Luminescence of nitrogen and neon atoms isolated in solid helium

Chemical Physics ELSEVIER

Luminescence

Chemical Physics 189 ( 1994) 367-382

of nitrogen and neon atoms isolated in solid helium

R.E. Boltnev a, E.B. Gordon a, V.V. Khmelenko a, I.N. Krushinskaya ‘, M.V. Martynenko a, A.A. Pelmenev a, E.A. Popov a, A.F. Shestakov b ainstitute of Energy Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation h Institute of Chemical Physics in Chernogolovka, Russian Academy of Sciences, I42432 Chemogolovka, Moscow Region, Russian Federation

Received 7 April 1994; in final form 8 September 1994

Abstract Investigations of metastable N( *D) and Ne(3P2) atoms isolated in solid helium have been carried out for the first time. The luminescence spectra and kinetics in the impurity helium solid phase (IHSP), with impurity centers N,, Ne, Ar, and Kr, show the sensitizing influence of a neighboring heavy particle on the forbidden N(*D-%) transition. The extremely long-lived (r= 10%) hyperbolically decaying afterglow and thermoluminescence of this transition have been observed in the IHSP. Thermoluminescence studies allowed the determination of the energy barrier for pair fusion of neighboring centers. In agreement with calculations the energy barrier of this process which determines the IHSP stability turns out to be El = 40 + 4 K. The activation energy of the long-lived afterglow stage was found to be E2 = 7 K, close to the energy of vacancy formation in solid helium. By using laser-induced fluorescence, Ne( 3P,) atoms have been detected in superfluid helium and in the IHSP for the first time. Within an experimental accuracy of 0.18 A their spectral lines were unshifted and unbroadened with respect to the gas-phase values.

1. Introduction

Studies of atoms, molecules, and ions in low temperature solids or “matrices” are an area of considerable current interest. The essential advantage of such matrix isolation is the possibility to study optical spectra for different particles, including reactive intermediates, stabilized as impurities in inert solids. Solid rare gases (RG), in view of their chemical inertness and weak polarizability, represent a particularly convenient medium for such studies. Rare gas crystals with embedded impurity particles are readily produced experimentally and are also accessible for theoretical analysis. A large number of various chemical species have been studied in solid RG - neon, argon, krypton, xenon 0301-0104/94/$07.00 Q 1994 Elsevier Science B.V. All rights reserved SSDlO301-0104(94)00337-8

and numerous reviews and books have summarized these studies [ 1,2]. Until recently it was assumed that helium, the most inert and least polarizable RG, was unavailable as matrix medium because its quantum-mechanical behavior prohibited it from being solid at normal pressure even at T= 0 K. Much of the information concerning the interactions of guest particles in a helium environment has been obtained by analyzing the luminescence of the particle immersed in bulk liquid helium. Probably Jortner et al. [3] were the first to observe the luminescence from species embedded in liquid helium: a-particle bombardment of liquid helium with N2 and O2 colloidal particles caused an and emission on the N, (A “2: +X ‘2:)

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02(C3Xz --+X3X;) m 0 1ecular transitions as well as the N(*D --f 4S) atomic one. The excitation of pure liquid helium by fast electrons produces metastable He( 2 ‘S) atoms and He, (a “2) molecules and their spectra have been observed [ 4-61. Both absorption and emission bands were shifted with respect to the free atomic and molecular lines, showing the influence of the helium environment on the excited states of helium atoms and molecules. Some spectral investigations of metastable He, (a “C) molecules in solid helium have been carried out as well [ 71. The observed lifetime of the He, (a ‘C) turned out to be close to the radiative one for the free molecule [ 81. Optical properties of alkali-earth (Ca, Sr, Ba) atoms and ions implanted into bulk superfluid helium have been extensively studied by Bauer et al. [ 9-121. The excitation bands of both atoms and ions were found to be blue-shifted for singlet transitions by 3-5 nm (for Ba’ ions by - 10 nm) and for triplet transitions by 13-20 nm with respect to the free atomic and ionic lines. They were asymmetric and broadened up to 5-7 nm in width. In contrast, the emission lines were narrower and unshifted within the experimental accuracy of 1 nm. The detection of metastable Ba atoms in the 5d6p 3P, state in bulk liquid helium has been reported in Ref. [ 121 and its lifetime has been estimated to be more than 55 ms. The shifting and broadening of the spectral lines of excited atoms observed in liquid helium has been explained in the framework of a model describing the atomic energy as a function of the radius of a helium bubble formed around the excited atoms [ 12-151. Recently investigations of solitary large He, (n = 104) clusters with immersed heavy particles have appeared [ 16,171. The IR spectral line of the SF, molecule embedded in a helium cluster was found to be red-shifted by - 1.5 cm ~ ’ [ 171. The spectral shift of SF, induced by interaction with the helium environment in acluster was calculated by Barnett and Whalley

[ISI. According to Kurten and Clark [ 191 a neutral heavy particle (Im) embedded in bulk liquid helium, as a rule, has to form a van der Waals complex ImHe, ‘. ’ Indeed, for an atom with an outer s-electron, for example Cs, the electron density is so high even at large distances that the Pauli repulsion arises much earlier than the van der Waals attraction. Therefore, for such atoms in bulk liquid helium a “bubble’‘-like state similar to the “bubble” around an electron [ 20 I forms [ 191.

Physics 189 (1994) 367-382

The formation enthalpy of the XeHe, (n = 14, 15) complex was calculated to be AH = - 300 K [ 191. We have shown [ 21,221 that the localization of helium atoms around a heavy particle in van der Waals clusters results in freezing out of such clusters and in the formation of an impurity-helium solid phase (IHSP). Essentially the suppression of the zero-point vibrational amplitude of helium atoms removes their quantum-mechanical behavior, so this solid phase can exist at pressures as high as 1 atm and up to temperatures as high as 7-8 K. This effect opens up the possibility for matrix isolation of impurity species by solid helium. Such matrix isolation was experimentally realized for N and Ne atoms initially [ 22 1, and for Ar, Kr, and Xe subsequently [ 2 11. Measurements of the elemental composition of the IHSP containing these model particles showed that for Ne or N2 there are S= 12-17 helium atoms per one impurity atom, where for heavier and more polarizable Ar and Kr atoms S = 40-60. Additionally, the high stability of the IHSP was proved: its samples exist for a long time (t> IO3 s) even after being taken out of the liquid helium at temperatures up to 6-8 K. According to a model calculation [ 231 the pair fusion of heavy centers of neighboring clusters demands the overcoming of an energy barrier of 28 cm-‘. However, destruction of an IHSP three-dimensional lattice by such fusion needs to have a significantly higher barrier because coherent movement of the entire local helium environment is necessary in this case [ 211. The IHSP discovery opens the way to investigations of various particles in helium matrices. Undoubtedly the IHSP is a novel quantum object and comprehensive investigations of its intrinsic properties are necessary. For this purpose it is reasonable to use N and Ne atoms since they are optically accessible and well studied in other RG solids. In this work the first results of experimental investigations of the metastable N atom luminescence as well as metastable Ne atom laser excitation spectra in the IHSP at temperatures 1.5-2 K are presented. From these experimental data the values of the energy barriers for processes determining the microscopic stability of the IHSP are obtained. 2. Experimental The experimental technique of the IHSP sample preparation has been described elsewhere [ 211. A

