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Combination of synchrotron radiation and laser for two-photon spectroscopy of BaF21
ELSPEC 3418
Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
Combination of synchrotron radiation and laser for two-photon spectroscopy of BaF 21 O. Arimoto a,*, S. Fujiwara a, T. Tsujibayashi b, M. Watanabe c, M. Itoh d, S. Nakanishi e, H. Itoh e, S. Asaka f, M. Kamada g a
Department of Physics, Okayama University, Okayama 700-8530, Japan Department of Physics, Osaka Dental University, Hirakata 573-1121, Japan c Department of Fundamental Sciences, Kyoto University, Kyoto 606-8501, Japan d Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553, Japan e Department of Advanced Materials Science, Kagawa University, Takamatsu 760-8526, Japan f Equipment Development Center, Institute for Molecular Science, Okazaki 444-8585, Japan g UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan b
Abstract We have carried out nonlinear spectroscopy by making use of tunable VUV light from synchrotron radiation (SR) together with intense light from a laser. The valence excitons of BaF 2 with a large band gap of ,11 eV were chosen as our target. A single crystal of BaF 2 was irradiated at 15 K by the two light pulses from the SR (6–9 eV) and an Nd : YAG laser (2.33 eV). Self-trapped exciton luminescence induced by two-photon excitation was detected with a time-gated photon-counting system developed for the present study. This zero-method technique is expected to be sensitive compared to early experiments by an Italian group monitoring transmittance changes. The obtained two-photon spectrum shows a distinct band below the band gap energy. This band includes two contributions, a two-step cascade process and a two-photon absorption process. The energy of the 2P exciton is estimated to be 10.6 eV at 15 K. q 1998 Elsevier Science B.V. All rights reserved Keywords: Synchrotron radiation; Laser; Two-photon spectroscopy; BaF 2; Excitons; Cascade excitations
1. Introduction One-photon spectroscopy with synchrotron radiation (SR) is a powerful and useful tool for studies of the electronic structures of various materials over a wide spectral range. On the other hand, laser has been utilized not only for one-photon spectroscopy, but also multi-photon spectroscopy because of its high power * Corresponding author. 1 Presented at the Todai Symposium 1997 and the 6th ISSP International Symposium on Frontiers in Synchrotron Radiation Spectroscopy, Tokyo, Japan, 27–30 October 1997.
and narrow spectral width. Combination of these two light sources is interesting and promising for the spectroscopy of solids in the VUV region, although only afew studies have been carried out so far [1–7]. In recent years, we have performed two-photon spectroscopy of insulating crystals, by making use of tunable VUV light from SR together with intense light from a laser. One-photon spectroscopy is limited by a selection rule: only odd-parity transitions are allowed. In contrast, two-photon spectroscopy, where even-parity transitions are allowed, provides additional and complementary information on the electronic structures. The valence excitons in BaF 2 crystals with a large
0368-2048/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 8 )0 0 12 5 -X
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O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
band gap of ,11 eV were chosen as our target, since its optical properties as well as electronic structures have been investigated in some detail. This crystal is also interesting since it shows the so-called Augerfree luminescence due to the radiative recombination of an electron in the valence band and a hole in the outermost core state [8]. The valence band of BaF 2 consists of a 2p orbital of F −, while the conduction band consists of 6s and 5d orbitals of Ba 2+. The excitons with an envelope function of the S type are dipole-allowed in BaF 2. A prominent peak due to the 1S excitons is observed at 10.0 eV in the reflection spectrum [9]. However, no information on P excitons is presently available. A preliminary result at room temperature was reported recently at SRI’97 [10]. In the present study, the experimental system has been improved to obtain definite results at low temperatures. The obtained two-photon spectrum shows distinct structures just below the band gap energy. The origin of these structures will be discussed.
