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Stokes and anti-stokes hyper-Raman scattering from benzene, deuterated benzene, and carbon tetrachloride
Volume 155, number 4,s
CHEMICAL PHYSICS LETTERS
STOKES AND ANTI-STOKES HYPE&RAMAN FROM BENZENE, DEUTERATED BENZENE,
10 March 1989
SCAlTERING AND CARBON TETRACHLORIDE
William P. ACKER, David H. LEACH and Richard K. CHANG Section ofApplied Physics and Centerfor Laser Diagnostics, Yale University, New Haven, CT 06520, USA
Received 13 December 1988
Hype+Raman spectra from liquid benzene, deuterated benzene, and carbon tetrachloride are observed using a cw pumped @ switched mode-locked Nd:YAG laser and a synchronously gated two-dimensional single-photon-counting detector. Both Stokes and anti-Stokes peaks arc observed and assigned to hyper-Raman active vibrational modes, some of which are forbidden in the Raman and infrared spectra of benzene and deuterated benzene. The effect of the stimulated Raman process in populating the vibrational levels and modifying the hyper-Raman spectra is discussed.
Hyper-Raman spectroscopy, first predicted [ 1 ] in 1959 and observed [2] in 1965, can provide vibrational mode information complementary to the information obtained from Raman and IR spectroscopy of molecules with inversion symmetry. Because the hyper-Raman signal is frequency shifted from the second harmonic of the incident laser radiation, hyper-Raman spectroscopy is helpful in studying lowfrequency rotational and vibrational modes which are difficult to measure in the IR and are overwhelmed by the strong elastic scattering from the laser in Raman spectroscopy. Although the advantages of hyper-Raman scattering are recognized, this spectroscopy has seen limited use primarily because of the extremely weak scattering cross section [ 1,2 1. The hyper-Raman signal intensity, which increases as the square of the incident laser intensity, is constrained by the breakdown threshold of the sample. Even at the prebreakdown input intensity, hyper-Raman scattering is typically five to seven orders of magnitude weaker than that of Raman scattering. Larger hyper-Raman signals are observed under resonant conditions [ 3,4] and in surface-enhanced hyper-Raman scattering [ 5,6], Technological advances have increased the ease of obtaining hyper-Raman signals. The initial spectra were generated with a Q-switched ruby laser operating at % 1 Hz and detected with a synchronously gated photomultiplier tube and subsequently with 0 009-2614/89/s (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
optical multichannel detection [ 7-9 1. The use of cw pumped acousto-optically Q-switched Nd:YAG lasers (repetition rates of 5 kHz), synchronously gated photomultiplier tubes [ 10 1, and synchronously gated two-dimensional single-photon-counting detectors [ 111 has improved the signal-to-noise ratio of hyper-Raman spectra. Studies in which these instruments were used have reported the first observation of a vibrational mode which is Raman and IR inactive in C&l4 liquid [ 1O] as well as the Stokes and anti-Stokes lines of soft mode and transverse optical modes in SrTi03 and other crystals [ 111. Modelocked pulses (at 82 MHz) from a Nd: YAG laser have recently been used in conjunction with a photomultiplier to observe the surface-enhanced hyperRaman signal from pyridine adsorbed on silver [ 61. The hyper-Raman scattering selection rules for a number of point groups have been presented [ 12141. The classical example of the complementary nature of hyper-Raman spectroscopy relative to Raman and IR spectroscopy is CsHb or C6Ds because of the many vibrational modes which are uniquely hyper-Raman active. The C6H6 and C6D6 molecules ( Dhh point group) have 30 normal vibrational modes with 20 unique frequencies ( 10 modes are doubly degenerate). Because the Dsb point group has inversion symmetry, an exclusion rule exists between modes which are Raman active and those which are IR and/or hyper-Raman active. Although all IR-acB.V.
