Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons

are studied with the focus on the role of diffusion controlled atom-atom recombination reaction

of oxygen in the relaxation cascades. The results of correlated in real time measurements of

thermally stimulated phenomena are presented. The experiments have been performed using activation

spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolved

thermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrapping

triggering the relaxation cascades is obtained

Belov, A.G.

Bondybey, V.E.

Gumenchuk, G.B.

Ponomaryov, A.N.

Savchenko, E.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Fizika Nizkikh Temperatur, 2006, v. 32, No. 11, p. 1417–1421Oxygen-driven relaxation processes in pre-irradiatedAr cryocrystalsE.V. Savchenko1, A.G. Belov1, G.B. Gumenchuk1, A.N. Ponomaryov2,and V.E. Bondybey2,31B. Verkin Institute for Low Temperature Physics and Engineering of the National Academyof Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, UkraineE-mail: savchenko@ilt.kharkov.ua2Institut fr Physikalische und Theoretische Chemie, TU Mnchen, Lichtenbergstr, 4,Garching 85747, Germany3University of California, Irvine 92697, USAReceived September 13, 2006Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy elec-trons are studied with the focus on the role of diffusion controlled atom-atom recombination reac-tion of oxygen in the relaxation cascades. The results of correlated in real time measurements ofthermally stimulated phenomena are presented. The experiments have been performed using acti-vation spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolvedthermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrap-ping triggering the relaxation cascades is obtained.PACS: 72.20.Jv, 79.75+g, 78.60.KnKeywords: thermally stimulated luminescence, thermally stimulated exoelectron emission, relaxationprocesses, radiation effects, radical recombination, cryocrystals.IntroductionOxygen is one of the most abundant elements in theuniverse that exhibits in many respects unique proper-ties attracting considerable interest of scientists overmany years. A.F. Prikhot’ko studied extensively ele-mentary excitation of solid oxygen [1] and a lot of thecredit must go to her research group which discovereda number of novel phenomena in solid oxygen. Thebrilliant findings of A.F. Prikhot’ko inspired us whenwe went to the heart of the matter on the role of oxy-gen in the relaxation pattern of pre-excited cryo-crystals. Exposure of insulating materials to ionizingradiation results in formation of electron-hole pairs,defects of structure, molecule fragments. Relaxationchannels following the primary electronic excitationinvolve such processes as electron-hole recombination,trapping or self-trapping of charge carriers, emissionof electrons and photons etc. Some fraction of energyabsorbed by solids under irradiation is then stored bycharged and neutral centers formed in the lattice.After the irradiation is completed a further relaxationcan be stimulated by heating the samples or optically— by photons. Both factors can induce relaxation pro-cesses in electronic and atomic subsystems. Clear un-derstanding of the radiation effects changing the solidproperties is of great importance for material and sur-face sciences as well as for radiation chemistry. Veryoften electronic and atomic relaxation processes occur-ring in pre-irradiated solids are considered separatelyignoring the interconnection between these channelsof relaxation. Our recent studies demonstrated thefundamental importance of the interconnection be-tween atomic and electronic processes in the formationand branching of relaxation paths [2].Ar cryocrystals were chosen as matrices for severalreasons: (i) because of a small atomic radius of oxygen(0.12 nm) it can be easily stabilized at low tempera-tures in Ar solids without any significant distortion ofthe lattice; (ii) solid Ar and O2 have close sublimationtemperatures (about 30 K); (iii) a high mobility and© E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey, 