Spectroscopy of Ionic Centers in Solid Xe

Journal of Low Temperature Physics, Vol. 111, Nos. 3/4, 1998

SPECTROSCOPY OF IONIC CENTERS IN SOLID XE E.V. Savchenko, N. Caspary*, A. Lammers* and V.E. Bondybey* Verkin Institute for Low Temperature Physics & Engineering, 47 Lenin Avenue, Kharkov 310164, Ukraine savchenkoCilt. kharkov. ua *Institut fur Physikalische und Theoretische Chemie der TU Munchen LichtenbergstraBe 4, 85747 Garching, Germany

Intrinsic ionic centers in solid Xe were studied by absorption and laserinduced fluorescence spectroscopy. The influence of the temperature, of lattice defects and of impurities were examined. Based on the analysis of the spectra in comparison with theoretical data the revealed bands were assigned to the transition between the ground state and the lower excited states of the Xe+ ion. PACS numbers: 72.20.Jv, 78.55.-m, 78.40.Ha

1.

INTRODUCTION

The study of the electronic structure and dynamics of intrinsic ionic centers in Rare Gas Solids (RGS) is of high current interest and provides an attractive and fundamental model framework to explore basic problems of hole self-trapping, energy relaxation and the formation of charge transfer states. Exposing RGS to ionizing radiation generates free and bound states of charge carriers (electrons, holes and excitons). The self-trapping of holes due to the hole-lattice interaction results in a formation of intrinsic ionic centers. The electronic structure of self-trapped holes in rare gas solids formed from rare gases heavier than He is assumed to be similar to that of dimer ions (Rg+)1; however, this topic is still under discussion. As long as the temperature is low enough to prevent the release of the electrons from their traps in the lattice, the self-trapped holes are stable and can be spectroscopically investigated. The self-trapped holes in solid Ar2, Kr3,5 and Xe4,5 have revealed themselves in recombination luminescence. But up to 693 0022-2291/98/0500-0693$15.00/0

© 1998 Plenum Publishing Corporation

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now there are no direct optical data on the electronic transitions between the ground state and the lower excited states arising from the lowest limit Rg( 1 S) + Rg+(2P) for any of the RGS. The paper presents the first direct spectroscopic study of intrinsic ionic centers in solid Xe.

2.

EXPERIMENT

A high-purity (99.998%) Xe gas was used. The ionic centers were generated by condensation of an ion-containing gas on the substrate KC1 window. The substrate was cooled by an APD Cryogenics HC-2D closed-cycle cryostat. During the deposition time the temperature was held below the recombination luminescence threshold temperature of 30 K4,5 to avoid a thermally induced release of electrons from their traps followed by a recombination with the positive charge centers. We have used an electrical discharge-pulsed supersonic jet source6 to produce the gas phase ions. The gas was ionized in the discharge fixture and expanded supersonically into the cryogenic cell. A repetition rate of 5-10 Hz resulted in a deposition rate of about 1-2 mmol/h. Each gas pulse (50 nmol) deposed approx. 20 monolayers. This technique generates well-annealed samples. The rapid cooling of the thin layers helps to suppress surface diffusion and results well-isolated species. Producing the ions just one microsecond prior to supersonic expansion has the advantage of minimizing the wall-induced recombination. The pulsed mode of the source provides samples of an excellent optical quality with a high concentration of ionic centers. A spectroscopic study was performed on 10 um samples. This thickness was obtained by observing the fringes of the spectra. Absorption and laser-induced fluorescence (LIF) spectra of solid Xe as well as excitation spectra were measured using a modified Bruker 120 IFS HR spectrometer operating in the spectral range of 500-40000 cm - 1 , lasers and continuum light sources. The measurements were carried out in the temperature range of 10-65 K.

3.