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tive modes are also hyper-Raman active, there are modes which are active only in the hyper-Raman process. Fig. 1 shows the low-lying vibrational energy levels for C6D6 grouped by their activity involving transitions from the ground state, i.e. Raman, IR, hyperRaman (HR), and none of the above which are grouped as silent. The numerical designation of the vibrational modes in fig. 1 follows Herzberg’s convention [ 15 1. Note that in the third column four modes with energy below 1500 cm- ’are only hyperRaman active and three modes are both IR and hyper-Raman active. Thus in the hyper-Raman spectra we expect to see seven Stokes peaks which are shifted in energy from the second harmonic frequency of the incident laser beam. Previous hyper-Raman results on C,H, were present orally (abstract form) [ 161. We report here on C6H6 and C6D6 hyper-Raman spectra containing Stokes peaks arising from vibrational modes which Deuterated
Benzene
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fl
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f
Fig. I Vibrational energy levels of C6D, (energy < 1600 cm-‘) grouped by their activity from the ground state, i.e. Raman, IR, or hyper-Raman (HR) Mode labeling follows Henberg’s notation [ 151. Modes which are not active in Raman, KR, or hyperRaman are grouped as silent.
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are both IR and hyper-Raman active as well as modes which are solely hyper-Raman active. The role of stimulated Raman scattering (SRS ) in selectively populating vibrational modes and the consequences to the hyper-Raman spectra were investigated because the generation of hyper-Raman scattering requires high input intensity radiation. An optical multichannel detection system was used to simultaneously measure the Stokes and anti-Stokes hyperRaman spectra of C6D6. Laser-induced selective population effects were not observed in C,D6 but were observed in CCL,, which because of its lack of inversion symmetry has modes which cannot be exclusively grouped as Raman or hyper-Raman active. The incident laser beam is the fundamental ( 1.064 pm) of a @switched mode-locked Nd:YAG laser (Quantronix 416). Each &switched pulse (260 ns duration at a rate of 1 to 6 kHz) consisted of a train of mode-locked pulses ( 100 ps pulse/ 12.5 ns) with an energy of = 1.5 mJ at 1 kHz. The incident beam propagating in the vertical direction was focused in a 4 cm cell. The scattered light was collected normal to both the propagation direction and the polarization of the incident laser with a f/l lens and imaged onto the entrance slit of a spectrograph (Spex 1870). The extremely weak scattered intensity prevented polarization analysis of the hyper-Raman radiation. A single-photon-counting two-dimensional detector (Mepsicron) [ 17 ] was placed at the exit focal plane of the spectrometer. In our experiment, the Mepsicron was gated on for 1.4 ps synchronously with the laser @switcher. This reduced the detected background noise (with the laser on and the slit open but without the sample in place) to < 8 x 1O-’ cps over the entire detector. In the optical multichannel analyzer configuration, the horizontal direction of the Mepsicron corresponds to the dispersed wavelength while the vertical direction corresponds to the spatial position in the cell with the appropriate optical magnification factors. Because the hyper-Raman signal is generated primarily in the waist of the focused incident, beam, it was necessary to accept the signal only in the vertical dimension of approximately one-quarter of the detector diameter. The full horizontal dimension was used to achieve the maximum wavelength coverage. Channels in the vertical direction were summed since we were not interested in spatial resolution but only
Volume 155,number 4,5
in using the vertical dimension of the detector to increase the totaI signal coIlected. This resulted in a total background noise of ~2 x 1OS5 cps/wavelength channel or an average of one count in each wavelength channel every 15 h. Fig. 2a shows the Stokes shifted hyper-Raman spectra from a cell containing C6H6 after 11 h of integration with the Mepsicron and the laser Qswitched at I kHz. Higher Q-switching rates led to greater thermal blooming in C6H6 as a result of the slight absorption at 1.064 pm, which is caused by the overtones of the C-H stretching modes [ 181. Even at 1 kHz, the laser beam was slightly defocused, thus reducing the hyper-Raman signal, which depends on the square of the incident laser intensity. The Stokes peaks in fig. 2a are labeled in accordance with Herzberg’s notation. The large peak at 790 cm-’ corresponds to the u4 mode (a,,), which is both IR and hyper-Raman active. The smaller peak at 400 cm-’ corresponds to vZo (e,,), which is solely hyper-Raman active. The problem of thermal blooming is much less
pronounced in C,D, since the wavelengths at which the C-D stretching mode overtones occur are longer than those of the C-H stretching mode overtones. With the laser Q-switched at 6 kHz, fig. 2b shows the Stokes hyper-Raman spectra from C,D, (99.5 at% D) after integrating the lower spectrum for 6 h and the upper spectrum for 10 h. The reduced thermal blooming allows a higher repetition rate and tighter focusing. Consequently, the signal-to-noise ratio of
