Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

Context. Formic acid (HCOOH) is the simplest organic carboxylic acid in chemical synthesis and the signiﬁcant species in interstellar chemistry. HCOOH has been abundantly detected in interstellar ices, dense molecular clouds and star-forming regions. Aims. Laboratory hydrogenation experiments of HCOOH molecules with H atoms were performed with two cryogenic ultra-high vacuum devices on amorphous solid water ices, and highly oriented pyrolytic graphite surfaces. The aim of this work is to study the reactivity of HCOOH molecules with H atoms at low surface temperature 10 K, low surface coverage of one monolayer to three layers, and low H-atom ﬂux of about 3 . 0 × 10 12 moleculecm − 2 s − 1 . Methods. HCOOH and H beams were deposited on cold surfaces held at 10 K, and the condensed ﬁlms were analyzed by in-situ Reﬂection Absorption InfraRed Spectroscopy and temperature programmed desorption (TPD) mass spectrometry technique by heating the sample from 10 to 200 K. Results. Using the temperature programmed during exposure desorption technique, we highlight the possible dimerization of HCOOH molecules at low surface temperatures between 10 and 100 K. In our HCOOH+H experiments, we evaluated a consumption of 20–30% of formic acid by comparing the TPD curves at m/z 46 of pure and H-exposed HCOOH ice. Conclusions. The hydrogenation HCOOH+H reaction is efﬁcient at low surface temperatures. The main products identiﬁed experimentally are carbon dioxide (CO 2 ) and water (H 2 O) molecules. CO bearing species CH 3 OH, and H 2 CO are also detected mainly on graphite surfaces. A chemical surface reaction route for the HCOOH+H system is proposed to explain the product formation.


Introduction
Formic acid (HCOOH), the simplest organic acid, is thought to play a major role both in atmospheric and interstellar environments, as well as human body (Cao et al. 2014) as intermediate in chemical synthesis of organic molecules and pre-biotic species. With its derivative formate group (-(C=O)OH), formic acid is considered as the basis compound for the formation of more complex carboxylic acids (RCOOH) and biomolecules used by life on Earth, including amino acids, glycine, and DNA bases (Pilling et al. 2011). HCOOH is also the smallest organic acid which has been detected in cold dark interstellar clouds L134 N with a relative abundance of 10 −10 with respect to H 2 (Irvine et al. 1990). It has been observed in the gas phase towards high-and low-mass star-forming regions (Woods et al. 1983;Bisschop et al. 2007a;Lefloch et al. 2017), and towards L1544 (Vastel et al. 2014) with a fractional gas-phase abundance [HCOOH]/[H 2 ] = 10 −11 -10 −9 . It has also been observed toward hot molecular cores in Orion Kleinmann-Low (KL) region of star formation at brightness temperatures between 100 and 200 K, where icy mantles undergo sublimation or destruction (Liu et al. 2001). HCOOH has been abundantly identified in interstellar ices before the onset of star formation (Knez et al. 2005;Keane et al. 2001) with abundances of 2-10% with respect to solid H 2 O (Knez et al. 2005). The earlier tentative detection of HCOOH towards the high-mass young stellar object (NGC 7538:IRS9) suggests that formic acid is a general component of ices in the vicinity of embedded high-mass young stellar objects (Schutte et al. 1999). In dense molecular clouds, solid HCOOH is likely mixed with abundant species in the ice like H 2 O, CO, CO 2 , and CH 3 OH. Moreover, formic acid together with methanol (CH 3 OH) and formaldehyde (H 2 CO) are considered to be the most abundant complex molecules detected in protoplanetary disks so far. Recently, Favre et al. (2018) reported the first detection of HCOOH towards the TW Hydrae protoplanetatry disk. The observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have shown the trans-HCOOH 6(1,6)-5(1,5) transition at 129 GHz for the lower energy formic acid conformer (Lopes et al. 2018), and the measurements revealed a disk-average gas phase trans-HCOOH column density of 2-4 × 10 12 cm −2 , as large as that of methanol CH 3 OH. Cis and trans-HCOOH planar isomer conformers refer to the relative position of the two H atoms in the molecule with respect to C-O bond (Szymafiski & Gillan 1996;Lopes et al. 2018). The radioastronomical observations acquired with the IRAM 30 m telescope revealed the detection of HCOOH even in the comet C/2013R1 (Lovejoy) of planetary system (Makochekanwa et al. 2014). A recent astrochemical model of Skouteris et al. (2018) shows a new gas-phase scheme of reactions for the formation of formic acid involving ethanol (CH 3 CH 2 OH) as a parent molecule.