2. Experimental procedures The experiments were done at the UVSOR facility, Institute for Molecular Science. An optical system for the two-photon spectroscopy was constructed at the beam line 1B. Fig. 1 shows a schematic diagram of the apparatus used in the present work. The SR light was dispersed through a monochromator of Seya–Namioka type with a 1-m focusing length. Three gratings can be exchanged in our VUV monochromator, covering the
spectral range from 30 to 650 nm. A typical photon flux of the incident SR was about 10 10 photons/s/mm 2 around 100 nm. Single crystals of BaF 2 supplied from Horiba Ltd. were cleaved from the ingot and mounted on the cold finger of a conduction-type cryostat. Monochromated SR light (6–9 eV; spectral resolution 50 meV, repetition rate ,90 MHz, pulse width ,400 ps) was introduced in a sample chamber along the opposite direction of the second harmonic light of an Nd : YAG laser (2.33 eV; repetition rate ,5 kHz, pulse width ,70 ns, average power ,12 W). The output power of the laser and the degree of its focus onto the sample were carefully adjusted to avoid sample damage, as well as the multi-photon processes due to the laser light itself. Both the SR and laser lights were linearly polarized, and their polarizations were set parallel to each other. The RF signal of 90 MHz from the master oscillator of the UVSOR electron storage ring under the multibunch operation was divided down to 5 kHz, which triggers the YAG laser. The time interval between successive SR pulses was about 11 ns, while the time width of the laser pulse was 70 ns. Therefore, the time coincidence between the SR and laser pulses was achieved automatically, because about six pulses of the SR (,70 ns/11 ns) were involved within the time duration of a single pulse of the laser. This is an advantage compared to the preliminary experimental system with a short-pulse laser [10], in which the temporal coincidence between the SR and laser pulses had to be carefully adjusted. Furthermore, the use of a high-repetition laser with a wide pulse duration and
Fig. 1. Schematic diagram of the experimental system for two-photon spectroscopy with SR and laser.
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
low peak power enabled us to detect weak two-photon signals more efficiently without any damage to the sample. The luminescence at about 4.1 eV, which is due to self-trapped excitons (STEs) produced by two-photon excitation, was observed by a micro-channel plate photomultiplier tube (MCP-PMT; Hamamatsu Photonics R2809U), with a combination of filters and a conventional monochromator (SPEX 270M) for eliminating the scattered light from the laser. The signals from the MCP-PMT were fed to a threechannel time-gated photon counter. The photon signals just before and after the incidence of the laser pulses were counted at channels 1 and 2, respectively: channel 1 (CH1) counts the background signal induced only by the SR light, while CH2 counts the signal due to the STE luminescence induced by the simultaneous irradiation with the SR and laser lights, in addition to the background signal. The gate widths for both counters were set to be 4 ms by referring to the lifetime of the STE luminescence [11]. For reference, the total signal was also counted at CH3 with the timegate fully open. The net luminescence signal induced by two-photon excitation was obtained by subtracting the count at CH1 from that at CH2. In order to get a sufficient net count (or a good signal-to-noise ratio), only a few tens of seconds were required to accumulate the counts at a fixed wavelength. These detection systems have been developed especially for the present experiments, the details of which will be reported elsewhere [12]. It is also noteworthy that our experimental technique is a kind of zero method, and therefore it will be more sensitive than the transmission method that was applied to observe two-photon absorption of KI, KCl and NaCl by Pizzoferrato et al. [2–4].
3. Results and discussion Fig. 2 shows the two-photon excitation spectrum of BaF 2 (open circles) measured by monitoring the STE luminescence at 15 K. For comparison, the onephoton excitation spectrum of the STE luminescence is also shown as a solid curve. The abscissa for the two-photon spectrum indicates the sum of photon energies of the laser (2.33 eV) and the SR, and that for the one-photon spectrum indicates the SR photon
221
Fig. 2. Two-photon excitation spectrum of BaF 2 at 15 K (open circles), obtained by monitoring the STE luminescence under excitation with both SR and laser. For comparison, the one-photon excitation spectrum obtained under excitation with SR only is also shown as a solid curve. The abscissa for the two-photon spectrum indicates the sum of photon energies of the laser (2.33 eV) and the SR, and that for the one-photon spectrum indicates the SR photon energy. The energy of the 1S exciton is indicated by an arrow.