2006free-like behavior of electrons in solid Ar [3] togetherwith a large electron escape depth of about 500 nm [4]enables one to gain information about relaxation pro-cesses in the bulk and at the surface; (iv) a negativeelectron affinity of solid Ar (–0.3 eV [3]) facilitates es-caping the electrons from the surface. Moreover, thefirst successful experiments on thermally stimulatedexoelectron emission (TSEE) [5] offered the prospectfor developing of low-temperature comprehensive acti-vation spectroscopy technique as applied to cryocrystals.We combined TSEE studies with measurements ofthe total and spectrally resolved yields of thermallystimulated luminescence (TSL). Partial yields in the vis-ible and in the vacuum ultraviolet (VUV) ranges weredetected. In addition, the experiments on photon-stimu-lated exoelectron emission (PSEE) were performed.Since both TSL and TSEE are quite sensitive to samplestructure and concentration of guest centers there is aclear need to make synchronous measurements of TSL,TSEE on the same sample. We present the results of«correlated activation spectroscopy» studies of relax-ation processes in pre-irradiated Ar solids doped withoxygen. The studies are aimed to elucidating in detailthe role of oxygen in the relaxation cascades.ExperimentalIn this study we used the low-temperature modifi-cation of activation spectroscopy technique developedrecently by our group, namely the real time-correlatedmeasurements of relaxation processes in cryogenic so-lids with simultaneous measurements of TSEE, spec-trally resolved TSL or partial TSL yields in a chosenwavelength range. The samples were grown from thegas phase by deposition on a metal substrate coatedwith a thin layer of MgF2. The substrate was cooledby a closed-cycle 2-stage Leybold RGD 580 cryostat.The high-purity (99.999%) Ar gas was used. A basepressure in the vacuum chamber was 10–8 mbar. Thesamples were doped with minute amounts of O2(10–4). The deposition was performed with a concur-rent irradiation by 500 eV electrons to generate chargecenters throughout the layer. The current density waskept at 30 A/cm2. A typical deposition rate was10–1 m/s. We deposited the Ar films of 50–100 mthickness. The sample thickness and the depositionrate were determined by measuring the pressure de-crease in a known volume of the gas-handling system.The temperature was measured with a calibrated sili-con diode sensor, mounted on the substrate. The rela-xation processes in the Ar samples were studied in thetemperature range from 7 to 42 K. The programmabletemperature controller LTC 60 allowed us to maintaindesired temperatures of deposition, irradiation andheating regimes. Two heating modes were used: linearheating at a constant rate of 3.2 K/min and step-wiseheating of the samples with a step of 2 K and a 5 mininterval between successive steps. After the samplepreparation the substrate was turned to the positionfor measurements. We measured simultaneously theyields of thermally TSEE from the samples and spec-trally-resolved TSL in the visible range or the yield ofTSL in the VUV range. The exoelectron yield wasmeasured with an Au-coated Faraday plate kept at asmall positive potential +9 V and connected to a cur-rent amplifier FEMTO DLPCA 100. A centrally lo-cated hole in the Faraday plate permitted us to detectspectra from the sample through an optical window.The spectra were recorded by a spectrometer (Multi-channel S2000 Spectrometer Ocean Optics based onCCD detectors) operating in the range 170–1100 nmwith a resolution not worse than 1.3 nm in themillisecond time window. This spectrometer allowsmonitoring the temporal evolution of the spectra inthe operating range. The program developed permit-ted us to detect synchronously spectra over the entireoperating range, TSEE current, temperature and pres-sure in the chamber.Results and discussionFigure 1 shows typical curves of the yields of ther-mally stimulated exoelectron emission and recombina-tion thermally stimulated luminescence in the VUVrange stemmed from the reactionAr Ar Ar Ar2 2      e E h  (9.7 eV). (1)The measurements were performed simultaneously onthe same sample in the temperature range 7–42 Kusing heating at a constant rate of 3.2 K/min. Theclearly seen structure of the curves reasonably agrees1418 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey5 10 15 20 25 30 35 40 4502040608010012014002468101214TSEETemperature, KVUV TSLVUVTSLintensity, arb. unitsTSEEcurrent,pAFig. 