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Hyper-Raman spectra of some cubic crystals
Volume
122, number
1,2
CHEMICAL
HYPER-RAMAN
SPECTRA
OF SOME
and
I. NAKAGAWA
Depormwrr OJ Chenlism:
Faculty 01 Science,
Y_ MORIOKA
Received 5 August
PHYSICS
CUBIC
LE?TERS
29 November
1965
CRYSTALS
Tohoku Universiry. Aohu. Aramuki,
Sencfai, 960 Jqx.w
1985
were obsewcd by a 1.06 pm line from nn acousm-opIic;llly Hyper-Rnman ~pecm or SrliO,. GC~CI,. Cd and N&l Q-switched Nd I YAG lnser and a mullichanncl deleclion syslem. The TO nnd LO frequencies or CsCaCl, are slighlly different Irom those obmined by Ihs rar-infrared rellecIion spectrum
1. Introduction Hyper-Raman scattering is known to be a spectroscopic tool which provides information more or less complementary to Raman sdattering. By hyperRaman scattering, one can obtain the frequencies of a silent mode (infrared and Raman inactive) as well as the transverse and longitudinal frequencies of infrared active modes in ionic crystals directly_ Another advantage in hyper-Raman scattering was demonstrated by Inoue et al. [1] and Vogt and Uwe [2] in the study of ferroelectric phase transition, where the ferroelectric soft mode in the low-frequency region plays an important role. Since hyper-Rayleigh scattering is forbidden in the centrosymmetric phase, strong disturbance such as the Rayleigh scattering in the case of Raman spectroscopy is absent, permitting detailed study on the very-low-frequency modes. Some pioneering works on the lattice vibration measurements by the hyper-Rarnan scattering have been reported on the crystals such as CsI [3], LiNbOg [4] and SrTi03 [5,6), in which singlechannel photon counting has mainly been used. Hyper-Raman scattering has an extremely small scattering cross section which makes the measurement time-consuming. The multichannel system essentially is an efficient method of detection and is expected to bring about the breakthrough in the hyper-Raman spectroscopy. Thus we have attempted the measurements of hyper-Raman spectra of crystal vibration by the multichannel detection. In the present Letter, the hyper-Raman spectra of 150
SrTi03, CsI, NaCl and CsCaCl, crystals measured by using the intensified photodiode array detector are reported. The crystal structures of NaCl and CsI are the face-centered cubic and the simple cubic respectively, and both of them contain one formula unit in a Bravais unit cell. SrTiOs and C.sCaC13 belong to the perovskite structure (simple cubic with one formula unit in a Bravais unit cell).
2. Experimental Single crystals of SrTi03, NaCl and CsI used for measurements were commercially available ones. Single crystals of CsCaC13 were grown from the melt by Bridgman method. Because CsCaCl, is strongly hygroscopic,
samples were sealed in quartz cell with
dry Nzgas.
The 1.06 pm line from an acousto-optically Q: switched Nd : YAG laser (NEC) was used for the excitation. The repetition rate was set to between 2 kHz (pulse width 150 ns) and 6 kHz (200 ns). A peak power between 3 and 50 kW in the immediate neighborhood of an output mirror of the laser was employed, depending on the samples. The laser beam was focused into the sample by af= 5 cm lens. The scattered light. in the 90° geometry was dispersed by a polychromator (JASCO TRS-501) and detected by the intensified photodiode array detector (Tracer Northern) with a controller (Seki-Shoji). The laser radiation and the collected radiation are not polarized_ The dispersion of the polychromator ensures a resolution of 0 009-2614/85/$ (North-Holland
0330
0 Elsevier Science Publishers B-V.
Physics Publishing Division)
Volume 122, number 112
CHEMICAL
PHYSICS
LETTERS
29 November
1985
0.5 cm-l/element at around 532 nm, and in this multichannel detection system the frequency region of approximately 230 cm-1 is measured simultaneously. The dark noise was reduced by gating the intensifier synchronously to the Q switch_ When hyper-Raman bands are weak (CsCaC13 and NaCI), &al is integrated on the photodiode array for up to 40 min minimizing the readout noise, followed by accumulation of several times on the digital memory of the conabout
troller.