Whereas, the gas-grain chemical simulations of Garrod et al. (2008) assumed that HCOOH is formed by the recombination of HCO and OH radicals on the grain surfaces. This assumption was recently contradicted by the model of Vasyunin et al. (2017), who considered that the reaction between HCO and OH is inefficient because both reactants do not diffuse in the surface at the low temperatures of prestellar cores. The authors thus suggest that HCOOH is formed through the radiative association reaction HCO + +H 2 O, and then accumulated in the ice via gas accretion. Formic acid is predominantly formed in dense interstellar clouds through surface reactions on grains. In the laboratory, HCOOH has been formed on cold surfaces (Ioppolo et al. 2011) by the co-deposition of H atoms and CO:O 2 mixtures through the hydrogenation of the HO-CO complex. This can confirm the presence of formic acid in dense cold clouds, at the beginning of the warm-up phase of a protostar. Pure HCOOH ice as well as HCOOH-H 2 O and HCOOH-CH 3 OH ice matrices have been widely characterized by infrared spectroscopy in previous works (Bisschop et al. 2007a;Bennett et al. 2011). Whereas HCOOH dimers and their different cis and trans conformers were recently studied experimentally by infrared spectroscopy in nitrogen matrix, and theoretically by abinitio calculations (Lopes et al. 2018). The heat of dimerization of formic acid by FTIR, reported in an early work (Henderson 1987) is ∆H D = (−48.0 ± 0.4) kJ mol −1 . The reactivity of formic acid with neutral atomic hydrogen was studied theoretically (Cao et al. 2014) and experimentally (Bisschop et al. 2007b;Cao et al. 2014) in the literature. Cao et al. (2014) have studied the reaction HCOOH + H in a rare-gas Kr matrix and characterized the isolated radical products using infrared spectroscopy and ab-initio quantum chemical calculations. Their theoretical studies have suggested three possible channels for the HCOOH + H reaction: (i) reaction with the oxygen atom of the hydroxyl group (-OH) of HCOOH leading to H 2 O formation with HCO radical, (ii) reaction with the oxygen atom of the carbonyl group (-C=O) leading to the formation of diols HC(OH) 2 , and (iii) reaction with the carbon atom of carbon-hydrogen group (-CH) leading to the formation of the radical H 2 COOH. See reactions (1), (2), and (3), respectively: The computational energetic predictions revealed, firstly, the formation of an intermediate radical trans-H 2 COOH (m/z 47) through the reaction (3), which converts to the more stable radical isomer trans-cis-HC(OH) 2 diol via hydrogen atom tunneling on a timescale of hours at 4.3 K. This simplest and lowest energetic diol, identified in their work as the final product, is expected to be very reactive, and further reaction channels are of particular interest. Previous cryogenic laboratory experiments performed in Leiden group of H-atom addition to pure HCOOH ice at 15 K on cold surfaces (Bisschop et al. 2007b) did not result in detectable reaction products within the experimental sensitivity of their set-up. In their work performed in the multilayer regime, the authors exposed thick films of pure HCOOH ice of 20 layers and a mixed (HCOOH+H 2 O) film of 40 layers on golden substrate with H-fluence of 7 × 10 14 molecule cm −2 s −1 .
In this paper, we experimentally study the reactivity of HCOOH molecules adsorbed on graphite and amorphous water ice surfaces with a typical H atom-flux of 3.0 × 10 12 molecule cm −2 s −1 , at low surface temperatures of 10 K and low surface coverage of HCOOH ice varying from one to three layers. We used two cryogenic setups with different configurations of molecular and atomic beam lines for the deposition of the reactants on the cold surfaces. The two setups allow us to study the effect of various substrates (gold, carbonaceous graphite and water ice) on the reaction HCOOH+H by using different experimental methods for the identification of the species adsorbed and desorbed from the surfaces, mainly Reflection Absorption InfraRed Spectroscopy (RAIRS), temperature programmed desorption (TPD), and temperature programmed during exposure desorption (TP-DED). This paper is organized as follows: Sect. 2 explains the experimental methods; Sect. 3 focuses on the data analysis for the dimerization and the reactivity of HCOOH with H atoms; Sect. 4 is open for the discussion of the experimental results; and Sect. 5 presents the astrophysical implications and summarizes the main conclusions of this work.