1. Yields of TSL in the VUV range and TSEE takenfrom the sample of Ar which was annealed at 25 K andthen irradiated at 7 K.with the previously measured yield of TSL detectedin the M-band [6,7] and the yield of TSEE [5] withthe only difference that the curves in Fig. 1 showmore distinct features. The low-temperature peak at11.5 K is related to the electron traps in a subsurfacelayer or at inner interfaces of the sample. The peak atabout 15 K exhibiting the pronounce dose dependencewas assigned to radiation-induced defects [5,6,8,9]. Arather strong TSEE maximum observed between 37and 41 K is presumably due to electrons trapped toodeep in the sample to be promoted to the conductionband, which are however released on the sample eva-poration. The corresponding maximum in the VUVTSL turned out to be quite weak.The origin of the broad 23 K peak in thermallystimulated phenomena was under discussion over10 years. This peak was detected in the total yield ofTSL [5,8] and in intrinsic recombination emission —spectrally resolved TSL in the M-band [6] taken fromAr pre-irradiated by a 1 keV electron beam as well asfrom Ar pre-irradiated by x-rays [7]. Note, that thepeak at 23 K was also registered in the thermally sti-mulated conductivity [9] and in thermally stimulatedexoelectron emission [5]. On the other hand, the in-tensity of this peak showed a clear connection withthe presence of oxygen in the samples enhancing TSLand TSEE peaks at 23 K with increasing oxygen con-tent. Monitoring of spectrally resolved TSL in the vi-sible range taken from pre-irradiated Ar solids re-vealed the emergence of O2 emission near 23 K inoxygen-containing samples. Different mechanisms ofthe 23 K peak formation were suggested to explainthis puzzling behavior of this peak in the yields ofthermally stimulated phenomena in Ar solids. For in-stance it has been suggested in [9] that this peakis caused by recombination of O– ions with neutralO atoms. Another scenario was discussed in [10,11]. Itwas supposed that neutral O atoms on heating startedto diffuse through the Ar lattice forming oxygen mole-cules O2 . The radiative transition of O2 into theground state was considered as a stimulating factor toelectron detrapping which is the primary step ofbranching relaxation channels.To verify the discussed above [9–11] mechanisms oftriggering relaxation cascades by recombination reac-tions we performed the experiments with synchronousmeasurements of thermally stimulated chemilumi-nescence of O2 stemmed from the diffusion-controlledatom-atom recombination reaction and TSEE yield aswell as yields of TSEE and TSL in the VUV rangeoriginated from the intrinsic charge recombination re-action (1). The samples were doped with O2 (10–4)and the generation of O atoms under irradiation wasmonitored spectroscopically. Figure 2 shows a part ofthe luminescence spectrum in the visible range re-corded under electron beam at low temperature (7 K).The molecular bands of oxygen (the C Xu g3 3 transition) became relatively weak just after 10 minirradiation. The strongest feature in the spectrum be-longs to the O atoms effectively formed in the sampleunder electron beam. An increase of the O line inten-sity with exposure time is illustrated in the insert.In order to study the origin of the 23 K peak we«removed» all the peaks at temperatures lower thanthe temperature of emerging the peak under study.For the purpose we used the so-called «cleaning tech-nique» [12] and irradiated oxygen containing samplesat 18 K. On completing the irradiation we recordedthe yield of TSEE and the TSL yield in the VUVrange shown in Fig. 3. Both curves exhibit a maxi-mum near 23 K thus demonstrating the correlation