energy. The intensity of the two-photon spectrum corresponds to the net count of the luminescence signal, as described in the previous section. The one-photon excitation spectrum shows a dip at around 10 eV. This corresponds to the reflection peak of the 1S exciton, whose resonance energy is 10.0 eV at 90 K [9], as marked by an arrow. It is to be noted that the onephoton excitation spectrum shown by the solid curve has weak peaks at 8.3 and 8.8 eV. These bands are probably due to excitons bound to some unknown lattice imperfection. As seen in the figure, the two-photon spectrum rises around 10.0 eV and shows a peak at 10.6 eV followed by another peak on the high energy side at 11.2 eV. From detailed analysis of the data described below, it is revealed that these two-photon excitation bands are made up of two contributions: one is a two-photon absorption process and the other is a two-step cascade process. The two-photon spectrum is illustrated by open circles in Fig. 3. As mentioned above, its intensity corresponds to the net luminescence count obtained by subtraction of the count at CH1 from that at CH2. The solid curve is the total count at CH3, which is properly normalized around 10 eV to the two-photon
222
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
Fig. 3. The two-photon excitation spectrum of BaF 2 at 15 K (open circles), which is reproduced from Fig. 2. The abscissa indicates the sum of photon energies of the laser (2.33 eV) and the SR. The spectrum includes two contributions: a two-photon absorption process and a cascade excitation process. The solid curve represents the spectrum due to the cascade excitation process, while the filled circles show that due to the two-photon absorption process. The energies of the 1S and 2P excitons are indicated by downward and upward arrows, respectively.
spectrum shown by the open circles. The data shown by the filled circles are obtained by subtracting the data shown by the solid curve from those shown by the open circles, the meaning of which will be given later. The count due to the net two-photon process, which is the difference between the count at CH1 and that at CH2, was at most 1% of that at CH3. Accordingly, the solid curve, the intensity of which is proportional to the total count at CH3, can be regarded as the one-photon spectrum induced by the SR alone. As is easily recognized in the figure, the two-photon spectrum shown by open circles resembles the onephoton spectrum shown by the solid curve. Such a similarity invokes the existence of cascade processes: real states (or their relaxed states) excited by one photon from the SR are subsequently re-excited to the higher states by the laser, which in turn contributes to the enhancement of the STE luminescence. This is in contrast to the usual two-photon absorption process, in which virtual intermediate states are involved. It is likely that the real excited states are the unknown bound exciton states mentioned before. In fact, the energy positions of the two peaks at 10.6 and 11.2 eV in Fig. 3 are in good agreement with those
of the weak peaks at 8.3 and 8.8 eV in the one-photon excitation spectrum shown in Fig. 2. The reason why the STE luminescence is efficiently enhanced by the cascade process is not known at present. Here, it is noted that we made careful checks, not only of the reproducibility of data, but also the instruments used, especially the time-gated counter, to make sure that the experimental data obtained are reliable enough. As mentioned above, the filled circles in Fig. 3 are the result of subtraction of the spectrum shown by the solid curve from that shown by the open circles. Since the former refers to the count due to the cascade process and the latter the net count due to the twophoton process, the resultant spectrum depicted by filled circles can be regarded as the spectrum due to the usual two-photon absorption process under the assumption that the efficiency of the cascade excitation is constant. The spectrum has a broad band peaking at 10.6 eV. This is 0.6 eV higher than the 1S exciton, whose energy position is indicated by a downward arrow. We assign this peak to the 2P exciton. Here, we should note that, from the present experiments, the relative intensity between the solid curve (the cascade excitation process) and the filled circles (the two-photon excitation process) cannot be determined uniquely, because the method of normalization for the solid curve mentioned before involves some arbitrariness. In this connection, it is also remarked that both the spectra shown by the open circles and the solid curve are very similar, but never agree exactly with each other. In fact, we tried several methods of subtraction and the 2P exciton band always remained at 10.6 eV, irrespective of the normalization factor. Therefore, it is certain that the 2P exciton band exists at 10.6 eV. Assuming a simple hydrogen model, the binding energy of the 1S exciton is thus estimated to be 0.8 eV, which gives the band gap energy as 10.8 eV. From measurements of reflection spectra, Rubloff has deduced a band gap energy of 11.0 eV [9], whereas Tomiki and Miyata have obtained a value of 10.6 eV [13]. It seems that the present value of 10.8 eV is more accurate because we observed a definite peak for the 2P exciton. Although the data points above 10.8 eV are somewhat scattered, we suppose that they originate from the two-photon interband absorption process.