Experimental
The experiments were performed with the FORmation of MOLecules in the InterStellar Medium (FORMOLISM) and Vers de NoUvelles Syntheses (VENUS) apparatus located in LERMA-Cergy laboratory. The setups are dedicated to study the reactivity of molecules and atoms on surfaces of astrophysical interest as well as the formation of simple and complex organic molecules, under temperature and pressure conditions similar to those encountered in the ISM. FORMOLISM apparatus is welldescribed in previous papers (Amiaud et al. 2007;Congiu et al. 2012). It is composed of an ultra-high vacuum chamber (UHV) stainless steel chamber with a base pressure lower than 10 −11 mbar. The sample holder located in the main chamber is thermally connected to a cold finger of a closed-cycle He cryostat. It is made of a 1-cm-diameter copper block covered with a highly oriented pyrolytic graphite (HOPG, ZYA-grade) slab. The temperature of the sample is measured in the range of 10-700 K by a calibrated silicon diode clamped to the sample holder. Before starting the experiments, the surface of the graphite substrate is heated from 10 to 700 K with a small heating rate of 2 K per minute by maintaining the pressure in the main UHV chamber close to 10 −9 -10 −10 mbar. The surface is then cooled down to 10 K for the exposure of the reactants. This operation is practiced to clean the surface of the graphite substrate from any volatile contaminants (CO, CO 2 , CH 4 ...) that might be trapped in the defects and modify the structure of the surface. This allows as to have reproducible profiles for the infrared and the TPD spectra of the studied molecules. The UHV chamber of FORMOLISM set-up is also equipped with an amovable quadrupole mass spectrometer (QMS) of mass spectra ranging from 1 to 100 atomic mass unit (AMU) with a Channeltron detector. It can be rotated and placed in front of each beam line to characterize the chemical gas composition of the HCOOH and the H beams. It can also be faced to the surface of the sample holder at 1 cm distance to perform TPD measurements. During the warm-up phase of the sample from 10 to 200 K, the species desorbing from the surface into the gas phase are detected with the QMS by their mass-to-charge ratio (m/z). The VENUS setup described in recent papers (Minissale et al. 2019;Nguyen et al. 2018) is composed of a main UHV chamber with a low pressure of 10 −10 mbar, where a gold-coated copper sample holder is placed in the central position of the main chamber and thermally connected to a Helium liquid closed cycle cryocooler. This main chamber itself is linked to an intermediate UHV chamber through five diaphragms of 3 mm in diameter materializing the direction of five beam-lines aimed to the center of the gold surface. Like FORMOLISM, VENUS setup is equipped with a QMS and a Fourier transform infrared (FTIR) spectrometer in RAIR configuration using an external mercury cadnium tellurite (MCT) detector cooled with liquid nitrogen. Contrarily to the FOR-MOLISM setup, the VENUS setup has the advantage of being able to record the infrared spectra in real time during the exposure of the surface to the molecular and atomic beams without moving the sample holder. The chemical components used in this work are liquids HCOOH (Sigma Aldrich Fisher Scientific 98% purity) and H 2 O (deionized), which are introduced in pyrex flasks. The H-beam is obtained through the dissociation of H 2 beam with a microwave source. The hydrogen atomic plasma is generated through the microwave discharge within the Surfatron cavity operating at 2.45 GHz. The hydrogen beam dissociation rate is measured by the QMS from H 2 signals during the discharge (ON) and the discharge (OFF) of the microwave source. The value of the dissociation rate varies between 60 and 75%, with an effective microwave power of 60 Watt. The flux of Hatoms coming from the gas phase and hitting the surface is measured from the so-called King and Wells method described in the reference (Amiaud et al. 2007).The estimated H-flux is (3.7 ± 0.5) × 10 12 atom cm −2 s −1 .
For the HCOOH beam, the flux of formic acid molecules is derived from TPD data, and defined as the amount of solid HCOOH that saturates the surface per unit time. The HCOOHflux value is estimated to 2.6 × 10 12 molecule cm −2 s −1 . The chemical composition of the HCOOH beam is checked by placing the QMS in front of the HCOOH beam line. The main cracking pattern ion fragments of gaseous HCOOH are m/z 46 (HCOOH + , 26%), m/z 45 (HCOO + , 17%), m/z 44 (COO + , 4%), m/z 29 (HCO + , 20%), m/z 28 (CO + , 10%), m/z 18 (H 2 O + , 13%). During the characterization of the HCOOH beam, the QMS did not detect masses higher than m/z 46.
