Charging Effects in an Electron Bombarded Ar ... - ACS Publications

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Charging Effects in an Electron Bombarded Ar Matrix and the Role of Chemiluminescence-Driven Relaxation Elena V. Savchenko,*,† Ivan V. Khyzhniy,† Sergey A. Uyutnov,† Galina B. Gumenchuk,‡ Alexey N. Ponomaryov,‡ Martin K. Beyer,§ and Vladimir E. Bondybey‡ †

Institute for Low Temperature Physics and Engineering NASU, 61103 Kharkov, Ukraine Department Chemie, Lehrstuhl 2 f€ur Physikalische Chemie, Technische Universit€at M€unchen, Lichtenbergstrasse 4, 85747 Garching, Germany § Institut f€ur Physikalische Chemie, Christian-Albrechts-Universit€at zu Kiel, Olshausenstrasse 40, 24098 Kiel, Germany ‡

ABSTRACT: The relaxation processes in pure and doped Ar films preirradiated by an electron beam are studied with the focus on charging effects. Correlated real time measurements have been performed applying current and optical activation spectroscopy methods. Thermally stimulated exoelectron emission and thermally stimulated luminescence are detected in the vacuum ultraviolet and visible range. An appreciable accumulation of electrons in the matrix is found, and prerequisites for negative space charge formation are ascertained. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron trap stability is elucidated. It is shown that laser-induced electron detachment from O
centers results in an enhancement of electron detrapping via the chemiluminescence mechanism, viz. neutralized and thermally mobilized O atoms recombine. Formation of O2* results in the emission of visible photons. These photons act as a stimulating factor for electron release and transport, terminating in exoelectron emission and charge recombination. Chemiluminescence therefore plays an important role in the decay of charged centers.

’ INTRODUCTION Electrostatic charging of insulating materials is of high fundamental and technological importance in solid-state physics and chemistry as well as in space science. Solidified gases, often referred to as “ices”, provide a unique possibility to study radiation effects because of their relative simplicity, characterized by weak interatomic forces and simple lattices. Different aspects of radiation physics and chemistry in atomic and molecular ices have been discussed in a number of books and reviews.1
7 The studies were centered on energy loss of charged particles in solids and in particular energy deposition and ejection of secondary particles during bombardment by charged particles. Radiation effects attract much current attention in space physics.4
6,8
10 Electrostatic charging in rare-gas ices caused by ion bombardment has been studied with the focus on processes occurring during bombardment.11
13 It is found that such a treatment results in an accumulation of uncompensated positive charge in rare-gas films. The physical background of the effect is charge trapping and emission of secondary electrons. The charging effect induced by electron beams could be different. First of all, there is no charge exchange between projectile and target. In contrast to ions slowing down, one cannot distinguish between a scattered electron and a secondary r 2011 American Chemical Society

electron. A low-energy electron is not able to produce a lattice defect via the knock-on mechanism, so the only way of defect production is through electronic mechanisms. The ionization capability of electrons depends on their energy, with the ionization cross section exhibiting a maximum in the energy range of 100
300 eV. For ions, the maximum is reached only at much higher energies.14 It is notable that the ionization cross section for charged particles is by an order of magnitude higher than that for photoionization. Radiation effects like sputtering and defect formation induced by an electron beam have been studied in rare-gas ices in detail (e.g., refs 3, 15, and 16 and references therein). However, there is still a significant gap in our knowledge on charging effects. The accumulation of positive charge in thin Ar films (20
180 monolayers) under bombardment by low-energy, Eirr = 5
30 eV, electrons is reported in ref 16. A quite different effect, the

Special Issue: J. Peter Toennies Festschrift Received: January 15, 2011 Revised: April 23, 2011 Published: May 23, 2011 7258