ofthe exoelectron emission and the emission of the VUVphotons of the intrinsic charge recombination reaction(1). The VUV TSL curve turn out to be much broaderthan the TSEE curve which is to say that some addi-tional electron traps are involved in the bulk recombi-nation channel. The corresponding VUV TSL curvetaken from the sample annealed at 25 K (Fig. 1) ex-hibits a more narrow peak which correlates with the23 K peak in TSEE. At the same time the correlated inreal time measurements of the TSEE yield and spec-trally resolved TSL in the operating range of the spec-trometer demonstrated in a conclusive way the corre-lation in the TSEE yield of the sample irradiated at18 K and the observed simultaneously photon emissionOxygen driven relaxation processes in pre-irradiated Ar cryocrystalsFizika Nizkikh Temperatur, 2006, v. 32, No. 11 1419300 400 500 600 70005101520253035550 560 5700102030405060Wavelength, nmIntensity,arb.units12Fig. 2. Luminescence spectrum of the Ar sample dopedwith O2 (10–4) recorded in the range of O2 and O emis-sions. The spectrum was detected during deposition underelectron beam. The O line detected after 10 (1) and 20 (2)min exposure is shown in the insert.of O2 (the C Xu g3 3  transition). Two cycles ofthe experiments were performed to check carefully thecorrelation (with liner and step-wise heating). Figure 4,adisplays the oxygen molecule progression in TSL mea-sured in step at 23 K. The simultaneously detectedTSEE current is depicted in Fig. 4,b together with thebehavior of the chosen band of the progression ( = 0, = 10). The experiment with the linear heating(Fig. 5) confirmed the correlation between the mole-cular oxygen emission and the yield of the TSEE cur-rent near 23 K.Our experimental data on relaxation processes inpre-irradiated Ar solids doped with oxygen enable usto restore the following scenario to explain the patternpuzzling at first sight.The charge recombination reactions in pre-irradi-ated rare-gas atomic cryocrystals are controlled by themobility of electrons. The weakest bound in the trapselectrons are mobilized at the lowest temperatures andescape the crystal or neutralize positive intrinsic(self-trapped holes) ions. These charge recombinationreactions result in the appearance of the VUV emis-sion. At a definite, higher temperature, the O atomspresent also start to diffuse, and a complex sequenceof events may take place. Recombination of neutral Oatoms results in the formation of molecular O2 inbound excited electronic states. This is followed by arapid relaxation, and eventually by radiative transi-tion into the ground state. The emitted visible rangephotons, in turn, can be absorbed and provide the en-ergy needed to detrap electrons from deeper traps pro-moting them to the conduction band. This is somekind of «internal photoeffect» followed by (i) ejec-tion of exoelectrons from the surface, (ii) conversionof visible light into VUV photons via recombinationof electrons with self-trapped holes by the reaction(1), or (iii) conversion of visible light into photons ofother range via recombination of electrons with someextrinsic positively charged centers. All these elec-tron-driven processes, which are outlined in thescheme below, were detected in our experiments.1420 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey5 10 15 20 25 30 3502040608010012014016018020001020304050TSEETemperature, KVUV TSLVUVTSLintensity, arb. unitsTSEEcurrent,pAFig. 3. Yields of TSL in the VUV range and TSEE takenfrom the oxygen-containing sample pre-irradiated at 18 K.16000 18000 20000 22000 24000 2600050101520253035404550Wavenumber , cm–10 1000 2000 3000 4000 50000102030405060700100200300400500TSEE39K37K35K33K31K29K27K25K23K21K19KTime, sTime,s17KO line2TSLTSEEcurrent,pATSLi, arb.unitsntensityIntensity, arb.unitsab28002500Fig. 4. TSL yield in the visible range and TSEE yieldmeasured at step-wise heating of the Ar sample dopedwith O2 (10–4). The sample was pre-irradiated at 18 K.The TSL spectrum measured at the 23 K step is shown in(a); (b) displays the temperature behavior of the 0–10 mo-lecular