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Spectroscopic studies of Hg2+ color centers in KI single crystal
J. Phys. Chem. So/ids Vol. 50. No. Printed in Great Britain.
I, PP. 69-74,
01X2-3697/89 $3.00 + 0.00 0 1988 Pergamon Press plc
1989
SPECTROSCOPIC STUDIES OF Hg2+ COLOR CENTERS IN KI SINGLE CRYSTAL S. W. LEE, S. K. Ju and J. G. Kmct Department of Chemistry, College of Natural Sciences, Chungnam National University, Daejeon 302-764, Republic of Korea (Received 13 April 1988; accepted 26 July 1988)
Abstract-The emission from KI : Hg2+ was studied at low temperature as a function of exciting photon energy. The absorption and emission spectra of KI: Hg2+ were also measured at several temperatures. The near UV excitation produces complex emission from KI: Hg*+ at low temperature (T < 80 K), which is very dependent on the exciting photon energy. A definitive model for relaxed excited states of Hg2+ in a cubic field is formulated in terms of the Jahn-Teller effect and the spin+rbit interaction with an application of the vibronic reduction. The corresponding configurational coordinate diagram accounts Neil for the experimental results. Keywork
Spectroscopic studies, KI : Hg2+. EXPERIMENTAL
INTRODUCTION
The samples containing cu 200 ppm of H8+ were grown in this laboratory using the vertical Bridgman method. Thin samples were cleaved from the ingot. The optical measurements were performed in an Oxford CF-1140 cryostat for the absorption and an AirProduct He refrigerator using an APD-B temperature controller for the emission. The absorbances were calculated from the transmittance spectra of KI and KI:Hg2+ crystals, which were individually measured at a given temperature. For the transmittance spectrum, the light from a 60W D, lamp was passed into a Jobin-Yvon H-20 monochromator with a 1200 grooves mm-’ grating. The incident beam was focused on the surface of the crystal mounted in the cryostat using two quartz lenses. The transmitted light was measured by a Hamamatsu R-955 photomultiplier tube placed in the parallel optical-arrangement. The emission and the excitation were measured at right angles to the incident beam from a 25OW Hg arc lamp. A UV cut-off filter assembly was placed before a 0.5 m Jarrel-Ash monochromator with a 1180 grooves mm-’ grating to eliminate unwanted stray light and the second-order emission light. A cooled signal which was amplified by an amplifier/ discriminator (PRA). Details of the optical arrangement have been given in previous work [6].
Unlike Tl+-like ions in alkali halide single crystals, Ag+-like ions with the d”’ electronic configuration show a number of absorption bands in the near UV [l-3]. These absorption bands are due to transitions from the ground state (IS,) to the terms (‘D and ‘D) arising from the d9s excited configuration. The d + s transitions are fundamentally parity-forbidden. However, when the impurity ion is centered on the lattice site, the transitions are allowed by odd-parity lattice vibrations. The oscillator strength of these transitions is very weak and increases with increasing temperature. On the other hand, if the ion is off-center in the host crystal, the crystalline Stark effect destroys the parity selection-rule and leads to strong transitions with fine structure. The oscillator strength of the off-center ion is found to decrease with increasing temperature [4]. Some authors have attempted to obtain satisfactory agreement with the observed absorption spectra by varying the crystal-field, the spin-orbit and the electrostatic parameters [4,5]. However, the electronic structure of the ion they considered is only qualitative. It is not very responsible for the emission, which may be due to the complex of the relaxed excited states (RESs) of the ion in the host crystal. To date, studies of Hg2+ ions in alkali halide single crystals have not been performed. The present work has been undertaken to investigate the emission from Hg2+ ions in a KI single crystal as a function of the exciting photon energy and temperature. We find that the Jahn-Teller effect plays a key role in interpreting the emission from KI : H8+. In this paper, taking into account the Jahn-Teller stabilization and the effect of the vibronic reduction on the spin-orbit coupling, we proposed the energy-level scheme and the configurational coordinate diagram of the RESs of Hg2+ in the KI crystal.
