Electronic Energy Transfer in Rare-Gas Solid Alloys Studied by

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Electronic Energy Transfer in Rare-Gas Solid Alloys Studied by Photon Stimulated Desorption† D. E. Weibel,‡ T. Nagai,‡ T. Hirayama,‡ I. Arakawa,*,‡ and M. Sakurai§ Department of Physics, Gakushuin University, Toshima, Tokyo 171, Japan, and Department of Physics, Kobe University, 1-1, Rokkodai, Nada, Kobe 657, Japan Received September 1, 1994X Metastable-particle desorption from Ne-Ar and Ne-Kr solid alloys stimulated by photon excitation were studied using synchrotron radiation as excitation source. Excitation of Ne excitons by optically allowed transitions led mainly to desorption of the other components of the alloy revealing a very efficient energy transfer process. On the other hand, the excitation of the partially allowed electronic transition at the surface (2p5 3p) of Ne showed almost no electronic energy transfer to the matrix and desorption of Ne metastables was observed. A sharp enhancement of the Ar metastable signal with respect to pure solid Ar was also observed in the study of Ne-Ar alloys in the energy range below the excitation of Ne excitons. Different mechanisms for desorption are suggested according to the energy region of excitation.

I. Introduction Electronic excitation of a rare-gas solid (RGS) leads to a conversion of the absorbed energy into photons, phonons, emission of electrons, ions, ground and excited atoms, or dimers. These events show the diversity of relaxation processes which occur in RGSs after electronic excitation and in recent years many efforts have been made to elucidate the microscopic details of the desorption dynamics.1-4 The excitons in RGSs play an important role in DIET (desorption induced by electronic transitions) of neutrals from the solid surface. DIET of neutrals from RGS multilayers occurs mainly through two accepted mechanisms:5 (a) the cavity-ejection mechanism (CE), i.e., a metastable atom at the surface is ejected by the repulsive force between the expanded electron orbit of the exciton and the negative electron affinity of the solid matrix; and (b) the molecular or excimer-dissociation mechanism (ED), i.e., the conversion of electronic excitation energy into kinetic energy occurs after an electronic transition to an antibonding state. The excitation of second or higher order excitons in solid Ne leads to the desorption of Ne metastables (Ne*) with the kinetic energies of about 0.2 and 1.1 eV for CE and ED mechanisms, respectively.6,7 In a similar way, the excitation of second or higher order Ar excitons produces the desorption of Ar metastables (Ar*) from solid Ar with kinetic energies of about 0.04 and 0.6 eV for CE and ED mechanisms, respectively,8-10 whereas for Kr and Xe only one component, the high kinetic energy, * To whom correspondence may be addressed at Department of Chemical Physics, Faculty of Chemical Sciences, University of Cordoba, Suc. 16, C.C. 61, 5016, Cordoba, Argentina. † Presented at the symposium on Advances in the Measurement and Modeling of Surface Phenomena, San Luis, Argentina, Aug 24-30, 1994. ‡ Gakushuin University. § Kobe University. X Abstract published in Advance ACS Abstracts, January 1, 1996. (1) Fugol’, I. Ya. Adv. Phys. 1978, 27, 1. (2) Schwentner, N.; Koch, E.-E.; Jorner, J. Electronic Excitations in Condensed Rare Gases; Springer-Verlag: Berlin, 1985. (3) Zimmerer, G. Excited-State Spectroscopy in Solids; Grassano, U. M., Terzi, N., Eds.; North-Holland: Amsterdam, 1987. (4) Desorption Induced by Electronic Transitions, DIET V; Burns, A. R., Stechel, E. B., Jennison, D. R., Eds.; Springer-Verlag: Berlin, Heidelberg, 1993. (5) Coletti, F.; Debener, J. M.; Zimmerer, G. J. Phys. Lett. 1984, 45, L467. (6) Kloiber, T.; Zimmerer, G. Radiat. Eff. Defects Solids 1989, 109, 219. (7) Kloiber, T.; Zimmerer, G. Phys. Scr. 1990, 41, 962.