R.E. Boltnev et d. /Chemical

Fig. 1. Experimental setup: ( 1) excimer laser; (2) movable mirror; (3) dye laser; (4) system of laser beam deflection; (5) dewars; (6) atom source; (7) glass cup with He II; (8) gas jet; (9) IHSP sample; ( 10) thermometer; ( 1 I ) netted disk; ( 12) thermomechanical pump; ( 13) condenser lens; (14) grating monochromator with photomultiplier; (15) objective with interference filter and photomultiplier.

major feature of the method is the ability to form in the gas phase a jet consisting of neutral particles (atoms, molecules) diluted by helium and to direct it into bulk superfluid helium (He II). The technique enables us to check the overall process of the IHSP formation: from precooling of impurity particles in the gas phase 1241 to the formation of small clusters and their sticking into micron-sized particles. The principles of our experiment are depicted in Fig. 1. The gaseous mixtures of atoms and molecules under investigation, Im, and helium atoms ( [ Im] : [He] = 1: ( lo’-10’); Im = Ne, Ar, Kr, N2) are injected from the liquid-nitrogen cooled discharge source through a small orifice, 0.75 mm in diameter, into the dewar containing liquid helium. High relative concentrations of nitrogen atoms could be achieved at the output of the atom source - up to 30% and more [ 221. Moreover, the observation of the luminescence arising due to nitrogen atom recombination shows that the concentrations of electronically excited N( *D) atoms were comparable with unexcited N(4S) ones in the jet [ 241. The pressure of He vapor in the helium dewar was maintained at 3-4 Torr by rotary pumping, and consequently the temperature of He II was = 1.5 K. A glass cup 4.5 cm in diameter was filled with He II, where the samples were formed, was placed under the atom source at a distance of 2.5 cm. The He II level in the cup was held constant during the experiment by a continuous He II stream. The stream was pumped from

Physics 189 (1994) 367-382

369

the bottom of the helium dewar based on the thermomechanical effect in supertluid helium. The gas jet from the atom source (total flux was - 5 X lOi s-i) was directed toward the He II surface in the cup so the condensate was formed in the bulk He II. The process of condensation was easily observable due to both nitrogen atom recombination luminescence in the jet and a strong emission of N( *D) atoms in the condensate [ 241. A macroscopic IHSP sample was formed mainly at the bottom of the cup and near its wall just beneath the helium surface; from time to time the latter ring-shaped condensate sank to the bottom and joined with the rest of the sample. Usually, the time of the sample storage was about 2 X 10” s, when the sample with a visible volume of 1-2 cm3 had been stored at the bottom. The IHSP sample forms a rather homogeneous, rigid and transparent substance. The samples grew on a specially netted movable disk placed at the bottom of the cup to allow removal of the samples from liquid helium for experiments with “dry” ones (see Fig. 1). A semiconductor thermometer for temperature determination was attached to the disk. A grating monochromator, equipped with a photomultiplier (see Fig. 1)) was used to record optical luminescence spectra during sample growth as well as afterglow spectra. The signal from the photomultiplier passed to a high-speed photon counter interfaced to a computer. The spectral resolution in the range of 40s 800 nm was about 1 A. A glass lens collected the emission from different parts of the jet or the condensate, and focused it on the entrance slit of the monochromator. The afterglow decay could be detected in two ways: (i) when high sensitivity was necessary, a photomultiplier with a suitable interference filter (typically FWHM = 8-13 nm) was applied; (ii) to obtain better spectral resolution, the monochromator-based system was used. The laser induced fluorescence (LIF) method proved to be a useful and powerful method of studying electronically excited species in matrix isolation experiments [ 1,2]. We have applied the LIF method to detect the metastable Ne(“P2) atoms isolated by helium atoms. A tunable dye laser, VL- 12, pumped by an excimer laser, ELI-91 (A= 308 nm, E= 100 mJ/pulse) (see Fig. l), was used to excite Ne metastables. Its output parameters were as follows: spectra1 range 260-800

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RX. Boltnev et al. /Chemical

nm; linewidth about 0.018 nm if scanned with a grating; pulse energy 0.1-2 mJ in UV and up to 10 mJ in the visible and neat-IR range; pulse duration l&25 ns; and repetition rate up to 50 Hz. For laser induced emission detection either an interference filter or monochromator was used. The metastable Ne(3P,) atoms produced in the discharge have long enough lifetimes [ 251 to be readily studied downstream of the jet and even in the bulk He II. For this purpose a laser beam of 2 mm in diameter could cross either the gas jet or the bulk He II in the cup.

3. Experimental

results

3. I. Luminescence

of nitrogen atoms

The luminescence spectra of nitrogen atoms in IHSP during sample growth have been investigated. Since the ratio of helium atoms to nitrogen atoms and molecules in such nitrogen-IHSP samples measured in previous experiments was as high as S= [He] / ([N2]+[N])-12-17[21,26],theIHSPwasproved to be mainly consisting of solitary atoms or molecules as the impurity centers. The luminescence spectrum in the range 500-700 nm recorded during condensation of the gas mixture [N2] : [He] = 1: 80 when the optical system was focused to the area of the IHSP sample, well off the jet, is shown in Fig. 2. The dominant component of the spectrum is the N atom c-u-group (N( *D *4S) transition). It is accompanied by the weaker nitrogen a’-group at 595 nm

5000

5200

5400

5600

5800

6000

w*v%x.mcm.

6200

6400

6600

6800

7000

A

Fig. 2. Luminescence spectrum of the nitrogen IHSP sample during condensation of the [N,] : [He ] = 1: 80 gas mixture.

Physics 189 (I 994) 367-382

[ 271. There are also weak diffusive bands near 560 nm connected with emission of contaminant oxygen atoms (p-group, 0(‘S + ‘D) transition), and bands of molecular nitrogen I+-system (N,(B 311g+ A ‘2: ) transition) in the spectrum. As for (Y,(Y’,and p emissions they undoubtedly originated from the IHSP sample. N, 1 +-system bands possess a rich rotational structure (corresponding to a rotational temperature of 80-100 K) and the vibrational bands (us = 7,8; B 311,) are rather intense in the spectrum. In the jet spectrum, recorded near the He II surface, the bands are narrow (T,<20 K) and the V, = 7, 8 band are absent [ 281. This proves that the observed 1 +-system spectrum arises from the IHSP sample as well. Since the rotational temperature of N, molecules in the jet, above the He II surface, was found to be already l&20 K [ 241, only the N2 (B “I&) molecules resulting from the N atom recombination in bulk liquid helium could have a rotational temperature as high as 80-100 K. Because of the relatively long radiative lifetime of the B 311gstate of 6 X 10m6 s [ 291 the high non-equilibrium rotational temperature displayed in the luminescence spectra can be associated with fast quenching of N2 (B ‘IIs) and/or extremely low rotational relaxation in the IHSP (contrary to the gas phase, where rotational equilibrium is achieved within the N2( B 311s) radiative lifetime). The cx-group spectra for different nitrogen-helium mixtures are shown in Fig. 3. For comparison the spectrum of the a-group in a normal N2 matrix excited by electrons [ 271 is also presented. Although the a-group of nitrogen isolated by helium has the same spectral components as the a-group in the N2 matrix there are noticeable differences - the spectral components with )I = 521 and 522 nm become dominant. The spectrum of the a-group in Fig. 3a is typical for samples formed by condensation of gas mixtures containing 0.2% of N2 or less. The presence of the a!‘-group, A=595 nm, in the spectrum is typical for all recorded spectra. Usually, its intensity is 100 times less than the a-group intensity. The &-group is an electronic N( ‘D --) 4S) transition accompanied by a simultaneous vibrational excitation u = 0 + u = 1 in the neighboring N2 (X lx.,‘) molecule [ 271. The ~1’spectrum observed in our experiments is shown in Fig. 4a where the 0~’spectrum in the normal N, matrix [ 271 is also presented for comparison. One can see that the CX’spectrum in the IHSP is broader and