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The Journal of Physical Chemistry A accumulation of excess electrons, was found in thick Xe films after irradiation by 500 eV electrons.17 Here, we report an investigation of charging effects induced by 500 eV electrons in nominally pure and doped Ar films. Since both charged and neutral defect centers can be created under irradiation, there are obvious benefits of applying the combination of current measurement and optical spectroscopy. The experiments were performed using cathodoluminescence, thermally stimulated exoelectron emission (TSEE), and thermally stimulated luminescence (TSL). These techniques are valuable tools for trap-level analysis and study of relaxation processes.18 In view of the high sensitivity of TSL and TSEE to the sample structure and impurity concentration, we monitored those phenomena simultaneously on the same sample. TSL was measured in the range of intrinsic charge recombination luminescence (vacuum ultraviolet or VUV) as well as in the visible range to elucidate the role of dopant-assisted reactions. Unlike the measurement of thermally stimulated conductivity, TSEE does not require strong external electric fields, which might perturb negative charge centers. Because of the high mobility of free electrons in rare-gas solids,1 TSEE measurements provide information not only on surface-related processes but also on the processes occurring in deeper layers of the films. Moreover, in solid Ar there is no barrier for an electron to escape due to the negative electron affinity Ea =
0.4 eV.1 Such a combined study of postirradiation processes, that is, processes observed after completing the irradiation, offers a means of monitoring the behavior of charged centers of both signs via concurrent measurement of electron current and recombination luminescence of self-trapped and trapped holes. The developed approach seems to be most adequate, since TSEE and TSL compete with each other.19 Being promoted to the conduction band, detrapped electrons either neutralize positively charged centers or escape from the sample. It is obvious that the presence of impurities or dopants can drastically alter the charge accumulation and stability. As an example, negative charge accumulates due to surface adsorbed oxygen.20 A strong effect of oxygen admixture on the relaxation of charged species was found.21 The scenario put forward is triggering of relaxation cascades by chemiluminescence. Here we present new results in favor of the suggested mechanism. The results elucidate the role of chemiluminescence-driven relaxation and address the question of the stability of charged centers over the temperature range of Ar ice existence.

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optically transparent. An irradiation by 500 eV electrons was performed either during deposition to generate charged centers throughout the film, or after the film preparation in the experiments aimed to elucidate the influence of annealing. A current density of about 30 μA cm
2 was kept. The dose behavior of cathodoluminescence spectra was monitored. On completion of the sample preparation and irradiation, the substrate was turned to the position for simultaneous measurement of postirradiation relaxation emission of photons and electrons. Decay curves of the “afterglow” and “afteremission current” were detected first at the irradiation temperature. When the afterglow and afteremission current had decayed to essentially zero, the thermally stimulated relaxation emission of photons, TSL, and electrons, TSEE, were measured upon heating. A digital programmable temperature controller Leybold LTC 60 allowed us to maintain desired temperatures of deposition, irradiation and heating regimes. Two heating modes were used, linear heating at a constant rate of 3.2 K min
1 and stepwise heating with a step of 2 K and a 20 min interval between successive steps. The temperature of the sample was monitored with a silicon diode sensor mounted on the substrate. The substrate was kept at ground electric potential. The TSEE yield was measured with an Au-coated Faraday plate kept under þ9 V and connected to a current amplifier FEMTO DLPCA 200. A special series of experiments was performed at a variable voltage (from þ9 up to
9 V) applied to the Faraday plate to probe a space charge accumulating in the Ar samples under electron beam. A central hole in the Faraday plate was used for the concurrent TSL measurements. The spectra were taken in the range from 200 to 1100 nm by an Ocean Optics S2000 spectrometer. The VUV luminescence in the correlation experiments was recorded by a Hamamatsu R5070 photomultiplier tube attached to a window covered by a film of C7H5NaO3 (sodium salicylate), which serves as a convertor of the VUV photons into photons of visible range. Control measurements of the cathodoluminescence spectra and spectrally resolved TSL in the VUV range were performed with a VUV monochromator. Additional experiments on electron detachment in the O2-doped Ar matrix were carried out with a laser. We used a COHERENT Ar ion laser (Innova 200). The laser light of wavelength needed was introduced into the sample chamber with an optical fiber. The power did not exceed 20 mW. In that case, TSL and TSEE were measured after a short exposure of the electron preirradiated sample to the laser beam.

’ RESULTS AND DISCUSSION ’ EXPERIMENTAL METHODS The experiments were carried out in the ultrahigh vacuum chamber with a base pressure in the range of 10
10 mbar. The details of the experimental techniques are described elsewhere.22 Samples of pure Ar (99.999%) and Ar matrix doped with O2 were grown in a vacuum chamber onto a metal substrate, an Au/MgF2 coated copper plate. The substrate was cooled by a two stage, closed cycle Leybold RGD 580 cryostat and during deposition was kept typically at 8 K. The gas mixture was prepared in a gashandling system previously pumped out and degassed by baking in order to decrease the amount of contaminations. The gas flow rate and the sample thickness were controlled by a Bronkhorst Gas Flow Controller. In the current experiment, we used films of 100 μm thicknesses. Annealing was performed at 28 K for 5 min to avoid sample sublimation. Thereafter the sample was slowly cooled down at a rate of 3 K min
1. The annealed films were