3. Results and discussion
HY PER-RAMAN
The hyper-Raman spectrum below 280 cm-l of SrTiOg is shown in fig. I_ The spectral pattern is in good agreement with previous single channel [5,6] and multichannel hyper-Raman studies [7]. The bands at 88, 175 and 266 cm-l are F1,(TOl), F1,(LOl) + F,,(T02) and FZu, respectively. SrTiO3 shows strong hyper-Raman scattering and reliable vibrational frequencies are available. Therefore we used this crystal for the adjustment and wavenumber calibration of our spectrometer. The intensity scale is arbitrarily but chosen in common within the measurements in this Letter. In CsCaCl+ five hyper-Raman bands were observed_ The selection rule predicts 3 FIU(TO), 3 F&LO) and 1 F, (silent mode)_ In table 1 observed frequencies are listed comparing with the previous data obtained from the far-infrared reflection spectrum and a
(cm’)
SH I FT
Fig. 1. Hypcr-Raman spectrum of SrTiOs. Peak power of the laser is below 3 kW, and repetition rate is 6 kHz. The time for recording this spectrum was 10 min. Ai to the scale of intcnsity. see test.
Table 1 Lattice mode frequencies ofCsCaC1~ This work l=,,
rO1 LO1 TO2 LO2 TO3 LO3
F2u
in cm-’ Far-IR a)
76
59 69
114 142 267 312
113 138 263 309
-
-
a) hlorioka and Nakagawa [ 81.
0.0.5 76
312
‘i % ul
Jf+a 267
350
250
HYPER-RAMAN Fig. 2. Hyper-Raman specQurn ofCsCaC13. recording the 230 cm-’ portion was 4 h.
SHIFT
( cm-l
)
Peak power of the laser is below 50 kW. and repetition rate is 2 kHz. The time for 151
Volume 122, number 1.2
CHEhiICAL
PHYSICS LETl-ERS
61
I
I
100
HVPER-RAMAN
( c m-l 1
Fig. 3. Hypcr-Raman spectrum of Csl. Peak power of the laser is below 7.5 kW. and repetition rate is 5 kHz. The time for recording this spectrum was 30 min.
Kramers-Kronig analysis [8]. The F,, mode and the lowest TO mode.of F,, symmetry were not observed. As for the observed frequencies shown in fig. 2 and listed in table 1, the hyper-Raman data might be more reliable than the far-infrared data, because (1) the reflectivity measurements depend on the condition of the crystal surface and (2) the data in the low-reflectivity region, where S/N is low, influence the accuracy of the dielectric function and the TO and LO frequencies calculated from reflectivity data by KramersKronig relation. In spite of the centrosymmetric crystal, a relatively strong hyper-Rayleigh scattering was observed. This is considered to be due to the local strains or impurities which reduce local symmetry. The hyper-Raman spectra of Csl and NaCl are shown in figs. 3 and 4, respectively. One TO and one LO of F,, species are expected_ The frequencies 61 cm-l (TO) and 89 cm-l (LO) of Csl are in good agreement with those of Vogt and Neumann measured by the single channel system, 61.4 and 87 cm-J [3] _The hyper-Rayleigh line is much weaker reflecting the centrosymmetr-y. The frequencies 165 cm-J (TO) and 270 cm-l (LO) of NaCl are also in good agreement with the recent study by Vogt and Presting [9]. The LO bands of CsI and NaCl are much broader than TO bands. This is ascribed to the fact that the LO frel quency of alkali halide crystals are close to the maxi152
HYPER-&N
0
SHIFT
165
270
I
I
29 November 1985
SHIFT
( cm-’
)
100
Fig. 4. Hypcr-Raman spectrum of NaCl. Peak power of the laser is below 30 kW, and rcpctition rate is 2 kHz. The time for recording this spectrum was 2 h.
mum of the two-phonon density of state, which is a main decay channel of optical phonon energy [9]. In conclusion the study of the hyper-Raman spectra by the multichannel detection system has proved to be a promising method for investigating the crystal lattice vibration_
Acknowledgement This study was supported by Grant-in-Aid Scientific Research (A) 58430005.