Amorphous solid water (ASW) ice is grown on top of the gold (or graphite) surface maintained at 110 K, by spraying water vapor from a micro-channel array doser located inside the ultrahigh vacuum chamber (UHV). The water vapor was obtained from deionized liquid water purified by several pumping cycles under cryogenic vacuum (Noble et al. 2012). The thickness of the water ice film is 10 layers, where one monolayer unit is defined as 1 ML = 10 15 molecule cm −2 . For the HCOOH+H experiments, we firstly deposited one to three layers of HCOOH on the surface held at 10 K, and then we exposed the film of formic acid to H beam at the same surface temperature for several exposure times. For comparison, co-deposition experiments of HCOOH and H beams on cold surfaces are also performed using two beam lines simultaneously. After the deposition phase of HCOOH or HCOOH + H films, an infrared spectra were recorded in the midinfrared region (4000-400 cm −1 or 2.5-25 µm) with a spectral resolution of 4 cm −1 and an average number of 400 scans. The TPD method is performed in the 10-200 K temperature range with a linear heating rate of 0.2 K s −1 . We also performed TP-DED experiments to study the dimerization of HCOOH on the surface of the sample holder using FORMOLISM setup. The TP-DED technique is well-described in Minissale et al. (2016a), and consists of exposing the sample to the molecular beam (here HCOOH) while slowly reducing the sample temperature from a higher temperature, where the molecules remain in the gas phase, to a lower surface temperature, where they stick on the surface. In these experiments, the QMS (see Fig. 1a) was placed close to the surface in the lower position, between the direction of the beam line and that of the surface, at a specified orientation angle of about 55 • relative to the normal to the sample holder, in order to collect the maximum amount of formic acid species released from the surface during the exposure of the surface to the HCOOH beam. The species promptly reflected from the surface, or desorbed into the gas phase after thermalization, were continuously measured by the QMS during the linear cooling of the sample from 180 to 100 K, using a small heating rate of 0.033 K s −1 . A4, page 3 of 12 A&A 636, A4 (2020)

Dimerization of HCOOH on the gold surface
Figure 1, panel b shows the TP-DED experiments in the 180-100 K range of m/z 45, 46, and 47 during the exposure of a gold sample to the HCOOH beam. For surface temperatures ranging between 180 and 160 K, formic acid molecules (detected at m/z 45 HCOO + and m/z 46 HCOOH + ) do not stick on the surface, promptly returning to the gas phase. For temperatures varying from 160 to 145 K, the signal at m/z 46 decreases indicating that HCOOH starts to stick efficiently on the surface. On the other hand, m/z 47 starts to increase. Such signal is likely result of the HCOOH dimers desorption that is cracked in the QMS head (H 2 COOH + , m/z 47). At 145-125 K, the signal of mass m/z 47 reaches a maximum of 80 count s −1 , while those of m/z 46 and m/z 45 remain at 30 count s −1 . As the temperature of the surface decreases to 100 K, the signals of m/z 47, m/z 46, and m/z 45 drop and reach the minimum values, because both monomers and dimers of formic acid molecules condensate on the surface, and their adsorption rate exceeds that of their desorption.  Figure 2 shows the cycle complex structure (HCOOH − HCOOH) of formic acid dimer, which can be formed on the surface through the association of two adsorbed HCOOH molecules linked together by double hydrogen bond (-O...H) modeling the noncovalent interaction between two HCOOH molecules. The average hydrogen bonding (-O...H) energy is 20 kJ mol −1 or 0.2 eV. A control experiment was performed by rotating the QMS face to the beam line of HCOOH in order to analyze the chemical composition of the HCOOH beam, by measuring the signals of the cracking patterns of the species coming from the beam. In this case, no signals of m/z 47 and other higher masses are provided by the QMS, meaning that the dimerization of HCOOH cannot occur in the beam at room temperature for very low HCOOH vapor pressure of 10 −5 -10 −8 mbar. The sharp peak in Fig. 1, panel b observed for masses 47, 46 and 45 of formic acid at a specific surface temperature 123 K has not been observed in the signals of other masses (44, 32, 31, 29, 28, 18, and 17) used in this experiment. In addition, a similar TP-DED experiment of HCOOH performed on a graphite (HOPG) substrate showed similar sharp peaks for the same masses 47, 46 and 45 at the same range of temperature 123-127 K. This leads us to consider that this peak is not a noise or a parasite signal caused by uncommon dysfunction of the QMS, but a real signal of the maximum desorption of formic acid dimers upon formation on the surface at 123 K from the adsorbed neighboring monomers. This means that the residence time of the newly formed dimer (HCOOH-HCOOH) molecules on the surface at 123 K remains shorter than the lifetime of the adsorbed HCOOH monomers, probably because of the local heat released from the exothermic dimerization of HCOOH on the surface (Henderson 1987). The dimerization excess energy would briefly heat the sample (Hellebust et al. 2007) at this surface temperature and provoke  the abrupt desorption of the amorphous solid formic acid dimers into the gas phase.

Adsorption-desorption of HCOOH on the gold surface
The TPD family curves of pure HCOOH pre-deposited on gold surface at 10 K are shown in Fig. 3, panels a and b for m/z 47 and m/z 46, respectively. For the exposure doses lower than 1 ML, the profiles of the TPD peaks of m/z 46 and m/z 47 are different. The desorption curves of HCOOH (m/z 46) present a large peak at 140-180 K corresponding to the sub-monolayer desorption regime of HCOOH monomer (cis and trans) molecules from the energetic adsorption sites of the golden surface. As the exposure dose increases from 0.5 ML to 1 ML, a second TPD peak grows at 138 K, and continues to increase for 1.3 ML. This peak corresponds to the multilayer desorption regime of HCOOH ice. TPD curves of m/z 47 follow an order zero, where the maximum of the desorption peaks shifts to the lower temperatures from 145 to 138 K with the increasing surface exposure dose of formic acid from 0.5 to 1.3 ML. In Fig. 3c, we see the evolution of the integrated areas below the TPD curves with the HCOOH exposure time. The data are more important for m/z 47 than 46 when the adsorbed formic acid surface density increases and the thickness of the ice grows, favoring therefore the association of HCOOH monomers into dimers.
