainties are evaluated by first considering the respective contributions of the D2 and CD4 integrated curves, normalized to their sum, and then measuring the respective contributions of the D2 and CD4 integrated curve normalized to the sum of all integrated curves up to mass 60. These two normalizations give us a minimum (including other masses) and a maximum (including only the desired signals) percentual abundance. We derived that D2 represents 95.8 ± 3.4% of all the species released from the film, while CD4 represents about 3.1 ± 2.1% The remaining signal is composed of other, mostly heavier, species under identification. This means that about 96% of the deuterium released by the destruction of C-D bonds is consumed by the formation of D2. We can deduce the total number of deuterated molecular hydrogen ND2 from the number of destroyed C-D bonds , where γD2 is the D2 production-efficiency factor per photolyzed CD bond. For a-C:H we also estimated the quantity of H2 produced from the film in the same way.

3.3. H2 quantum yield

We define the differential quantum yield YH2 of molecular hydrogen production as the ratio of the number of produced molecules NH2 formed from the sample per second per cm2 to the number of photons Φ absorbed by the sample per second per cm2, The integral quantum yield is defined as the ratio of the total number of product molecules formed in the sample over the irradiation time, thus corresponding to .

Figure 10 presents the variation of differential quantum yields YH2 as a function of time. It decreases during irradiation. This decrease is explained not only by consumption (depletion) of CH3 and CH2 groups, but also by accumulation of conjugated photolysis products, which are able to “protect” the deeper layers by a mechanism of electronic energy transfer (e.g., Skurat 2003).

For thin films the photon flux is quite constant during propagation inside the film. We can accept in this case that the quantum yield is constant and close to the value when irradiation of the film is started, where H2 is produced mainly by the layers close to the surface; YH2 ≃ 0.025. Absorbed photons do not all contribute to C-H bond destruction. Part of the absorbed energy is dissipated by mechanisms like relaxation to electronic ground state, photoluminescence (Godard et al. 2010)2, vibrational relaxation, or conical intersection of electronic states.

thumbnail Fig. 10

Quantum yields of H2 production from a-C:H film, as a function of irradiation time.

4. Astrophysical implications

Carbonaceous dust in the diffuse ISM is observed in emission via the PAH infrared emission bands and in absorption by a set of CH bond features at 3.4 μm for the stretchings and 6.85 and 7.25 μm for the bendings, associated with HACs. According to current models, PAHs require about 4–5% of the cosmic carbon abundance (e.g., Draine & Li 2007) to exhibit the observed emission, whereas a-C:Hs (HACs) contain 5 to 30% of the cosmic carbon.

In our analysis of a-C:H submitted to VUV photolysis, we evaluated the effect of the decrease of the mean intensity of the photons in the film as a function of depth (e.g., Cottin et al. 2003, see their Fig. 4) using our films VUV absorption and the lamp spectral flux profile (Fig. 2), correcting for nonlinearities, and using different film thicknesses. The destruction cross-section is evaluated to be of the order of σdes = 3 ± 0.9 × 10-19 cm2. This value is roughly 25 times higher than that obtained for a-C:H irradiation with much lower energy photons (≈σdes = 1.2 × 10-20 cm2) estimated by Mennella (2001) from the experiments reported by Ida et al. (1984).

It can be compared with the VUV irradiation of various hydrocarbon species (Muñoz Caro et al. 2001) and hydrogenated carbon grains produced by condensation of carbon vapour obtained by striking an arc discharge between two carbon rods in a hydrogen atmosphere (Mennella et al. 1999, 2001). For the latter, a UV photodestruction cross-section of σdes = 1 to 5 × 10-19 cm2 per photon was estimated from the IR, assuming a simple two layer hydrocarbon model, decoupling an unprocessed zone at high depth from a UV irradiated one closer to the surface. We thus find similar destruction cross-sections for the aliphatic C-H bonds in interstellar analogs made under different conditions. In this work, we show by using a mass-spectrometric detection method that the main released product of a-C:H photolysis is molecular hydrogen, with a cross-section of H2 production of σH2prod ≈ 1.4 × 10-19 cm2/photon.

These high destruction cross-sections entail a short lifetime for hydrogenated carbonaceous dust grains, which are not expiated to survive the diffuse-medium UV field, if one only takes into account the destruction. The presence of these grains in the diffuse medium therefore implies that there must be rehydrogenation or reformation pathways for the CH bonds in space, otherwise there would be no interstellar detection of aCH. Mennella (1999) found that the C-H mode can be reactivated (i.e., the grain rehydrogenated) by exposure to a beam of H atoms. He showed by a simple modeling based on these experiments that the observed a-C:H is expected be hydrogenated constantly in the diffuse medium, otherwise, as stated above, the photolysis time-scale of HACs would rapidly dehydrogenate them. Possible reactivation processes for CH bonds were also discussed in Muñoz Caro et al. (2001) and Mennella et al. (2001). Mennella (2006) performed experiments to determine an activation energy of about 70 K.