R.E. Boltnev et al. / Chernicul Physics 189 (1994) 367-382

C

1

5210

5220

5230

WAVELENGM

5240

5250

5;

0

,%

Fig. 3. Structure of the cY-group spectra (N( ‘D-%) transition): (a) in the nitrogen IHSP sample, condensed mixtured [Nz] : [He] = I : 2000; (b) in the nitrogen IHSP sample, condensed mixture[N,]:IHe]=1:60;(c)inthenitrogenmatrix[25].

more complex compared to the single narrow line of N, matrices. The validity of the CY’assignment was tested by “N2 experiments: the cr’-group, save for its shape, was shifted to the blue by - 80 cm- ‘. This shift corresponds to a reduction of the first vibrational quantum of the X ‘2: state when 14N2 is replaced by “N2. At first glance the presence of the cl’-group in the spectra is rather surprising, because most of the nitrogen atoms and molecules stabilized in solid helium are separated by a distance of about 11 A [ 2 I]. Since the a’-group appearance unambiguously demands the existence of at least a two-body complex N(*D)-N,, the following assumption arises: would the presence of a “heavy” particle near the excited atom inside the helium cluster be responsible for the observed luminescence? In other words, the intensive luminescence can arise only from those helium clusters which contain in every core both aN( ‘D) atom and a “heavy” neigh-

371

bor to help break the optical transition prohibition. To check this assumption a set of experiments with heavy rare gases added to the condensed nitrogenhelium mixture have been carried out. The RG atom content was much higher than the nitrogen content so there the most probable neighbor of N( *D) in the center of helium cluster should be a RG atom. For N2 : RG : He (where RG is Ne, Ar, Kr) mixtures the luminescence spectra of IHSP contain an intense cygroup and rather weak bands of the P-group; the intensity of the 1 + -system and the cx’-group were significantly weaker than for N, : He mixtures. An important point is that RG addition to the heliumnitrogen mixture, when the total flow as well as the N,/ He ratio ( [N2] : [He] = 1:2000) are kept constant, leads to a strong enhancement of the o-group intensity. For example, addition of 20 parts of Ar to the mixture mixture [N2] : [Ar]: [He] = 1:20:2000) (the increases the cl-group intensity by a factor of 5-6, while the same amount of Kr increases it by a factor of 40. Addition of Ar or Kr to the mixture probably did not change the dissociation degree of N2 molecules and the yield of excited N( *D) atoms in the discharge, because metastable Ar and Kr atoms do not have enough energy to ionize or break N2 molecules and they can only excite the latter to a bound state. Therefore the corresponding increase in a-group intensity provides, in our opinion.

WAVEL5NG54.

11

Fig. 4. @‘-group spectra: (a) in the nitrogen IHSP sample, condensed mixture [N2] : IHe] = 1 : 100; (b) in the nitrogen matrix [25].

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R.E. Bolmev er al. /Chemical

Physics 189 (I 994) 367-382

The spectra of the a-group obtained during the condensation of N, : RG : He gas mixtures presented in Fig. 5 show that the addition of heavy rare gases leads to significant changes in the spectra. They begin to resemble the nitrogen a-group spectra in the corresponding rare-gas solids although the IHSP samples mainly consist of helium (their stoichiometric ratio was found tobeS= [He]/[RG] ~40-60 [21]).AsinRGmatrices [ 30-331 as one goes from Ne to Kr the a-group spectra shift to the red with respect to the gas-phase line (A = 520 nm). The shifts for Ne, Ar, and Kr are 30.5, 88, and 145 cm-‘, correspondingly. 3.2. Nitrogen atom luminescence

D WAVELFNCTH,

%

Fig. 5. The o-group in RG-IHSP samples for different condensed mixtures: (a) IN?] : [Ne] : [He] = I :20: 1700; (b) [NJ : [Ar] : [He]=1:20:2000;(c) [N2]:]Kr]:[He]=I:20:4000.

conclusive evidence that the a-group luminescence observed comes only from IHSP pairs of a N( ‘D) atom and a N, molecule or a RG atom. Indeed the addition of a large amount of heavy RG atoms into the nitrogenhelium mixture would significantly increase the probability of their clustering with a N( 2D) atom into the same helium cluster and consequently considerably enhance the luminescence yield. A similar effect occurs by enrichment of the Nz content in nitrogen-helium mixtures.

kinetics

The luminescence decay curves of the different components of the a-group obtained after cessation of the sample storage were satisfactorily fitted by single exponential or double exponential curves. The corresponding lifetimes are listed in Table 1. For the nitrogenhelium gaseous mixtures with a relatively high nitrogen content ( = 2-5%) the decay curves are well fitted by the sum of two exponents. As is known [ 271, the different components of the a-group in a N2 matrix have different decay times. But in the case of nitrogen atom isolation by solid helium, the overall decay time for the different spectral components appears to approach 15 s (see Table 1). The a’-group behavior is similar to that of the a-group in the afterglow of the nitrogenIHSP samples. For the IHSP samples obtained by the condensation of N2 : RG : He mixtures the measured decay times of the a-group are collected in Table 2. Characteristic decay times for Kr- and Ar-containing IHSP samples are noticeably longer than those in the corresponding RG matrices. For Ne-containing IHSP samples, besides

Table I

Times of exponential Wavelength

decay for o-group

spectrum components

Condensed

in nitrogen IHSP and NZ matrix, 7 (s)

gas mixture ] N? ] : ] He ]

Nz-matrix a d

(nm)

521 522 523

I:60

I : 100

1:200

1:lOOO

1 : 5000

7;16h 15;24 30

9; 18 7;21 17;33

13 14 16

14 15 16

13 13 _

9 5;25 37

“Ref. [29]. h At some wavelengths the luminescence decay was best described by a double exponential law. This was associated with the presence of two spectral components with different lifetimes and approximately the same wavelength. The uncertainty of the decay time definition was k 20%.

