J. Phys. Chem. B 1997, 101, 6301-6303
6301
Low-Energy (5-40 eV) Electron-Stimulated Desorption of Atomic Hydrogen and Metastable Emission from Amorphous Ice G. A. Kimmel and T. M. Orlando* EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MS/N K8-88, Richland, Washington 99352
P. Cloutier and L. Sanche* MRC Group in Radiation Sciences, Faculty of Medicine, UniVersity of Sherbrooke, Quebec, Canada J1H 5N4 ReceiVed: October 11, 1996X
Low-energy (5-40 eV) electron-stimulated desorption (ESD) of D (1 2S) from amorphous D2O water films has been studied using laser resonance-enhanced multiphoton ionization spectroscopy. The D (1 2S) desorption product has a ∼6.5 ( 0.3 eV threshold energy, relative to the vacuum level, and a low velocity distribution. ESD of electronically excited D (n g 2 2S) was not detected using photoionization schemes. A small metastable (n g 2 2P) desorption yield and/or emission of ultraviolet (UV) photons (hν g ∼6 eV) was detected at incident electron energies >25 eV, using a multichannel plate detection technique. We attribute the ground state D desorption to exciton decay and associate the small metastable/UV photon emission yield with dissociation of doubly excited states and efficient autoionization.
Introduction The interaction of ionizing radiation with water is a subject which has received a large amount of attention, and several reviews have been published.1,2 This general interest is due, in part, to the importance of understanding radiation-induced transformations in biological systems and astrophysical environments.3,4 It is well-known that the interaction of high-energy radiation with condensed matter produces large numbers of lowenergy (∼5-100 eV) secondary electrons.2,5,6 The interactions of both the primary particle and these secondary electrons generate cations, anions, and radicals primarily via ionization,1,2 dissociative electron attachment (DEA),5,7 and dissociative excitation,8-14 respectively. Several gas-phase electron-impact dissociative excitation1,8-11 and vacuum-ultraviolet (VUV) photodissociation studies12-14 have been reported in the energy range from 5 to 200 eV. In general, there is much less information concerning electronimpact and VUV excitation of ice. A detailed study of lowenergy (15-50 eV) electron-stimulated desorption (ESD) of protons from water ice reported an H+ ESD threshold of ∼21 eV and a monotonic increase in the yield above the threshold.15 This threshold has been assigned to two-hole-two-electron excitations (1b1-24a12 and 1b1-13a1-14a12) by Noell et. al15 and two-hole-one-electron states (1b1-24a1 and 1b1-13a1-14a1) by Ramaker.16 Recent work on proton ESD from nanoscale ice thin films by Sieger et al.17 corroborates the ∼21 eV threshold energy and also reports a second threshold at ∼40 eV. The ESD of D- (H-) ions via DEA has also been reported by Rowntree et. al7 for incident electron excitations between 5 and 15 eV. Since neutral desorption is the dominant ESD channel, some low-energy electron-impact studies of ice have concentrated on ESD of neutral fragments. For example, Prince et. al18 reported ESD of excited OD (OH) radicals (A 2Σ+) and described electron (50-200 eV) induced fluorescence of D2O (H2O) ice films at 77 K. More recent studies have demonstrated the * Authors to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1997.