Using the TPD data of HCOOH on gold surface for m/z 46 and the set of independant Polanyi-Wigner equations described in the paper (Chaabouni et al. 2018), we estimated the desorption energy distribution of HCOOH on gold surface for HCOOH exposure dose N = 1.3 ML. Figure. 3d shows the desorption energy distribution E i (in kJ mol −1 ) of HCOOH as a function of the surface population N i (in ML) of formic acid molecules adsorbed on the gold substrate. The surface exposure dose N (in ML) of HCOOH molecules is the sum of the surface fraction exposure doses N i (N = n i=1 N i = N 1 + N i ...).The desorption parameters (E i , N i ) are calculated using the Arrhenius desorption rate Eq. (4): where N i is the surface population of HCOOH molecules desorbing at a surface temperature T (in K) with a desorption energy barrier E i (in kJ mol −1 ), A is the pre-exponential factor for desorption (in s −1 ), and k B is the Boltzmann constant (k B = 1.38 × 10 −23 J K −1 ). The desorption energy distribution (45-65 kJ mol −1 ) of HCOOH from gold surface shown in Fig. 3d is calculated with the best-fit pre-exponential factor A =1 × 10 18 s −1 of the m/z 46 TPD data. This range of desorption energies includes the value 55 kJ mol −1 (or 6615 K) reported in the work of Burke et al. (2015) for the desorption energy of carboxylic acid (RCOOH) from graphite substrate using a pre-exponential factor 8 × 10 17 s −1 . We note that the best-fit preexponential factor (A) for desorption of HCOOH found in this work is similar to that of formamide (NH 2 (C = O)H) molecules calculated from graphite surface in a previous paper (Chaabouni et al. 2018). The high value of A is likely to be related to the chemical carbonyl group (-C = O) of these complex organic molecules.

HCOOH+H system on graphite (HOPG) surface
The reactivity of HCOOH with H atoms has been performed in the laboratory on the graphite (HOPG) surface using the FOR-MOLISM setup and two beam lines for HCOOH and H atoms. The experiments consist firstly of depositing 2 ML of HCOOH ice on the HOPG surface held at 10 K, and then exposing the film to the atomic hydrogen beam for 50 min at the same surface temperature. Figure 4 compares the RAIR spectra of the solid HCOOH deposited on the HOPG surface before and after H atom addition. The RAIR spectrum a of Fig. 4 shows the IR absorption bands of pure (2 ML) solid HCOOH on HOPG, mainly at 1740 cm −1 for the stretching vibration mode ν S (C = O), 1397 cm −1 for the antisymmetric deformation modes δ(OH) or δ(CH), 1237 cm −1 for the stretching vibration mode ν S (C-O), and 1080 cm −1 for the bending mode ν B (H-C-O) out of plane representing molecular vibrations oscillating perpendicular to the plane of the formic acid molecule, and occurring perpendicularly to the substrate surface (Bisschop et al. 2007a;Hellebust et al. 2007). These IR measurements describe the adsorption of HCOOH in its monomer and dimer forms. We observed a large IR band centering at 3200 cm −1 which corresponds to the OH stretching vibration mode ν S (O-H) of pure HCOOH and some H 2 O molecules that come from the liquid aqueous phase. The presence of water is confirmed by the small IR band at 1620 cm −1 for the bending vibration mode ν B (HOH). The RAIR spectrum b in Fig. 4 shows the significant decrease of the IR band at 1080 cm −1 to a greater than 80%, and the growth of new infrared absorption bands, mainly at 2342 cm −1 , 1535 cm −1 , and 1029 cm −1 . The IR band at 2342 cm −1 is attributed to the asymmetrical stretching vibration bond ν S (C-O) of CO 2 molecules, which at 1535 cm −1 is attributed to ν S (CH 2 ) of H 2 CO (Bouilloud et al. 2015), and the band at 1029 cm −1 to the stretching vibration mode ν S (C-O) of CH 3 OH molecules (Martín-Doménech et al. 2014). These new species are likely to be formed by the reaction of HCOOH molecules with H atoms on the graphite surface at 10 K. However, the spectrum b does not show a noticeable decrease in the intensity of the main IR band of HCOOH at 1740 cm −1 after 50 min of H atom addition to HCOOH film, probably because the ν S (C = O) mode of HCOOH ice not consumed by H atoms overlaps with the ν S (C = O) mode of H 2 CO molecules newly formed by the reaction HCOOH+H. Moreover, the widening of the IR band at 1740 cm −1 of the HCOOH in the spectrum b can be explained by the formation of HCOOH-H 2 O complex between the HCOOH molecules adsorbed on the surface and the neighboring H 2 O molecules formed by the reaction HCOOH+H on the surface. These newly formed H 2 O molecules are identified by their