Early studies (Gould & Salpeter 1963; Hollenbach & Salpeter 1971a; Hollenbach et al. 1971b; Jura 1975) provided the first estimates of H2 formation rates in the diffuse ISM, concluding that grain surface chemistry is an unavoidable route for efficient hydrogen formation. These early hydrogen-formation rate numbers are within small factors still the ones used today. Hydrogen has the highest cosmic abundance. Several mechanisms are responsible for the formation of molecular hydrogen in the ISM; their importance varies depending on the environmental conditions.

Several laboratories have worked on H atom irradiation and adsorption on diverse surfaces such as graphite and other carbonaceous material, silicates, and ice mantles (mainly water ice), followed by chemistry leading to H2 production (e.g. Gavilan et al. 2012; Vidali et al. 2009; Mennella 2008; Amiaud et al. 2007; Perets 2007; Creighan et al. 2006, and references therein). Many of these studies focused principally on physisorbed atoms and are in general poorly efficient in producing large H2 abundances at temperatures above about 30 K. The observation of H2 formation in astronomical environments with gas at high kinetic temperatures has led to the proposition of a mechanism that necessarily involves chemisorbed H atoms (Habart et al. 2005; Cazaux et al. 2011; Le Bourlot et al. 2012) to overcome the barrier of higher temperature formation, in particular in PDR environments. The rather constant hydrogen rate coefficient (Rc) deduced in astronomical source lines of sight with degrees of excitation varying over a wide range is a severe constraint for thermally activated processes.

Alternative studies have searched for the influence of cosmic rays on the H2 formation in denser regions (e.g., Pirronello et al. 1997), showing that in shielded regions this is a valid competing way for producing H2. Mennella et al. (2003) and Godard et al. (2011) have also followed the destruction cross-section by cosmic rays. These processes also produce H2, but are probably less efficient than the UV in the diffuse medium and the PDR interfaces.

Many of the studies cited above have been performed ignoring the energetic impact of ultraviolet photons, or considering their energy input almost solely for the destruction rate (photodissociation) of H2 in the gas phase, to balance the formation rate equation and deduce it from the observations.

4.1. Comparison with the surface formation rate coefficient

The observation of the so-called 3.4 μm band in the diffuse ISM also results from the balance between destruction, with H2 as a byproduct, and hydrogenation. The mean penetration depth for the VUV photons produced by a hydrogen discharge lamp (120–160 nm) is approximately 80 nm, thus clearly affecting the bulk of the grains. It allows the formation of H2 mediated via the bulk of the grains as opposed to pure surface schemes.

From our measurements, a photolytic destruction rate of CH bonds in grains can be evaluated, where is in cm2 photon-1 CH-1 and φ(λ) in photon cm-2 s-1 Å-1,

Because most of the VUV photon penetration length is lower than or equal to the grain size, this can be approximated by (see also Mennella 2002) where σCH des(FUV) and φ(FUV) are the integrated destruction cross-section in cm2 photon-1 CH-1 and UV flux in photon cm-2 s-1,

The corresponding photolytic H2 formation rate reads where γH2 is the H2 production efficiency factor per photolyzed CH bond, φISRF(FUV) is the standard interstellar radiation field FUV flux photon cm-2 s-1, χ is a scaling factor (Draine1978), , the fraction of interstellar carbon abundance locked into a-C:H, and ntot = n(H) + 2nH2 is the total number density of hydrogen atoms, and e−τd is the dust extinction.

The seminal equation for the H2 formation rate derives generally from a consensus that the molecule forms from the recombination on the surface of dust grains from gas-phase incoming H atoms (e.g., Hollenbach et al. 1971b). Therefore it is by definition proportional to the atomic H number density and the total number density, with an assumed gas-to-grain ratio. The formation rate is the product of a rate coefficient times these number densities. The formation rate can be rewritten to extract a rate coefficient for this VUV-induced mechanism that can be set in correspondence to the one used in the rate equation of hydrogen recombination on a surface, This formulation allows performing more direct comparisons of rate coefficients of H2 formation generated in the bulk by VUV with those obtained from hydrogen recombination on the surface of dust grains due to H atoms coming from the gas-phase. Therefore, we can assess the importance of the bulk photolysis that is the aim of this investigation. Proceeding from the previous equations, then , , ,