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
4. Conclusion In the present study, we have undertaken twophoton spectroscopy on the valence excitons in BaF 2, by making use of a combination of tunable VUV light from SR and intense light from the second harmonics of a YAG laser. It is shown that the detection of luminescence signals by a time-gated photoncounting technique, instead of the transmittance changes, is a very sensitive and useful method. The two-photon spectrum obtained shows distinct structures just below the band gap energy. Careful examination of the experimental data reveals that the observed bands have two different origins: one is the cascade excitation process via real excited states and the other is the usual two-photon absorption process via virtual intermediate states. From the latter, the energy of the 2P exciton in BaF 2 is estimated to be 10.6 eV at 15 K. Finally, it should be stressed that two-step excitation processes like the present case are not yet detectable by the use of usual lasers, since their onephoton energies fall into the transparent region of the materials with a wide band gap, so that the real excited states cannot be excited. It is also to be pointed out that the present experimental system with SR and laser for two-photon spectroscopy can be extended to investigations of various relaxation processes of the highly excited states produced by the one-photon process with SR, in which the laser is used as a probe
223
light. Experiments along these lines are now in progress.
References [1] V. Saile, Appl. Optics 19 (1980) 4115. [2] R. Pizzoferrato, M. Casalboni, R. Francini, U.M. Grassano, Europhys. Lett. 2 (1986) 571. [3] R. Pizzoferrato, M. Casalboni, J. Phys. E: Sci. Instrum. 20 (1987) 896. [4] M. Casalboni, C. Cianci, R. Francini, U.M. Grassano, M. Piacentini, N. Zema, Phys. Rev. B 44 (1991) 6504. [5] T. Mitani, H. Okamoto, Y. Takagi, M. Watanabe, K. Fukui, S. Koshihara, C. Ito, Rev. Sci. Instrum. 60 (1989) 1569. [6] J.P. Long, H.R. Sadeghi, J.C. Rife, M.N. Kabler, Phys. Rev. Lett. 64 (1990) 1158. [7] D.L. Ederer, J.E. Rubensson, D.R. Mueller, R. Shuker, W.L. O’Brien, J. Jai, Q.Y. Dong, T.A. Calcott, G.L. Carr, G.P. Williams, C.J. Hirschmugl, S. Esemad, A. Inam, D.B. Tanner, Nuc. Instrum. Meth. A 319 (1992) 250. [8] M. Itoh, S. Hashimoto, S. Sakuragi, S. Kubota, Solid State Commun. 65 (1988) 523. [9] G.W. Rubloff, Phys. Rev. B 5 (1972) 662. [10] M. Kamada, S. Hirose, S. Asaka, T. Tsujibayashi, M. Watanabe, O. Arimoto, S. Fujiwara, S. Nakanishi, H. Itoh, M. Itoh, Proc. 6th Int. Conf. Synchrotron Radiation Instrum., Himeji, August 1997 (in press). [11] R.T. Williams, M.N. Kabler, W. Hayes, J.P. Scott, Phys. Rev. B 14 (1976) 725. [12] S. Asaka, S. Nakanishi, H. Itoh, M. Kamada, M. Watanabe, O. Arimoto, T. Tsujibayashi, S. Fujiwara, M. Itoh, Rev. Sci. Instrum. (in press). [13] T. Tomiki, T. Miyata, J. Phys. Soc. Jpn. 27 (1969) 658.
Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
Combination of synchrotron radiation and laser for two-photon spectroscopy of BaF 21 O. Arimoto a,*, S. Fujiwara a, T. Tsujibayashi b, M. Watanabe c, M. Itoh d, S. Nakanishi e, H. Itoh e, S. Asaka f, M. Kamada g a
Department of Physics, Okayama University, Okayama 700-8530, Japan Department of Physics, Osaka Dental University, Hirakata 573-1121, Japan c Department of Fundamental Sciences, Kyoto University, Kyoto 606-8501, Japan d Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553, Japan e Department of Advanced Materials Science, Kagawa University, Takamatsu 760-8526, Japan f Equipment Development Center, Institute for Molecular Science, Okazaki 444-8585, Japan g UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan b
Abstract We have carried out nonlinear spectroscopy by making use of tunable VUV light from synchrotron radiation (SR) together with intense light from a laser. The valence excitons of BaF 2 with a large band gap of ,11 eV were chosen as our target. A single crystal of BaF 2 was irradiated at 15 K by the two light pulses from the SR (6–9 eV) and an Nd : YAG laser (2.33 eV). Self-trapped exciton luminescence induced by two-photon excitation was detected with a time-gated photon-counting system developed for the present study. This zero-method technique is expected to be sensitive compared to early experiments by an Italian group monitoring transmittance changes. The obtained two-photon spectrum shows a distinct band below the band gap energy. This band includes two contributions, a two-step cascade process and a two-photon absorption process. The energy of the 2P exciton is estimated to be 10.6 eV at 15 K. q 1998 Elsevier Science B.V. All rights reserved Keywords: Synchrotron radiation; Laser; Two-photon spectroscopy; BaF 2; Excitons; Cascade excitations
1. Introduction One-photon spectroscopy with synchrotron radiation (SR) is a powerful and useful tool for studies of the electronic structures of various materials over a wide spectral range. On the other hand, laser has been utilized not only for one-photon spectroscopy, but also multi-photon spectroscopy because of its high power * Corresponding author. 1 Presented at the Todai Symposium 1997 and the 6th ISSP International Symposium on Frontiers in Synchrotron Radiation Spectroscopy, Tokyo, Japan, 27–30 October 1997.
and narrow spectral width. Combination of these two light sources is interesting and promising for the spectroscopy of solids in the VUV region, although only afew studies have been carried out so far [1–7]. In recent years, we have performed two-photon spectroscopy of insulating crystals, by making use of tunable VUV light from SR together with intense light from a laser. One-photon spectroscopy is limited by a selection rule: only odd-parity transitions are allowed. In contrast, two-photon spectroscopy, where even-parity transitions are allowed, provides additional and complementary information on the electronic structures. The valence excitons in BaF 2 crystals with a large
0368-2048/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 8 )0 0 12 5 -X
220
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
band gap of ,11 eV were chosen as our target, since its optical properties as well as electronic structures have been investigated in some detail. This crystal is also interesting since it shows the so-called Augerfree luminescence due to the radiative recombination of an electron in the valence band and a hole in the outermost core state [8]. The valence band of BaF 2 consists of a 2p orbital of F −, while the conduction band consists of 6s and 5d orbitals of Ba 2+. The excitons with an envelope function of the S type are dipole-allowed in BaF 2. A prominent peak due to the 1S excitons is observed at 10.0 eV in the reflection spectrum [9]. However, no information on P excitons is presently available. A preliminary result at room temperature was reported recently at SRI’97 [10]. In the present study, the experimental system has been improved to obtain definite results at low temperatures. The obtained two-photon spectrum shows distinct structures just below the band gap energy. The origin of these structures will be discussed.
2. Experimental procedures The experiments were done at the UVSOR facility, Institute for Molecular Science. An optical system for the two-photon spectroscopy was constructed at the beam line 1B. Fig. 1 shows a schematic diagram of the apparatus used in the present work. The SR light was dispersed through a monochromator of Seya–Namioka type with a 1-m focusing length. Three gratings can be exchanged in our VUV monochromator, covering the
spectral range from 30 to 650 nm. A typical photon flux of the incident SR was about 10 10 photons/s/mm 2 around 100 nm. Single crystals of BaF 2 supplied from Horiba Ltd. were cleaved from the ingot and mounted on the cold finger of a conduction-type cryostat. Monochromated SR light (6–9 eV; spectral resolution 50 meV, repetition rate ,90 MHz, pulse width ,400 ps) was introduced in a sample chamber along the opposite direction of the second harmonic light of an Nd : YAG laser (2.33 eV; repetition rate ,5 kHz, pulse width ,70 ns, average power ,12 W). The output power of the laser and the degree of its focus onto the sample were carefully adjusted to avoid sample damage, as well as the multi-photon processes due to the laser light itself. Both the SR and laser lights were linearly polarized, and their polarizations were set parallel to each other. The RF signal of 90 MHz from the master oscillator of the UVSOR electron storage ring under the multibunch operation was divided down to 5 kHz, which triggers the YAG laser. The time interval between successive SR pulses was about 11 ns, while the time width of the laser pulse was 70 ns. Therefore, the time coincidence between the SR and laser pulses was achieved automatically, because about six pulses of the SR (,70 ns/11 ns) were involved within the time duration of a single pulse of the laser. This is an advantage compared to the preliminary experimental system with a short-pulse laser [10], in which the temporal coincidence between the SR and laser pulses had to be carefully adjusted. Furthermore, the use of a high-repetition laser with a wide pulse duration and
Fig. 1. Schematic diagram of the experimental system for two-photon spectroscopy with SR and laser.