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important role of exciton dissociation in the ESD of H2(1Σg+), D(2S), O(3P2,1,0), and O(1D2) from amorphous ice.19-21 In this paper, we discuss the atomic hydrogen yield as a function of incident energy and demonstrate that, although the ground state D (1 2S) ESD yield is relatively high, the metastable/ultraviolet (UV) photon emission yield is very low. We attribute the low metastable/UV emission yield in the condensed phase to efficient and rapid autoionization of the highly excited states known to produce H* (n g 2) in the gas-phase. Experimental Section The experiments were performed in two ultrahigh vacuum (∼1-2 × 10-10 Torr) systems: one at the Environmental Molecular Sciences Laboratory of Pacific Northwest National Laboratory (PNNL) and the other at the University of Sherbrooke. The system at PNNL has been previously described19-21 and is equipped with a low-energy electron gun, a quadrupole mass spectrometer, a cryogenically cooled Pt(111) surface, an effusive gas doser, and a time-of-flight (TOF) spectrometer for laser resonance-enhanced multiphoton ionization (REMPI) detection of neutrals. D2O multilayers (∼50-100 layers) were prepared by dosing at ∼88 K under conditions that have been shown to produce amorphous ice. The ice samples were irradiated with a variable pulsed (200 ns to 25 µs) electron beam which had an energy spread of ∼0.3 eV, a typical current density of ∼1010 electrons/cm2/pulse, and a beam spot size of ∼1.5 mm. The neutral D atoms desorbing from the surface were detected using (2+1) REMPI spectroscopy via the 3s rr 1s 2S, twophoton transition, at 205.1 nm. Nonresonant detection of the long-lived metastable (n g 2 2S) hydrogen is possible by simply detuning the laser to either the red or blue of the 3s rr 1s 2S transition. The laser wavelengths necessary for this detection scheme were generated by frequency tripling the output of a Nd:YAG pumped dye laser with KDP-C and β-barium borate wavelength extension crystals. Typical laser pulse powers were ∼0.5-1.5 mJ/pulse, and our estimated detection efficiencies were ∼106 atoms/cm3/quantum state. All experiments were done in the TOF mode in which the neutrals produced by the © 1997 American Chemical Society
6302 J. Phys. Chem. B, Vol. 101, No. 32, 1997
Figure 1. Desorption yield of D (1 2S) as a function of incident electron energy. The D (1 2S) was detected using 2+1 resonance-enhanced multiphoton ionization via the 3s rr 1s 2S two-photon transition. The intensities were measured using a 15 µs electron-beam pulse which integrates the time-of-flight distribution presented in Figure 2.
Figure 2. Time-of-flight distributions for D (1 2S). The distributions, which have been normalized at the peak, have the same shape, independent of the excitation energy. The peak of the distribution corresponds to an energy of ∼85 meV.
pulsed electron beam were resonantly ionized ∼4 mm above the surface by the focused laser beam. State-specific TOF distributions were obtained by varying the delay time between the electron-beam pulse and laser pulse. The Sherbrooke apparatus has also been previously described.22 Briefly, the ultrahigh vacuum apparatus contains a well-collimated low-energy (1-35 eV) electron beam, which impinges on a Pt(111) surface at 18° with respect to the surface normal. The electron beam has an intensity of 5 nA and an energy resolution of 60 meV full-width at half-maximum. Multilayer H2O ice films were condensed on a Pt(111) crystal at 20 K under conditions that form amorphous ice. Desorbed metastable particles and UV photons are detected with a large area microchannel plate array which is superimposed on a position-sensitive anode. The threshold energy for detection of metastable particles or UV photons is estimated to be ∼6 eV.22 Results and Discussion The D (1 2S) yield versus incident electron energy, Ei, is shown in Figure 1. This data was obtained using a 15 µs electron-beam pulse width which integrates the entire velocity distribution presented in Figure 2. The D (1 2S) yield has an apparent threshold at ∼6.5 eV (relative to the vacuum level), which is well below the 21 eV threshold reported for ESD of H+ from ice.15,17 We note that if the incoming electron is accelerated by the gradient associated with the bulk V0 inner potential, (∼0.9-1.0 eV23), the actual electron collision energy, and hence threshold energy, is approximately ∼7.4 ( 0.3 eV. Above threshold, the D (1 2S) intensity increases rapidly until
