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D− ion desorption from condensed CD4, C2D2, C2D4, C2D6 and C3D8 molecules induced by electron impact
ARTICLE IN PRESS
Radiation Physics and Chemistry 68 (2003) 215–219
D ion desorption from condensed CD4; C2 D2; C2D4 ; C2D6 and C3D8 molecules induced by electron impact Pawe" Mo’zejko, Luc Parenteau, Andrew D. Bass, Le! on Sanche* Groupe des Instituts de Recherche en Sant!e du Canada en Sciences des Radiations, Facult!e de M!edecine, Universit!e de Sherbrooke, 3001, 12 Avenue Nord, Sherbrooke, Que., Canada J1H5N4
Abstract The electron stimulated desorption of D anions from deuterated hydrocarbons condensed on a platinum substrate has been studied experimentally. D anion yield functions are reported for incident electron energies ranging from 0 to 20 eV: For each molecule studied, the yield function exhibits a single dissociative attachment resonance at approximately 10 eV: Above 15 eV; a linearly increasing D ion signal due to dipolar dissociation was recorded. No heavier polyatomic anion fragments were observed to desorb. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Electron-stimulated desorption (ESD); Dissociative electron attachment (DEA); Electron collisions; Electron scattering; Condensed molecules; Molecular solids; Dipolar dissociation (DD)
1. Introduction Processes initiated by low-energy electron impact, including the desorption of anions from molecular solids (i.e. multilayer (ML) films of molecular gases condensed onto a metallic substrate) have a practical importance for many research fields including plasma physics, radiation physics, physics of biomolecules and astrophysics (Sanche, 1992). For example, recent experiments indicate that electron scattering via transient anions provides an efficient pathway for secondary electron damage to DNA (Boudaiffa et al., 2000) while dissociative electron attachment (DEA) to chlorofluorocarbons condensed on ice crystals in the Earth’s atmosphere, provides a plausible contributory mechanism to ozone depletion (Lu and Sanche, 2001). In the gas phase, DEA (Oster et al., 1989; Hashemi et al., 1990) is an efficient decay channel for transient negative ions (TNI) formed by attachment of low-energy (0–20 eV) electrons i.e., e þ M-M * -X þ Y :
ð1Þ
*Corresponding author. Tel.: +1-819-3461110; fax: +1-8195645442. E-mail address: [email protected] (L. Sanche).
In the DEA process, the TNI, M * ; is created by temporary capture of the incident electron in an unoccupied molecular orbital. Attachment of the electron leads to the distortion of the molecule and to modification of particular molecular bonds; if the lifetime of the TNI is sufficiently long and its configuration dissociative in the Franck-Condon region, dissociation of the anion can occur forming both a stable anionic, X ; and neutral, Y ; fragment. At higher electron energies (usually above 15 eV) production of negative ions may proceed via dipolar dissociation (DD): e ðEÞ þ M-ðMÞn þ e ðE 0 Þ-X þ Y þ þ e ðE 0 Þ:
ð2Þ
In the above nonresonant process the electronically excited neutral intermediate state M n fragments by dipolar dissociation, into the anion X and cation Y þ : As in the gas phase, the creation of negative ions by electron impact on condensed molecules occurs mainly via DEA and DD processes (Sanche, 1990, 1991, 1993; Illenberger, 1994). However for DEA occurring in the condensed phase, the transient anion state is often affected by the surrounding medium i.e. the neighboring molecules and surface (Huels et al., 1994). These perturbations can alter the energy and lifetime of the TNI, the energy of the dissociation limits, relative
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00285-8
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probability of decay via allowed channels and the dynamics of the overall process. Moreover, it was shown that the magnitude of the desorption yields from condensed molecules strongly depended on the morphology of the substrate (Huels et al., 1994; Simpson et al., 1997; Azria et al., 1999; Bass et al., 2000, 2001). Electron stimulated desorption from condensed Cn H2nþ2 ðn ¼ 0; 2; 4; 5; 6Þ molecules has been studied by Sanche and Parenteau (Sanche and Parenteau, 1987) for electron beam energies ranging from 5 to 20 eV: Anion yield production by low-energy electron impact on some simple saturated (Cn H2nþ2 ; n ¼ 1–9) and unsaturated (Cn H2n ; n ¼ 2–4) hydrocarbons in the condensed phase has also been studied (Rowntree et al., 1991b). It was found (Sanche and Parenteau, 1987; Rowntree et al., 1991b) that the dominant dissociation channel for the intermediate anion states is H elimination. The larger anion fragments, CH ; CH 2 and CH 3 were also observed but their abundance, in comparison with H ; was much lower. Electron stimulated desorption of D ions from C2 D6 and C6 D6 condensed on the platinum foil has been studied by Rowntree et al. (Rowntree et al., 1993) with the same apparatus as used in the present study. The aim of the present work is to study electron stimulated desorption of negative ions from thin solid films made of some simple saturated and unsaturated deuterated hydrocarbons. While one may expect that the mechanism and resonant states involved in ions ESD from condensed deuterated hydrocarbons films are very similar to those observed for nondeuterated hydrocarbons, isotope effects must also be considered. In the case of condensed molecules, isotope effects can be intrinsic due to different stabilization times for H and D ions, and extrinsic due to the different escape velocities relative to the substrate for desorbed H and D ions. The effect of the intrinsic characterization upon isotope substitution on negative ion production, has been observed in gas phase experiments for H2 ; HD and D2 molecules (Rapp et al., 1965; Schulz and Asundi, 1965) and CH4 and CD4 (Sharp and Dowell, 1967). From a practical point of view, the study of the D desorption rather than H allows us to eliminate from the ESD signal, hydrogen anions originating from water, which is a common vacuum contaminant and facilitates the further study of electron attachment to deuterated molecules on substrates containing hydrogen (such as water (Mo’zejko et al., 2003)).