R.E. Boltnev et (11./Chemical Table 2 Times of exponential decay for cu-group spectrum rare gases IHSP and RG matrices

components

in

Condensed gas mixture

Lifetime in RG-matrix

7

Wavelength

A

(nm)

Lifetime 7 (s)

(s)

IN,l:INel:IHe] =1:20:2000

519.9 520.1 521.0

260 50 350

lNz]:[Arl:IHe] =1:20:2000

521 522

26; 54 c 26; 70

IN21 : [Krl: [He] =1:20:4000

522

12; 2s

Ne matrix 340 a 3.50 h

Ar matrix 18 >’ Kr matrix Ia

“Ref. [30]. ‘Ref. 1311. ’ At some wavelengths the luminescence decay was best described by a double exponential law. This was associated with the presence of two spectral components with different lifetimes and approximately the same wavelength. The uncertainty of the decay time definitions was + 20%.

105

1

v1

i

1-,,-,-_-,_,-,_,c5

102

20bo

40bo

6060

80’00

lOdO

TIME, S

373

Physics 189 (1994) 367-382

under ordinary matrix isolation of nitrogen atoms I30331, we have found an extremely long-lived green afterglow (LLA) that lasts over three hours. The afterglow was so long and its intensity was found to decay so slowly that the time of its observation was limited only by the liquid helium reservoir in our cryostat. The dependence of the afterglow intensity on the time for the nitrogen-IHSP sample is shown in Fig. 6. The afterglow was observed through an interference filter (A,=521 nm, FWHM = 13 nm) at a temperature T- 1.47 K for the sample in bulk He II. It is clearly seen that the fast decay stage (which was described in Sections 3.1 and 3.2) is followed by a considerably slower one with a characteristic decay time of 7~8x10~s. The time dependence of the LLA intensity, Z, is best approximated by a hyperbolic law, Z- r - I. The LLA was detected for samples both in bulk liquid helium and taken out of the liquid. The samples with a visible volume of l-2 cm’ were prepared by condensation of the [ N2] : [He] = 1 : 100 gas mixture during N 2 X lo” s. In the spectral range 400-800 nm only the broad band with a maximum at A=522 nm has been observed (see Fig. 7), independently of the replacement of 14N2 with “N,, which allows us to assign the LLA to the emission of nitrogen atoms trapped in solid helium. Under condensation of [ N2] : [ RG] : [He] = 1: 20 : 2000 (RG = Ar or Kr) mixtures the LLA absolute intensity for samples with the same volume as nitrogen-IHSP samples turned out to be 20 times less for Ar-containing IHSP, and for Kr-containing IHSP samples it was below the detection threshold. Since the absolute nitrogen content of these samples was 20 times less than in the nitrogen-IHSP ones, the N(2D) atom

Fig. 6. Temporal behavior of the nitrogen-IHSP sample afterglow at T= 1.47K (dark count rate of - 70 per second).

the well-known component with a decay time of T= 350 s [ 311, new components with lifetimes of r=50sath=520. 1 nmandT=%OsatA=519.9nm appear. .’

3.3. Long-lived afterglow

1__

5150

-i&Y

-a~--

5210

WAVELENGTH.

Besides the above described nitrogen-IHSP sample afterglow with characteristics similar to those obtained

7

I

5230

5250

1

5270

1

5;

IO

it

Fig. 7. cu-group spectrum at a long-lived afterglow stage. Note the 2 nm red-shift from the gas phase line.

374

R.E. Bolrnev et (11./Chemical

Physics 189 (1994) 367-382

- b)

Ic

0

1000

1 2000

3000

4000

5000

6000 Time (s)

Fig. 8. Kinetics of the long-lived afterglow intensity for nitrogen IHSP sample (a) under step heating from curves obtained by computer fitting of experimental results in accordance with Eq. (5)).

luminescence yield of Ar-containing IHSP samples seems to be nearly the same. At high concentrations of heavy RG, a large percentage of N atoms will be in helium clusters containing an RG atom. In this case the total number of excited N(‘D) atoms provided, as will be seen below, for the LLA has to diminish significantly due to their radiative quenching in the fast stage of the afterglow. This may explain the lowering of the LLA intensity in kryptoncontaining mixtures. 3.4. Low temperature

thermoluminescence

During the long-lived afterglow stage even a slight increase in temperature (AT< 0.1 K) of IHSP samples with nitrogen atoms led to a significant rise in luminescence. The thermoluminescence has been observed beginning at 1.47 K - the lowest achievable temperature in our experiments. As far as we know this is the lowest temperature of matrix thermoluminescence ever observed. To study the influence of temperature on the LLA intensity the following experiments were performed. The nitrogen-containing IHSP samples after storage were held in bulk He II at T= 1.47 K for a time of - 1 hour. After decay of the luminescence, the temperature was increased. The experiments on the intensity-time dependence require the sample temperature

1.47to 1.93 K (b) ( 1,2,3-hyperbolic

to be kept constant with a high accuracy, therefore they were mainly carried out in the temperature range 1.472.0 K with samples submerged in He II. The temperature was set and held constant by control of the helium vapor pressure in the dewar. Luminescence observation at higher temperatures (up to N 5 K) were carried out for “dry” (taken out of He II) samples. A temperature rise up to 7 or 8 K resulted in an explosive-like destruction of the sample [ 211. A typical dependence of the luminescence intensity on temperature for a nitrogen-containing IHSP sample upon two-stage step heating from To = 1.47 K to T, = 1.73 K and then to T2 = 1.95 K is presented in Fig. 8. One observes that the temperature rise in both cases leads to strong emission enhancement. If, thereafter, the temperature was held constant the afterglow decayed according to a hyperbolic law. The kinetics of the LLA are described by this law for the whole investigated temperature interval from 1.47 to 2.0 K. Lowering the temperature causes a sharp depression of the afterglow (see Fig. 9). After repeated heating of the sample the luminescence began to increase and reached its former value only when the temperature approached the maximum temperature of the previous heating (see Fig. 9). Using an interference filter (A,= 599 nm, FWHM = 10 nm) , the thermoluminescence near

R.E. Bdtnev

et al. /Chemical

2.2

x

2.0

ii

1.8

i!

LX 1.4 0

200

400

600

800

TIUE CS)

Fig. 9. Annealing

effect for lHSP samples.

2.4

700

1

600

1

1 2.2

Physics 189 (1994) 367-382

37s

dence that N( ‘D) atoms during the LLA emit only in the vicinity of N, molecules. The of-group intensity decay is not described by either a hyperbolic or an exponential law, although this decay proceeds slowly enough with r,,* = 70 s. A weak thermoluminescence in the blue spectral range (interference filter with h, =450 nm, FWHM = 9 nm) has also been observed (see Fig. 10). At constant temperature its intensity also decayed first by a complex law with 7i ,2 = 40 s and after 100-200 s it reaches the long-lived stage. The heating of Ar- and Kr-IHSP containing nitrogen atoms also resulted in the appearance of a-group luminescence. For Ar-IHSP samples obtained by condensation of [ N2] : [ Ar] : [He] = 1 : 20 : 2000 mixtures, the decay of the LLA intensity after termination of the sample storage, as well as after step heating, was described by a hyperbolic law. 3.5. Laser stimulated emission of the IHSP containing nitrogen atoms

0

250

500

750

Time

1500

1250

1000

1750

2000

(s)

Fig. 10. Kinetics of blue molecular luminescence (A - 450 nm) of nitrogen IHSP sample at step heating from I .47 to 1.93 K.

I

5180

5200

I

I

WAVELENGTH.