Kimmel et al. a distinct plateau is reached for ∼14 eV e Ei e 21 eV. The signal increases more gradually for electron energies above ∼21 eV. Although information concerning electronic excitation24 and electron-impact ionization cross sections of amorphous ice is not available over the entire energy range investigated, the photoionization yield of liquid water has been calculated for 8 eV < Ei < 100 eV.6 The calculations indicate that above the threshold, the photoionization/excitation yield, like the D (1 2S) yield, increases rapidly, has a plateau from ∼15 to 21 eV, and has an approximately linear increase at higher energies. This is in accord with our very low yield below the conduction band at ∼10 eV24 and the assertion that the excited states (excitons) at 10.4 and 14.5 eV (i.e. 1,3A1 and 1,3B2) undergo rapid autoionization.24 For ∼14 eV e Ei e 21 eV, the probability that an incident electron ionizes or electronically excites one substrate water molecule is constant and results in the plateau. Above ∼21 eV, the desorption yield increases again since the incident electron has enough energy to ionize or excite two or more molecules. The normalized TOF distributions for D (1 2S) at Ei ) 25 and 50 eV are displayed in Figure 2. The Boltzmann velocity distribution for D atoms that have thermalized to the 88 K surface temperature is indicated by the dotted line. Assuming negligible time delay between the incident electron pulse and the D (1 2S) desorption, the peaks of the TOF distributions correspond to a translational energy of ∼85 meV. These nonthermal components are due to atoms ejected directly from the surface without interacting with surrounding molecules. The shoulders starting at ∼3 µs also have contributions from atoms which have accommodated the surface temperature prior to desorption. Although new excitation channels are associated with higher Ei, the shapes of the D (1 2S) TOF distributions do not change as Ei increases. This indicates that the forces in the final dissociative states are independent of the initial excitation and that most higher energy excitations autoionize and/or relax to excited states near the bottom of the conduction band prior to dissociation. However, due to the large (∼10 eV) band gap in ice,24 the energy of these low-lying excited states (excitons) is most effectively dissipated via dissociation.21 Note that the detection efficiency of this experiment is limited to desorbates with translational energies (Etrans) less than ∼1.0 eV. No signal was detected within this kinetic energy range when detuning the laser from the 3s rr 1s 2S transition at 205.1 nm. Because of conservation of angular momentum and parity, the n g 2 2S state of hydrogen is very long-lived (∼1/7 s) and only decays to the ground state via simultaneous emission of two photons.25 Our results therefore indicate that no metastable hydrogen in the n g 2 2S levels desorbs from ice with Etrans e 1.0 eV. The primary electronically excited fragments produced during photon and electron-impact excitation of gas-phase water are OH*, O*, and H*, and one of the strongest emissions observed is the Lyman-R line at 1215.7 Å. The threshold for gas-phase Lyman-R emission has been observed at 15.3 eV and involves single-electron nR (n g 4) Rydberg states which converge to the 2B2 (1b2)-1 state of the water ion.8-13 It has been suggested that the single-electron configuration, (1b2)-1(4sR)1, predissociates via curve crossings with the (3a1-1)(1b2)-1(4a1)1(nR)1 doubly excited molecular states.8 Figure 3 demonstrates that a small but reproducible metastable desorption/UV (hν g 6 eV) emission occurs only at Ei > ∼ 25 eV. These results indicate that single-electron superexcited Rydberg states, such as the one mentioned above, autoionize in ice due to the extended nature of the n g 4 high Rydberg wave functions. Whereas photodissociation of Rydberg states into excited neutral fragments
Low-Energy ESD of Atomic Hydrogen
J. Phys. Chem. B, Vol. 101, No. 32, 1997 6303 amorphous D2O water films has been studied using laser resonance-enhanced multiphoton ionization spectroscopy and multichannel plate detection techniques. The D (1 2S) desorption product has a desorption threshold energy ∼6.5 ( 0.3 eV, relative to the vacuum level, and a low velocity distribution. Electron-stimulated desorption of electronically excited D (n g 2 2S) was not detected using photoionization schemes, though a small metastable (n g 2 2P) desorption yield and/or emission of UV photons (hν g 6 eV) was detected at Ei g ∼25 eV. We attribute the ground state D yield to exciton decay and the general lack of metastable desorption from amorphous water ice to efficient autoionization of highly excited states. At Ei g ∼25 eV, dissociation of doubly excited states likely contribute to the small metastable/UV emission yield.
Figure 3. Yield of metastable/ultraviolet photon (hν g ∼6 eV) emission from 20 layers of water adsorbed on Pt(111) at 20 K. The emission yield is detected with a multichannel plate array which is superimposed on a position-sensitive anode. Note the background count rate from clean Pt(111) indicates that the total signal from ice is quite small. The threshold value ∼25 eV is associated with doubly excited configurations which primarily autoionize in ice.