RESULTS
The absorption spectrum of KI: Hg2+ at liquid N, temperature and room temperature is shown in Fig. 1. The absorption spectrum is very complex and the oscillator strength is very weak in the near UV region. The hypothesis of an off-center Hg*+ in the KI crystal can be ruled out in the theoretical consideration, since the absorbance increases with increasing temperature and the sizes of K+ and Hg2+ ions are comparable.
t To whom all correspondence should be addressed. 69
S. W. LEE et al. (eV)
400)
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Wavelength
(nm 1
200
220
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260 280 300 Wavelength (nml
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Fig. 1. Absorption curves of KI : Hg2+ at LNT and RT. The absorbance was calculated from the transmittance spectra of KI and KI:Hg*+ at the given temperature.
Fig. 3. Excitation spectra of the 750 nm excitation (full line) and the 475 nm emission (broken line) at T = 35 K.
In this work, we concentrated on the emission from KI:Hg*+ to permit a more definitive RESs of Hg’+. The emission from KI : Hg*+ excited in the near UV region was measured at T = 35 K. As shown in Fig. 2, the emission from KI : Hg2+ is very dependent on the exciting photon energy and is very broad. The whole emission spectrum can be simply classified into two groups: the low-energy emission ranging from 900 to 600 nm and the high-energy emission ranging from 600 to 300nm. The 300nm excitation produces the low-energy emission peaking at 690 nm and the highenergy emission with relatively weak intensity. With increasing exciting photon energy, the intensity of the
low-energy emission decreases, whereas that of the high-energy emission increases. At 270 nm (4.59 eV) excitation, the intensity of the high-energy emission attains a maximum and the low-energy emission appears merely as traces. The excitation above 4.59 eV produces the reverse trend in the emission. With increasing exciting energy emission revives and gains a maximum intensity at 240 nm (5.17 eV) excitation, while the high-energy emission decreases and disappears at 230 nm (5.39 eV) excitation. The excitation spectra of the 750 and 475 nm emission bands were measured at T = 35 K. As shown in Fig. 3, the excitation spectrum of the low-energy
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Fig. 2. Emission spectra of KI: Hg*+ (T = 35 K) excited at (a) I; 300 nm, 2; 290 nm, 3; 280 nm, 4; 270 nm and (b) I; 260 nm, 2; 250 nm, 3; 240 nm. 4; 230 nm.
900
Spectroscopic studies of Hgr+ color centers
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71
nm
Wavelength
(nml
Fig. 4. Resolution of the emission from KI:Hg2+ at T = 35 K as two sets of three overlapping Gaussian bands. The continuous lines show the individual resolved bands and their sum, and the circles show the experimental data. is clearly different from that of the highenergy emission. In the case of the low-energy emission, the excitation spectrum has two separate bands peaking at 4.2 and 5.1 eV, which probably correspond to the bands A and C in Fig. 1, respectively. It is not surprising if one takes into account the spectrum of the Hg lamp used as the light source. Furthermore, it is indicative of the possibility that non-radiative transitions between the two states responsible for bands A and C may contribute to the low-energy emission. The shape of the emission spectrum shows that both the low-energy and the high-energy emissions consist of several Gaussian bands. In the region of a spectrum where two or more bands overlap, the total intensity & will be the sum of the individual values of I which is computed as a function of the wavenumber
emission
IeV)
25
3.5
of the maximum 1’,, , half-width of the band at halfheight and I,,,, at f,,,,,,. We started with estimated
values of the three parameters of the individual band and then refined these by some least square procedure using the Taylor series approximation until the experimental and calculated values of I were the best possible fit. It should be noted that the number of bands given is the most effective parameter for the best fit. The computer plot of a resolved spectrum and the experimental data of the corresponding emission spectrum with background subtracted is shown in Fig. 4. Both resolve well into three overlapping Gaussian bands. In addition, we also measured the temperature effect on the emission from KI :Hg*+. As shown in Fig. 5, with increasing temperature the emission drastically decreases and disappears above T = 80 K.
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Fig. 5. Temperature dependence of the emission from KI:Hg2+ excited at (a) 240 nm and (b) 290 nm: T= 1; 36K, 2; SOK, 3; 80K.