2. Experimental The apparatus and experimental procedure used in the reported experiment were described in detail elsewhere (e.g. Huels et al., 1994; Rowntree et al., 1994). Briefly the multilayer molecular films are condensed on a
clean polycrystalline, cryogenically cooled ðT ¼ 20 KÞ thin platinum foil. This foil is cleaned by resistive heating. The film thickness is determined by a volumetric dosing procedure (Sanche, 1979) with an uncertainty in the thickness of no more than 50%. The thin films are bombarded with a monoenergetic ðFWHM ¼ 80 meVÞ electron beam (0–20 eV) produced by a hemispherical electron monochromator. The electron beam of about 1 nA current strikes the target surface at an angle of 70 to the normal. The absolute energy scale of the incident electrons is determined to within 70:3 eV with respect to the vacuum level, by observing the onset of current transmission to the platinum metal as a function of electron energy. The desorbed anions are mass selected with a quadruple mass spectrometer, which is positioned at 20 from the surface normal, and detected by an electron multiplier. The measured ion yield functions contain an experimental uncertainty of about 12%. The apparatus is housed in an ultra high vacuum chamber maintained at a background pressure of 1010 Torr by a combination of a ion pump and closed-cycle refrigerated cryopump. The residual magnetic field in the region of electron and ion optics as well as the electron monochromator and interaction region is reduced to less than 15 mG using a double m-metal shield. We used pure sample gases from Cambridge Isotope Laboratories with a minimum isotopic purity of 99% for deuterated methane ðCD4 Þ and acetylene ðC2 D2 Þ and 98% in the case of deuterated ethylene ðC2 D4 Þ; ethane ðC2 D6 Þ; and propane ðC3 D8 Þ; respectively.
3. Results Fig. 1 displays the ESD D yields for 5–6 multilayers of the studied deuterated hydrocarbons. The energies of the observed resonant features, as well as peak widths are listed in Table 1, where they are compared with values for the corresponding nondeuterated hydrocarbon molecules. Generally the shape of the ESD signal is very similar for each of the studied compounds. Each contains a single strong broad resonant maximum which is centered around 10 eV for CD4 and C2 D6 ; 9 eV for C2 D2 ; and 9.7 and 9:9 eV for C2 D4 and C3 D8 molecules, respectively. The onset for D formation starts at about 7 eV for each compound which is very close to those observed for nondeuterated hydrocarbons. Based on experimental investigations of DEA to small nondeuterated (Sharp and Dowell, 1967; Von Trepka and Neuert, 1963; Azria and Fiquet-Fayard, 1972; Rutkowsky et al., 1980; Dressler and Allan, 1987; Andric! and Hall, 1988) and deuterated (Sharp and Dowell, 1967; Azria and Fiquet-Fayard, 1972) hydrocarbons in the gas phase, we can state that formation of D from condensed saturated deuterated hydrocarbons at low
ARTICLE IN PRESS P. Mo’zejko et al. / Radiation Physics and Chemistry 68 (2003) 215–219 16000
14000
−
D ionyield[counts]
12000
5ML CD4
10000
8000
6ML C2D6
6000
6ML C3D8
4000
x4
5ML C2D4
2000
x4
5ML C2D2 0 0
5
10
15
20
Electron energy [eV] Fig. 1. Comparison of D signal desorbed from condensed CD4 ; C2 D2 ; C2 D4 ; C2 D6 and C3 D8 molecules on Pt substrate. Table 1 Resonance energies and DA peak width (in eV) for D and H anions desorbed from deuterated and nondeuterated hydrocarbon films by electron impact Parent Energy molecule
FWHM
Parent Energy molecule
CD4 C2 D 2 C2 D 4 C2 D 6 C3 D 8
2.5 1.9 3.1 2.3 2.3
CH4
10.8a
3.4a
C2 H 4 C2 H 6
9.0a 9.9a
3.3a 2.4a
a
10.1 9.2 9.7 10.0 9.9
FWHM
Rowntree et al. (1991b).