I

5240

5220

I

I

5260

A

Fig. Il. Luminescence spectrumof nitrogen IHSP sample stimulated by laser irradiation ( He-Ne laser with P = S mW).

h= 595 nm (were the a’-group is located) was recorded. Its intensity was approximately 100 times weaker than for the o-group. The relation of intensities of (Yand CI’groups during thermoluminescence is similar to that during condensation. This is taken as evi-

The irradiation of the nitrogen-IHSP samples by a narrow, 0.2 cm diameter, laser beam caused an increase in green emission along the beam path. Surprisingly, the effect had practically the same efficiency over the 500-700 nm spectral range of laser irradiation. It is interesting to note that a CW He-Ne laser (A = 6328 A) with the same average power ( =: 5 mW) induced a similar luminescence intensity. The intensity of the induced glow decayed non-exponentially with or ,2 - 2&30 s. After exposure to laser irradiation during several hundred seconds the glow spread over the whole sample. The spectrum of the emission is shown in Fig. I 1. This spectrum looks like the nitrogen o-group spectrum obtained during the sample storage (see Fig. 3). The prolonged ( 1500 s) laser irradiation led to the development of a dark (nonluminescent) zone along the laser beam path. After termination of the laser irradiation the sample emission became visually uniform over the whole sample in 200 seconds. 3.6. Laser excitation spectra of metastable Ne(‘PJ atoms in gaseous and condensed helium The first results of our studies of matrix-isolated metastable Ne atoms by the LIF technique are presented

376

R. E. Bnltnev et al. /Chemical

Physics 189 (1994) 367-382

mergy t?V 3p’I

9.73

l/2

I, L

\

18.71

16.85

16.62

3s[3’2

I-

( =P* ) metastable

Fig. 12. Scheme of the Ne electron excite LIF from metastable levels.

levels and transitions

used to

here. The following transitions of Ne( ‘P,) metastable atoms (see also Fig. 12) were used for laser excitation: 3s[3/2];(‘P,)

-+3p’[ l/2],,

h=5881.89

A,

3s[3/2];(‘P2)

-+3p’[3/2]*,

h=5944.83

A,

and for luminescence

monitoring

3p’[1/2],

+3s’[1/2]:,

A=6599

3p’[3/2],

+3s’[1/2]:,

h=6678A.

I

A,

Fig. 13 displays the excitation spectra of the Ne metastables in various media in our experiment: in the gas phase, in liquid helium, and in the IHSP,sample. All spectra were obtained during the condensation of the [ Ne] : [He] = 1: 20 mixture. An intereference filter (A “, = 660 nm and FWHM = 10 nm) was used to isolate the laser induced luminescence. The gas phase spectrum was recorded when the laser beam intersected the gas jet about 1 cm above the liquid helium surface. To obtain the Ne( 3P,) LIF spectrum in liquid helium an induced emission from a laser irradiated volume being 2 cm under the liquid helium surface was collected. To obtain the spectrum of Ne(‘P,) atoms isolated by solid helium the emission from the laser-excited Ne-IHSP sample formed at the bottom of the cup ( -5 cm under the liquid helium surface (see

30 5944.55

I

I

5944.80

5945.05

WAVELINGM.

A

594

30

Fig. 13. Laser excitation spectra of the Ne 3~[3/2]~(~P~) + 3p’[ 3/2], transition: (a) in the gas phase; (b) in superfluid helium; (c) in a neon IHSP sample.

Fig. 1) ) was collected. In all cases the excitation line widths, within the experimental accuracy defined by the laser linewidth (0.18 A), turned out to be the same.

4. Discussion As has been mentioned, the bulk of the obtained IHSP represents a solid where the sublattice of “guest” particles consists of atoms and molecules matrix-isolated by solid helium. Indeed, dimerization and clustering of impurity centers in the IHSP should cause a

R.E. Boltnev et al. /Chemical

significant lowering of the experimentally measured stoichiometric ratio, S, in comparison with the value calculated for mono-particle impurity centers. It did not take place; for instance, while the theoretical S value for the nitrogen- or neon-IHSP is [He] / [Im] = 12 [ 231, the experimental S values are as high as 13-17 [ 211. This has to be true also for both unexcited N( 4S) and metastable N( *D) atoms trapped in the IHSP, the latter causing the characteristic green glow due to N( 2D-4S) transition. In the gas phase the wavelengths of these forbidden transitions are N( 2D,:,-4S,,,): 520.0 nm, N(2D,,2-4S,,2) : 5 19.8 nm and the lifetimes, r,, are 1.6 X 10”s and 4.4X 104s, respectively [25]. Despite the fact that N( ‘D) metastable in the IHSP are mainly matrix isolated by helium the characteristics of the luminescence observed in our experiments distinctly indicate the presence of “heavy” neighbors near the N( 2D) ( N2 molecule or RG atoms as in the case of N2 : RG : He mixtures). The perturbing influence of the heavy atoms on the N( 2D-4S) transition is well known. In the N, or heavy RG matrices this transition (the socalled o-group) is shifted, as a rule, to the red and its lifetime is dramatically shortened due to interaction with the crystalline field and lattice phonons. For different matrices these effects were distinct and they were thoroughly studied in a number of works [ 27,30-331. Similar effects were observed in our experiments as well. The a-group spectra are shown in Fig. 3, and the lifetimes of their components are collected in Table 1. At large N,/He ratios 2-5%, where the probability of formation of N2 macroclusters is high, the observations are similar to that known for N(*D) luminescence in N2 matrix [27]. When the mixtures were diluted by He, to reduce the dimerization probability of nitrogen atoms and molecules, the characteristics of the o-group changed, as described in Sections 3.1 and 3.2. In comparison with the N( 2D) spectra in the normal N, matrix this u-group displays a strong suppression of the phonon-induced wing (lines near 523 nm), an increase in the relative intensity of the band at the 521 nm, and the appearance of a single lifetime, 7% 15 s for all components. In the case of nitrogen-IHSP samples the existence of the a’-group in the luminescence spectra is evident for a perturbing particle near the emitting N( *D) . Taking into account the large values of the stoichiometric ratio (S = 12-17 [ 261) the observed &-group as well as the o-group is most probably connected with the