0743-7463/96/2412-0193$12.00/0

originated by the ED mechanism can be expected because the CE mechanism depends on the electron affinity of the solid matrix. Ne and Ar have negative electron affinity but Kr and Xe have a positive one; therefore ejection of Kr and Xe metastables (Kr* and Xe*, respectively) by CE was not found.8,11,12 Apart from the studies on pure RGSs, several papers have been published related to doped RGS and thin films of one RGS covered by another different one. For example, Schwentner et al.13a measured the energy distribution of the emitted electrons by excitation with photons of thin films of solid Ar and Ne doped with 1 atom % Xe. They found that the energy of the free excitons is transferred rather than the energy of the bound (self-trapped) excitons and there is a difference in the relaxation process between the Ne and Ar matrix. Sanche et al.11 excited Kr films covered with a monolayer of Xe with monochromatic lowenergy electrons. They interpreted their results of the desorption in terms of two different mechanisms related to the kind of excitons formed and the atomic environment around it. Weibel et al.14 studied the photon stimulated desorption (PSD) of Ne* from thin Ne layers on solid Ar, Kr, and Xe. They concluded that the relative change in the kinetic energy of Ne* desorbed by the excitation of the first-order surface exciton of Ne films on solid Ar, Kr, and Xe can be correlated to the lattice distortion energy and also to the mean cohesive energies at the surface. It means that the CE mechanism still works in those absorbed Ne systems. The above studies showed that many nonradiative electronic transitions and structural changes take place between the excitation of the solid and the emission of light or desorption of particles. Concerning the desorption process, the features of exciton-lattice interaction can lead to localization or conversion of electronic excitation (8) Arakawa, I.; Takahashi, M.; Takeuchi, K. J. Vac. Sci. Technol. 1989, A7, 2090. (9) Arakawa, I.; Sakurai, M. Desorption Induced by Electronic Transitions, DIET IV; Betz, G., Varga, P., Eds.; Springer-Verlag: Berlin, Heidelberg, 1990; p 246. (10) O’Shaughnessy, D. J.; Boring, J. W.; Cui, S.; Johnson, R. E. Phys. Rev. Lett. 1988, 61, 1635. (11) Mann, A.; Leclerc, G.; Sanche, L. Phys. Rev. B 1992, 46, 9683. (12) Buller, W. T.; Johnson, R. E. Phys. Rev. B 1991, 43, 6118. (13) (a) Schwentner, N.; Koch, E. E. Phys. Rev. B 1976, 14, 4687. (b) Hahn, U.; Schwentner, N. Chem. Phys. 1980, 48, 53. (c) Schwentner, N. Appl. Opt. 1980, 19, 4104. (14) Weibel, D. E.; Hoshino, A.; Hirayama, T.; Sakurai, M.; Arakawa, I. Desorption Induced by Electronic Transitions, DIET V; Burns, A. R., Stechel, E. B., Jennison, D. R., Eds.; Springer-Verlag: Berlin, Heidelberg, 1993; p 333.

© 1996 American Chemical Society

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Figure 1. Schematic diagram of the experimental apparatus: S, sapphire rods; E, synchrotron radiation; T, thermocouple; MCP, microchannel plates; Vr, retarding voltage; HV, high voltage.