energies occurs via the dissociation of a core-excited anion state of Rydberg character, created by electron capture into an empty molecular orbital involving the sn symmetry (Rowntree et al., 1991a):
* -D þ C D e ð> 6 eVÞ þ Cn D2nþ2 -Cn D2nþ2 n 2nþ1 :
ð3Þ
A similar situation occurs in the case of condensed unsaturated deuterated hydrocarbons (Dance and Walker, 1974; Van Veen and Plantenga, 1976):
e ð> 6 eVÞ þ Cn D2n -Cn D2n* -D þ Cn D2n1 :
ð4Þ
The lowest shape resonance in acetylene, observed around 2:5 eV in gas phase experiments and corresponding to the capture of an electron into the first empty
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orbital, pg (Azria and Fiquet-Fayard, 1972; Dressler and Allan, 1987; Andri!c and Hall, 1988) was not seen in the present study. It is very likely that any C2 D and D ions produced from the decay of this resonant state in the gas phase (Azria and Fiquet-Fayard, 1972) would have insufficient kinetic energy to escape the surface. The similar shape resonance, observed for ethylene at 1:7 eV (Sanche and Schulz, 1973), does not lead to negative-ion formation through dissociative attachment but rather decays via autodetachment alone (Von Trepka and Neuert, 1963). Hence this state cannot be seen in the present data. The D anion yields recorded in the present experiments are generally similar to those observed for H desorption from condensed nondeuterated hydrocarbons (Rowntree et al., 1991b). Unfortunately, direct comparison of the intensity of D anion yields with those of H is impossible, due to the different transmission of H and D in the mass spectrometer and the different electron beams densities and apparatus arrangements used in the two sets of experiments. It is clear, however that for deuterated hydrocarbons, due to the intrinsic isotope effect, the heavier mass and slower motion of the D component requires a longer time for stabilization of the negative ion which provides a greater opportunity for electron autodetachment than in nondeuterated hydrocarbons. Therefore one may expect that formation of D should be smaller than H : Moreover, it is noticeable that yield distributions for unsaturated hydrocarbons are very similar in both form and intensity to those observed for the saturated hydrocarbon species while in the case of deuterated molecules a drop in the yield signal intensity is observed. Some differences in resonant peak positions and peak widths between data for D and H desorption can also be seen. The most intense D anion yield is observed for CD4 molecules, those for C2 D6 and C3 D8 being less intense at the resonant maximum than for the CD4 molecule by factors 1.25 and 2.2, respectively. Desorption of the D from C2 D4 and C2 D2 is much less intense than for the saturated deuterated hydrocarbons. At the peak maximum, anion yields are at least 6 and 10 times lower for deuterated ethylene and acetylene, respectively, than for the CD4 molecule. This drop in the intensity of the D anion yield can be partially attributable to the competition between complementary channels in TNI decay in the DEA to hydrocarbons (Von Trepka and Neuert, 1963). Furthermore, it was observed (Rutkowsky et al., 1980) that the cross section for dissociative attachment, sDA obeys following inequality sDA ðCH4 Þ > sDA ðC2 H6 Þ > sDA ðC2 H2 Þ > sDA ðC3 H8 Þ > sDA ðC2 H4 Þ; which suggests that electron attachment to hydrocarbons depends not only on electron energy, but also on molecular size and slightly on the degree of saturation. In comparison with nondeuterated methane,
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the resonance peak position for the CD4 molecule is shifted towards lower energies by about 0:6 eV: The energy of the C2 D6 maximum is close to that for C2 H6 while for the C2 D4 molecule is shifted towards higher energies by 0:7 eV relative to C2 H4 : Thus, if it is assumed that the energy of the resonance in corresponding deuterated and nondeuterated hydrocarbons is almost the same, due to their near identical electronic structure, it is evident that in the case of D anions desorbed from C2 D2 and C2 D4 molecules, larger total kinetic energy release in dissociation is required for desorption than in the case of H anions. Such extrinsic isotope effects can be responsible for observed differences in negative ion yields intensities. It is worth noting that in the case of condensed molecules, desorption of an ionic fragment can strongly depend on the orientation of the uppermost molecules which can also be reflected in the observed yield intensities. The widths of recorded resonant peaks for deuterated hydrocarbons are slightly narrower than those observed for nondeuterated hydrocarbons. Since in their ground vibrational state, deuterated hydrocarbons possess a shorter C–D distance than the nondeuterated molecules, their Franck–Condon width is narrower. Thus the region for allowed Franck–Condon transitions (which create the TNI) is narrower in the case of DEA to deuterated molecules and is reflected in the observed width of the D resonant peaks. For films of deuterated acetylene, ðC2 D2 Þ two additional, weak, resonant-like features are present at 12 and 15 eV: In gas phase experiments (Dance and Walker, 1974; Van Veen and Plantenga, 1976) some higher Rydberg core excited resonant states were observed for acetylene molecule, and these may be involved in DEA production of D anion. It is also possible that these additional peaks are due to multiple electron collisions in the film through which the projectile loses a portion of its kinetic energy and then attaches to a molecule. Further work is required to discriminate between these possible mechanisms. The gas phase experiments on DEA to nondeuterated (Von Trepka and Neuert, 1963; Azria and FiquetFayard, 1972; Rutkowsky et al., 1980; Dressler and Allan, 1987) and deuterated (Azria and Fiquet-Fayard, 1972) hydrocarbons report, in addition to H and D creation, the observation of complementary decay channels of the TNI. These channels yield CH and CH 2 and CH3 from saturated hydrocarbons and C2 H (C2 D ) anion from unsaturated hydrocarbons. However, and contrary to our measurements for nondeuterated hydrocarbons, no heavier polyatomic anion fragments are observed to desorb in the present experiments. Nevertheless, it is worth noting that in measurements for nondeuterated condensed hydrocarbons (Rowntree et al., 1991b), the resonant desorption of CH ; CH 2 ; CH3 and C2 H while observed, repre-
sented less than 1% of the intensity of the H signal. Elementary considerations show that in the partitioning of the total kinetic energy during dissociation, large fragment anions are created with low kinetic energy, often lower than that required to escape the charge/ image-charge induced polarization of the film (Sanche, 1991). Therefore the desorption signal for polyatomic ions is generally weaker than for atomic ions. In the case of deuterated hydrocarbons, one may expect that production of heavier polyatomic anion fragments is additionally less efficient with comparison with nondeuterated hydrocarbons due to the intrinsic isotope effect. It was found in the gas phase experiment (Sharp and Dowell, 1967) that the cross section for methylene, CH 2 anion production is 100 times higher than for production of CD 2 (Sharp and Dowell, 1967). This is attributable to the fact that the dissociation process requires more time for heavier isotopes (longer stabilization time of the electron on a fragment) so that even the yield of negative atomic ions is lower for the heavier isotopes (Rapp et al., 1965; Schulz and Asundi, 1965). It is therefore not surprising to find that in the condensed phase the desorption of polyatomic anions from deuterated hydrocarbons is completely suppressed since in this case both intrinsic and extrinsic effects reduce the D yields. Above 15 eV for C2 D4 ; 17 eV for C2 D2 ; C2 D6 and C3 D8 ; and 19 eV for CD4 molecules, a linearly increasing D ion signal was recorded. This latter is strictly connected with the dipolar dissociation process (Sanche, 1991).