Pure Argon Samples. Irradiation of the films with electrons of several hundred electronvolts, close to the maximum of the ionization cross section, efficiently generates electron
hole pairs. The energy Ei needed for creation of an electron
hole pair in Ar is about Ei = 27 eV.23 The practical range of 500 eV electrons in solid Ar was found to be about 7 nm.24 The profiles of energy deposited by electrons calculated in refs 25 and 26 appeared to be quite broad with a nearly Gaussian energy distribution and an extended high-energy tail, ranging up to 60 nm at Eirr = 1 keV. The ionization by primary and secondary electrons generates an equal number of holes and electrons if the film is transparent for the primary beam. The balance between their numbers is governed by the mobility of charge carriers and their trapping or self-trapping. As mentioned before, excess electrons are in a freelike state and exhibit high carrier mobility. The holes in rare-gas solids become self-trapped on a short time scale of 7259

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Figure 1. Dose dependence of TSEE yield from unannealed Ar film preirradiated by 500 eV electron beam.

10
12 s,1 forming intrinsic cationic centers of dimer configuration Ar2þ, which remain stable at low temperatures as long as the electrons are trapped elsewhere. There are three groups of electron traps: (i) traps related to pre-existing lattice defects, that is, growth defects, (ii) radiationinduced defects, and (iii) dopants, impurities or their fragments produced under irradiation. In view of the negative Ea of solid Ar, the most probable trapping sites are vacancies, vacancy components of Frenkel pairs, vacancy clusters, grain boundaries, and impurity centers with a positive electron affinity. When electrons are detrapped optically or by heating, they are able to travel for large distances. In solid Ar, a large electron escape depth of about 500 nm was found.27 The TSEE active layer estimated in ref 19 was found to be ∼1000 nm. The competing channel, recombination of self-trapped holes with electrons 

Ar2 þ þ e
f Ar2 ð3 Σu þ Þ f Ar2 ð1 Σg þ Þ f Ar þ Ar þ hvðVUVÞ þ ΔE yields a stimulated luminescence in the VUV range. This is a wellknown M-band at 9.8 eV stemming from the radiative decay of excimer centers Ar2* via the bound-free transition 3Σuþf1Σgþ. Analysis of TSEE and TSL yields in relation to the sample structure, irradiation condition and presence of dopants provide us with information not only on the local defect levels within the band gap but also on charge accumulation, charge redistribution and branching of the relaxation paths. The TSEE yield from an Ar film measured as a function of exposure time in the temperature range of 8
45 K is shown in Figure 1. In this experiment,the film was deposited at 8 K and then irradiated by a 500 eV electron beam. The total yield of electron emission steeply increased up to about 10 min exposure and then saturated. The yield of TSL arising from the recombination reaction between the self-trapped holes and electrons showed the tendency to saturation at a much longer exposure time of about 1 h, owing to its essentially bulk nature. Note that for annealed samples the saturation effect occurs at a shorter exposure time. The TSEE curve taken after short exposure of the sample to the beam exhibits a clearly distinguished structure.

Long irradiation resulted in a broadening of the peaks and loss of the TSEE curve structure. The influence of annealing for short and long exposure times is demonstrated in Figure 2a,b. The TSEE yield detected from the annealed films is much lower than that from unannealed ones. In contrast, the yield of intrinsic recombination luminescence increased for the annealed samples. Annealing strongly suppressed the peak at 12 K, which was identified in the first report of TSEE from solid Ar28 as related to surface traps and traps at inner interfaces in the sample. In the present experiment, we observed in addition a narrow peak at 9 K prevailing at a lower energy, 100
200 eV beam excitation. As shown in Figure 2a, this peak completely disappeared after annealing resulting in recrystallization with a crystal grain growth and could not be distinguished in the TSEE curve taken after a long-time exposure (Figure 2b). This suggests that the low-temperature peak at 9 K originates from shallower surface-related traps with a trap depth Ed ≈ 1 meV. However, at present we are not able to specify the associated sites. The bulk peak at 15 K clearly seen in the TSEE yield of the annealed Ar samples is assigned to radiation-induced defects, Frenkel pairs.28 The close association of this peak with radiation
induced defects in Ar was demonstrated in ref 29. Their structure was supposed to be the split Æ100æ “dumbbell” configuration of interstitial atom and vacancy. Because of the negative Ea, the vacancy component of a Frenkel pair serves as a trap. The trap depth of Ed = 14 meV estimated by different methods of activation spectroscopy is in close agreement with the results reported previously.29,30 Note that a similar peak at about 16 K was observed in the experiments on TSL and thermally stimulated conductivity of solid Ar doped with Au and Ag preirradiated by X-ray and photons.31 There the estimated Ed = 36.5 meV31 is higher than that extracted for Ar solids preirradiated by an electron beam. Such a divergence of Ed can be caused by a different trap distribution and charging effects. As demonstrated before, long-time irradiation conceals the TSEE curve structure. This tendency is kept for both annealed and unannealed samples (Figure 2b). Such a peak broadening is ascribed not only to a wide distribution of pre-existing traps, but also to radiation-induced traps and formation of a “Coulomb 7260