900
12
S. W. Lzz et al. DlSCUSSION
In connection with the question of which excited states are involved in the optical transitions, it is of interest to compare the position of the excited states in the free ion with those in the crystal. The d9s excited co~guration gives rise to 3Ds,,, and ‘4 terms. According to the atomic energy-level of the free ion [7], E(3D3) and E(3D2) are ca 5.31 and 5.71 eV, respectively. Compared with these two states, the other two states lie above 7.0 eV: E( ‘D,) = 7.24 eV and E(‘L),) = 7.57 eV. In fact, since the free-ion states are somewhat close to those in the KI crystal, the 3D, and ‘D, states may be involved in the observed optical transitions. When the ion is introduced into the octahedral crystal-field, it results in the splitting of the representation DJ into components, each of which is an irreducible representation of the 0, symmetry: I), = Tlg, I)r = Eg i- Tzg and D, = A,, + T,, + Tig (hereafter, the subscript g is omitted). If the crystal-field effect is taken into account, the degeneracy of each state will be removed, depending on the magnitude of the crystal-field splitting [4,5]. As mentioned earlier, this consideration is only qualitative. One finds that the emission from KI : Hg*+ needs more parameters to be interpreted. For the detailed assignment of the emission from KI : H$+, we shall discuss the energy-level scheme of the RESs in terms of the Jahn-Teller effect coupling to the tetragonal EJQz, Q3) and the trigonal Tk (Q4, Qs, Q,) vibronic modes. Since the Jahn-Teller energy EJTz i Stokes shift, one expects from the observed absorption and emission spectra a very strong Jahn-Teller effect. The Jahn-Teller effect on the triply degenerate T, or 7; state and the doubly degenerate E state is well understood [S-10]. Since Tz = A,, + Eg + T, in Oh symmetry, the T electronic state is coupled to the E, and Tzgmodes: this problem is accordingly specified as T @(E, + Tzg). In this problem, all the matrix elements of the linear terms of the vibronic interaction are expressed by means of the two constants FE and FT which represent the strength of the couplings to the E, and the Tzgmodes, respectively. When the coupling to the Eg modes is much stronger than that to the Tzgmodes, i.e. FE>bFT (specified as T @ E,), the Jahn-Teller stabilization is given by [8]
FE = Fr, there are six orthorhombic depth being given by:
In this case, they exist only as saddle points. In the case of the doubly degenerate Estate, the Jahn-Teller active modes are only the Ex modes. Only two components of the Eg displacements are retained as the elements of the vibronic interaction. The resulting Jahn-Teller stabilization energy is the same as E& Accordingly, it can be seen from these Jahn-Teller stabilization energies that the stabilization of the T, and Estates is dependent on the vibronic mode, When the Jahn-Teller interaction with the Eg modes takes place, the T2 and E states are equally reduced by the amount of E$-. On the other hand, when coupling to the Tzg modes predominates, the T2 level becomes more stable than the E level. Accordingly, the JahnTeller effect coupling to the Tzg modes can split the five-fold 3D, state into a T2 state below an E state. For the seven-fold ‘D, state, no matter whether the coupling is to the Eg or Tzgmodes, the T, and T2states become equally stable. In addition to the Jahn-Teller stabilization of the degenerate states, the important effect of the vibronic interaction on these states is the reduction of the physical quantities of the electronic origin. According to the theorem of the vibronic reduction known as the Ham effect [ll], the vibronic interaction essentially reduces the spin-orbit splitting of the states, sometimes by several orders of magnitude. In the particular case of the T @ Tzgproblem, the electronic orbital angular momentum which belongs to T, or E is strongly quenched by the strong Jahn-Teller effect, compared with the case for T2. When the vibronic reduction is absent, the magnitude of the spin-orbit coupling constant of the T, state is the same as that of the T2 state. On the other hand, the presence of the Jahn-Teller effect coupling to the Tz8 modes could reduce the spin-orbit coupling of the T, state, much more than that of the T2 state. Accordingly, for the 3D3 state the vibronic reduction splits the T2 state to lower energy and the Ti state to higher energy through the reduction of the spin-orbit interaction. Furthermore, the spin-orbit interaction in the offdiagonal elements [12] mixes the ‘D, and ‘D2 states leading to the following eigenstates ]‘r:>=
where & is the force constant of the lattice for the E, modes. On the other particular case of T ~3 T,, the Jahn-Teller wells in the space of the trigonal coordinates (Q4, Q,, Q6) have four minima lying on the C,-axes of the cubic system. The depth of the minima is given by
where K7 is the force constant of the Tzglattice. When
points with their
-v/lI-,)+/f131-,>
where Ii represents the T2 or E state and p and v are mixing coefficients (,u* + v 2= 1). The mixing results in lowering the triplet-spin states and raising the singletspin states. This additional stabili~tion of the tripletspin states may cause the 3Tf wells to lie just above the ‘T2 wells. The resulting energy-level scheme and the corre-
Spectroscopic studies of Hg2+ color centers
13
Fig. 6. (a) Energy-ievel scheme of the Hgr+ RESs, formulated in terms of the Jahn-Teller e&t coupling to the Tzg modes and the spin-orbit interaction, whose off-diagonal elements lead to ‘7’$ and 3,fZE,L states. Here, the spinorbit interaction could be reduced by the strong Jahn-Teller effect. (b) Configurational coordinate diagram for the RESs. The first two columns are for the free-ion state and for the 0, crystal-field (51.