4. Summary Formation of the D ion from electron interaction with condensed deuterated hydrocarbon molecules occurs via dissociative electron attachment and dipolar dissociation processes. For each of the studied compounds a single resonant maximum was observed. The energy of these core-excited resonances are very close to those observed for nondeuterated condensed hydrocarbons as well as to those observed in the gas phase experiments. No desorbing heavier polyatomic ions were observed due to partitioning of the kinetic energy and isotope effects in production of negative ions.
Acknowledgements This work is supported by the Canadian Institutes of Health Research.
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References Andri!c, L., Hall, R.I., 1988. Resonance phenomena observed in electron scattering from acetylene. J. Phys. B 21 (2), 355–366. Azria, R., Fiquet-Fayard, F., 1972. Attachement e! lectronique dissociatif sur C2 H2 et C2 D2 : J. Phys. (Paris) 33 (7), 663–667. Azria, R., Le Coat, Y., Lachgar, M., Tronc, M., Parenteau, L., Sanche, L., 1999. Effects of morphology in electronstimulated desorption: O from O2 condensed on D2 O films grown at 15–150 K on Pt. Surf. Sci. 436 (1–3), L671–L676. Bass, A.D., Parenteau, L., Weik, F., Sanche, L., 2000. The effects of temperature and morphology on electron transmission and stimulated desorption of H from thin hydrocarbon films. J. Chem. Phys. 113 (9), 8746–8752. Bass, A.D., Parenteau, L., Weik, F., Sanche, L., 2001. Effects of morphology on the low-energy electron stimulated desorption of O from O2 deposited on benzene and water ices. J. Chem. Phys. 115 (10), 4811–4818. Boudaiffa, B., Cloutier, P., Hunting, D., Huels, M.A., Sanche, L., 2000. Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons. Science 287, 1658–1660. Dance, D.F., Walker, I.C., 1974. Threshold electron energy-loss spectra for some simple alkynes. J. Chem. Soc. Faraday Trans. II 70, 1426–1434. Dressler, R., Allan, M., 1987. A dissociative electron attachment, electron transmission, and electron energy-loss study of the temporary negative ion of acetylene. J. Chem. Phys. 87 (8), 4510–4518. Hashemi, R., Kuhn, . A., Illenberger, E., 1990. Electron capture induced processes in molecules and molecular aggregates. Int. J. Mass Spectrom. Ion Process. 100, 753–784. Huels, M.A., Parenteau, L., Sanche, L., 1994. Substrate dependence of electron-stimulated O yields from dissociative electron attachment to physisorbed O2 : J. Chem. Phys. 100 (5), 3940–3956. Illenberger, E., 1994. Anion formation in low energy electron impact to gaseous and condensed molecules. In: Christophorou, L.G., Illenberger, E., Schmidt, W.-F. (Eds.), Linking the Gaseous and Condensed Phases of Matter. Plenum Press, New York, pp. 49–72. Lu, Q.-B., Sanche, L., 2001. Effects of cosmic rays on atmospheric chlorofluorocarbon dissociation and ozone depletion. Phys. Rev. Lett. 87(7), 078501-1–078501-4. Mo’zejko, P., Parenteau, L., Bass, A.D., Sanche, L., 2003. To be published. Oster, T., Kuhn, . A., Illenberger, E., 1989. Gas phase negative ion chemistry. Int. J. Mass Spectrom. Ion Process. 89 (1), 1–72. Rapp, D., Sharp, T.E., Briglia, D.D., 1965. Large isotope effect in the formation of H or D by electron impact on H2 ; HD, and D2 : Phys. Rev. Lett. 14 (14), 533–535. Rowntree, P., Parenteau, L., Sanche, L., 1991a. Dielectric polarization invariance in dissociative electron attachment
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from condensed saturated hydrocarbons. J. Phys. Chem. 95 (2), 523–524. Rowntree, P., Parenteau, L., Sanche, L., 1991b. Anion yields produced by low-energy electron impact on condensed hydrocarbon films. J. Phys. Chem. 95 (12), 4902–4909. Rowntree, P., Sambe, H., Parenteau, L., Sanche, L., 1993. Formation of anionic excitations in the rare-gas solids and their coupling to dissociative states of adsorbed molecules. Phys. Rev. B 47 (8), 4537–4554. Rowntree, P., Sanche, L., Parenteau, L., Meinke, M., Weik, F., Illenberger, E., 1994. Dissociative electron attachment to condensed and adsorbed halomethanes. J. Chem. Phys. 101 (5), 4248–4259. Rutkowsky, J., Drost, H., Spangenberg, H.J., 1980. Untersuchung der