The standard FUV (6 < hν< 13.6 eV) interstellar radiation field from Draine (1978) corresponds to φISRF(FUV) = 1.94 × 108 photon cm-2 s-1 (2.68 × 10-3 erg s-1 cm-2, that is about 1.7 times G0, the standard ISRF from Habing 1968), If only 10% of the carbon abundance is locked into HACs, , and This rate must be compared with the adopted rate coefficient of Rc ≈ 3 × 10-17 cm3 s-1 from observations in the diffuse medium (e.g., Jura 1974; Gry et al. 2002) with χ/n(H) ≈ 1/30, and to PDRs3, where Rc can rise to ≈1.5 × 10-16 cm3 s-1 (e.g., Habart et al. 2004). The χ/n(H) in these regions reaches values of up to 0.25 (e.g., Orion Bar).

The photolytic bulk production of H2 with carbonaceous HAC dust grains is thus able to sustain a very large portion of the contribution to the H2 formation, and probably all the necessary rate formation in some PDRs.

4.2. Implications for molecular abundances

The equations were rewritten above to compare the derived FUV formation rate with previous works. There is a significant difference between models assuming a surface reaction scheme (either physisorbed or chemisorbed) with models of FUV photolysis. In the seminal equation equilibrating the formation and destruction rates, when H2 is produced by UV photolysis in the bulk of the grains, the dust grain UV flux attenuation appears on both sides of the equation, and thus, the dust extinction equally affects the formation and destruction rate: where RdH2(0) is the unshielded photodissociation rate per H2 for χ = 1, given by ~5 × 10-11 s-1, see, for example Eq. (1) in Habart et al. (2004) and fs(N(H2)) the H2 self-shielding factor at a given N(H2) [cm2] column density from the FUV illuminating source. The molecular H2 abundance is therefore given by with the above numbers. This implies for the very diffuse unshielded ISM that the smallest molecular fraction is with the fraction of interstellar carbon abundance locked into a-C:H. This can be compared with the observations for atomic diffuse clouds, such as high-latitude lines of sights (e.g., Wakker 2006; Richter et al. 2003).

The FUV photolysis of bulk carbonaceous dust grains is not only able to provide the ISM with H2 molecules, but, as shown in our experiments, also delivers small hydrocarbons such as methane to the gas phase. This is accompanied by a network cross-linking and structural modification (e.g., Gadallah et al. 2011). This gas-phase released CH4 will be efficiently photodissociated (photodissociation rate ≈1.2 × 10-9 s-1, van Dishoeck et al. 2006). The small hydrocarbons produced from this methane release will contribute to the abundance observed in the diffuse ISM (e.g., Liszt et al. 2012), a consequence of the erosion of a-C:H grains under the effect of the FUV photolysis.

The steady-state molecular CH4 abundance can be estimated, as for H2, from where RdCH4(0) is the unshielded photodissociation rate per CH4 for χ = 1. This implies for the very diffuse unshielded ISM that the smallest molecular fraction reaches, with , The photoproduced CH4 is released from the a-C:H network at a higher temperature than H2 in the TPD experiments. Given this slower diffusion rate, CH4 may be photodissociated while diffusing in the grain over astrophysical time scales. This matter deserves additional experimental investigation.

5. Conclusion

We have experimentally investigated the VUV photolysis of HACs, analogs of the interstellar a-C:H. We showed by combining IR to mass spectrometry that the FUV irradiation leads to the efficient production of H2 molecules, but also to small hydrocarbons such as CH4, released from the a-C:H film.

The species are produced by VUV photolysis not only at the surface, but principally within the bulk of the a-C:H carbonaceous network and diffuse out at higher temperatures than the purely physisorbed species made by the recombination of H atoms on surfaces. This provides efficient formation in environments where the residence time scale for H atoms prevents an efficient formation on the surface. This mechanism provides high H2 formation rates at low to high grain temperatures.

In an interstellar context where both H atoms and FUV photons are present, this reaction can be viewed as a catalytic production with a repeated atomic H-addition and VUV H-abstraction on the same grains. This photolytic process will progressively erode the carbon network, however, and the carbon loss is not fully reversible because hydrocarbons molecules are produced and ejected in the photochemical process.

At the interface of PDRs, an interesting consequence on the observed rate coefficient is the grain hydrogenation balance driven by how fast the rehydrogenation of the grains proceeds and/or how fast the process is advective, that is processes newly exposed grains, because real PDRs are not in a steady state, as assumed in most models.