sponding configurational coordinate diagram for the RESs of Hgz+ in the KI single crystal are shown in Fig. 6. This model and the diagram give a more definitive assignment of the emission from KI: Hg2+ as follows: the low-energy emission could be attributed
to the transition from the low-lying T2 wells to the ground state (referred to as A, emission) and the high-energy emission to the transition from the J’i wells to the ground state (referred to as B,, emission). This assignment accounts well for the characteristic features of the emission from KI: H$+. The A-band excitation could mainly produce the A, emission. With increasing the exciting photon energy, the 7’, wells become more populated than the T, wells, which may cause the increase of the B, emission and the decrease of the A, emission. The excitation above the B-band may populate the T: wells, lying just above the T2 wells due to the mixing effect. The nonradiative transition from the upper 2’; wells to the lower T, wells could occur and produce the A, emission. In addition, the non-radiative transition from the RESs’ wells to the ground state well may
occur, to which the temperature dependence of the emission from KI : Hg*+ can be attributed. The strong dependence of the emission on temperature may be determined to a large extent on the tunneling from the upper wells to the ground well. Similar effects are sometimes observed in KI single crystal f13]. The band shape of the emission spectrum is indicative of the nature of the vibronic coupling. According to theoretical calculations of the band shape conducted by Toyoxawa and Inoue [lo] and Cho [14], the absorption or emission band of the A +-+ T transitions has three humps and the band is split into three components if vibronic coupling occurs to the Tzs modes. If the coupling to the Eg modes is predominant, no splitting of the band is P.C.S. xl,,--E.
observed, although the three-fold T state is split. When the coupling to the A,, and Es modes increases, the three-humped curve becomes essentially smoothed. The observed shape of the A, and BT, emission bands is in good agreement with the theoretical band analysis. CONCLUSION The observed complex-feature of the emission from KI:H$+ is well explained by the RESs treated in terms of the Jahn-Teller effect couphng to the Tzs modes and its vibronic reduction in the spin-orbit interaction. The present analysis leads to the assignment of the emission from Hg2+ as follows: the 600~900 nm emission referred to as the A, and the 300-600 nm emission referred to as the Br, can be attributed to transitions from the 3T2,(3D,) and the ‘T,,( 3D,) states to the ground state, respectively. The 3Tg wells arising from the 3B2state of the free ion may be lying just above the ‘T2, wells and the excitation of the ‘T$ produces the A, emission via the nonradiative ‘Tzf
transition
from these wells to the low-lying
wells.
Acknowledgement-This
Research Foundation,
work was supported by the Korean 1986.
REFERENCES Oggioni R. and Scarmelli P., Phys. Status Solidi 9,411 (1965).
Fussganger K., Martienssen W. and Bilz H., Phys. Status Solidi 12, 383 (1965). Fowler W. B., Physics of Color Centers (Edited by W. B. Fowler). Academic, New York (1968). Matsuyama T., Saidoh M. and Itoh N., J. phys. Sot. Jupun 39, 1486 (1975).
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S. W. LEE et al.
5. Knox R. S., J. phys. Sot. Japan 18, Suppl. II, 268 (1963). 6. Kang J. G., Ju S. K., Gill Y. H., Shim I. K. and Chang C. c, J. Phys. Chem. Solids 49, 813 (1988). I. Moore C. E.. Atomic Enerav Levels. NSRDS-NBS 35. Vol. III, 191’(1971). -. 8. Bersuker I. B., The Jahn-Teller Effect and Vibronic Interactions in Modern Chemistry. Plenum Press, New York (1984). 9. t)pik U. and Pryce M. H. L., Proc. R. Sot. Lond. Ser. A238,425 (1957).