absorption bending mode ν B (HOH) at about 1680 cm −1 (Bisschop et al. 2007a;Bouilloud et al. 2015) as a shoulder in the main absorption band of HCOOH. In the same spectrum b of Fig. 4, we observe an increase in the intensity of the large ν S (O-H) band at 3200-3500 cm −1 , which may correspond to HCOOH ice mixed with CH 3 OH and H 2 O newly formed on the surface. The RAIR spectra of pure HCOOH and HCOOH+H films in Fig. 4 show a very small IR feature at 2149 cm −1 with very low absorbance value (<10 −4 ), which may be attributed to background gaseous CO degassing from the cryocooler, and then accumulated on the surface during the deposition phase of HCOOH at 10 K. The intensity of this IR peak in spectrum b of Fig. 4 does not change significantly after H-atom bombardment of the HCOOH film. The infrared wave numbers of HCOOH and HCOOH+H systems found in this work on the graphite surface are summarized in Table 1. Figure 5 shows the TPD curves of some masses for pure HCOOH ice (in black) and HCOOH+H system (in red) on HOPG surface. In Fig. 5a, we observe a decrease in the TPD peak of the HCOOH multilayer (m/z 46) centering at 159 K and a shift to 148 K after H-exposure, probably due to presence of water ice on the surface during the exposure phase. In parallel, in Fig. 5b we see the growth of a desorption peak at 82 K for the film of HCOOH+H, which may correspond to the desorption of CO 2 molecules formed by the reaction of HCOOH with H on the surface. The growth of the desorption of CO 2 correlates with A4, page 5 of 12  Fig. 4. RAIRS spectra of HCOOH and HCOOH + H films on HOPG surface using two beam lines. (a) RAIR spectrum of 2 ML HCOOH ice on graphite (HOPG) surface at 10 K; (b) RAIR spectrum of (2 ML HCOOH + 50 min H atoms) system on graphite (HOPG) surface at 10 K.
the small IR absorption band at 2342 cm −1 observed in the RAIR spectrum b of Fig. 4 at 10 K for the HCOOH+H film. In addition, Fig. 5 panels c and d show the growth of two TPD peaks centering at 143 K for masses m/z 32 and m/z 30, which are likely to be assigned to CH 3 OH and H 2 CO formed by the reaction of HCOOH+H, respectively. In Fig. 5e, we observe the growth of a TPD peak of mass m/z 28 at 45 K after H atom addition to the HCOOH film, which may correspond to CO molecules formed by the reaction HCOOH + H and mixed with CO contaminant coming from the cryocooler. Figure 5f shows the growth of a TPD peak at 143 K for mass m/z 18, which likely to be attributed to H 2 O ice formed by the HCOOH+H surface reaction. It is noted that similar HCOOH+H experiments (not shown in this work) have been performed on a bare gold surface, firstly by depositing two layers of HCOOH ice at 10 K and then adding H atoms for 45 min using tow beam lines. QMS-TPD results have shown a significant decrease in the intensities of the TPD peaks of masses m/z 47 and m/z 46 of HCOOH by 40 and 60%, respectively after H beam addition to HCOOH pre-deposited film. However, despite the efficient consumption of HCOOH by H atoms on the gold surface, no detectable products except m/z 18 for H 2 O molecules have been observed by TPD measurements in these experiments. The lack of products in these experiments can be explained by the effect of the chemical desorption of the products upon formation on the gold surface through the exothermic surface reactions.

HCOOH+H system on ASW ice surface
We performed similar HCOOH+H experiments on ASW ice surfaces. Figure 6 displays three RAIR spectra of HCOOH and HCOOH+H systems on an ASW ice: a an IR spectrum of pure 3 ML HCOOH ice on ASW ice surface at 10 K, b an IR spectrum of 3 ML HCOOH co-deposited simultaneously with H atoms during 11 min at 10 K, and c an IR spectrum with additional exposure time of H atoms for 120 min on the pre-deposited (HCOOH+H) film at 10 K. The RAIR spectrum a shows the absorption bands of pure HCOOH ice on the ASW ice surface at positions: 1070,1216,1394,1729,2589,2743,2934, and 3218 cm −1 . The labels and the vibrational modes of these IR peaks are given in Table 1. In this spectrum, we see a small redshift of the IR band at 1729 cm −1 with respect to that observed previously on graphite at 1740 cm −1 (Fig. 4, spectrum a). In the RAIR spectrum b of Fig. 6, we see the growth of a very small IR peak at 2345 cm −1 during the first 11 min of HCOOH and H co-deposition on water ice surface at 10 K. This IR band that continues to grow with the additional H-exposure time for 120 min in the spectrum c is attributed to the stretching vibrational mode of CO 2 likely formed by the reaction HCOOH+H.