Physics I89 (1994) 347-382

371

emission of N(*D) from two-body N(*D)-N2 conplexes. First of all the probability of two-body complex formation is much higher than three- or multi-body ones, and the presence of even one heavy neighbor is enough to reduce the N( ‘D) lifetime by several orders of magnitude (the effect of prohibition breakdown is in the best case proportional to the number of heavy neighbors). The idea of N( 2D)-N2 emitting complexes is also supported by the following consideration. The ratio of (Y’- to o-group intensities in the normal N2 matrix where N(2D) atoms have twelve neighbors is I,, /I, =: 0.1 [ 27,321 while in the nitrogen IHSP this value is about ten times less: Z,, /I, = 0.01. This fact can be easily explained by the same decrease in the number of N( 2D) neighbors in the IHSP in comparison with the N2 matrix. More evidence that the whole N( ‘D) atom emission originated from N(‘D)-N, complexes is the resemblance of a-group spectra observed in our experiments in the N,-IHSP to the known spectrum of o-group in the Ne matrix with a relatively high content of N2 ( N 7%) [ 3 11. Both spectra had a luminescence maximum near h N 522 nm and decay times of 15 s. Taking into account the inertness of the Ne matrix and the high content of N2 leading to the high ( -0.6) probability of N and N, dimers, the authors of Ref. [ 3 1] also tried to explain the o-group features by the influence of the N2 molecule on the N( ‘D) atom. Our experiments on the condensation of nitrogenhelium mixtures with admixture of RG atoms gives reliable evidence for a profound effect of any heavy neighboring particle on the emission of N(‘D) in the IHSP. As expected, the a-group spectra strongly depend on the type of heavy impurity particles. In Ne-, Ar-, and Kr-containing IHSP the centers of (Ygroup spectra were red-shifted by -30, - 88 and - 145 cm ‘, respectively, as compared to the N( ‘D4S) transition in the gas phase. These results, being in reasonable agreement with those for the luminescence of N(2D) in the corresponding RG matrices [ 30,3 11, display also a linear dependence of the o-group spectral shifts on the polarizability of the neighboring heavy particle (see Fig. 14); this supports our interpretation. The differences of the a-group features observed in our experiments apparently result from distinctions of the environment of the N( 2D) metastable atom in the IHSP and in ordinary Ne, Ar, and Kr matrices.

37x

R.E. Boltnev et al. / Chmical

O[,,,,,,,,,,!,,,li 0

1

.o

2.0 Polarizability,

3.0 A’

4.0

F1.g. 14. Shift of the a-group center versus the polarizability of the neighboring heavy particle (N, or RG). (Treatment of the spectra presented in Figs. 3 and 5 was done).

The strong influence of the RG matrix environment on the breakdown of the N(2D-4S) transition prohibition and spectral shifts, although well known [ 321, has not yet obtained any serious theoretical consideration. For pair interaction of the N(‘D) atom with NZ, in accordance with calculations [ 32,341, the splitting, the red-shift, and prohibition breakdown of the N( *D4S) transition can be explained by the existence of a quadrupole moment on the N, molecule. Since RG atoms do not possess a quadrupole moment the only possible explanation of the relatively strong RG atom’s action on the N(‘D-4S) transition could be the formation of excimer-like states, as found for Xe atoms [35]. The exponential decay of the N( *D) emission after cessation of sample formation and its characteristic lifetimes being close to that in corresponding RG-matrices is reliable evidence that this luminescence is connected with those N( “D) atoms which were previously formed in the HF-discharge and were then trapped together with an N2 molecule or a heavy RG atom in the same helium cluster during the IHSP growth. Correlations of the cl-group spectra features in RG-containing IHSP and in corresponding RG matrices as well as the observed behavior of a’-group luminescence being synchronous with o-group also support this idea. The experiments and discussions described above show that the observed luminescence results from the N( ‘D) atom disturbed by a heavy neighbor in the same helium cluster. Moreover, our attempts to detect the luminescence of solitary N(‘D) atoms, which was

Physics I89 (1994) 367-382

expected to be a narrow unshifted line with a radiative lifetime close to the gas phase value, were to our surprise unsuccessful. One can suppose that the slow stage of afterglow is related to the solitary N(2D) atom matrix isolated by solid helium. It seems however that the hyperbolic law of its kinetics cannot be explained in terms of radiative decay. Nevertheless the hyperbolic kinetics may in principle be valid for radiative decay in the case of a wide distribution of radiative lifetimes, rr. If one supposes, for instance, an equal yield of emission for centers having radiative lifetimes in the range of rr = r0 to TV= T,, the time dependence of luminescence intensity will be governed by

where p = rr- ’ . For times t x=- T,, the dependence is very close to hyperbolic. A more essential argument against the radiative origin of LLA decay is its temperature dependence. Therefore, apparently some thermoactivated process forms the emission centers. The key problem here is: Could new excited N( 2D) atoms appear, for example, by the Edwards mechanism [ 321 during heating? The primary act in this mechanism consists of N(4S) atoms recombining followed by formation of N, molecules electronically excited in the A ‘2: and ‘2: states: N(4S) +N(4S)

--+N;.

The excited N( 2D) atoms can arise in the matrix due to energy transfer from excited Nz molecules to N( 4S) atoms, N; +N(4S)

-N(2D)

+N,.

This mechanism was used to explain experiments on o-group thermoluminescence in solid nitrogen. However, such thermoluminescence occurs at higher temperatures (T> 10 K) [ 321 and this is extra evidence that the luminescence in our experiments takes place in the IHSP, but not, for example, in microparticles of the molecular nitrogen matrix. One can hardly imagine that the Edwards mechanism is effective in the IHSP where most of the atoms and molecules are reliably isolated by a helium “coat”. Meanwhile the generation of N( 2D) atoms requires “triple” collisions because just after Nz excited molecule formation the N( 4S) atom

R. E. Boltnev et al. /Chemical

must arrive during the lifetime of the Nz . This mechanism seems to be particularly negligible in the IHSP samples obtained by condensation of mixtures with dominant content of heavy RG atoms ( [ RG] : [ N2] = 20 : 1) . In this case the helium cluster with nitrogen atom as a core is most likely surrounded by helium clusters containing RG atoms in the center. Nonetheless, the LLA intensity of nitrogen IHSP samples and, for example, argon-nitrogen IHSP ones, taking into account the strong dilution of nitrogen by argon for the latter, was found to be comparable. The following mechanism of thermoluminescence and the LLA is preferable: the metastable N( 2D) atoms formed in the discharge and trapped in the IHSP sample have an extremely long lifetime being surrounded by a helium “coat” and can radiate only due to their disturbance by nearby heavy particles. For this the fusion of centers of two neighboring helium clusters could be regarded as an elementary process of IHSP destruction [22]. Since the efficiency of this process is slightly dependent on whether nitrogen atoms in the ground or excited state fuse either into a Nz molecule or the N(*D)-Im van der Waals complex, respectively, the recombination emission from N, molecules has to be synchronous with the atomic luminescence. In Ar matrices, [ N2] : [ Ar] = 1: 20, intense emission in the blue and UV region due to N2(A “2: -+ X ‘cl ) transition (VK system) has previously been observed [30]. This emission, as an afterglow, is observed in the IHSP samples as well. It is also temperature dependent and when the temperature rises from 1.5 to 1.9 K the emission intensity increases and then decays with characteristic time ~,,~=40 s. Such behavior, similar to that of the o-group luminescence, implies that the molecular luminescence arises as a result of thermoactivated fusion of neighbor centers, similar to the one responsible for atomic luminescence sensitization, But in this case it arises from recombination of N( 4S) atoms to form N, molecules. It appears reasonable that both recombination of nitrogen atoms and their fusion with RG atoms or nitrogen molecules occurs first in the defective sites of the lattice or at the surface of the sample. Indeed, the phase stability is determined by the process of pair fusion in such sites, whereas inside a perfect lattice this process requires a coherent rearrangement of the whole local environment, requiring the expenditure of more energy [23]. This is supported by the fact that in spite of the