10. Toyozawa Y. and Inoue M., J.phys. Sot. Japan 20,1289 (1965); 21, 1663 (1966).
11. Ham F. S., Phys. Reo. 138, 1727 (1965); Elecfron Paramagnetic Resonance (Edited by S. Geschwind). Plenum Press, New York (1972). 12. Condon E. U. and Shortley G. H., The Theory of Atomic Spectra. Cambridge, London (1979). 13. Kang J. G., Cuss0 F., Belliveau T. F. and Simkin D. J., J. Phys. C: Solid St. Phys. 18, 4753 (1985). 14. Cho K., J. phys. Sot. Japan 25, 1372 (1968).
I, PP. 69-74,
01X2-3697/89 $3.00 + 0.00 0 1988 Pergamon Press plc
1989
SPECTROSCOPIC STUDIES OF Hg2+ COLOR CENTERS IN KI SINGLE CRYSTAL S. W. LEE, S. K. Ju and J. G. Kmct Department of Chemistry, College of Natural Sciences, Chungnam National University, Daejeon 302-764, Republic of Korea (Received 13 April 1988; accepted 26 July 1988)
Abstract-The emission from KI : Hg2+ was studied at low temperature as a function of exciting photon energy. The absorption and emission spectra of KI: Hg2+ were also measured at several temperatures. The near UV excitation produces complex emission from KI: Hg*+ at low temperature (T < 80 K), which is very dependent on the exciting photon energy. A definitive model for relaxed excited states of Hg2+ in a cubic field is formulated in terms of the Jahn-Teller effect and the spin+rbit interaction with an application of the vibronic reduction. The corresponding configurational coordinate diagram accounts Neil for the experimental results. Keywork
Spectroscopic studies, KI : Hg2+. EXPERIMENTAL
INTRODUCTION
The samples containing cu 200 ppm of H8+ were grown in this laboratory using the vertical Bridgman method. Thin samples were cleaved from the ingot. The optical measurements were performed in an Oxford CF-1140 cryostat for the absorption and an AirProduct He refrigerator using an APD-B temperature controller for the emission. The absorbances were calculated from the transmittance spectra of KI and KI:Hg2+ crystals, which were individually measured at a given temperature. For the transmittance spectrum, the light from a 60W D, lamp was passed into a Jobin-Yvon H-20 monochromator with a 1200 grooves mm-’ grating. The incident beam was focused on the surface of the crystal mounted in the cryostat using two quartz lenses. The transmitted light was measured by a Hamamatsu R-955 photomultiplier tube placed in the parallel optical-arrangement. The emission and the excitation were measured at right angles to the incident beam from a 25OW Hg arc lamp. A UV cut-off filter assembly was placed before a 0.5 m Jarrel-Ash monochromator with a 1180 grooves mm-’ grating to eliminate unwanted stray light and the second-order emission light. A cooled signal which was amplified by an amplifier/ discriminator (PRA). Details of the optical arrangement have been given in previous work [6].
Unlike Tl+-like ions in alkali halide single crystals, Ag+-like ions with the d”’ electronic configuration show a number of absorption bands in the near UV [l-3]. These absorption bands are due to transitions from the ground state (IS,) to the terms (‘D and ‘D) arising from the d9s excited configuration. The d + s transitions are fundamentally parity-forbidden. However, when the impurity ion is centered on the lattice site, the transitions are allowed by odd-parity lattice vibrations. The oscillator strength of these transitions is very weak and increases with increasing temperature. On the other hand, if the ion is off-center in the host crystal, the crystalline Stark effect destroys the parity selection-rule and leads to strong transitions with fine structure. The oscillator strength of the off-center ion is found to decrease with increasing temperature [4]. Some authors have attempted to obtain satisfactory agreement with the observed absorption spectra by varying the crystal-field, the spin-orbit and the electrostatic parameters [4,5]. However, the electronic structure of the ion they considered is only qualitative. It is not very responsible for the emission, which may be due to the complex of the relaxed excited states (RESs) of the ion in the host crystal. To date, studies of Hg2+ ions in alkali halide single crystals have not been performed. The present work has been undertaken to investigate the emission from Hg2+ ions in a KI single crystal as a function of the exciting photon energy and temperature. We find that the Jahn-Teller effect plays a key role in interpreting the emission from KI : H8+. In this paper, taking into account the Jahn-Teller stabilization and the effect of the vibronic reduction on the spin-orbit coupling, we proposed the energy-level scheme and the configurational coordinate diagram of the RESs of Hg2+ in the KI crystal.