into motional energy of atoms. The study of the dependence of the desorption yield on the excitation energy and on the composition of the matrix allows a systematic variation of the electronic and structural parameters under analogous conditions and a better picture of the processes which occur between the excitation of the solid and the desorption of particles can be inferred. In this paper we are mainly concerned with the study of Ne-Ar and NeKr alloys. By using monochromatic radiation, we were able to excite a specific exciton in the solid matrix and then investigate the processes of electronic energy transfer to the other RGSs and desorption of neutrals. II. Experimental Section The experiments were carried out in an ultrahigh vacuum chamber with a base pressure of about 1 × 10-8 Pa. The apparatus is schematically shown in Figure 1. Their details have been published elsewhere15 though some modifications were made for the purpose of the present study. RGS films were condensed on the (111) surface of a Pt single crystal held at a temperature of about 6 K or less which was attached to a liquid helium cryostat. The Pt crystal was electrically insulated by sapphire rods. The sample and the cryostat are surrounded by a radiation shield except for the surface (∼8 mm in diameter) on which RGS is condensed. The thickness of the RGS films, usually more than 2000 atomic layers thick, was calculated from the exposure assuming a sticking coefficient of rare gas of unity and taking into account the gauge sensitivity and the collision number per area or fluences for each rare gas.16 Mixtures of rare gases were prepared in the gas handling system and then condensed on Pt crystal. Solid samples were excited by monochromatic VUV light from the beam line BL5B of the UVSOR facility at the Institute for Molecular Science. The pulse beam, which was obtained by a rotating disk with slits, was 15 µs in width and in a 2-ms interval. The kinetic energy distribution of metastables was measured by using the time-of-flight (TOF) technique with a maximum resolution of 5 ns/channel. The distance between the substrate and the detector was 67 mm. The detector was a position-sensitive and single event counting device which consists of two microchannel plates (MCP, Galileo Electro-Optics Corp., 75 mm in diameter) mounted in front of a resistive anode (Quarter Corp.). To repel the charged-particle signals, a positive potential of about 20 V was applied to the grids in front of the detector so that only light and metastable particles can pass through the grids to the MCP. The metastable species detected in the present experiment are metastable atoms in 3P0 and 3P2 states whose lifetimes are much longer17 than their transit times in vacuum, and therefore it is not necessary to be weighted by exp(t/τ), where t is the flight time and τ the lifetime of the metastable atom. (15) Sakurai, M.; Hirayama, T.; Arakawa, I. Vacuum 1990, 41, 217. (16) Menzel, D.; Fuggle, J. C. Surf. Sci. 1978, 74, 321. (17) Van Dyck, R. S., Jr.; Johnson, C. E.; Shugart, H. A. Phys. Rev. A 1972, 5, 991.

Figure 2. TOF spectra of Ne* (λ ) 65.2 nm), Ar* (λ ) 94.5 nm), and metastables (λscan ) 55-110 nm) desorbed from pure solid Ne, pure solid Ar, and a Ne-Ar solid alloy with 25% of Ne, respectively.

III. Results and Discussion A. Ne-Ar Alloy. Typical TOF spectra of the PSD particles from the surface of pure solid Ne, pure solid Ar, and a Ne-Ar solid alloy are shown in Figure 2. The wavelength of light (λ) corresponds to the excitation of a higher order surface exciton (S′: 2p53p) for pure solid Ne and Ar, which are partially allowed at the surface as a consequence of the reduced symmetry. For the Ne-Ar alloy the wavelength was scanned from 50 to 110 nm. The intense signal at TOF ) 0 µs corresponds to photons emitted from the RGS layers during the decay of excited states within the bulk of the films at the surface or after ejection into the vacuum. Reflections of the synchrotron light are also included in this signal whose position was chosen as the origin of the flight time. The ED and CE peaks correspond to desorbed metastables by the excimerdissociation and the cavity-ejection mechanisms, respectively. In the Ne-Ar alloy the metastable signal with high kinetic energy has a contribution mainly of Ne*(CE), but as the time resolution is not enough, Ar*(ED) signal is included in this peak. The dependence of the metastable PSD yield on the photon energy for pure solid Ne and Ar is shown in Figure 3. These spectra display a correlation between the metastable yield and the excitation of surface and bulk excitons in pure solid Ne6,7 and Ar.9,18 Figure 4 shows the relation between the desorption yield of Ar* and the excitation wavelength in the range 50-110 nm for pure solid Ar and various Ne-Ar alloys. During these experiments the data collection was restricted to a time window between 130 and 230 µs (see Figure 2, bottom). Therefore, (18) Saile, V.; Skibowski, M.; Steinmann, W.; Gurtler, P.; Koch, E. E.; Kozevnikov, A. Phys. Rev. Lett. 1976, 37, 305.