RESULTS AND DISCUSSION

The absorption spectra of the ion-containing samples exhibit a strong asymmetric band with a full width at half the maximum (FWHM) of 0.1 eV near 3.8 eV. Fig. 1 shows the absorption spectra measured at different temperatures. When the samples were excited in the region of the absorption band, a strong fluorescence was observed. It consists of a broad (FWHM 0.27 eV) and asymmetric band near 2.15 eV reproduced in Fig. 2. The excitation spectra of the emission followed closely the measured absorption

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Fig. 1. UV-absorption spectra of solid Xe produced by condensation of Xe gas with a pulsed discharge valve. All spectra were recorded at a temperature of 12 K. Trace a) shows the spectrum of the "fresh" sample. Spectra b), c), and d) were recorded after subsequent heating cycles of 5 minutes duration to 44 K, 54 K, and 64 K, respectively.

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Fig. 2. LIF spectra of solid Xe produced by condensation of Xe gas with a pulsed discharge valve. The emission was excited with an Ar+-laser at 3.72 eV. Spectrum a) shows the spectrum of the unannealed sample at 12 K. Spectra b), c), and d) were recorded after heating followed immediately by recooling the same sample to 50 K, 55 K, and 65 K, respectively. The gap in the spectrum near 15800 cm-1 is caused by the bandpass filter used to block the internal He-Ne laser of the Fourier transform spectrometer.

curves. There are good reasons to believe that both the 3.8 eV absorption and the 2.15 eV emission bands are due to closely related carriers. Both appear only in discharged or photoionized Xe samples and show a similar behavior. To check the intrinsic nature of the carriers we performed control experiments with a Xe sample grown by a pulsed deposition but without discharging, as well as with an Ar sample grown from a pulsed discharge. In both cases the above mentioned bands were not observed and are therefore not caused by impurities of the the gas phase or by impurities arising because of electrode sputtering. Since the measured LIF peak position was close to the LIF band previously observed in a Cl doped Xe matrix7, the


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presence of Cl has been carefully checked. Our samples show no signs of Cl or any other halogen contamination. The doping of an Ar matrix with 2.5% Xe resulted in a LIF band shown in Fig. 3. This band is shifted by approx.

Fig. 3. The LIF spectrum of a sample produced by discharging a Xe/Ar 2.5/100 mixture (trace a)) is compared with the spectrum of a sample produced with pure Xe. Both emissions were excited at 3.72 eV.

0.03 eV to lower energies with respect to its position in pure solid Xe. These 2.5% are sufficient to form dimers of Xenon in Argon. Therefore the main features in the absorption and LIF spectra can be unambiguously assigned to intrinsic ionic centers. The assignment of the revealed spectral features to intrinsic ionic centers in solid Xe is supported by experiments with an "injection" of electrons. This was done by a controlled heating of the discharge-grown samples, resulting in electrons released from their traps followed by pronounced thermoluminescence. This is a valuable test of charged species and recombination processes in solids. During the heating we observed a steplike suppression of the LIF band. The band decreased irreversibly in samples annealed above

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65 K as figure 2 shows. The absorption band showed the same steplike behavior (Fig. 1). A pronounced intensity decrease of the relevant bands started at 37 K; a value near the threshold temperature of the thermoluminescence glow curve. A subsequent cooling of the sample restored neither the absorption nor the LIF band. The measured thermoluminescence maxima correlated well with those observed in nominally pure solid Xe irradiated by a synchrotron source of VUV photons8. Those arose due to a recombination of electrons released from shallow traps with self-trapped holes. Irradiation of the sample grown from discharge with visible and near UV light, which dislodges electrons from deep traps, resulted in a decrease of the 2.15 eV band emission during exposure time (Fig. 4). A shift of the exciting laser

Fig. 4. Several subsequently recorded emission spectra with laser excitation at 3.42 eV with 100 mW are compared to demonstrate the effect of bleaching by excitation at this frequency. The trace displaying the strongest emission was recorded first, the following spectra were recorded after intervals of 160 s.