Physics 189 (1994) 367-382

379

presence of blue luminescence, resulting from recombination of nitrogen atoms, their total concentration monitored by the ESR method does not fall noticeably either in time or on heating processes up to 2 K [ 221. The annealing effect observed confirms this assumption. Recombination thermoluminescence of N2 molecules seems to be the simplest one to realize. Excited nitrogen molecules in the A “&+ state are formed during recombination and their radiative lifetime in vacuum is 1.9 s [ 291 (in an Ar matrix it is about 0.4 s [ 30]), less than the observed typical times of emission decay after the temperature jump, ri ,2 = 40 s (see Fig. 10). Consequently, the observed dependence reflects the kinetics of nitrogen atom recombination in the IHSP complicated however by the effects of local overheating in the neighborhood of the recombination site. Atomic luminescence during the process of the IHSP storage and immediately after its cessation, as already mentioned, represents the emission of preliminarily excited N(‘D) atoms, bound to a heavy particle (N, or RG) in the core of the helium cluster. Accordingly the emission intensity decay has an exponential character with times close to that realized in the corresponding matrix (N2 or RG). Thermostimulated atomic luminescence (o-group), like molecular luminescence, is caused by convergence of impurity centers, but with one important difference: in contrast to molecular luminescence, sensitization of atomic luminescence by heavy particles appears to be possible at distances, exceeding the equilibrium distance in N-N, or N-RG van der Waals molecules. Since the nature of the sensitizing effect of RG atoms, as already mentioned, is not clear so far, let us consider the pair N-N2 only. Since molecular nitrogen possesses a quadrupole moment Q.+ = 1.13 au, there is an electric field - Qi.,, /R 4 in its vicinity.

Its value for

equilibrium distance R = R, - 6 au in the van der Waals complex N,-N( *D) is rather high, - 10e4 au - lo6 V/ cm, and because of the rather slow field decay with distance, it is noticeable even for R - 20 au. One should note that because of the high symmetry of the helium environment around the N2 molecules in the impurityhelium cluster such molecule must rotate practically free, as was confirmed by observation of the rotation structure during sample growth (see Fig. 2). Therefore

380

R.E. Boltnev et al. /Chemical

nitrogen molecules in rotationally excited states are active molecules ‘. An energy 2B = 6 K (B is the rotational constant) is necessary for excitation to the N2 (U = 0, J = 1) state. This leads to a weak temperature dependence of the luminescence intensity. An evident physical reason for 2D-4S transition prohibition breakdown in the N, molecular field consists in parity conservation rule violation for N atoms: an admixture of odd terms to the 2D term increases the probability of radiative transitions. Since the contribution of the odd terms is proportional to QNz/R4, the transition probability depends on distance, by ignoring exchange effects, as only

W= W, -t Wb(R,/R)X,

(2)

where W, is the free atom transition probability and W,, is the N-N? van der Waals complex transition probability (form experiment W,,= l/ 15 s-‘). As W,,(*D,,,) =2.5X lo-’ SC’ and hence W,/W,,= 4 X 10p4, the influence of the N2 molecule on N( 2D) atom radiative transitions will be noticeable up to distances R = 10 A. So one can expect the presence of a spectrum of radiative times which corresponds to various distances N(2D)-N, and extends from 7,=4x 104s to T(]‘15 s. It is important that the proximity mechanism of quenching involves all bulk N( ‘D) atoms in the emission processes, since there are one or more N2 molecules in the environment of any N atom due to their incomplete dissociation in the discharge, [N] / [N,] = 1. In comparison with the emission of N( 2D) atoms in N,-N( 2D) van der Waals complexes formed in the thermoactivated recombination process where an activation energy value EL = 40 K has been calculated [ 23 I, the proximity mechanism must possess a much weaker temperature dependence. Indeed N, rotational state excitation demands an energy of only EK = 2B = 6 K, and the activation energy of molecular mobility thawing, which can be estimated as the energy of a vacancy initiation in solid helium, is about E, = 9.5 K [361. 'Lnprinciplethe ground state of the N2 molecule could also activate the N(‘D-%) transition by the electron-rotation transitions NZ(O, O)N(‘D) + N,(O, J)N(“S) but the efficiency of the process has not been estimated so far.

Physics 189 (1994) 367-382

2.54d ,

0 0

I

I 40

TirnZ’

I

I 120

160

(s)

Fig. 15. Comparison of experimental dependence of the w-group thermoluminescence intensity for the nitrogen IHSP under the temperatureincrease from ( 1.5to 1.8 K ( 1) with the theoreticalcurve (2) calculated using Eq. (4). Just after the temperature jump in thermoluminescent experiments the main part of the luminescence originated from the van der Waals pair N( 2D)-Im formation by a strongly temperature dependent fusion mechanism in the defective places of a regular IHSP lattice. Supposing an Arrhenius dependence of the pair formation rate, dNldt=Aexp(

-EllkT)

,

(3)

and taking into account that the heating time was comparable with the radiative lifetimes of luminescence 7” ranged from 15 to 50 s (see Table 1 and 2) - the experimental data has been fitted in accordance with the following dependence:

Z(t) =I,

exP

+

(4)

0 where Z, is the initial value of luminescence intensity just before heating, and T(t) is the experimental temporal dependence of temperature. The simulated thermoluminescence kinetic curves turn out to be extremely sensitive to the choice of activation energy E, and rather sensitive to the value of TV. This assures the reliable matching of parameters El, 7. and N which resulted in the best fit to the first stage of thermoluminescence curves corresponding to the fastest temperature change (see Fig. 15). The results obtained are collected in Table 3. The processing of data yields an activation energy value E, =40+4 K, and life-time values for all cases (molecular lumines-

R.E. Boltnev et al. /Chemical

381

Physics 189 (1994) 367-382

Table 3 Results of analysis of experimental

kinetic curves of IHSP samples thermoluminescence

Condensed gas mixture

Inter. filter a A

Temperature range of the heating AT

(nm)

(initial stage) Energy barrier E, (K)

Lifetime 7 (s)

17 17 10

(K)

l&l : [HeI

521 (o-group) 600 (c&group) 460 (V-K system)

1.5- 1.I5 1.5-1.71 1.5- 1.82

39 40 39

1.5-1.77 b 2.2-2.5 b

40 40-44

lNzl : [Ne] : [He] =1:100:10000

521

1.5-1.86

40

50

[NJ : LAr] : [HeJ =1:20:2000

521

1.55-1.78

40

50

=l:lOO

2.5 2.5

(a-group)

(o-group)

a Filters with A,, =599 nm and FWHM= 10 nm; h,= 521 nm and FWHM = 13 nm; A, =450 nm and FWHM=9 ‘Two successive step heating of the sample, first 1.5 to 1.77 K and then 2.2 to 2.5 K.

cence, o-group atomic emissions in N,- as well as RGcontaining matrices), as is seen in Tables 1 and 2, in very good agreement with the afterglow decay times measured just after the sample storage cessation. The activation energy El correlates well with the previously estimated barrier of 40 K for fusion of two impurityhelium clusters [ 231. When the temperature of the sample being preliminary held at T= T, was first decreased to T, and then was raised up to T2 > To, the effective activation energy, measured for the heating stage from T, to T2, was found

nm were used.

to be higher than 40 K. This readily points to an annealing effect (luminescence intensity approached its initial value only after the temperature reached the initial temperature To). One can notice for all thermoluminescent kinetic curves a sharp drop in effective activation energy at a given time (for the curve of Fig. 15, at t= 130 s). This time corresponds to the beginning point of the hyperbolic decay of atomic luminescence being typical for the LLA. It seems quite natural to consider that all pairs N( 2D)-Im separated by rather intimate distances when direct fusion is possible, have just depopulated at this time and only the above mentioned proximity mechanism of atomic luminescence sensitization takes place. The factor favoring the kinetic data processing is the sample temperature constancy on this stage. Since the LLA stage is described well by a hyperbolic law to obtain the activation energy, one can simply describe the emission intensity kinetics as I-

N(T) t+At’

0.5

0.6 l/T

Fig. 16. Dependence curves from Fig. 8).