RESULTS
The absorption spectrum of KI: Hg2+ at liquid N, temperature and room temperature is shown in Fig. 1. The absorption spectrum is very complex and the oscillator strength is very weak in the near UV region. The hypothesis of an off-center Hg*+ in the KI crystal can be ruled out in the theoretical consideration, since the absorbance increases with increasing temperature and the sizes of K+ and Hg2+ ions are comparable.
t To whom all correspondence should be addressed. 69
S. W. LEE et al. (eV)
400)
(200
Wavelength
(nm 1
200
220
240
260 280 300 Wavelength (nml
320
3iO
Fig. 1. Absorption curves of KI : Hg2+ at LNT and RT. The absorbance was calculated from the transmittance spectra of KI and KI:Hg*+ at the given temperature.
Fig. 3. Excitation spectra of the 750 nm excitation (full line) and the 475 nm emission (broken line) at T = 35 K.
In this work, we concentrated on the emission from KI:Hg*+ to permit a more definitive RESs of Hg’+. The emission from KI : Hg*+ excited in the near UV region was measured at T = 35 K. As shown in Fig. 2, the emission from KI : Hg2+ is very dependent on the exciting photon energy and is very broad. The whole emission spectrum can be simply classified into two groups: the low-energy emission ranging from 900 to 600 nm and the high-energy emission ranging from 600 to 300nm. The 300nm excitation produces the low-energy emission peaking at 690 nm and the highenergy emission with relatively weak intensity. With increasing exciting photon energy, the intensity of the
low-energy emission decreases, whereas that of the high-energy emission increases. At 270 nm (4.59 eV) excitation, the intensity of the high-energy emission attains a maximum and the low-energy emission appears merely as traces. The excitation above 4.59 eV produces the reverse trend in the emission. With increasing exciting energy emission revives and gains a maximum intensity at 240 nm (5.17 eV) excitation, while the high-energy emission decreases and disappears at 230 nm (5.39 eV) excitation. The excitation spectra of the 750 and 475 nm emission bands were measured at T = 35 K. As shown in Fig. 3, the excitation spectrum of the low-energy
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Fig. 2. Emission spectra of KI: Hg*+ (T = 35 K) excited at (a) I; 300 nm, 2; 290 nm, 3; 280 nm, 4; 270 nm and (b) I; 260 nm, 2; 250 nm, 3; 240 nm. 4; 230 nm.
900
Spectroscopic studies of Hgr+ color centers
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Fig. 4. Resolution of the emission from KI:Hg2+ at T = 35 K as two sets of three overlapping Gaussian bands. The continuous lines show the individual resolved bands and their sum, and the circles show the experimental data. is clearly different from that of the highenergy emission. In the case of the low-energy emission, the excitation spectrum has two separate bands peaking at 4.2 and 5.1 eV, which probably correspond to the bands A and C in Fig. 1, respectively. It is not surprising if one takes into account the spectrum of the Hg lamp used as the light source. Furthermore, it is indicative of the possibility that non-radiative transitions between the two states responsible for bands A and C may contribute to the low-energy emission. The shape of the emission spectrum shows that both the low-energy and the high-energy emissions consist of several Gaussian bands. In the region of a spectrum where two or more bands overlap, the total intensity & will be the sum of the individual values of I which is computed as a function of the wavenumber
emission
IeV)
25
3.5
of the maximum 1’,, , half-width of the band at halfheight and I,,,, at f,,,,,,. We started with estimated
values of the three parameters of the individual band and then refined these by some least square procedure using the Taylor series approximation until the experimental and calculated values of I were the best possible fit. It should be noted that the number of bands given is the most effective parameter for the best fit. The computer plot of a resolved spectrum and the experimental data of the corresponding emission spectrum with background subtracted is shown in Fig. 4. Both resolve well into three overlapping Gaussian bands. In addition, we also measured the temperature effect on the emission from KI :Hg*+. As shown in Fig. 5, with increasing temperature the emission drastically decreases and disappears above T = 80 K.