unelastischen Wchselwirkung langsamer monoenergetischer Elektronen mit einfachen KohlenwasserstoffMolekulen. . Ann. Phys. (Leipzig) 37 (4), 259–270. Sanche, L., 1979. Transmission of 0–15 eV monoenergetic electrons through thin-film molecular solids. J. Chem. Phys. 71 (12), 4860–4882. Sanche, L., 1990. Low-energy electron scattering from molecules on surfaces. J. Phys. B 23 (10), 1597–1624. Sanche, L., 1991. Low energy electron stimulated desorption from condensed molecules. Comments At. Mol. Phys. 26 (6), 321–332. Sanche, L., 1992. Dissociative attachment and surface reactions induced by low-energy electrons. J. Vac. Sci. Technol. B 10 (1), 196–200. Sanche, L., 1993. Surface reactions and desorption by electron attachment. In: Burns, A.R., Stechel, E.B., Jennison, D.R. (Eds.), Desorption Induced by Electron transitions DIET V. Springer, Berlin, Heidelberg, pp. 3–14. Sanche, L., Parenteau, L., 1987. Ion-molecule surface reactions induced by slow (5–20 eV) electrons. Phys. Rev. Lett. 59 (1), 136–139. Sanche, L., Schulz, G.J., 1973. Electron transmission spectroscopy: resonances in triatomic molecules and hydrocarbons. J. Chem. Phys. 58 (2), 479–493. Schulz, G.J., Asundi, R.K., 1965. Formation of H by electron impact on H2 at low energy. Phys. Rev. Lett. 15 (25), 946–949. Sharp, T.E., Dowell, J.T., 1967. Isotope effects in dissociative attachment of electrons in methane. J. Chem. Phys. 46 (4), 1530–1531. Simpson, W.C., Sieger, M.T., Orlando, T.M., Parenteau, L., Nagesha, K., Sanche, L., 1997. Dissociative electron attachment in nanoscale ice films: temperature and morphology effects. J. Chem. Phys. 107 (20), 8668–8677. Van Veen, E.H., Plantenga, F.L., 1976. Low-energy electronimpact excitation spectra of acetylene. Chem. Phys. Lett. 38 (3), 493–497. . Von Trepka, L.V., Neuert, H., 1963. Uber die Entstehung von negativen Ionen aus einigen Kohlenwasserstoffen und Alkoholen durch ElektronenstoX. Z. Naturforschg. 18a, 1295–1303.
Radiation Physics and Chemistry 68 (2003) 215–219
D ion desorption from condensed CD4; C2 D2; C2D4 ; C2D6 and C3D8 molecules induced by electron impact Pawe" Mo’zejko, Luc Parenteau, Andrew D. Bass, Le! on Sanche* Groupe des Instituts de Recherche en Sant!e du Canada en Sciences des Radiations, Facult!e de M!edecine, Universit!e de Sherbrooke, 3001, 12 Avenue Nord, Sherbrooke, Que., Canada J1H5N4
Abstract The electron stimulated desorption of D anions from deuterated hydrocarbons condensed on a platinum substrate has been studied experimentally. D anion yield functions are reported for incident electron energies ranging from 0 to 20 eV: For each molecule studied, the yield function exhibits a single dissociative attachment resonance at approximately 10 eV: Above 15 eV; a linearly increasing D ion signal due to dipolar dissociation was recorded. No heavier polyatomic anion fragments were observed to desorb. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Electron-stimulated desorption (ESD); Dissociative electron attachment (DEA); Electron collisions; Electron scattering; Condensed molecules; Molecular solids; Dipolar dissociation (DD)
1. Introduction Processes initiated by low-energy electron impact, including the desorption of anions from molecular solids (i.e. multilayer (ML) films of molecular gases condensed onto a metallic substrate) have a practical importance for many research fields including plasma physics, radiation physics, physics of biomolecules and astrophysics (Sanche, 1992). For example, recent experiments indicate that electron scattering via transient anions provides an efficient pathway for secondary electron damage to DNA (Boudaiffa et al., 2000) while dissociative electron attachment (DEA) to chlorofluorocarbons condensed on ice crystals in the Earth’s atmosphere, provides a plausible contributory mechanism to ozone depletion (Lu and Sanche, 2001). In the gas phase, DEA (Oster et al., 1989; Hashemi et al., 1990) is an efficient decay channel for transient negative ions (TNI) formed by attachment of low-energy (0–20 eV) electrons i.e., e þ M-M * -X þ Y :
ð1Þ
*Corresponding author. Tel.: +1-819-3461110; fax: +1-8195645442. E-mail address: [email protected] (L. Sanche).