6
10 March 1989
CHEMICAL PHYSICS LETTERS
1
(a)
the spectra ,is substantially improved. Comparing these spectra to the predicted modes in fig. 1, we observe vZOat 340 cm-’ and vq at 495 cm-‘, both appropriately down shifted from those of C,H,. The origin of the peak at 660 cm-’ (denoted by an asterisk) is unknown as it does not correspond to a hyper-Raman-allowed fundamental mode of C6D6. However, this peak has the proper energy to correspond to a hyper-Raman process involving the pll mode, which is Raman active only from the ground state. It is doubtful that the collisional or high incident intensity perturbation to the selection rules is sufficient to account for the relatively large magnitude of the peak. In fig. 2b, the peak at 8 10 cm-’ corresponds to v,~, which is hyper-Raman active, and/or vN4,which is both IR and hyper-Raman active. The peak at 950 cm-’ can be assigned to the us mode, which is hyper-Raman active only. We did not detect any signal corresponding to the v19 mode, the next hyper-Raman active mode (see fig, 1). Two closely spaced peaks are observed at 1290 and 1330 cm-‘. The origin of the former peak is unknown. The latter corresponds to the IR and hyper-Raman-active v,~ mode. The small peak at 1450 cm-’ probably corresponds to the v2 + v, combination mode which has been observed with IR spectroscopy and is both IR and hyper-Raman active. Fig. 3 shows the Stokes and anti-Stokes hyper-Raman spectra for C6D, in the low frequency shift region (0 to 600 cm - ’). These spectra were obtained
Anti-Stokes
0.20
I ma Hyper-Raman shift (mi’)
Fig. 2. Hyper-Raman spectra of C,H, integrated for 11h with a Nd: YAG laser Q-switched at 1kHz (a) and of C,D, integrated for 6 h in the lower spectrum and 10 h in the upper spectrum, both with the laser Q-switched at 6 kHz (b).
t---
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HypwRayleigh
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i
i
i
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Deuterated nenzeoe
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200 shift (cm-‘)
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Fig. 3. Stokes and anti-Stokes hyper-Raman spectra of C,D, in the low-frequency shift region (0 to 600 cm-‘) integrated for 5 h with a Nd:YAG laser @switched at 6 kHz. The broad hyperRayleigh peak is observable at 0 cm-’ shift.
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with the laser Q-switched at 6 kHz and an integration time of 5 h. Hyper-Raman peaks associated with modes v4 and v20 are readily detected in both Stokes and anti-stokes spectra. The relative intensities between the Stokes and anti-Stokes peaks are in accord with a Bose-Einstein population distribution at 25 “C (slightly above the laboratory temperature). The broad hyper-Rayleigh peak, centered at the secondharmonic frequency (0 cm- ’), is the result of intermolecular interactions in liquid C6Ds which remove the inversion symmetry of an isolated C6Ds molecule and thereby allow some second-harmonic generation. The hyper-Rayleigh intensity from liquid CC& is several orders of magnitude greater than that of liquid CsDh because an isolated Ccl, molecule lacks inversion symmetry. Figs. 4a and 4b show the Stokes and anti-Stokes hyper-Raman spectra for CCL, measured at two different input intensities, where the enormous hyper-Rayleigh peak is not displayed. The three hyper-Raman active modes ( vl, v3, and vq) of CCL, are present in the Stokes and anti-Stokes spectra, All three of these modes are also Raman active. The anti-Stokes hyper-Raman process involves a three-photon transition starting from an excited vibrational mode ( vI, v3, or v,) and ending at the ground state. At lower incident laser intensity, the ratios of hyper-Raman intensity for the anti-Stokes and Stokes peaks correspond approximately to the Bose-Einstein population distribution at a slightly elevated temperature (see fig. 2a). However, at higher incident laser intensity (see fig. 4b), the antiStokes-to-Stokes intensity ratios (0.6, 0.3, and 0.35 for modes vl, v3, and Y,, respectively) far exceed those expected from the room temperature population distribution. The laser-dependent anti-Stokes-to-Stokes intensity ratio can be explained by considering the selective population associated with the SRS process. Since v,, vj, and vq are Raman active, the SRS process taking place in the IR wavelength region can populate these levels. During the input laser pulse, the selective pumping rate of the v, mode exceeds that of the v3 and v4 modes because the spontaneous Raman cross section of the v, mode is known to be larger than that of the v3 and v4 modes. In addition, the spontaneous Raman cross section of the v4 mode is larger than that of the v3 mode. Although the anti494
10 March 1989
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Hyper-Raman shift (cm” ) Fig. 4. Stokes (top) and anti-Stokes (bottom) hyper-Raman spectra of Ccl4 at low (a) and high (b) incident laser intensity. The population distribution deduced from the relative anti-Stokes and Stokes intensities is in accord with a near room temperature Bose-Einstein distribution for the low incident laser intensity only.