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Fig. 5. Temperature dependence of the emission from KI:Hg2+ excited at (a) 240 nm and (b) 290 nm: T= 1; 36K, 2; SOK, 3; 80K.
900
12
S. W. Lzz et al. DlSCUSSION
In connection with the question of which excited states are involved in the optical transitions, it is of interest to compare the position of the excited states in the free ion with those in the crystal. The d9s excited co~guration gives rise to 3Ds,,, and ‘4 terms. According to the atomic energy-level of the free ion [7], E(3D3) and E(3D2) are ca 5.31 and 5.71 eV, respectively. Compared with these two states, the other two states lie above 7.0 eV: E( ‘D,) = 7.24 eV and E(‘L),) = 7.57 eV. In fact, since the free-ion states are somewhat close to those in the KI crystal, the 3D, and ‘D, states may be involved in the observed optical transitions. When the ion is introduced into the octahedral crystal-field, it results in the splitting of the representation DJ into components, each of which is an irreducible representation of the 0, symmetry: I), = Tlg, I)r = Eg i- Tzg and D, = A,, + T,, + Tig (hereafter, the subscript g is omitted). If the crystal-field effect is taken into account, the degeneracy of each state will be removed, depending on the magnitude of the crystal-field splitting [4,5]. As mentioned earlier, this consideration is only qualitative. One finds that the emission from KI : Hg*+ needs more parameters to be interpreted. For the detailed assignment of the emission from KI : H$+, we shall discuss the energy-level scheme of the RESs in terms of the Jahn-Teller effect coupling to the tetragonal EJQz, Q3) and the trigonal Tk (Q4, Qs, Q,) vibronic modes. Since the Jahn-Teller energy EJTz i Stokes shift, one expects from the observed absorption and emission spectra a very strong Jahn-Teller effect. The Jahn-Teller effect on the triply degenerate T, or 7; state and the doubly degenerate E state is well understood [S-10]. Since Tz = A,, + Eg + T, in Oh symmetry, the T electronic state is coupled to the E, and Tzgmodes: this problem is accordingly specified as T @(E, + Tzg). In this problem, all the matrix elements of the linear terms of the vibronic interaction are expressed by means of the two constants FE and FT which represent the strength of the couplings to the E, and the Tzgmodes, respectively. When the coupling to the Eg modes is much stronger than that to the Tzgmodes, i.e. FE>bFT (specified as T @ E,), the Jahn-Teller stabilization is given by [8]
FE = Fr, there are six orthorhombic depth being given by:
In this case, they exist only as saddle points. In the case of the doubly degenerate Estate, the Jahn-Teller active modes are only the Ex modes. Only two components of the Eg displacements are retained as the elements of the vibronic interaction. The resulting Jahn-Teller stabilization energy is the same as E& Accordingly, it can be seen from these Jahn-Teller stabilization energies that the stabilization of the T, and Estates is dependent on the vibronic mode, When the Jahn-Teller interaction with the Eg modes takes place, the T2 and E states are equally reduced by the amount of E$-. On the other hand, when coupling to the Tzg modes predominates, the T2 level becomes more stable than the E level. Accordingly, the JahnTeller effect coupling to the Tzg modes can split the five-fold 3D, state into a T2 state below an E state. For the seven-fold ‘D, state, no matter whether the coupling is to the Eg or Tzgmodes, the T, and T2states become equally stable. In addition to the Jahn-Teller stabilization of the degenerate states, the important effect of the vibronic interaction on these states is the reduction of the physical quantities of the electronic origin. According to the theorem of the vibronic reduction known as the Ham effect [ll], the vibronic interaction essentially reduces the spin-orbit splitting of the states, sometimes by several orders of magnitude. In the particular case of the T @ Tzgproblem, the electronic orbital angular momentum which belongs to T, or E is strongly quenched by the strong Jahn-Teller effect, compared with the case for T2. When the vibronic reduction is absent, the magnitude of the spin-orbit coupling constant of the T, state is the same as that of the T2 state. On the other hand, the presence of the Jahn-Teller effect coupling to the Tz8 modes could reduce the spin-orbit coupling of the T, state, much more than that of the T2 state. Accordingly, for the 3D3 state the vibronic reduction splits the T2 state to lower energy and the Ti state to higher energy through the reduction of the spin-orbit interaction. Furthermore, the spin-orbit interaction in the offdiagonal elements [12] mixes the ‘D, and ‘D2 states leading to the following eigenstates ]‘r:>=