Energy Transfer in Rare-Gas Solid Alloys

Figure 3. Dependence of the total Ne* (broken line) and Ar* (solid line) signal intensity with the wavelength of light desorbed from pure solid Ne and Ar, respectively: S, nth order surface exciton; B, nth order bulk exciton. The wavelength was scanned between 50 and 75 nm and between 80 to 110 nm for Ne and Ar, respectively.

the metastable signal shown in Figure 4 corresponds mainly to Ar* desorbed by the CE mechanism. At higher concentrations of Ne some contribution of Ne* coming from a tail or broad peak observed by excitation of Ne bulk excitons7,19 may also contribute to the Ar*(CE) signal. Figure 4 shows an unstructured enhancement of Ar*(CE) signal when the RGS contains Ne and this signal increase is specially strong when the amount of Ne is higher than 10%. The signal enhancement starts at around 90 nm and covers the entire region of Ne excitonic excitation. When the concentration of Ne is higher than about 30%, some peaks can be observed in the yield of Ar*(CE) which can be correlated with the excitation of surface Ne excitons. The data collection in Figure 5 was restricted to a time window between 35 and 85 µs to collect only the Ne* signal desorbed by the CE mechanism. The contribution of Ar*(ED) to this signal cannot be totally ruled out, but it is negligibly small. The threshold internal energy for the detection of metastable particles was estimated to be around 7 eV from the detection of N2 (A).20 The lowest metastable atomic levels of Ne and Ar (3P2) have energies of 16.62 and 11.55 eV, respectively.1 Until now, few data have been published on the detection efficiency of the MCP for ions and specially for neutrals.21,22 For example, in the case of the electron detection efficiency, it is a function of its angle of incidence (defined relative to MCP front surface normal) and the electron energy, and this efficiency (19) Weibel, D. E.; Hirayama, T.; Arakawa, I. Surf. Sci. 1993, 283, 204. (20) Leclerc, G.; Bass, A. D.; Mann, A.; Sanche, L. Phys. Rev. B 1992, 46, 4865. (21) Fraser, G. W. Nucl. Instrum. Methods 1983, 206, 445. (22) Gao, R. S.; Gibner, P. S.; Newman, J. H.; Smith, K. A.; Stebbings, R. Rev. Sci. Instrum. 1984, 55, 1756.

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Figure 4. PSD yield of Ar* desorbed by the CE mechanism measured in a broad spectra range: 50-110 nm for pure solid Ar and various Ne-Ar solid alloys. The concentration of Ne in each alloy corresponds to the amount estimated in the solid and is indicated in the figure.

strongly depends on the energy of the incident particle, specially at energies near the threshold.21 The energy of Ar* is considerably much closer to the threshold than Ne* and this fact turns into a quantum efficiency much lower in the case of Ar* than Ne*. This difference in quantum efficiency was clearly observed when we used a ceratron as detector (see below) in which it was possible to detect Ne* but not Ar*. Therefore, the Ar* signal can be discarded as an important contribution to the signal observed in Figure 5. From these results three main facts can be pointed out: (i) the almost absence of Ne* signal desorbed by excitation of Ne bulk excitons (B1, 70.5 nm; B2, 61.2 nm) at a percentage of Ne smaller than 50%; (ii) the absence of Ne* signal desorbed by first-order surface excitation of Ne (S1, 72.2 nm) when the amount of Ne in the alloy is smaller than 13%, together with a shift from about 72.9 to 72.5 nm when the concentration of Ne in the alloy changes from 13 to 55%, respectively, and (iii) the Ne* signal obtained by S′ excitation of Ne (λ ) 65.2 nm) can be observed even at less than 13% of Ne with no shift in the excitation wavelength. Figure 6 shows TOF spectra of metastables desorbed from a Ne-Ar alloy by excitation of surface and bulk excitons of Ne. The TOF spectrum of metastables desorbed by S1 excitation of Ne excitons in the alloy resembles the TOF spectrum of Ne* desorbed from pure solid Ne. On the other hand, the TOF spectrum of metastables desorbed by B1 excitation of Ne excitons from the alloy differs from the corresponding spectrum for pure solid Ne. The metastable signals desorbed from the solid alloy by B1 excitation of Ne can be related to the desorption of Ar* through ED and CE mechanisms by comparison with the TOF spectra of Ar* desorbed from pure solid Ar (Figure 2).