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Figure 2. (a) TSEE yields taken from annealed and unannealed Ar samples after 1 min exposure to 500 eV electron beam. (b) TSEE yields taken from annealed and unannealed Ar samples after 40 min exposure to 500 eV electron beam.

Figure 3. TSEE (red) and VUV TSL (blue) yields taken from annealed Ar sample preirradiated by 500 eV electron beam.

landscape” resulting from the long-range Coulomb interaction between charged centers in preirradiated samples.22 Figure 3 shows the simultaneously measured TSEE current and VUV TSL glow curve taken from an annealed Ar sample preirradiated with 500 eV electrons for 5 min. A coincidence of peak positions is observed, pointing to a common primary process, the release of electrons from the same kind of traps. However, the intensity distribution of VUV TSL and TSEE curves appeared to be different, demonstrating a competition between the charge recombination channel and exoelectron emission. The efficiency of the relaxation channels is controlled by several factors, like sample thickness and morphology, charge distribution, electron trap-cation separation, concentration and type of retrapping centers, and so forth. TSEE dominates for lowtemperature and high-temperature traps. The TSEE intensity distribution for the 12 K peak closely matches the intensity distribution of VUV TSL glow curve. The corresponding trap depth Ed = 12 meV agrees very well with Ed estimated in ref 30 for the samples of solid Ar grown in a closed cell at 60 K and

irradiated then by a 1 keV electron beam. A difference of the intensity distribution of TSEE and VUV TSL curves in the range of 13
20 K related to radiation-induced defects is worthy of notice. This feature is relatively broad, especially in the VUV TSL yield and slightly shifted in yields of electrons and photons, pointing to the spatial trap distribution. Note that TSEE arises substantially from a relatively thin surface layer of the sample, whereas all traps throughout the whole thickness of the sample contribute to TSL. A pronounced maximum at 23 K in VUV TSL yield deserves special attention. Discussion of its origin extends over ten years (e.g., refs 19, 21, and 29
31 and references therein). This maximum was also observed in thermally stimulated chemiluminescence of oxygen in Ar matrix.31
34 The feature emerged in any relaxation emission: intrinsic charge recombination emission (VUV TSL),21,35 thermally stimulated conductivity TSC,31 and TSEE.19,21 A high sensitivity of this peak to the presence of oxygen when measuring intrinsic relaxation emissions was found. A discussion of dopant-induced mechanisms is given further below. Upon heating we observed in the TSEE yield a peak over the range 35
43 K. This peak of nonelementary structure emerges at temperatures higher than the sublimation temperature of solid Ar (30 K). As the sample sublimates, successively deeper layers contribute to the TSEE yield. This “high-temperature” feature does not show any appreciable increase with exposure to the electron beam. In the experiments with a variable pre-exposure of the substrate in the vacuum chamber, the “high-temperature” TSEE peak increases. An enhanced TSEE feature was observed in N2-doped Ar.36 Note that an intense TSEE peak at about 35 K was observed from solid N2 preirradiated by an electron beam.37 Taking into account that N2 constitutes the greater part of air (roughly about 78%) one could suggest that the “high-temperature” feature at least partially is due to N2. However, the specific origin of this feature calls for further investigation. High TSEE currents observed from preirradiated Ar films give reason to expect an accumulation of negative charge upon irradiation of thick films with 500 eV electrons. In order to investigate the charging effect, we measured the TSEE yield from a 20 min preirradiated Ar film by applying different voltages to the Faraday plate. The film was annealed at 25 K during 5 min before the irradiation cycle. The results obtained are shown in 7261

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Figure 5. Afteremission current detected at different voltages applied to the Faraday plate. Figure 4. TSEE yields detected at different voltages applied to the Faraday plate.