The IR spectrum c also shows the disappearance of the peak at 1070 cm −1 for the bending (HCO) mode of HCOOH, and even the broadening and the increasing of the IR band at 1729 cm −1 . The broadening of the band could be affected by the bending mode ν B (HOH) of H 2 O at about 1655 cm −1 , and the ν S (CH 2 ) mode of H 2 CO at about 1535 cm −1 (Bouilloud et al. 2015), which may be formed by the reaction HCOOH+H on the surface. The increase of the 1729 cm −1 IR band can be explained by the overlapping of the C = O stretching bond of H 2 CO molecules (Bennett et al. 2011) with that of unreacted HCOOH. Spectrum c also shows the growth of two infrared bands at 1020 cm −1 and 3218 cm −1 with the increasing time of H atoms on the surface. The thin infrared absorption band at 1020 cm −1 is attributed to ν S (CO) mode of CH 3 OH molecules (Bennett & Kaiser 2007), while the large infrared band at 3218 cm −1 is attributed to the stretching vibration mode OH of H 2 O molecules mixed with HCOOH and CH 3 OH species. In this experiment performed on ASW ice, RAIR spectra of pure HCOOH and HCOOH+H films on ASW ice in Fig. 6 don't show a clear IR band at around 2140 cm −1 for CO ice, but a large absorption band centered on 1997 cm −1 , which increases with H atom addition to HCOOH ice. This unexpected broad IR feature observed in RAIR spectra Notes. v S refers to the symmetric and antisymmetric stretching mode, δ refers to antisymmetric deformation mode, v B refers to the bending mode (Bisschop et al. 2007b).
References. a, b and c of Fig. 6 and increasing with H atom addition can result from the interaction of HCOOH with the water molecules of the substrate, according to the reference (Hellebust et al. 2007). In their work, the authors indeed observed a broad 2100-1900 cm −1 feature centered on 2000 cm −1 in their RAIR spectrum of thick mixed amorphous HCOOH and water ices at 110 K deposited on gold substrate. They attributed this IR absorption band to the presence of a small amount of oxonium H 3 O + and formate HCOO − ions, indicating the ionization of the acid in a solid water matrix and not on the ice surface. For the authors, the ionization of formic acid is also evidenced by low frequency shoulder on the ν S (OH) mode of water band at 3220 cm −1 . The infrared wave numbers observed for HCOOH+H system on ASW ice are given in Table 1. Figure 7 shows the TPD curves of 3 ML HCOOH pure ice (in black) and after exposure to H atoms (in red) on amorphous water ice. Figure 7, panels b and c show a 30% decrease in the intensities of the HCOOH multilayer TPD peaks of m/z 47 and m/z 46 at 147 K for HCOOH + H system with comparison to TPD peaks of pure HCOOH ice. This decrease indicates the consumption of HCOOH species by H atoms from the water ice surface. Whereas, TPD peaks at 158 K correspond to the fraction of formic acid co-desorbing with the crystalline water ice substrate. In parallel, we see the growth of a new TPD peak at 85 K for mass m/z 44 corresponding to CO 2 molecules likely formed by the reaction of HCOOH with H atoms, as has been previously observed by RAIR spectroscopy in Fig. 6, spectrum c.
A4, page 7 of 12 A&A 636, A4 (2020) However, contrary to the previous experiment performed on graphite surface (Sect. 3.3.1), TPD curves of HCOOH+H on water ice did not show clear growth in the peaks of m/z 32, m/z 31 and m/z 30 at 120-150 K, corresponding to CO-bearing species (H 3 COH, H 3 CO, and H 2 CO). In this TPD experiment on ASW ice, we did not observe a new TPD peak of m/z 32 at 120 K-150 K, which could correlate with the IR band at 1020 cm −1 , observed in the spectrum c of Fig. 6. Moreover, we observe the growth of small TPD peaks at 137 and 150 K for m/z 30 of the system (2 ML HCOOH +130 min H) in Fig. 6e, which may correspond to the cracking pattern H 2 CO + . In addition, we observe the increase of two TPD peaks of m/z 48 at 148 K and 158 K in Fig. 7, panel a after H addition to the HCOOH film, which may correspond to the desorption of diol (H 2 C(OH) 2 ), m/z 48, formed by the reaction of HCOOH 2 radicals (m/z 47) with H atoms both on the surface and in the bulk of the water ice film. Figure 8 shows the kinetic evolution of HCOOH and CO 2 present on the surface as a function of the co-deposition time of HCOOH and H atoms on the amorphous water ice surface held at 10 K. These data correspond to the integrated areas below the RAIRS bands of HCOOH ice at 1729 cm −1 and that of CO 2 ice at 2345 cm −1 , measured during the simultaneous deposition phase of HCOOH and H on the ASW ice substrate. The experimental data of CO 2 are scaled by a factor of 10 to be observed in Fig. 8. We see the exponential growth of the HCOOH film and the slow increase in the amount of CO 2 on the surface with the exposure time of HCOOH and H beams during 55-min periods.

Kinetic of (HCOOH + H) on ASW ice
These kinetics indicate the slow hydrogenation reaction of solid HCOOH by H atoms on the water ice at low surface temperature 10 K, probably caused by the high activation energy barriers of the hydrogenation reactions.