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Figure 5. Dependence of the Ne* signal desorbed by the CE mechanism with the wavelength of light between 55 and 75 nm for various Ne-Ar solid alloys. The concentration of Ne estimated in each alloy is indicated in the figure.

The experimental results on PSD of metastables from Ne-Ar alloys can be explained according to two energy regions: (a) excitation of Ne excitons, 72.5 g λ g 58.5 nm; (b) excitation of Ar bulk excitons and Ar+ formation, 90.0 g λ g 75.0 nm. The main exciton states and excitation energies in solid Ne and Ar are compiled in Table 1. (a) Excitation of Ne Excitons. The n ) 1 and n ) 2 exciton of Ne lie below the band gap energy of Ne but above that of Ar. The direct excitation of Ar at these wavelengths cannot produce the enhancement of the metastable signal as can be seen in Figure 4 (solid line). Energy transfer from the Pt substrate yields almost no contribution to the desorption process due to the thick layers of RGSs used in these experiments. The excitation of Ne excitons contributes therefore to desorption of Ar* only by secondary processes. A possible mechanism that can explain the above experimental results is

Ne* + Ar f Ne + Ar+ + e (ArR)**

Ar+ + R + e 98 R + Ar* + Ek

(1) (2)

or (ArR)**

Ne* + Ar 98 Ne + Ar* + Ek + hν

(3)

where R ) Ne or Ar; Ek ) kinetic energy. The Ne exciton transfers the electronic energy to a neighbor Ar atom resulting in the ionization of Ar according to step 1. A similar process was observed by

Figure 6. TOF spectra of metastables desorbed by excitation of S1 and B1 excitons of Ne from pure solid Ne and a Ne-Ar solid alloy with 25% of Ne. Table 1. Excitation Energies of nth Order Surface (S) and Bulk (B) Excitons in Solid Ne and Ar2 a Ne

Ar

excitons

E (eV)

λ (nm)

E (eV)

λ (nm)

S1 B1 S′ B2 B3 Egap

17.15 17.5 19.0 20.4 21.0 21.6

72.3 70.5 65.3 61.2 59.1 57.5

11.7 12.1 13.0 13.6 13.9 14.2

106 102 95.4 91.1 89.2 87.3

a Only the energies corresponding to j ) 3/2 are given. E gap ) band gap energy.

Baudon et al.23 which measured the differential elastic cross sections for Ne*(3P0,2)-Ar collisions in the cross beam experiments at low collision energies ( 75.0 nm) the energy is not enough to excite Ne excitons but sufficient to produce Ar+ (see Table 1). The enhancement of the Ar* signal observed in Figure 4 can be interpreted according to the electron-hole recombination process of step 2 which leads to the desorption of Ar*. Figure 4 also shows that the enhancement of the Ar* signal intensity starts at wavelengths longer than the band gap energy of Ar (λ ≈ 87.3 nm). In studies of photoionization of Ne-rare gas dimers below the atomic ionization threshold but above the molecular ionization (26) Chang, R. S. T.; Setser, D. W. J. Chem. Phys. 1978, 69, 3885. (27) Chang, R. S. T.; Setser, D. W. J. Chem. Phys. 1980, 72, 4099. (28) Cui, S.; Johnson, R. E.; Cummings, P. T. Phys. Rev. B 1989, 39, 9580. (29) Foster, T. Mod. Quantum Chem. 1965, 3, 93.

