A&A 386, 743-747 (2002)
J. B. A. Mitchell1 - C. Rebrion-Rowe1 - J.-L. LeGarrec1 - G. Taupier1 - N. Huby1 - M. Wulff2
1 - Equipe dAstrochimie Expérimentale, P.A.L.M.S. , UMR 6627 du CNRS, Université de Rennes I, 35042 Rennes Cedex, France
2 - European Synchrotron Radiation Facility (ESRF), BP 220, 38043 Grenoble cedex, France
Received 9 November 2001 / Accepted 18 February 2002
X-ray absorption by nanometer sized soot particles in an ethylene flame has been studied using a beam from the ESRF synchrotron. This absorption appears to lead to the destruction of the particles. Application of this phenomenon to the release of molecules from the surface of interstellar dust grains is discussed.
Key words: methods: laboratory - ISM: dust, extinction - X-rays: ISM
Interstellar dust particles can act as active surfaces for the catalytic formation of molecular species. The very cold (10 K) interstellar dust particles can also act as condensation surfaces onto which interstellar molecules will deposit and thus be depleted from the gas phase. A question that remains poorly answered is how molecules once formed or deposited could be released from the surface of the cold dust grains and thus appear as free molecular species that are detected by astronomical observations. A number of workers have performed experimental and theoretical studies of molecule formation and desorption from surfaces (Watson & Salpeter 1972; Léger et al. 1985; Duley 1996) but many uncertainties still remain concerning such processes. One hypothesis is that the energy released by exothermic chemical reactions on the surface of the particles can serve to project the molecules so formed out into space (Hollenbach & Salpeter 1970; Vittadini & Selloni 1995). Since the dust particles are often bombarded by X-rays and cosmic ray particles, capable of penetrating the vast interstellar clouds, it is also natural to assume that there may be processes whereby this energy can be absorbed by the dust and result in molecular desorption. Such processes involve the heating of the dust particles leading to evaporation (Aanestad et al. 1979; Gauger et al. 1990; Voit 1991) or even the triggering of explosive reactions involving reactive radicals that eject condensed species (d'Hendecourt et al. 1982; Shalabiea & Greenberg 1994). The photoemission of electrons from irradiated dust particles has also been examined by astrophysicists (Watson 1972; Verstraete et al. 1990; Dwek & Smith 1996) as it may represent a heating mechanism for interstellar clouds. A recent synchrotron radiation experiment bears direct relevance to this problem and may have identified a new mechanism for the release of molecular species from grain surfaces into the interstellar medium.
The basic idea behind the experiment was to pass an X-ray beam from a synchrotron
through a cylindrically symmetric ethylene diffusion flame and to detect any ionisation
produced by X-ray absorption by soot particles, using an electrically biased wire probe,
located just above where the beam passed through the flame. Ethylene diffusion flames
have been very well characterised and it is known that typical soot particle densities within
particle-rich zones in such flames are of the order of 1011 cm-3 and that their diameters range
from 10-200 nm (Santoro et al. 1983). They represent therefore a very inexpensive system for
the production and study of nano-particles.
The structure of soot particles can be compared with that of carbonacious grains found
in the interstellar medium (Mathis & Whiffen 1988; Dwek 1997; Vaidya et al. 2001). Soot particles, sampled from hydrocarbon flames, have been
subjected to a wide range of chemical and physical analytical techniques including Rayleigh
scattering and visible light depolarisation measurements (Santoro et al. 1983) to determine
particle density, size and gross structure, and scanning (Saito et al. 1991) and transmission
(Vander Wal 1997; Ishiguro 1997) electron microscopy for fine details of the structure. Laser
microprobe mass spectrometry (Dobbins et al. 1996), laser desorption mass spectrometry
(Majidi et al. 1999) and real time mass spectrometry (Reilly et al. 2000) have been used for chemical analysis of the particles. Young soot particles
take the form of spherical particles with diameters of a few nm and
contain a range of aromatic and polyaromatic hydrocarbon compounds. As the particles age in
the flame, they lose hydrogen and the material becomes more graphite-like. The particles
agglomerate into fractal-like forms (Filippov et al. 2000) consisting of small spherical particles connected by
branches, these structures having dimensions from tens of nanometres up to microns in size. It is also possible (Mitchell & Miller 1989) to
dope soot particles by introducing metallic additives into the fuel and so one can produce soot
particles that contain, for example, iron oxide in their cores. One can also create
silicon dioxide and titanium dioxide nanoparticles in flames and in fact that is a major
industrial technique for the manufacture of such species (Pratsinis 1998; Wooldridge 1998).
|Figure 1: Schematic of the X-ray absorption experiment showing the burner assembly that can be moved horizontally and vertically so that the soot density in the flame can be mapped. The electrically biased probe is used to collect the ionisation products formed naturally in the flame and due to ionisation of soot particles and background air molecules by X-ray absorption.|
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|Figure 2: Measured ionisation current to a positive probe at height of 32 mm above the burner. The crosses show current due to X-ray ionisation of background air without flame present. Open triangles and open squares are ionisation current with and without the X-ray beam respectively. The solid circles represent the difference in these two measurements with the contribution due to air absorption subtracted.|
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|Figure 3: Measured ionisation current to a positive probe at height of 2 mm above the burner. Open triangles and open squares are ionisation current with and without the X-ray beam respectively. The solid circles represent the difference in these two measurements with the contribution due to air absorption subtracted.|
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High energy X-ray photons are absorbed primarily by atomic inner-shell electrons
(rather than outer-shell valence electrons as is the case for ultra-violet absorption). The
electrons are ejected from the atom and thus a primary ionisation event occurs that leaves an inner-shell vacancy in the target atom. This vacancy is then filled by an electron
from a higher level either of the target atom or of a neighbouring atom. In this event a high
energy photon can be emitted (fluorescence) or the energy release can lead to the
ejection of another electron from the atom (the Auger effect). If a lower lying electron is
ejected then a second such Auger process can occur etc. This is known as an Auger cascade
though in fact for carbon only one 262.4 eV Auger electron is released (Dwek & Smith 1996).