Discussion
Our present experiments performed in the sub-monolayer regime of 1-3 ML showed that the reactivity of HCOOH with H atoms at low surface temperature (10 K) depends on the substrate used in this work (graphite HOPG and ASW ice). The main products formed by the reaction HCOOH+H on the graphite surface, and observed by RAIRS and TPD are CO, H 2 CO, CH 3 OH, and H 2 O. While in the case of the ASW ice surface, the main product formation identified by RAIRS and TPD is CO 2 . Our results contradict the experimental results of the Leiden group (Bisschop et al. 2007b) for the inefficient hydrogenation reaction of HCOOH with H atoms in the multilayer regime. In their work (Bisschop et al. 2007b), the authors bombarded thick (20 ML) pure HCOOH films covering gold substrate at surface temperature 12 K, using two H-fluxes of 7.8 × 10 13 atom cm −2 s −1 and 5.2 × 10 14 atom cm −2 s −1 , which are 10-100 orders of magnitude higher than our current fluxes. The hydrogenations were performed under UHV conditions in which less than a monolayer of background gas is accreted during the timescale of the experiments. In their two difference RAIR spectra for v S (C = O) stretch, obtained between pure HCOOH and HCOOH+H ices spectra, recorded at two H atom deposition temperatures of 12 K and 40 K, they observed a small decrease (<0.1 ML) on the blue side of the stretching mode at 1710 cm −1 of HCOOH (seen at 1750 cm −1 ) and an increase of a peak at 1730 cm −1 . They explained their IR observations by a slight change in the profile of the HCOOH band due to some restructuring of the HCOOH ice surface by H bombardment. Because of the lack of new IR bands at 1500 cm −1 , in their difference spectra, they did not attribute the band at 1730 cm −1 to v S (C = O) mode of H 2 CO species. They also considered the measured low level of H 2 CO formation observed in their experiment to have originated from the hydrogenation of background gaseous CO that is accreted on the surface during the experiments. Their CO contamination resulting from the degassing of the metal parts, was observed by an increase in the TPD signal of mass 28. In addition, in their TPD hydrogenation experiments, the authors did not detect masses 48 (CH 2 (OH) 2 ), 32 (CH 3 OH), 31 (H 3 CO) and 30 (H 2 CO), and thus confirmed the inefficient reaction of HCOOH+H in the ice at 12 K. The nondetection of CH 2 (OH) 2 in their experiment disagrees with our TPD results performed on water ice in the sub-monolayer coverage, clearly showing the desorption peak of mass 48 both from pure HCOOH and (HCOOH+H) systems at 148 K. This desorption peak is likely attributed in our experiments to the cracking pattern HCOOH 2 + of HCOOH dimers in the head of the QMS for pure HCOOH ice, and to the reaction product (CH 2 (OH) 2 ) of H atom with the energetically stable radical HCOOH 2 * expected to be formed on the water ice. The authors also confirmed the non efficiency of formic acid hydrogenation by the high classical activation barrier (2.4 eV) of the HCOOH+2H → H 2 CO+H 2 O reaction in the ice, but they did not check the efficiency of the hydrogenation reaction by tunneling process using the isotopic species rich in a deuterium element. In both sub-monolayer and multilayer regimes, the hydrogenation reaction of HCOOH competes with the H+H barrierless recombination reaction leading to H 2 formation (Amiaud et al. 2007). In the work of the Leiden group (Bisschop et al. 2007b), the H atom mobility is likely to be reduced in the bulk of the ice, making hydrogenation reactions less efficient. While in our work, H atom diffusion on the surface sites is favored by the substrate of HOPG, which can itself be partially hydrogenated by the impinging hydrogen atoms. In this case, the presence of H 2 molecules already adsorbed on the surface of HOPG at 10 K may increase the sticking coefficient and the residence time of hydrogen atoms on the surface (Amiaud et al. 2007), favoring therefore its reaction either with another adsorbed H atom or with a neighboring condensed HCOOH molecule. In our experiments, we evaluated a consumption of 20-30% of formic acid by comparing the TPDs curves at m/z 46 of pure and H-exposed HCOOH ice. Our RAIR spectra show that the peak at 1080 cm −1 (1070 cm −1 ) in spectrum a of Fig. 4 and Fig. 6 is significantly reduced after H atom bombardment. This peak attributed to the bending vibration mode ν B (HCO) of HCOOH molecules is the only peak that disappears in RAIR data, probably because it does not overlap with any vibration modes of other molecules (CH 3 OH, H 2 CO, CO, H 2 O, and CO 2 ) observed in these experiments, contrary to the stretching mode v S (C = O) at 1740 cm −1 (1729 cm −1 ) of HCOOH, which overlaps with that of H 2 CO at 1730 cm −1 , and even with ν B (HOH) mode of H 2 O at about 1655 cm −1 .
Here, we propose a chemical routes for the reaction HCOOH+H to explain the products formation observed in this work.