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
low peak power enabled us to detect weak two-photon signals more efficiently without any damage to the sample. The luminescence at about 4.1 eV, which is due to self-trapped excitons (STEs) produced by two-photon excitation, was observed by a micro-channel plate photomultiplier tube (MCP-PMT; Hamamatsu Photonics R2809U), with a combination of filters and a conventional monochromator (SPEX 270M) for eliminating the scattered light from the laser. The signals from the MCP-PMT were fed to a threechannel time-gated photon counter. The photon signals just before and after the incidence of the laser pulses were counted at channels 1 and 2, respectively: channel 1 (CH1) counts the background signal induced only by the SR light, while CH2 counts the signal due to the STE luminescence induced by the simultaneous irradiation with the SR and laser lights, in addition to the background signal. The gate widths for both counters were set to be 4 ms by referring to the lifetime of the STE luminescence [11]. For reference, the total signal was also counted at CH3 with the timegate fully open. The net luminescence signal induced by two-photon excitation was obtained by subtracting the count at CH1 from that at CH2. In order to get a sufficient net count (or a good signal-to-noise ratio), only a few tens of seconds were required to accumulate the counts at a fixed wavelength. These detection systems have been developed especially for the present experiments, the details of which will be reported elsewhere [12]. It is also noteworthy that our experimental technique is a kind of zero method, and therefore it will be more sensitive than the transmission method that was applied to observe two-photon absorption of KI, KCl and NaCl by Pizzoferrato et al. [2–4].
3. Results and discussion Fig. 2 shows the two-photon excitation spectrum of BaF 2 (open circles) measured by monitoring the STE luminescence at 15 K. For comparison, the onephoton excitation spectrum of the STE luminescence is also shown as a solid curve. The abscissa for the two-photon spectrum indicates the sum of photon energies of the laser (2.33 eV) and the SR, and that for the one-photon spectrum indicates the SR photon
221
Fig. 2. Two-photon excitation spectrum of BaF 2 at 15 K (open circles), obtained by monitoring the STE luminescence under excitation with both SR and laser. For comparison, the one-photon excitation spectrum obtained under excitation with SR only is also shown as a solid curve. The abscissa for the two-photon spectrum indicates the sum of photon energies of the laser (2.33 eV) and the SR, and that for the one-photon spectrum indicates the SR photon energy. The energy of the 1S exciton is indicated by an arrow.
energy. The intensity of the two-photon spectrum corresponds to the net count of the luminescence signal, as described in the previous section. The one-photon excitation spectrum shows a dip at around 10 eV. This corresponds to the reflection peak of the 1S exciton, whose resonance energy is 10.0 eV at 90 K [9], as marked by an arrow. It is to be noted that the onephoton excitation spectrum shown by the solid curve has weak peaks at 8.3 and 8.8 eV. These bands are probably due to excitons bound to some unknown lattice imperfection. As seen in the figure, the two-photon spectrum rises around 10.0 eV and shows a peak at 10.6 eV followed by another peak on the high energy side at 11.2 eV. From detailed analysis of the data described below, it is revealed that these two-photon excitation bands are made up of two contributions: one is a two-photon absorption process and the other is a two-step cascade process. The two-photon spectrum is illustrated by open circles in Fig. 3. As mentioned above, its intensity corresponds to the net luminescence count obtained by subtraction of the count at CH1 from that at CH2. The solid curve is the total count at CH3, which is properly normalized around 10 eV to the two-photon
222
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
Fig. 3. The two-photon excitation spectrum of BaF 2 at 15 K (open circles), which is reproduced from Fig. 2. The abscissa indicates the sum of photon energies of the laser (2.33 eV) and the SR. The spectrum includes two contributions: a two-photon absorption process and a cascade excitation process. The solid curve represents the spectrum due to the cascade excitation process, while the filled circles show that due to the two-photon absorption process. The energies of the 1S and 2P excitons are indicated by downward and upward arrows, respectively.