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spot to a new position on the sample restored the LIF band intensity. To our knowledge, there are no theoretical calculations on optical transition energies between the ground state and the lower electronically excited states of intrinsic ionic centers in solid Xe. Since the discussed bands appear only at a Xe concentration sufficiently high to build Xe dimers, we used the Xe+ potential energy curves to interpret the observed absorption and LIF bands. Xe dimer ions arise from Xe+ (2P) and Xe (1S0) states. Its spectroscopy is governed by a group of six low lying electronic states. In the heavy Xe atoms the spin-orbit splitting is very high (1.31 eV) and the j-j coupling (Hund's case c) describes much better the electronic states than the 1-s coupling. The relevant electronic states are shown in figure 5. There are three dipole-allowed transitions from the ground state of Xe+: 1(1/2)u -> 1(3/2) g , 1(1/2)u -> 1(1/2)g and 1(1/2)u -> 2(1/2) g . Only the 1(1/2)u -> 2(1/2)g transition has a high oscillator strength (0.3)10. The measured absorption band energy can be fitted to the dipole-allowed transition 1(1/2)u -> 2(1/2)g energy and is only slightly higher than the photoabsorption cross section maxima for Xe2 in the gas phase (3.53 eV)11. Assuming both, the 3.8 eV absorption and the broad, red shifted fluorescence bands were assigned to the Xe2 centers, one has to consider how the 2.15 eV emission maximum fits into the Xe2 electronic structure. It cannot be an emission from one of the two lower states, 1(1/2)g or 1(3/2) g , because they are expected to be in the infrared range of spectrum9,10. There are also reasons to believe that nonradiative relaxations of higher 2P1/2 levels into the lower manifold of 2P3/2 are inefficient in solids. For instance, the relaxation between fine structure spin orbit components of the 2P ground states of the I atom isoelectronic to the Xe ion, as well as that of Tl, appears to be radiative in solid Xe12. The only reasonable assignments appear to be the transitions from two states of the higher 2P1/2 level, viz. 2(1/2)g and 2(1/2)u. The 2(1/2)g state has a predicted shallow minimum at about 0.03 eV9 in the gas phase. In the matrix it could be stabilized by polarization interaction of the ionic center with neighboring atoms. The transition from the 2(1/2)g state to the ionic ground state is dipole-allowed. The 2(1/2)u state is predicted to be bound (dissociation energy De=0.17 eV)9 and can be populated via nonradiative transitions as a result of energy relaxation. From this state there exists a dipole-allowed 2(1/2)u -> 1(3/2)g transition, shifted to lower energy. The transition from 2(1/2)u to the ground state is forbidden in the free Xe 2 . In matrices however the selection rules are often violated due to the symmetry break caused by the lattice distortions about ionic centers. A broad emission (FWHM=0.3 eV) similar to the LIF band revealed in our experiments has also been observed in nominally pure solid Xe at 2.16 eV under excitation by VUV radiation13. The clear threshold at


Fig. 5. A schematic sketch of the potential energy curves of Xe2 restored by the data from 9.


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the band gap energy of Eg = 9.33 eV in the excitation spectrum indicates the ionic nature of the emitting centers. It shows that the primary step for producing this emission in pure solid Xe is the creation of free electron-hole pairs. The coincidence of the LIF band with that observed under ionizing radiation demonstrates that a pulsed discharge technique of sample preparation in combination with a complex optical study (absorption, fluorescence and excitation spectroscopy) provides a very valuable tool for a further investigation of the hole self-trapping problem. It is interesting that a broad emission band centered at 1.9 eV has been observed from Xe cluster ions14 and was assigned to a radiative transition related to the 2P1/2 ->2 P3/2 transition of Xe ions. It was also shifted to even higher energies of 0.3 eV with respect to the position predicted by theoretical calculations 10,15. The implication of larger ionic clusters was suggested by Kanaev et al.14 Our current study of small ionic clusters in different matrices will certainly be useful to obtain more detailed insights.

4.

ACKNOWLEDGMENT

We also thank Prof. K. S. Song, Prof. G. Zimmerer and Dr. P. Gurtler for useful discussions. Financial support for this research provided by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

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