for
References 111 K. Inoue,.N. Asai and T_ Sameshima. J. Phys. Sot. Japan 50 (1981) 1291. I21 H. Vogt and H. Uwc. Phys. Rev. B29 (1984) 1030. 131 H. Vogt and G. Neumann, Opt. Commun. 19 (1976) 108. r41 V.N. Denisov, B.N. Man-in, V.B. Pedobedov and Kh.E. Sterin, Opt. Common 26 (1978) 372. [51 K. Inouc and T. Samesbima. J. Phys Sot. Japan 47 (1979) 2037. 161 H. Vogt and G. Neumann, Phys. Stat Sol. (b) 92 (1979) 57. r71 V.N. Denisov, B-N. Mavrin. V.B. Pedobedov and J.F. Scott, J. Raman Spectry. 14 (1983) 276. IS1 Y. Morioka and I. Nakagawa, Bull. Chem. Sot. Japan 51 (1978) 2467. 6731. I91 H. Vogt and H. Prerting, Phys. Rev. B31(1985)
122, number
1,2
CHEMICAL
HYPER-RAMAN
SPECTRA
OF SOME
and
I. NAKAGAWA
Depormwrr OJ Chenlism:
Faculty 01 Science,
Y_ MORIOKA
Received 5 August
PHYSICS
CUBIC
LE?TERS
29 November
1965
CRYSTALS
Tohoku Universiry. Aohu. Aramuki,
Sencfai, 960 Jqx.w
1985
were obsewcd by a 1.06 pm line from nn acousm-opIic;llly Hyper-Rnman ~pecm or SrliO,. GC~CI,. Cd and N&l Q-switched Nd I YAG lnser and a mullichanncl deleclion syslem. The TO nnd LO frequencies or CsCaCl, are slighlly different Irom those obmined by Ihs rar-infrared rellecIion spectrum
1. Introduction Hyper-Raman scattering is known to be a spectroscopic tool which provides information more or less complementary to Raman sdattering. By hyperRaman scattering, one can obtain the frequencies of a silent mode (infrared and Raman inactive) as well as the transverse and longitudinal frequencies of infrared active modes in ionic crystals directly_ Another advantage in hyper-Raman scattering was demonstrated by Inoue et al. [1] and Vogt and Uwe [2] in the study of ferroelectric phase transition, where the ferroelectric soft mode in the low-frequency region plays an important role. Since hyper-Rayleigh scattering is forbidden in the centrosymmetric phase, strong disturbance such as the Rayleigh scattering in the case of Raman spectroscopy is absent, permitting detailed study on the very-low-frequency modes. Some pioneering works on the lattice vibration measurements by the hyper-Rarnan scattering have been reported on the crystals such as CsI [3], LiNbOg [4] and SrTi03 [5,6), in which singlechannel photon counting has mainly been used. Hyper-Raman scattering has an extremely small scattering cross section which makes the measurement time-consuming. The multichannel system essentially is an efficient method of detection and is expected to bring about the breakthrough in the hyper-Raman spectroscopy. Thus we have attempted the measurements of hyper-Raman spectra of crystal vibration by the multichannel detection. In the present Letter, the hyper-Raman spectra of 150
SrTi03, CsI, NaCl and CsCaCl, crystals measured by using the intensified photodiode array detector are reported. The crystal structures of NaCl and CsI are the face-centered cubic and the simple cubic respectively, and both of them contain one formula unit in a Bravais unit cell. SrTiOs and C.sCaC13 belong to the perovskite structure (simple cubic with one formula unit in a Bravais unit cell).
2. Experimental Single crystals of SrTi03, NaCl and CsI used for measurements were commercially available ones. Single crystals of CsCaC13 were grown from the melt by Bridgman method. Because CsCaCl, is strongly hygroscopic,
samples were sealed in quartz cell with
dry Nzgas.
The 1.06 pm line from an acousto-optically Q: switched Nd : YAG laser (NEC) was used for the excitation. The repetition rate was set to between 2 kHz (pulse width 150 ns) and 6 kHz (200 ns). A peak power between 3 and 50 kW in the immediate neighborhood of an output mirror of the laser was employed, depending on the samples. The laser beam was focused into the sample by af= 5 cm lens. The scattered light. in the 90° geometry was dispersed by a polychromator (JASCO TRS-501) and detected by the intensified photodiode array detector (Tracer Northern) with a controller (Seki-Shoji). The laser radiation and the collected radiation are not polarized_ The dispersion of the polychromator ensures a resolution of 0 009-2614/85/$ (North-Holland
0330
0 Elsevier Science Publishers B-V.
Physics Publishing Division)
Volume 122, number 112
CHEMICAL
PHYSICS
LETTERS
29 November
1985
0.5 cm-l/element at around 532 nm, and in this multichannel detection system the frequency region of approximately 230 cm-1 is measured simultaneously. The dark noise was reduced by gating the intensifier synchronously to the Q switch_ When hyper-Raman bands are weak (CsCaC13 and NaCI), &al is integrated on the photodiode array for up to 40 min minimizing the readout noise, followed by accumulation of several times on the digital memory of the conabout
troller.