spectrum shown by the open circles. The data shown by the filled circles are obtained by subtracting the data shown by the solid curve from those shown by the open circles, the meaning of which will be given later. The count due to the net two-photon process, which is the difference between the count at CH1 and that at CH2, was at most 1% of that at CH3. Accordingly, the solid curve, the intensity of which is proportional to the total count at CH3, can be regarded as the one-photon spectrum induced by the SR alone. As is easily recognized in the figure, the two-photon spectrum shown by open circles resembles the onephoton spectrum shown by the solid curve. Such a similarity invokes the existence of cascade processes: real states (or their relaxed states) excited by one photon from the SR are subsequently re-excited to the higher states by the laser, which in turn contributes to the enhancement of the STE luminescence. This is in contrast to the usual two-photon absorption process, in which virtual intermediate states are involved. It is likely that the real excited states are the unknown bound exciton states mentioned before. In fact, the energy positions of the two peaks at 10.6 and 11.2 eV in Fig. 3 are in good agreement with those
of the weak peaks at 8.3 and 8.8 eV in the one-photon excitation spectrum shown in Fig. 2. The reason why the STE luminescence is efficiently enhanced by the cascade process is not known at present. Here, it is noted that we made careful checks, not only of the reproducibility of data, but also the instruments used, especially the time-gated counter, to make sure that the experimental data obtained are reliable enough. As mentioned above, the filled circles in Fig. 3 are the result of subtraction of the spectrum shown by the solid curve from that shown by the open circles. Since the former refers to the count due to the cascade process and the latter the net count due to the twophoton process, the resultant spectrum depicted by filled circles can be regarded as the spectrum due to the usual two-photon absorption process under the assumption that the efficiency of the cascade excitation is constant. The spectrum has a broad band peaking at 10.6 eV. This is 0.6 eV higher than the 1S exciton, whose energy position is indicated by a downward arrow. We assign this peak to the 2P exciton. Here, we should note that, from the present experiments, the relative intensity between the solid curve (the cascade excitation process) and the filled circles (the two-photon excitation process) cannot be determined uniquely, because the method of normalization for the solid curve mentioned before involves some arbitrariness. In this connection, it is also remarked that both the spectra shown by the open circles and the solid curve are very similar, but never agree exactly with each other. In fact, we tried several methods of subtraction and the 2P exciton band always remained at 10.6 eV, irrespective of the normalization factor. Therefore, it is certain that the 2P exciton band exists at 10.6 eV. Assuming a simple hydrogen model, the binding energy of the 1S exciton is thus estimated to be 0.8 eV, which gives the band gap energy as 10.8 eV. From measurements of reflection spectra, Rubloff has deduced a band gap energy of 11.0 eV [9], whereas Tomiki and Miyata have obtained a value of 10.6 eV [13]. It seems that the present value of 10.8 eV is more accurate because we observed a definite peak for the 2P exciton. Although the data points above 10.8 eV are somewhat scattered, we suppose that they originate from the two-photon interband absorption process.
O. Arimoto et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 219–223
4. Conclusion In the present study, we have undertaken twophoton spectroscopy on the valence excitons in BaF 2, by making use of a combination of tunable VUV light from SR and intense light from the second harmonics of a YAG laser. It is shown that the detection of luminescence signals by a time-gated photoncounting technique, instead of the transmittance changes, is a very sensitive and useful method. The two-photon spectrum obtained shows distinct structures just below the band gap energy. Careful examination of the experimental data reveals that the observed bands have two different origins: one is the cascade excitation process via real excited states and the other is the usual two-photon absorption process via virtual intermediate states. From the latter, the energy of the 2P exciton in BaF 2 is estimated to be 10.6 eV at 15 K. Finally, it should be stressed that two-step excitation processes like the present case are not yet detectable by the use of usual lasers, since their onephoton energies fall into the transparent region of the materials with a wide band gap, so that the real excited states cannot be excited. It is also to be pointed out that the present experimental system with SR and laser for two-photon spectroscopy can be extended to investigations of various relaxation processes of the highly excited states produced by the one-photon process with SR, in which the laser is used as a probe
223
light. Experiments along these lines are now in progress.
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