The a-C:H material feeds the ISM with small hydrocarbons, which contribute to the formation of small carbonaceous radicals after being dissociated by the UV photons in the considered environment.

We experimentally investigated only the first small hydrocarbon, CH4, produced during the VUV photolysis of a-C:H. UHV experiments are currently conducted to extend the analyses to a quantitative information on heavier species that are released into the gas phase and feed the backend of the ladder for the formation of more complex hydrocarbons or carbon-based radicals that are detected at radio wavelengths.

2

High values for the photoluminescence quantum yields ηPL have been measured in the near UV and visible region for a-C:H films (ηPL = 7% at 250 nm).

3

Dividing by two , to take into account that only half the space receive the photon flux for a semi-infinite cloud and an isotropic impinging radiation field, as explained in Le petit et al. (2006).

Acknowledgments

This work was supported by the ANR COSMISME project, grant ANR-2010-BLAN-0502 of the French Agence Nationale de la Recherche. Part of the equipment used in this work has been financed by the ANR and French INSU-CNRS program “Physique et Chimie du Milieu Interstellaire” (PCMI). G.A.C.D. and G.M.M.C. were financed by Spanish MINECO projects AYA2011-29375 and CONSOLIDER grant CSD 2009-00038. We thank the anonymous referee for the comments that helped to improve the manuscript.

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All Tables

Table 1

Characterization of different irradiated a-C:H, a-C:D and polyethylene films.

In the text

All Figures

thumbnail Fig. 1

Schematic view of the SICAL experiment for VUV irradiation, mainly composed of a vacuum chamber containing several windows for FTIR measurement of the solid sample and VUV irradiation beam and quadrupole mass-spectrometer (QMS) for the detection of volatile species in the gas.
In the text
thumbnail Fig. 2

Emission spectrum of the H2 discharge VUV lamp between 110 and 200 nm at different pressures, through one MgF2 window, which has an cutoff at 115 nm. The spectra were recorded by Cruz-Diaz et al. 2013 under a hydrogen pressure of 0.6 mbar (blue dashed line) and 1.0 mbar (black continuous line). The expected spectrum of our lamp for 0.75 mbar (red continuous line) is the interpolation of these two curves.
In the text
thumbnail Fig. 3

a) Integrated absorption of the double bond in trans-vinylene group (–HC=CH–) arising at 965 cm-1, as a function of dose, for irradiated polyethylene films placed in the substrate holder. The curve is the mean of two independent measurements with the corresponding error bars. b) Corresponding spectra showing the band increasing with fluence over the irradiation dose. These measurements ensure a regular actinometric calibration of the VUV lamp flux (see text for details).
In the text
thumbnail Fig. 4

a) Transmittance spectra of a-C:H films deposited on a ZnSe window at 10 K. b) Transmittance spectra of a-C:D. c) Fits performed on the asymmetric CH3 and CH2 stretching modes. d) Fits performed on the asymmetric CD3 and CD2 stretching modes.
In the text
thumbnail Fig. 5

Optical-depth decrease corresponding to different times of irradiation for a) a-C:H film in the 3100–2700 cm-1 range. b) a-C:D film in the 2300–2000 cm-1 range. Each curve corresponds to an additional irradiation time of 20 min. The asterisk * denotes solid CO contamination around 2133 cm-1.
In the text
thumbnail Fig. 6

Number of destroyed C-H bonds NC − H and C-D bonds NC − D as a function of the photon dose.
In the text
thumbnail Fig. 7

a) TPD spectra of a-C:D film after 18 h of irradiation. D2 released from a-C:D film and the mass m/z = 5 u as background mass channel. b) TPD spectra of a-C:D film after 5 h of irradiation. c) TPD spectra of a-C:H film after 5 h of irradiation. At mass m/z = 4 u the signal is quite negligible and overlaps the background signal.
In the text
thumbnail Fig. 8

Irradiation and TPD spectra of a-C:D film at 100 K (upper panel), 75 K (middle panel), and 50 K (lower panel). The irradiation sequences are shown with the upper bar and the blue curve gives the film temperature. See text for details.

In the text
thumbnail Fig. 9

a) TPD spectra of CD4 released from a-C:D film and the mass m/z = 21 u as background mass channel. b) TPD spectra of a-C:D film after 5 h of irradiation. c) TPD spectra of a-C:H film, the two masses m/z = 20 u, (m/z = 18)/500 and the mass m/z = 21 u as background mass channel.

In the text
thumbnail Fig. 10

Quantum yields of H2 production from a-C:H film, as a function of irradiation time.

In the text

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