Figure 4. The TSEE current was detected even at negative voltages up to
3 V and was completely suppressed only at
6 V. For quench-condensed unannealed samples, the cutoff voltage appeared to be even higher (about
11 V). Taking into account the negative electron affinity of solid Ar (Ea =
0.4 eV), one could expect that a conduction electron will experience a finite increase of kinetic energy when it exits the surface. However, the cutoff voltage appeared to be an order of magnitude higher than Ea. The finding directly points to the accumulation of uncompensated negative space charge in thick Ar films preirradiated by low-energy electrons. It is clear from Figure 2 that the concentration of trapped electrons is much higher in quench-condensed unannealed films, corresponding to the higher number of electron traps. The TSEE yield from films grown by layered deposition under electron beam irradiation exceeded that detected from the samples irradiated after deposition. Integrating the TSEE current over the detection time we estimated a lower limit on the number of extracted electrons, which appeared to be up to 1015 electrons cm
3 in samples with a high space charge. Interestingly, a thin layer of neutral O2 deposited on top of the sample did not suppress the TSEE yield appreciably. This suggests accumulation of negative charge. Only electrons of near-thermal energies will be effectively attached to O2 species. The accumulated space charge results in acceleration of the released electrons, facilitating their escape through the O2 thin film to be detected by the Faraday plate. Additional evidence of negative charge accumulation is the observation of electron “afteremission” exemplified by Figure 5. This electron emission was detected from preannealed and then preirradiated samples after the primary electron beam had been removed. As the voltage applied to the Faraday plate was stepped

down, the afteremission current decreased. Nevertheless, it was still measurable at negative voltages up to
3 V and blocked only at
6 eV. The afteremission decayed nearly exponentially with a time constant τe ∼ 1 min. Particularly long τe of a few minutes were found in Ar films grown by layered deposition under electron beam irradiation. The negatively charged Ar sample can be considered as a capacitor with the charge distributed within the Ar layer. At a negative voltage applied to the Faraday plate, only fast electrons reach the collector. Faster electrons are supposed to originate from the surface layer that has less capacitance and therefore shorter decay time constant. The afterglow detected synchronously in the band of intrinsic VUV recombination emission exhibited similar behavior. The observed afteremission resembles the field-enhanced emission from insulating layers, the Malter effect.38 However, in our case the electrons from the substrate do not come into play. Special experiments with an O2 layer as spacer between the substrate and the sample showed that the substrate had no effect on the results of TSEE measurements. Laser irradiation of the sample in the course of afteremission resulted in a strong enhancement of the electron yield, followed by an exponential decay. The afteremission from Ar films doped with nitrogen was driven by the afterglow of N atoms via the well-known 2Df4S transition, which could be considered as an internal source of light stimulating electron detrapping.39 Oxygen-Doped Argon Samples. We now turn our attention to the role of oxygen as a dopant. The photochemistry of oxygen, cage-effect, atom dynamics in matrices and clusters attracted considerable interest [refs 31
34 and 40
44 and references therein]. The energy transfer from Ar to oxygen centers in Ar clusters of variable size was studied in detail using selective excitation and spectrally resolved emission.44 It was suggested that the excitation energy transfer is most likely to occur from Ar exciton states to O2þ centers. Then formation of ArO* is the result of the dissociative charge recombination 7262

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Figure 6. (a) Cathodoluminescence spectrum of O2-doped solid Ar in the range of oxygen emissions; (b) TSL spectrum taken at 23 K from O2-doped solid Ar preirradiated by 500 eV electron beam.

reaction: ArO2þ þ e
f ArO*(1 S) þ O (1D). Spectroscopic signature of this process is the emission at 2.2 eV with a small matrix red shift (0.02 eV). Figure 6a shows a typical example of the cathodoluminescence spectra of solid Ar doped with oxygen (10
4) measured in the range of oxygen emissions at 7 K. The line of atomic oxygen dominates the spectrum. As the concentration of O2 increases its intensity decreases. Temperature quenching of the O line is observed with increasing temperature. Along with this emission, the spectrum exhibits a series of molecular bands, which was identified by isotope analysis45 as the C3Δu f X3Σg
transition. We were able to detect the vibrational bands from (0
5) up to (0
13). The progression observed in our experiments is in general agreement with previously published data.34,45 The spectrum detected in cluster experiments44 shows redistribution of the intensities in favor of molecular emission. No atomic emission was registered in TSL spectra shown in Figure 6b. The spectrally resolved TSL in the visible range consists of the same vibrationally relaxed bands of the C3Δu f X3Σg
progression. It stems from thermally induced adiabatic recombination of oxygen atoms 