In the DEA process, the TNI, M * ; is created by temporary capture of the incident electron in an unoccupied molecular orbital. Attachment of the electron leads to the distortion of the molecule and to modification of particular molecular bonds; if the lifetime of the TNI is sufficiently long and its configuration dissociative in the Franck-Condon region, dissociation of the anion can occur forming both a stable anionic, X ; and neutral, Y ; fragment. At higher electron energies (usually above 15 eV) production of negative ions may proceed via dipolar dissociation (DD): e ðEÞ þ M-ðMÞn þ e ðE 0 Þ-X þ Y þ þ e ðE 0 Þ:
ð2Þ
In the above nonresonant process the electronically excited neutral intermediate state M n fragments by dipolar dissociation, into the anion X and cation Y þ : As in the gas phase, the creation of negative ions by electron impact on condensed molecules occurs mainly via DEA and DD processes (Sanche, 1990, 1991, 1993; Illenberger, 1994). However for DEA occurring in the condensed phase, the transient anion state is often affected by the surrounding medium i.e. the neighboring molecules and surface (Huels et al., 1994). These perturbations can alter the energy and lifetime of the TNI, the energy of the dissociation limits, relative
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00285-8
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probability of decay via allowed channels and the dynamics of the overall process. Moreover, it was shown that the magnitude of the desorption yields from condensed molecules strongly depended on the morphology of the substrate (Huels et al., 1994; Simpson et al., 1997; Azria et al., 1999; Bass et al., 2000, 2001). Electron stimulated desorption from condensed Cn H2nþ2 ðn ¼ 0; 2; 4; 5; 6Þ molecules has been studied by Sanche and Parenteau (Sanche and Parenteau, 1987) for electron beam energies ranging from 5 to 20 eV: Anion yield production by low-energy electron impact on some simple saturated (Cn H2nþ2 ; n ¼ 1–9) and unsaturated (Cn H2n ; n ¼ 2–4) hydrocarbons in the condensed phase has also been studied (Rowntree et al., 1991b). It was found (Sanche and Parenteau, 1987; Rowntree et al., 1991b) that the dominant dissociation channel for the intermediate anion states is H elimination. The larger anion fragments, CH ; CH 2 and CH 3 were also observed but their abundance, in comparison with H ; was much lower. Electron stimulated desorption of D ions from C2 D6 and C6 D6 condensed on the platinum foil has been studied by Rowntree et al. (Rowntree et al., 1993) with the same apparatus as used in the present study. The aim of the present work is to study electron stimulated desorption of negative ions from thin solid films made of some simple saturated and unsaturated deuterated hydrocarbons. While one may expect that the mechanism and resonant states involved in ions ESD from condensed deuterated hydrocarbons films are very similar to those observed for nondeuterated hydrocarbons, isotope effects must also be considered. In the case of condensed molecules, isotope effects can be intrinsic due to different stabilization times for H and D ions, and extrinsic due to the different escape velocities relative to the substrate for desorbed H and D ions. The effect of the intrinsic characterization upon isotope substitution on negative ion production, has been observed in gas phase experiments for H2 ; HD and D2 molecules (Rapp et al., 1965; Schulz and Asundi, 1965) and CH4 and CD4 (Sharp and Dowell, 1967). From a practical point of view, the study of the D desorption rather than H allows us to eliminate from the ESD signal, hydrogen anions originating from water, which is a common vacuum contaminant and facilitates the further study of electron attachment to deuterated molecules on substrates containing hydrogen (such as water (Mo’zejko et al., 2003)).
2. Experimental The apparatus and experimental procedure used in the reported experiment were described in detail elsewhere (e.g. Huels et al., 1994; Rowntree et al., 1994). Briefly the multilayer molecular films are condensed on a
clean polycrystalline, cryogenically cooled ðT ¼ 20 KÞ thin platinum foil. This foil is cleaned by resistive heating. The film thickness is determined by a volumetric dosing procedure (Sanche, 1979) with an uncertainty in the thickness of no more than 50%. The thin films are bombarded with a monoenergetic ðFWHM ¼ 80 meVÞ electron beam (0–20 eV) produced by a hemispherical electron monochromator. The electron beam of about 1 nA current strikes the target surface at an angle of 70 to the normal. The absolute energy scale of the incident electrons is determined to within 70:3 eV with respect to the vacuum level, by observing the onset of current transmission to the platinum metal as a function of electron energy. The desorbed anions are mass selected with a quadruple mass spectrometer, which is positioned at 20 from the surface normal, and detected by an electron multiplier. The measured ion yield functions contain an experimental uncertainty of about 12%. The apparatus is housed in an ultra high vacuum chamber maintained at a background pressure of 1010 Torr by a combination of a ion pump and closed-cycle refrigerated cryopump. The residual magnetic field in the region of electron and ion optics as well as the electron monochromator and interaction region is reduced to less than 15 mG using a double m-metal shield. We used pure sample gases from Cambridge Isotope Laboratories with a minimum isotopic purity of 99% for deuterated methane ðCD4 Þ and acetylene ðC2 D2 Þ and 98% in the case of deuterated ethylene ðC2 D4 Þ; ethane ðC2 D6 Þ; and propane ðC3 D8 Þ; respectively.