3. Results and discussion
HY PER-RAMAN
The hyper-Raman spectrum below 280 cm-l of SrTiOg is shown in fig. I_ The spectral pattern is in good agreement with previous single channel [5,6] and multichannel hyper-Raman studies [7]. The bands at 88, 175 and 266 cm-l are F1,(TOl), F1,(LOl) + F,,(T02) and FZu, respectively. SrTiO3 shows strong hyper-Raman scattering and reliable vibrational frequencies are available. Therefore we used this crystal for the adjustment and wavenumber calibration of our spectrometer. The intensity scale is arbitrarily but chosen in common within the measurements in this Letter. In CsCaCl+ five hyper-Raman bands were observed_ The selection rule predicts 3 FIU(TO), 3 F&LO) and 1 F, (silent mode)_ In table 1 observed frequencies are listed comparing with the previous data obtained from the far-infrared reflection spectrum and a
(cm’)
SH I FT
Fig. 1. Hypcr-Raman spectrum of SrTiOs. Peak power of the laser is below 3 kW, and repetition rate is 6 kHz. The time for recording this spectrum was 10 min. Ai to the scale of intcnsity. see test.
Table 1 Lattice mode frequencies ofCsCaC1~ This work l=,,
rO1 LO1 TO2 LO2 TO3 LO3
F2u
in cm-’ Far-IR a)
76
59 69
114 142 267 312
113 138 263 309
-
-
a) hlorioka and Nakagawa [ 81.
0.0.5 76
312
‘i % ul
Jf+a 267
350
250
HYPER-RAMAN Fig. 2. Hyper-Raman specQurn ofCsCaC13. recording the 230 cm-’ portion was 4 h.
SHIFT
( cm-l
)
Peak power of the laser is below 50 kW. and repetition rate is 2 kHz. The time for 151
Volume 122, number 1.2
CHEhiICAL
PHYSICS LETl-ERS
61
I
I
100
HVPER-RAMAN
( c m-l 1
Fig. 3. Hypcr-Raman spectrum of Csl. Peak power of the laser is below 7.5 kW. and repetition rate is 5 kHz. The time for recording this spectrum was 30 min.
Kramers-Kronig analysis [8]. The F,, mode and the lowest TO mode.of F,, symmetry were not observed. As for the observed frequencies shown in fig. 2 and listed in table 1, the hyper-Raman data might be more reliable than the far-infrared data, because (1) the reflectivity measurements depend on the condition of the crystal surface and (2) the data in the low-reflectivity region, where S/N is low, influence the accuracy of the dielectric function and the TO and LO frequencies calculated from reflectivity data by KramersKronig relation. In spite of the centrosymmetric crystal, a relatively strong hyper-Rayleigh scattering was observed. This is considered to be due to the local strains or impurities which reduce local symmetry. The hyper-Raman spectra of Csl and NaCl are shown in figs. 3 and 4, respectively. One TO and one LO of F,, species are expected_ The frequencies 61 cm-l (TO) and 89 cm-l (LO) of Csl are in good agreement with those of Vogt and Neumann measured by the single channel system, 61.4 and 87 cm-J [3] _The hyper-Rayleigh line is much weaker reflecting the centrosymmetr-y. The frequencies 165 cm-J (TO) and 270 cm-l (LO) of NaCl are also in good agreement with the recent study by Vogt and Presting [9]. The LO bands of CsI and NaCl are much broader than TO bands. This is ascribed to the fact that the LO frel quency of alkali halide crystals are close to the maxi152
HYPER-&N
0
SHIFT
165
270
I
I
29 November 1985
SHIFT
( cm-’
)
100
Fig. 4. Hypcr-Raman spectrum of NaCl. Peak power of the laser is below 30 kW, and rcpctition rate is 2 kHz. The time for recording this spectrum was 2 h.
mum of the two-phonon density of state, which is a main decay channel of optical phonon energy [9]. In conclusion the study of the hyper-Raman spectra by the multichannel detection system has proved to be a promising method for investigating the crystal lattice vibration_
Acknowledgement This study was supported by Grant-in-Aid Scientific Research (A) 58430005.
for
References 111 K. Inoue,.N. Asai and T_ Sameshima. J. Phys. Sot. Japan 50 (1981) 1291. I21 H. Vogt and H. Uwc. Phys. Rev. B29 (1984) 1030. 131 H. Vogt and G. Neumann, Opt. Commun. 19 (1976) 108. r41 V.N. Denisov, B.N. Man-in, V.B. Pedobedov and Kh.E. Sterin, Opt. Common 26 (1978) 372. [51 K. Inouc and T. Samesbima. J. Phys Sot. Japan 47 (1979) 2037. 161 H. Vogt and G. Neumann, Phys. Stat Sol. (b) 92 (1979) 57. r71 V.N. Denisov, B-N. Mavrin. V.B. Pedobedov and J.F. Scott, J. Raman Spectry. 14 (1983) 276. IS1 Y. Morioka and I. Nakagawa, Bull. Chem. Sot. Japan 51 (1978) 2467. 6731. I91 H. Vogt and H. Prerting, Phys. Rev. B31(1985)