Stokes-to-Stokes intensity ratio will also depend on the vibrational relaxation time of the particular modes and saturation effects in the stimulated Raman pumping process, the ratios are in qualitative agreement with those expected from the relative R.aman cross sections of the modes. At high input intensity, we did not observe additional hyper-Raman peaks in the anti-Stokes or Stokes spectra as a result of transitions from the selectively populated mode3 to levels other than the ground state. Determination of numerical values for the various hyper-polarizability tensor elements from the detected hyper-Raman signal intensities is limited by the lack of polarization analysis and the degree of accuracy to which we can estimate the laser intensity within the cell when thermal blooming is present. We
Volume 155, number 4,5
CHEMICAL
PHYSICS LETTERS
assumed that the interaction volume is a cylinder with a diameter of 200 pm and a height of 3 mm and that the laser intensity is 1 GW/cmZ. The hyper-Raman scattering cross section, i.e. the number of hyper-Raman scattered photons per incident photon per molecule, is estimated to be 1O-36 and 10M3’for the Y, and vzo modes in C6D6, respectively. In conclusion, the hyper-Raman spectra of C,H, and C6Ds were readily observable when the combination of a Q-switched mode-locked laser and a gated Mepsicron was used. A much better signal-to-noise ratio in the hyper-Raman Stokes and anti-Stokes spectra was obtainable from C,D, because thermal b@oming in C6D6 is a less serious problem than in C&I,. The solely hyper-Raman active modes ( vzo, v,) and the IR and hyper-Raman active Ul d ? and modes (u,, u14, and Y,~) have been identified in the C6D6 hyper-Raman spectra. The intensity ratio of the anti-Stokes to the Stokes peaks in Ccl, hyperRaman spectra at higher input intensity was noted to be larger than the ratio at lower input intensity. This, increase above the room-temperature BoseEinstein distribution is a result of selective laser-induced population (via the SRS process) of the higher vibrational levels which are both Raman and hyperRaman active for noncentrosymmetric molecules such as Ccl,. No such input intensity dependence was noted for C6D6 because the vibrational levels that can be selectively populated by the SRS process are mutually exclusive to those vibrational levels that are involved in the hyper-Raman transitions for centrosymmetric molecules such as C6D6 and C6H6. We gratefully acknowledge the partial support of this research by the US Department of Energy, En-
IO March 1989
ergy Conversion and Utilization Technologies gram (Contract No. DE AS04-87AL42879).