band of the C Xu g3 3  transition.5 10 15 20 25 30050100150200250300350400511 nmTSL OTSEETemperature, KTSLintensity,arb. unitsTSEEcurrent,pA040801201602002402Fig. 5. Yields of O2 emission in TSL and TSEE takenfrom the oxygen-containing sample pre-irradiated at 18 K.where Rg is a rare-gas atom.The results obtained provide solid evidence for theelectronic relaxation stimulation via chemiluminescentreactions. It is worthy of note that in contrast to thelocal energy release under nonradiative radical recom-bination the revealed radiative mechanism provides along-range energy transfer for hundreds of lattice con-stants. Based on these results other radiative mecha-nisms of relaxation cascades triggering in pre-irra-diated solids can be predicted, e.g., stimulation ofrelaxation by radiative transitions of metastable atomsor molecules. The first results demonstrating reliabi-lity of this mechanism are presented in [14]. The ex-periments on stimulation of relaxation processes by la-ser light [2,15] provide an additional proof of themechanisms discussed.SummaryThe combination of correlated in real time measure-ments of spectrally-resolved thermally stimulated lu-minescence and exoelectron emission from pre-irradi-ated Ar solids doped with O2 has given conclusiveevidence in favour of relaxation cascades stimulationvia chemiluminescent reactions. The verified mecha-nism provides a long-range energy transfer and dem-onstrates the existence of a new relaxation channel(triggering of electronic relaxation processes by diffu-sion controlled atom-atom recombination reaction fol-lowed by light emission).1. Yu.B. Gaididei, V.M. Loktev, A.F. Prikhot’ko, andL.I. Shanskii, in: Cryocrystals: Elementary Excitationsof Solid Oxygen, B.I Verkin and A.F. Prikhot’ko (eds.),Naukova Dumka, Kiev (1983), p. 231.2. E.V. Savchenko and V.E. Bondybey, Phys. StatusSolidi A202, 221 (2005).3. K.S. Song and R.T. Williams, Self-Trapped Excitons,Ser. Solid-State Sci., v. 105, Springer, Berlin, Heidel-berg (1996).4. E. Gullikson, Phys. Rev. B37, 7904 (1988).5. E.V. Savchenko, O.N. Grigorashchenko, A.N. Ogurtsov,V.V. Rudenkov, G.B. Gumenchuk, M. Lorenz, A. Lam-mers, and V.E. Bondybey, J. Low Temp. Phys. 122, 379(2001).6. E.V. Savchenko, A.N. Ogurtsov, O.N. Grigorashchenko,and S.A. Gubin, Fiz. Nizk. Temp. 22, 1210 (1996)[Low Temp. Phys. 22, 929 (1996)].7. M. Kink, R. Kink, V. Kisand, J. Maksimov, and M.Selg, Nucl. Instrum. Methods B122, 668 (1997).8. A.N. Ogurtsov, E.V. Savchenko, O.N. Grigorashchenko,S.A. Gubin, and I.Ya. Fugol’, Fiz. Nizk. Temp. 22,1205 (1996) [Low Temp. Phys. 22, 922 (1996)].9. A. Schrimpf, C. Boekstiegel, H.-J. Stkman, T. Borne-mann, K. Ibbeken, J. Kraft, and B. Herkert, J. Phys.:Condens. Matter 8, 3677 (1996).10. E.V. Savchenko, O.N. Grigorashchenko, A.N. Ogurtsov,V.V. Rudenkov G.B. Gumenchuk, M. Lorenz, A.M.Smith-Gicklhorn, M. Frankowski, and V.E. Bondybey,Surface Review and Letters 9, 353 (2002).11. E.V. Savchenko, O.N. Grigorashchenko, G.B. Gume-nchuk, A.G. Belov, E.M. Yurtaeva, M. Frankowski,A.M. Smith-Gicklhorn, and V.E. Bondybey, Surface Sci-ence 528, 266 (2003).12. D.R. Vij, in: Luminescence of Solids: Thermolu-minescence, D.R. Vij (ed.), Plenum Press, New Yorkand London (1998), p. 271.13. E.V. Savchenko, I.V. Khyzhniy, G.B. Gumenchuk,A.N. Ponomaryov, and V.E. Bondybey, RadiationPhysics and Chemistry (accepted to publication).14. G.B. Gumenchuk, M.A. Bludov, A.G. Belov, A.N.Ponomaryov, V.E. Bondybey, and E.V. Savchenko,Phys. Status Solidi (accepted to publication).15. G.B. Gumenchuk, M.A. Bludov, and A.G. Belov,Fiz. Nizk. Temp. 31, 237 (2005) [Low Temp. Phys.31, 179 (2005)].Oxygen driven relaxation processes in pre-irradiated Ar cryocrystalsFizika Nizkikh Temperatur, 2006, v. 32, No. 11 1421Photon-stimulated exoelectron emission              (i)eO + O  O2 O2 + h  e  Rg2 + e  Rg2 Rg + Rg + h1(VUV) + E (ii)eGuest+ + e  Guest*  Guest + h2,                 (iii)

Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals

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Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)