Oð3 PÞ þ Oð3 PÞ f O2 ð3 Δu Þ f O2 ð3 Σg
Þ This reaction is frozen at low temperatures up to 14 K. Upon further heating, the molecular bands are observed to rise and peak at 23 K. The TSL glow curve plotted at the energy of the most intense (0
10) band of the molecular progression is presented in the inset of Figure 7. The curve is not elementary, namely, two features can be distinguished, a less intense one at about 19 K and the pronounced maximum at 23 K. These features were assigned as emerging from thermally induced diffusion of O atoms detrapped from different lattice sites followed by O2* formation.33 It was suggested that two sites, substitutional (0.375 nm) and interstitial octahedral (0.156 nm), contribute to the glow curve. Analysis of the curve using different activation spectroscopy methods yield activation energies Ea = 21 ( 5 meV and Ea = 45 ( 5 meV in good agreement with ref 33. Ea for the 23 K peak correlates with the value reported by Schrimpf et al.31

Figure 7. TSL yield taken from O2-containing Ar sample (0.1%) in the VUV band of intrinsic charge recombination emission. Thermally stimulated chemiluminescence of O2 plotted in (0
10) vibrational band is shown in the inset.

Note that the recombination of neutral atoms is described by the same formalism as the recombination of charged carriers. Taking into account the high positive Ea of oxygen, Ea = 0.44 eV for O2 and 1.46 eV for O, one may expect an effective formation of deep traps upon irradiation of matrices doped with oxygen. High binding energies of electrons make these anion centers thermally disconnected. Moreover, in view of the low mobility of ionic species in rare-gas solids,46 the thermal diffusion of O
centers is not expected to contribute to the electron release from oxygen traps. To bring into play the mechanism of triggering relaxation by the chemiluminescent reaction O þ O f O2* f O2 þ hν, followed by the photon emission in the visible range via the C3Δu f X3Σg
transition, one needs a high enough concentration of neutral atomic O centers in the matrix. Figure 7 shows the yield of TSL in the VUV band originating from Ar2þ neutralization. The curve was taken from an oxygen-containing 7263

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Figure 9. TSEE currents taken from annealed O2-doped Ar sample immediately after irradiation by electron beam (red) and after short exposure to the laser light (blue).

Figure 8. The decaying parts of the VUV glow curve at 19, 21, and 23 K.

Ar film (0.1% O2) deposited at 17 K to “clean” the glow curve by removing the low-temperature peaks and thereafter irradiated with 500 eV electrons. The heating was performed in a stepwise mode with 2 K steps and 20 min interval between the successive temperature ramps. The distinctive feature of this curve is a nonmonotonic background in the temperature range 17
25 K. After each temperature step the intensity of VUV TSL did not reach its zero level. Note that in the experiments performed with nominally pure Ar samples such an effect was absent.22 The background falls within the temperature range where thermally stimulated chemiluminescence of oxygen (Figure 6 b) was observed. The chemiluminescence yield plotted at the energy 2.43 eV, which corresponds to the (0
10) band of the vibrational progression C3Δu f X3Σg
of O2*, is shown in the inset. The glow curve was taken at the heating rate 3.2 K min
1. The VUV