Pro-
References [ I] J.C. Decius and J.E. Rauch, Ohio State Symposium
on Molecular Structure and Spectroscopy, Paper 48 (June 1959). [2] R.W. Terhune, P.D. Maker and C.M. Savage, Phys. Rev. Letters 14 (1965) 681. [3] M.A. Washington, A.Z. Genack, H.Z. Cummins and R.H. Bruce, Phys. Rev. B 15 (1977) 2145. [4] L.D. Ziegler and J.L. Roebber, J. Chem. Phys. 136 (1987) 371. [5]D.V. Murphy, KU. von Raben, R.K. Chang and P.B. Dorain, Chem. Phys. Letters 85 (1982) 43. [6] J.T. Golab, J.R. Sprague, K.T. Carron, G.C. Schatz and R.P. Van Duyne, .I. Chem. Phys. 88 (1988) 7942. [7] CM. Savage and P.D. Maker, Appl. Opt. 10 (1971) 965. [8] M.J. French and D.A. Long, J. Raman Spectry. 3 (1975) 391. [9] J.H. Verdieck, S.H. Peterson, C.M. Savage and P.D. Maker, Chem. Phys. Letters 7 ( 1970) 2 19. [lo] W.J. Schmid and H.W. Schriitter, Chem. Phys. Letters 45 (1977) 502. [ll]H.Vogt,Phys.Rev.B36(1987)5001. [12] S.J. Cyvin, J.E. Rauch and J.C. Decius, J. Chem. Phys. 43 (1965) 4083. [ 131 J.H. Christie and D.J. Lockwood, J. Chem. Phys. 54 (1971) 1141. [ 141 D.L. Andrews and T. Thirunamachandran, J. Chem. Phys. 68 (197&j 2941. [ 15 ] G. Henberg, Molecular spectra and molecular structure, Vol. 2. Infrared and Raman spectra of polyatomic molecules (Van Nostrand Reinhold, New York, 1945) p. 118. [ 161 P.D. Maker, 4th International Raman Conference, Brunswick, Maine (August 1974). [ 171 W.P. Acker, B. Yip, D.H. Leach and R.K. Chang, J. Appl. Phys. 64 (1988) 2263. [ 18 ] K.V. Reddy, D.F. Heller and M.J. Berry, J. Chem. Phys. 76 (1982) 2814.
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CHEMICAL PHYSICS LETTERS
STOKES AND ANTI-STOKES HYPE&RAMAN FROM BENZENE, DEUTERATED BENZENE,
10 March 1989
SCAlTERING AND CARBON TETRACHLORIDE
William P. ACKER, David H. LEACH and Richard K. CHANG Section ofApplied Physics and Centerfor Laser Diagnostics, Yale University, New Haven, CT 06520, USA
Received 13 December 1988
Hype+Raman spectra from liquid benzene, deuterated benzene, and carbon tetrachloride are observed using a cw pumped @ switched mode-locked Nd:YAG laser and a synchronously gated two-dimensional single-photon-counting detector. Both Stokes and anti-Stokes peaks arc observed and assigned to hyper-Raman active vibrational modes, some of which are forbidden in the Raman and infrared spectra of benzene and deuterated benzene. The effect of the stimulated Raman process in populating the vibrational levels and modifying the hyper-Raman spectra is discussed.
Hyper-Raman spectroscopy, first predicted [ 1 ] in 1959 and observed [2] in 1965, can provide vibrational mode information complementary to the information obtained from Raman and IR spectroscopy of molecules with inversion symmetry. Because the hyper-Raman signal is frequency shifted from the second harmonic of the incident laser radiation, hyper-Raman spectroscopy is helpful in studying lowfrequency rotational and vibrational modes which are difficult to measure in the IR and are overwhelmed by the strong elastic scattering from the laser in Raman spectroscopy. Although the advantages of hyper-Raman scattering are recognized, this spectroscopy has seen limited use primarily because of the extremely weak scattering cross section [ 1,2 1. The hyper-Raman signal intensity, which increases as the square of the incident laser intensity, is constrained by the breakdown threshold of the sample. Even at the prebreakdown input intensity, hyper-Raman scattering is typically five to seven orders of magnitude weaker than that of Raman scattering. Larger hyper-Raman signals are observed under resonant conditions [ 3,4] and in surface-enhanced hyper-Raman scattering [ 5,6], Technological advances have increased the ease of obtaining hyper-Raman signals. The initial spectra were generated with a Q-switched ruby laser operating at % 1 Hz and detected with a synchronously gated photomultiplier tube and subsequently with 0 009-2614/89/s (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
optical multichannel detection [ 7-9 1. The use of cw pumped acousto-optically Q-switched Nd:YAG lasers (repetition rates of 5 kHz), synchronously gated photomultiplier tubes [ 10 1, and synchronously gated two-dimensional single-photon-counting detectors [ 111 has improved the signal-to-noise ratio of hyper-Raman spectra. Studies in which these instruments were used have reported the first observation of a vibrational mode which is Raman and IR inactive in C&l4 liquid [ 1O] as well as the Stokes and anti-Stokes lines of soft mode and transverse optical modes in SrTi03 and other crystals [ 111. Modelocked pulses (at 82 MHz) from a Nd: YAG laser have recently been used in conjunction with a photomultiplier to observe the surface-enhanced hyperRaman signal from pyridine adsorbed on silver [ 61. The hyper-Raman scattering selection rules for a number of point groups have been presented [ 12141. The classical example of the complementary nature of hyper-Raman spectroscopy relative to Raman and IR spectroscopy is CsHb or C6Ds because of the many vibrational modes which are uniquely hyper-Raman active. The C6H6 and C6D6 molecules ( Dhh point group) have 30 normal vibrational modes with 20 unique frequencies ( 10 modes are doubly degenerate). Because the Dsb point group has inversion symmetry, an exclusion rule exists between modes which are Raman active and those which are IR and/or hyper-Raman active. Although all IR-acB.V.