3. Results Fig. 1 displays the ESD D yields for 5–6 multilayers of the studied deuterated hydrocarbons. The energies of the observed resonant features, as well as peak widths are listed in Table 1, where they are compared with values for the corresponding nondeuterated hydrocarbon molecules. Generally the shape of the ESD signal is very similar for each of the studied compounds. Each contains a single strong broad resonant maximum which is centered around 10 eV for CD4 and C2 D6 ; 9 eV for C2 D2 ; and 9.7 and 9:9 eV for C2 D4 and C3 D8 molecules, respectively. The onset for D formation starts at about 7 eV for each compound which is very close to those observed for nondeuterated hydrocarbons. Based on experimental investigations of DEA to small nondeuterated (Sharp and Dowell, 1967; Von Trepka and Neuert, 1963; Azria and Fiquet-Fayard, 1972; Rutkowsky et al., 1980; Dressler and Allan, 1987; Andric! and Hall, 1988) and deuterated (Sharp and Dowell, 1967; Azria and Fiquet-Fayard, 1972) hydrocarbons in the gas phase, we can state that formation of D from condensed saturated deuterated hydrocarbons at low
ARTICLE IN PRESS P. Mo’zejko et al. / Radiation Physics and Chemistry 68 (2003) 215–219 16000
14000
−
D ionyield[counts]
12000
5ML CD4
10000
8000
6ML C2D6
6000
6ML C3D8
4000
x4
5ML C2D4
2000
x4
5ML C2D2 0 0
5
10
15
20
Electron energy [eV] Fig. 1. Comparison of D signal desorbed from condensed CD4 ; C2 D2 ; C2 D4 ; C2 D6 and C3 D8 molecules on Pt substrate. Table 1 Resonance energies and DA peak width (in eV) for D and H anions desorbed from deuterated and nondeuterated hydrocarbon films by electron impact Parent Energy molecule
FWHM
Parent Energy molecule
CD4 C2 D 2 C2 D 4 C2 D 6 C3 D 8
2.5 1.9 3.1 2.3 2.3
CH4
10.8a
3.4a
C2 H 4 C2 H 6
9.0a 9.9a
3.3a 2.4a
a
10.1 9.2 9.7 10.0 9.9
FWHM
Rowntree et al. (1991b).
energies occurs via the dissociation of a core-excited anion state of Rydberg character, created by electron capture into an empty molecular orbital involving the sn symmetry (Rowntree et al., 1991a):
* -D þ C D e ð> 6 eVÞ þ Cn D2nþ2 -Cn D2nþ2 n 2nþ1 :
ð3Þ
A similar situation occurs in the case of condensed unsaturated deuterated hydrocarbons (Dance and Walker, 1974; Van Veen and Plantenga, 1976):
e ð> 6 eVÞ þ Cn D2n -Cn D2n* -D þ Cn D2n1 :
ð4Þ
The lowest shape resonance in acetylene, observed around 2:5 eV in gas phase experiments and corresponding to the capture of an electron into the first empty
217
orbital, pg (Azria and Fiquet-Fayard, 1972; Dressler and Allan, 1987; Andri!c and Hall, 1988) was not seen in the present study. It is very likely that any C2 D and D ions produced from the decay of this resonant state in the gas phase (Azria and Fiquet-Fayard, 1972) would have insufficient kinetic energy to escape the surface. The similar shape resonance, observed for ethylene at 1:7 eV (Sanche and Schulz, 1973), does not lead to negative-ion formation through dissociative attachment but rather decays via autodetachment alone (Von Trepka and Neuert, 1963). Hence this state cannot be seen in the present data. The D anion yields recorded in the present experiments are generally similar to those observed for H desorption from condensed nondeuterated hydrocarbons (Rowntree et al., 1991b). Unfortunately, direct comparison of the intensity of D anion yields with those of H is impossible, due to the different transmission of H and D in the mass spectrometer and the different electron beams densities and apparatus arrangements used in the two sets of experiments. It is clear, however that for deuterated hydrocarbons, due to the intrinsic isotope effect, the heavier mass and slower motion of the D component requires a longer time for stabilization of the negative ion which provides a greater opportunity for electron autodetachment than in nondeuterated hydrocarbons. Therefore one may expect that formation of D should be smaller than H : Moreover, it is noticeable that yield distributions for unsaturated hydrocarbons are very similar in both form and intensity to those observed for the saturated hydrocarbon species while in the case of deuterated molecules a drop in the yield signal intensity is observed. Some differences in resonant peak positions and peak widths between data for D and H desorption can also be seen. The most intense D anion yield is observed for CD4 molecules, those for C2 D6 and C3 D8 being less intense at the resonant maximum than for the CD4 molecule by factors 1.25 and 2.2, respectively. Desorption of the D from C2 D4 and C2 D2 is much less intense than for the saturated deuterated hydrocarbons. At the peak maximum, anion yields are at least 6 and 10 times lower for deuterated ethylene and acetylene, respectively, than for the CD4 molecule. This drop in the intensity of the D anion yield can be partially attributable to the competition between complementary channels in TNI decay in the DEA to hydrocarbons (Von Trepka and Neuert, 1963). Furthermore, it was observed (Rutkowsky et al., 1980) that the cross section for dissociative attachment, sDA obeys following inequality sDA ðCH4 Þ > sDA ðC2 H6 Þ > sDA ðC2 H2 Þ > sDA ðC3 H8 Þ > sDA ðC2 H4 Þ; which suggests that electron attachment to hydrocarbons depends not only on electron energy, but also on molecular size and slightly on the degree of saturation. In comparison with nondeuterated methane,