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threshold, Pratt and Dehmer30 found that the photoionization cross sections of all rare gas dimers studied exhibited a sharp rise at thresholds, which is due to a combination of autoionization of molecular Rydberg states and associative ionization. In the particular case of NeAr dimer the results showed that associative ionization accounted for most of the structure in the energy region below the Ar 2P0,3/2 ionization limit according to

Ne + Ar* f NeAr+ + e

(4)

Some calculations31 confirmed the main spectral features experimentally observed and showed that the molecular photoionization spectrum in this energy range is not simply a weakly perturbed atomic spectrum. In Ne-Ar solid alloys in the energy range below the band gap of Ar but above the second order exciton of Ar, the enhancement of Ar* yield observed can be explained by a process similar to the associative ionization in NeAr dimers. B. Ne-Kr Alloy. In the study of the Ne-Ar alloys (section III.A) it was found that Ne* almost did not desorb by excitation of S1 of Ne when its amount in the alloy was smaller than 13% (see Figure 5). On the other hand, when the wavelength of excitation corresponded to the excitation of S′ excitons of Ne, desorption of Ne* by the CE mechanism was detected. To check if these results depend on the kind of RGSs or on the electronic excitation involved, some experiments were carried out with about 1000 atomic layer Kr film covered by few monolayers of Ne. These results are shown in Figure 7. Similar to the experiments of NeAr alloys, desorption of Ne* through first-order surface excitation of Ne was not observed when few monolayers of Ne are adsorbed on Kr. But desorption of Ne* was found even at less than 10 monolayers of Ne adsorbed on solid Kr when the wavelength corresponds to the excitation of S′ exciton of Ne. As in the Ne-Ar alloys, Figure 7 shows a shift in the excitation wavelength of S1 exciton of Ne but not in the wavelength of S′ exciton of Ne. By excitation of S1 exciton of Ne the energy can be transferred efficiently to Kr but by S′ of Ne there is not an efficient energy transfer process to Kr. The same arguments used in section III.A.a can be applied here. These results show an interesting phenomenon: the photon selectivity allowed us to discriminate at the surface between desorption and energy transfer processes. Besides, the desorption process could be studied by excitation of S′ excitons in RGSs, for example by the CE mechanism, together with a systematic variation of the surface composition without being affected by secondary processes. IV. Conclusions The study of Ne-Ar alloys showed a very efficient electronic energy transfer process between Ne excitons and the Ar matrix which led to desorption of Ar* through the formation of a probable heteronuclear center (NeAr)**. From electronic relaxation of this center at the surface, a fast Ar* can desorb. If the dissociation of (NeAr)** occurs in the bulk, Ar* can migrate to the surface and then desorb by the CE mechanism. When surface excitons of Ne are (30) Pratt, S. T.; Dehmer, P. M. J. Chem. Phys. 1982, 76, 3433. (31) Du, N. Y.; Greene, C. H. J. Chem. Phys. 1989, 90, 6347.

Figure 7. Dependence of the Ne* desorbed by the CE mechanism with the wavelength of light between 55 and 75 nm from thick Kr films covered by various thicknesses of Ne films.

excited, an interesting effect was found in Ne-Ar and Ne-Kr alloys. A competition between desorption of Ne* and electronic energy transfer to the other RGS of the alloy was observed by excitation of S1 excitons of Ne, while only desorption of Ne* was seen by S′ excitation of Ne excitons. These results are related to the efficiency in the electronic energy transfer process for dipole-allowed transitions. The desorption of Ne* by S′ excitation is suggested as a good candidate to study the CE mechanism dynamics because desorption by this mechanism occurs before any other process. In the energy range below Ne excitons and above the band gap of Ar matrix, a high desorption yield of Ar* was also found in the study of Ne-Ar alloys. These results were explained by the decay of (NeAr)** after the repulsive recombination of a (NeAr)+ with an electron, which leads to desorption of Ar*. Finally, in the energy range below the band gap of Ar but above the second order bulk excitation of Ar, the enhancement of Ar* signal is explained by a process similar to the associative ionization observed in photoionization experiments of NeAr gas dimers below the atomic ionization threshold of Ar. Acknowledgment. The authors wish to thank Dr. K. Mitsuke and M. Kanno for help in the experiments. D.E.W. gratefully acknowledges financial support from the Gakushuin University. LA940691A