TSL background maximum and the maximum of the O2* glow curve are in close agreement. A similar background was observed in the TSEE yield taken from an O2-doped Ar matrix upon stepwise heating. Analysis of the isothermal decay parts of the VUV TSL yield shows that they could be almost perfectly fitted by double exponentials as can be seen in Figure 8. Short τ1 (about 12 s) are the same as detected in nominally pure Ar films,22 while the second exponential τ2 (130
200 s) appeared to be much longer than that for pure Ar (80
120 s). The findings point to a slow process of diffusion, which is thought to be thermally stimulated diffusion of O atoms, resulting in O2* formation. We performed a special experiment to facilitate the chemiluminescent reaction in the Ar matrix via an increase of the neutral O atom concentration. For this purpose, we exposed the preirradiated Ar film enriched with O2 to laser light at a wavelength of 476 nm. The energy of these photons is sufficiently high for electron detachment from the O
centers. The binding energy Eb of electrons is estimated taking into account the polarization energy (Ep =
1.15 eV47) of an Ar matrix Eb = Ea
Ep = 2.6 eV. The sample was annealed and irradiated by 500 eV electron beam prior to laser illumination. The TSEE yield detected after 1 min exposure to the 20 mW laser beam is shown in Figure 9 in comparison with the yield taken from the sample not exposed to the laser. An enhancement of the 23 K peak, which correlates with the peak of chemiluminescence due to recombination of neutral O atoms is clearly seen. The 29 K peak was observed in the TSEE yield at the sublimation temperature of Ar. In a previous study,19 it was assigned to stimulation by chemiluminescent reaction, N atom recombination leading to formation of N2*. A detailed study of N2-doped Ar films48 shows that the “cold” vibrationally relaxed v0 = 0 A3Σuþ N2 emission in TSL from samples preirradiated by an electron beam is observed up to 42 K, with a maximum near 25 K and a shoulder between 30 and 35 K. The yield of intrinsic VUV TSL closely follows N2* emission in line with our recent study.36 The suggested radiative mechanism of electron detrapping provides a long-range energy transfer through the solid transparent to visible light. The observed “conversion” of low-energy chemiluminescence photons into high-energy photons of recombination 7264

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The Journal of Physical Chemistry A luminescence clearly demonstrates a prominent role of chemiluminescent reactions on the branching of relaxation paths and their effect on the stability of charged centers.

’ CONCLUSION Charge accumulation and relaxation processes in electron beam preirradiated films of pure and doped solid Ar were studied, applying concurrently the combination of current and optical activation spectroscopy methods. Thermally stimulated exoelectron emission TSEE was simultaneously detected with luminescence TSL over a wide spectral range. The primary states for the processes under study are considered to be self-trapped holes Ar2þ, trapped electrons and radicals formed under electron beam irradiation. Two modes of controlled sample heating were used: linear heating at a constant rate and stepwise. Different quality films were grown and the influence of annealing on the TSEE and TSL yields was ascertained. The yields of electrons and photons were monitored with exposure time and at variable voltage of both polarities applied to the Faraday plate. Solid evidence of a negative charge accumulation under exposure to an electron beam is presented. The contribution of dopants and impurities to charge accumulation and relaxation processes is discussed. The experiments with laser-induced electron detachment from O
centers demonstrated an enhancement of the relaxation emissions, electrons and VUV photons via the reaction O þ O f O2* followed by the emission of visible light. Chemiluminescent reactions can be viewed as an efficient “internal source” of photons to drive charge relaxation. Our new data support the long-range radiation mechanism of relaxation triggering in preirradiated rare-gas matrices and contribute a fresh aspect, interconnecting atomic and electronic subsystems in insulating materials. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors cordially thank Professors K.S. Song, D. Comins, G. Strazzulla, and E. Seperuelo Duarte for helpful discussions. E.V.S., I.V.Kh., and S.A.U. thank Deutsche Forschungsgemeinschaft for the travel grant. ’ REFERENCES (1) Song, K. S.; Williams, R. T. Self-trapped Excitons. In Springer Series in Solid State Science, 2nd ed.; Springer-Verlag: Berlin, 1996; Vol. 105. (2) Sanche, L. Primary interactions of low-energy electrons in condensed matter. In Excess Electrons in Dielectric Media; Ferradini, Ch., Jay-Gerin, J.-P., Eds.; CRC Press: Boca Raton, FL, 1991; p 1. (3) Johnson, R. E.; Schou, J. Mat. Fys. Med. K. Dan. Vidensk. Selsk. 1993, 43, 403. (4) Baragiola, R. A. Planet. Space Sci. 2003, 51, 953. (5) Leroux, H. Dust modification under photon, electron and ion irradiation. In Interstellar Dust from Astronomical Observations to Fundamental Studies; Boulanger, F., Joblin, C., Jones, A., Madden, S., Eds.; EAS Publications Series: Les Ulis Cedex, France, 2009; Vol. 35, p 153. (6) Cassidy, T.; Coll, P.; Raulin, F.; Carlson, R. W.; Johnson, R. E.; Loeffler, M. J.; Hand, K. P.; Baragiola, R. A. Space Sci. Rev. 2010, 153, 299.

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