Since the target atom is located within a solid matrix, the departing primary
photoelectron and the Auger electron must traverse this material in order to escape. One can
determine the distance needed to stop an electron in the material using the range equation
Electron microscopic and mass spectrometric studies (Saito 1991; Ball & Howard 1971; Dobbins et al. 1996; Filippov et al. 2000) of soot particles have shown that they display a fractal-like, agregated structure and at medium heights in the flame they consist of both graphite like and aromatic or polyaromatic materials. It is very likely therefore that if strong electric fields can be induced within such agregated particles by x-ray absorption as discussed above, that this would lead not only to runaway electron and positive ion emission but also to subsequent prompt disruption of the aggregate (Draine & Salpeter 1979; Chang et al. 1987; Ball & Howard 1971; Fruchter et al. 2001). Indeed a series of experiments has been performed by Grün and co-workers (Svestka & Grün 1992; Cermak et al. 1995) in which aggregate particles have been stored in an radiofrequency quadrupole trap and bombarded by ion or electron beams, and these experiments have shown that such fragmentation does occur. Field emission of electrons and ions has also been observed in these experiments.
In order for fragmentation to occur, the electrostatic stress
induced in the particles due to charging must exceed the tensile strength
of the material. (Most of the discussions in the astrophysical literature have used c.g.s units for such comparisons and so we shall also). Known values of tensile strength for relevent materials are
dyne cm-2 for graphite,
106-108 dyne cm-2 for silicates 1992. Pinter et al. (1989) have examined the tensile strength of a synthesized fluffy aggregate material consisting of spherical glass cores (<m diameter) with a hydrocarbon (N-pentacosane) mantle and measured a tensile strength ranging from
106-108 dyne cm-2). The electrostatic stress is given by:
|Figure 4: Schematic of the proposed electron emission process. The absorption of the incoming X-ray yields a primary high energy electron that leaves the particles without further reaction. An Auger electron is also released that makes further collisions with the aggregate yielding secondary electrons some of which also leave the particle. This results in a buildup of positive charge and subsequent high electric fields between the primary particles. These fields induce field emission of tertiary electrons causing further positive charge buildup and eventual Coulomb induced fragmentation of the aggregate.|
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Some fraction of interstellar dust particles are believed to be carbonaceous in form (Mathis et al. 1977; Mathis & Whiffen 1988) and to have irregular structures. In a recent article, Dominik & Tielens (1997) have examined how such structures can be formed. As noted above, such particles are often subject to X-ray irradiation and therefore similar ionisation processes can be expected to occur with these particles as have been seen in the synchrotron radiation experiment discussed here. If these particles are coated with molecular species, either formed in-situ or condensed from the gas phase, then such disruptive phenomena will be very effective in releasing these molecules back into space. We believe that this is the first time that actual experimental evidence for such an X-ray induced disruptive process in free nano-particles has been reported. It should be mentioned that a recent article by Najita et al. (2001) has discussed the thermal release of molecules from the surface of fluffy aggregated grains. This is caused by the spot heating of poorly thermally connected aggregated structures. Again this would be an example of a phenomenon that relies upon the aggregate nature of the particle.
There is other experimental information that shows that small particles can give rise to anomalous electron emission effects. In a series of experiments, Schmidt-Ott and co-workers (Schmidt-Ott et al. 1980; Burtscher et al. 1984, 1985; Müller et al. 1988a) observed a large enhancement of photoemission yields from 5 nm diameter silver particles, irradiated by 10 eV photons. This enhancement could not be explained by photoemission theory (Müller et al. 1988b). Anomalous electron emission has also been seen when magnesium is burned in air (Markstein 1967; Mitchell & Miller 1989) and when granular deposits of magnesium oxide are irradiated by ultra-violet light (Feist 1968). It seems reasonable to suppose that these observations can also be interpreted in terms of a runaway field emission phenomenon though in none of these experiments was the resulting state of the emitting particle studied.
A new apparatus is under construction that will address this point. Experiments are planned where time-of-flight mass spectrometry will be used to determine the masses of the positive particles produced during the absorption process. It is also planned to study the process as a function of incident photon energy, extending the measurements into the vacuum ultra-violet (VUV) region of the spectrum.
The financial support of the European Office of Aerospace Research and Development (EOARD), Air Force Office of Scientific Research (AFOSR) and of the European Synchrotron Radiation Facility, (ESRF) are gratefully acknowledged. Thanks are also due to the European Synchrotron Radiation Facility for granting us the beamtime necessary to perform the experiment and to Daniel Travers and René Jaffré of the University of Rennes for the construction of the apparatus. Useful conversations with Eli Dwek, Bruce Draine and Eberhard Grün are also acknowledged.