where & is the force constant of the lattice for the E, modes. On the other particular case of T ~3 T,, the Jahn-Teller wells in the space of the trigonal coordinates (Q4, Q,, Q6) have four minima lying on the C,-axes of the cubic system. The depth of the minima is given by
where K7 is the force constant of the Tzglattice. When
points with their
-v/lI-,)+/f131-,>
where Ii represents the T2 or E state and p and v are mixing coefficients (,u* + v 2= 1). The mixing results in lowering the triplet-spin states and raising the singletspin states. This additional stabili~tion of the tripletspin states may cause the 3Tf wells to lie just above the ‘T2 wells. The resulting energy-level scheme and the corre-
Spectroscopic studies of Hg2+ color centers
13
Fig. 6. (a) Energy-ievel scheme of the Hgr+ RESs, formulated in terms of the Jahn-Teller e&t coupling to the Tzg modes and the spin-orbit interaction, whose off-diagonal elements lead to ‘7’$ and 3,fZE,L states. Here, the spinorbit interaction could be reduced by the strong Jahn-Teller effect. (b) Configurational coordinate diagram for the RESs. The first two columns are for the free-ion state and for the 0, crystal-field (51.
sponding configurational coordinate diagram for the RESs of Hgz+ in the KI single crystal are shown in Fig. 6. This model and the diagram give a more definitive assignment of the emission from KI: Hg2+ as follows: the low-energy emission could be attributed
to the transition from the low-lying T2 wells to the ground state (referred to as A, emission) and the high-energy emission to the transition from the J’i wells to the ground state (referred to as B,, emission). This assignment accounts well for the characteristic features of the emission from KI: H$+. The A-band excitation could mainly produce the A, emission. With increasing the exciting photon energy, the 7’, wells become more populated than the T, wells, which may cause the increase of the B, emission and the decrease of the A, emission. The excitation above the B-band may populate the T: wells, lying just above the T2 wells due to the mixing effect. The nonradiative transition from the upper 2’; wells to the lower T, wells could occur and produce the A, emission. In addition, the non-radiative transition from the RESs’ wells to the ground state well may
occur, to which the temperature dependence of the emission from KI : Hg*+ can be attributed. The strong dependence of the emission on temperature may be determined to a large extent on the tunneling from the upper wells to the ground well. Similar effects are sometimes observed in KI single crystal f13]. The band shape of the emission spectrum is indicative of the nature of the vibronic coupling. According to theoretical calculations of the band shape conducted by Toyoxawa and Inoue [lo] and Cho [14], the absorption or emission band of the A +-+ T transitions has three humps and the band is split into three components if vibronic coupling occurs to the Tzs modes. If the coupling to the Eg modes is predominant, no splitting of the band is P.C.S. xl,,--E.
observed, although the three-fold T state is split. When the coupling to the A,, and Es modes increases, the three-humped curve becomes essentially smoothed. The observed shape of the A, and BT, emission bands is in good agreement with the theoretical band analysis. CONCLUSION The observed complex-feature of the emission from KI:H$+ is well explained by the RESs treated in terms of the Jahn-Teller effect couphng to the Tzs modes and its vibronic reduction in the spin-orbit interaction. The present analysis leads to the assignment of the emission from Hg2+ as follows: the 600~900 nm emission referred to as the A, and the 300-600 nm emission referred to as the Br, can be attributed to transitions from the 3T2,(3D,) and the ‘T,,( 3D,) states to the ground state, respectively. The 3Tg wells arising from the 3B2state of the free ion may be lying just above the ‘T2, wells and the excitation of the ‘T$ produces the A, emission via the nonradiative ‘Tzf
transition
from these wells to the low-lying
wells.
Acknowledgement-This
Research Foundation,
work was supported by the Korean 1986.
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