remains an important channel above the ionization potential in gas-phase H2O,12 autoionization likely dominates in the ice. A second threshold for Lyman-R emission has been observed at ∼23.5 eV during electron impact excitation of gas-phase water.9,11 Since this threshold has not been observed in photoexcitation studies, this highly excited state is likely to be of triplet character and/or most likely involves a two-electron process. Doppler profile measurements of gas-phase Balmer-R emission as a function of electron-impact energy also demonstrated that fast, Etrans > 4.0 eV, H (n ) 3) fragments were produced at Ei g ∼25 eV.8 Note that ESD of protons with translation energies greater than ∼4.0 eV was attributed to excitation of the lowest energy doubly excited state, (1a2)2(2a1)2(1b2)2(3a1)-1(1b1)-1(4a1)2 at 21 eV.15 Since cascading to n ) 2 levels can also contribute to Lyman-R emission, the same doubly excited configurations that lead to proton ESD may lead to both Balmer-R and Lyman-R emissions. Taking into account the fact that the dissociation dynamics in ice favor a branching fraction to ground state and ionic fragments, we suggest that the small metastable/UV emission signal observed at Ei g 25 eV arises from the decay of doubly excited configurations. Since the longest lived excited state usually dominates the desorption yield, we also suggest that dissociation of the low-energy (‚‚‚1b1-13s:4a11) 1,3B1 exciton is primarily responsible for desorption of D (1 2S) from D2O ice. Conclusions Low-energy (5-40 eV) electron-stimulated desorption of D (1 2S) and metastable/UV photon (hν g 6 eV) emission from
Acknowledgment. Work at Pacific Northwest National Laboratory was supported by the Department of Energy, Office of Basic Energy Sciences, Chemical Physics Program. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. L.S. acknowledges support from the Medical Research Council of Canada. References and Notes (1) Dixon, R. S. Radiat. Res. ReV. 1970, 2, 237. (2) Kaplan, I. G.; Miterev, A. M. AdV. Chem. Phys. 1987, 68, 255. (3) Brown, W. L.; Lanzeroetti, L. J.; Johnson, R. E. Science 1982, 218, 525. (4) Johnson, R. E. ReV. Mod. Phys. 1996, 68, 305. (5) Sanche, L. Radiat. Phys. Chem. 1989, 34, 487. (6) Pimblott, S. M.; Mozumder, A. J. Phys. Chem. 1991, 95, 7291. (7) Rowntree, P.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1991, 94, 8570. (8) Kouchi, N.; Hatano, Y.; Oda, N.; Tsuboi, T. Chem. Phys. 1979, 36, 239. (9) Morgan, H. D.; Mentall, J. E. J. Chem. Phys. 1974, 60, 4734. (10) Beenakker, C. I. M.; De Heer, F. J.; Krop, H. B.; Mo¨hlmann, G. R. Chem. Phys. 1974, 6, 445. (11) Ogawa, T.; Yonekura, N.; Tsukada, M.; Ihara, S.; Yasuda, T-o.; Tomura, H.; Nakashima, K.; Kawazumi, H. J. Phys. Chem. 1991, 95, 2788 and references therein. (12) Dutuit, O.; Tabche-Fouhaile, A.; Nenner, I.; Frohlich, H.; Guyon, P. M. J. Chem. Phys. 1985, 83, 584. (13) Wu, C. Y.; Judge, D. L. J. Chem. Phys. 1981, 75, 172. (14) Mentall, J. E.; Mo¨hlmann, G. R.; Guyon, P. M. J. Chem. Phys. 1978, 69, 3735. (15) Noell, J. O.; Melius, C. F.; Stulen, R. H. Surf. Sci. 1985, 157, 119. (16) Ramaker, D. Chem. Phys. 1983, 80, 183. (17) Sieger, M. T.; Simpson, W. C.; Orlando, T. M. Phys. Rev. B. In press. (18) Prince, R. H.; Sears, G. N.; Morgan, F. J. J. Chem. Phys. 1976, 64, 3978. (19) Kimmel, G. A.; Orlando, T. M.; Vezina, C.; Sanche, L. J. Chem. Phys. 1994, 101, 3282. (20) Kimmel, G. A.; Tonkyn, R. G.; Orlando, T. M. Nucl. Instrum. Methods Phys. Res. B 1995, 101, 179. (21) Kimmel, G. A.; Orlando, T. M. Phys. ReV. Lett. 1995, 75, 2606; Phys. ReV. Lett. 1996, 77, 3983. (22) Mann, A.; Clouthier, P.; Lui, D.; Sanche, L. Phys. ReV. B. 1995, 51, 7200. (23) Baron, B.; Hoover, D.; Williams, F. J. Chem. Phys. 1978, 68, 1997. (24) Michaud, M.; Clouthier, P.; Sanche, L. Phys. ReV. A 1991, 44, 5624. (25) Williams, J. F.; Heck, E. L. J. Phys. B 1988, 21, 1627.