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the resonance peak position for the CD4 molecule is shifted towards lower energies by about 0:6 eV: The energy of the C2 D6 maximum is close to that for C2 H6 while for the C2 D4 molecule is shifted towards higher energies by 0:7 eV relative to C2 H4 : Thus, if it is assumed that the energy of the resonance in corresponding deuterated and nondeuterated hydrocarbons is almost the same, due to their near identical electronic structure, it is evident that in the case of D anions desorbed from C2 D2 and C2 D4 molecules, larger total kinetic energy release in dissociation is required for desorption than in the case of H anions. Such extrinsic isotope effects can be responsible for observed differences in negative ion yields intensities. It is worth noting that in the case of condensed molecules, desorption of an ionic fragment can strongly depend on the orientation of the uppermost molecules which can also be reflected in the observed yield intensities. The widths of recorded resonant peaks for deuterated hydrocarbons are slightly narrower than those observed for nondeuterated hydrocarbons. Since in their ground vibrational state, deuterated hydrocarbons possess a shorter C–D distance than the nondeuterated molecules, their Franck–Condon width is narrower. Thus the region for allowed Franck–Condon transitions (which create the TNI) is narrower in the case of DEA to deuterated molecules and is reflected in the observed width of the D resonant peaks. For films of deuterated acetylene, ðC2 D2 Þ two additional, weak, resonant-like features are present at 12 and 15 eV: In gas phase experiments (Dance and Walker, 1974; Van Veen and Plantenga, 1976) some higher Rydberg core excited resonant states were observed for acetylene molecule, and these may be involved in DEA production of D anion. It is also possible that these additional peaks are due to multiple electron collisions in the film through which the projectile loses a portion of its kinetic energy and then attaches to a molecule. Further work is required to discriminate between these possible mechanisms. The gas phase experiments on DEA to nondeuterated (Von Trepka and Neuert, 1963; Azria and FiquetFayard, 1972; Rutkowsky et al., 1980; Dressler and Allan, 1987) and deuterated (Azria and Fiquet-Fayard, 1972) hydrocarbons report, in addition to H and D creation, the observation of complementary decay channels of the TNI. These channels yield CH and CH 2 and CH3 from saturated hydrocarbons and C2 H (C2 D ) anion from unsaturated hydrocarbons. However, and contrary to our measurements for nondeuterated hydrocarbons, no heavier polyatomic anion fragments are observed to desorb in the present experiments. Nevertheless, it is worth noting that in measurements for nondeuterated condensed hydrocarbons (Rowntree et al., 1991b), the resonant desorption of CH ; CH 2 ; CH3 and C2 H while observed, repre-
sented less than 1% of the intensity of the H signal. Elementary considerations show that in the partitioning of the total kinetic energy during dissociation, large fragment anions are created with low kinetic energy, often lower than that required to escape the charge/ image-charge induced polarization of the film (Sanche, 1991). Therefore the desorption signal for polyatomic ions is generally weaker than for atomic ions. In the case of deuterated hydrocarbons, one may expect that production of heavier polyatomic anion fragments is additionally less efficient with comparison with nondeuterated hydrocarbons due to the intrinsic isotope effect. It was found in the gas phase experiment (Sharp and Dowell, 1967) that the cross section for methylene, CH 2 anion production is 100 times higher than for production of CD 2 (Sharp and Dowell, 1967). This is attributable to the fact that the dissociation process requires more time for heavier isotopes (longer stabilization time of the electron on a fragment) so that even the yield of negative atomic ions is lower for the heavier isotopes (Rapp et al., 1965; Schulz and Asundi, 1965). It is therefore not surprising to find that in the condensed phase the desorption of polyatomic anions from deuterated hydrocarbons is completely suppressed since in this case both intrinsic and extrinsic effects reduce the D yields. Above 15 eV for C2 D4 ; 17 eV for C2 D2 ; C2 D6 and C3 D8 ; and 19 eV for CD4 molecules, a linearly increasing D ion signal was recorded. This latter is strictly connected with the dipolar dissociation process (Sanche, 1991).
4. Summary Formation of the D ion from electron interaction with condensed deuterated hydrocarbon molecules occurs via dissociative electron attachment and dipolar dissociation processes. For each of the studied compounds a single resonant maximum was observed. The energy of these core-excited resonances are very close to those observed for nondeuterated condensed hydrocarbons as well as to those observed in the gas phase experiments. No desorbing heavier polyatomic ions were observed due to partitioning of the kinetic energy and isotope effects in production of negative ions.
Acknowledgements This work is supported by the Canadian Institutes of Health Research.
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