0.7 (K-l)

of the N(T) parameter

on 1/T (treatment

of

where N(T) is represented by the Arrhenius dependence N( T) = No exp( - E,/RT) and At is a time-fitting parameter which is necessary to account for the finite time of the temperature jump and beginning of the afterglow stage. For each of the three temperatures 1.47, 1.73, 1.93 K the fitting was done (see Fig. 8) and the parameters N and At were determined and then the

R. E. Boltnev et al. /Chemical Physics 189 (1994) 367-382

382

long-lived stage activation energy, E2, was calculated (see Fig. 16). It is interesting that the value E,=7 K thus obtained is close both to the vacancy formation energy in solid helium and to the rotational N2 quantum. The fact that the appropriate kinetics must be nonexponential can be rather well understood, but the course of its closeness to hyperbolic decay in any case is not clear; moreover the kinetics of recombination emission decay is neither exponential nor hyperbolic. Probably the distribution of radiative life-times as mentioned above (see Eq. ( 1) ) leads to the appearance of hyperbolic decay, and plays a decisive role in the formation of this dependence.

Acknowledgement We are grateful to Dr. O.F. Pugachev for helpful participation in the initial stages of the optical investigations and fruitful discussions. The research described in this publication was made possible in part by Grant No. RED 000 from the International Science Foundation and Grant No. 94-03-08433 from the Russian Foundation for Basic Research.

References I I 1 L. Andrewsand M. Moskovits, eds., Chemistry and physics of matrix-isolated

species (North-Holland,

Amsterdam, 1989). of matrixisolated species (Wiley, New York, 1989).

121R.J. Clark and R.E. Hester, eds., Spectroscopy

I-71J. Jortner, L. Meyer, S.A. Rice and E.G. Wilson, Phys. Rev. Letters 12 (1964) 415. I41 J.W. Keto, F.J. Soley, M. Stockton and W.A. Fitzsimmons, Phys. Rev. A 10 (1974) 872. IS I J.W. Keto, F.J. Soley, M. Stockton and W.A. Fitzsimmons, Phys. Rev. A 10 (1974) 887. 161 W.S. Dennis, E. Durbin, W.A. Fitzsimmons, 0. Heybey and G.K. Walters, Phys. Rev. Letters 23 ( 1969) 1083. I71 D.B. Koperliovich, A.Y. Parshin and C.V. Pereverzev, Zh. Eksp. Teor. Fiz. 96 (1989) 1122. IX] CF. Chabalowski, J.O. Jensen, D.R. Yarkony and B.H. Lengsfild, J. Chem. Phys. 90 (1989) 2504.

[ 91 H. Bauer, M. Hausmann, R. Mayer, H.J. Reyher, E. Weber and A. Winnacker, Phys. Letters A 110 ( 1985) 279. [lOI H. Bauer, M. Beau, A. Bemhardt, B. Fried1 and H.J. Reyher, Phys. Letters A 137 (1989) 217. [ 111 H.J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafter and A. Winnacker, Phys. Letters A 11.5 ( 1986) 238. [ 12J H. Bauer, M. Beau, B. Friedl, C. Marchand, K. Miltner and H.J. Reyher, Phys. Letters A 146 (1990) 134. [ 131 J.P. Hansen and E.L. Pollock, Phys. Rev. A 5 (1972) 2214. [ 141 A.P. Hickman and N.F. Lane, Phys. Rev. Letters 26 (1971) 1216. [ 151 J. Wisdom, T.W. Hartquist and N.F. Lane, Phys. Rev. B 14 (1976) 4205. [ 161 S. Goyal, D.L. Schutt and G. Stoles, J. Phys. Chem. 97 (1993) 2236. [ 171 A. Scheidemann, B. Schillingand J.P. Toennies, J. Phys. Chem. 97 (1993) 2128. 1181R.N. Barnettand K.B. Whaley, J. Chem. Phys. 96 ( 1992) 2953. [ 191 K.E. Kurten and J.W. Clark, Phys. Rev. B 32 (1985) 2952. [20] J. Poitrenaud and F.I.B. Williams, Phys. Rev. Letters. 32 (1974) 1213. 1211 E.B. Gordon, V.V. Khmelenko, A.A. Pelmenev, E.A. Popov, O.F. Pugachev and A.F. Shestakov, Chem. Phys. 170 (1993) 411. I22 ] E.B. Gordon, V.V. Khmelenko, A.A. Pelmenev, E.A. Popov and O.F. Pugachev, Chem. Phys. Letters 155 (1989) 301. [23] E.B. Gordon, A.A. Pelmenev, E.A. Popov, O.F. Pugachev, V.V. Khmelenkoand A.F. Shestakov, Sov. J. Low Temp. Phys. 18 (1992) 952. 1241 E.B. Gordon, A.A. Pelmenev, O.F. Pugachev and V.V. Khmelenko, Chem. Phys. 61 (1981) 35. 1251 A.A. Radtsig and B.M. Smimov, Handbook of parameters of atoms and atomic ions (Energoatomizdat, Moscow, 1986). [26] R.E. Boltnev,E.B. Gordon, I.N. Krushinskaya,A.A. Pelmenev, E.A. Popov, O.F. Pugachev and V.V. Khmelenko, Sov. J. Low. Temp. Phys. 1X (1992) 576. [27] 0. Ochler, D.A. Smith and K. Dressler, J. Chem. Phys. 66 (1977) 2097. [28] E.B. Gordon, M.B. Martynenko, A.A. Pelmenev, O.F. Pugachev and V.V. Khmelenko, Khim. Fiz. 13 (1994) 15. 129) A. Lofthus and P.H. Krupenie, J. Phys. Chem. Ref. Data 6 (1977) 113. [ 301 D.S. Tinti and G.W. Robinson, J. Chem. Phys. 49 ( 1968) 3229. 1311 R.J. Sayer, R.H. Princeand W.W. Duley, Phys. Stat. Sol. (b) 106 (1981) 249. [ 32 I A.M. Bass and H.P. Broida, eds., Formation and trapping of free radicals (Academic Press, New York, 1960) [33] M. Peyron and H.P. Broida, J. Phys. Chem. 30 (1959) 139. [ 341 F.L. Kunsch and K. Dressler, J. Chem. Phys. 68 (1978) 2550. [ 351 A.F. Vilesov, J. Wildt and E.H. Fink, Chem. Phys. 153 ( 1991) 531. [ 361 A.F. Andreev, Usp. Fiz. Nauk 118 ( 1976) 25 1.