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tive modes are also hyper-Raman active, there are modes which are active only in the hyper-Raman process. Fig. 1 shows the low-lying vibrational energy levels for C6D6 grouped by their activity involving transitions from the ground state, i.e. Raman, IR, hyperRaman (HR), and none of the above which are grouped as silent. The numerical designation of the vibrational modes in fig. 1 follows Herzberg’s convention [ 15 1. Note that in the third column four modes with energy below 1500 cm- ’are only hyperRaman active and three modes are both IR and hyper-Raman active. Thus in the hyper-Raman spectra we expect to see seven Stokes peaks which are shifted in energy from the second harmonic frequency of the incident laser beam. Previous hyper-Raman results on C,H, were present orally (abstract form) [ 161. We report here on C6H6 and C6D6 hyper-Raman spectra containing Stokes peaks arising from vibrational modes which Deuterated
Benzene
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h 'E s t
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_--Raman
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fl
Silent
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4
f
Fig. I Vibrational energy levels of C6D, (energy < 1600 cm-‘) grouped by their activity from the ground state, i.e. Raman, IR, or hyper-Raman (HR) Mode labeling follows Henberg’s notation [ 151. Modes which are not active in Raman, KR, or hyperRaman are grouped as silent.
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are both IR and hyper-Raman active as well as modes which are solely hyper-Raman active. The role of stimulated Raman scattering (SRS ) in selectively populating vibrational modes and the consequences to the hyper-Raman spectra were investigated because the generation of hyper-Raman scattering requires high input intensity radiation. An optical multichannel detection system was used to simultaneously measure the Stokes and anti-Stokes hyperRaman spectra of C6D6. Laser-induced selective population effects were not observed in C,D6 but were observed in CCL,, which because of its lack of inversion symmetry has modes which cannot be exclusively grouped as Raman or hyper-Raman active. The incident laser beam is the fundamental ( 1.064 pm) of a @switched mode-locked Nd:YAG laser (Quantronix 416). Each &switched pulse (260 ns duration at a rate of 1 to 6 kHz) consisted of a train of mode-locked pulses ( 100 ps pulse/ 12.5 ns) with an energy of = 1.5 mJ at 1 kHz. The incident beam propagating in the vertical direction was focused in a 4 cm cell. The scattered light was collected normal to both the propagation direction and the polarization of the incident laser with a f/l lens and imaged onto the entrance slit of a spectrograph (Spex 1870). The extremely weak scattered intensity prevented polarization analysis of the hyper-Raman radiation. A single-photon-counting two-dimensional detector (Mepsicron) [ 17 ] was placed at the exit focal plane of the spectrometer. In our experiment, the Mepsicron was gated on for 1.4 ps synchronously with the laser @switcher. This reduced the detected background noise (with the laser on and the slit open but without the sample in place) to < 8 x 1O-’ cps over the entire detector. In the optical multichannel analyzer configuration, the horizontal direction of the Mepsicron corresponds to the dispersed wavelength while the vertical direction corresponds to the spatial position in the cell with the appropriate optical magnification factors. Because the hyper-Raman signal is generated primarily in the waist of the focused incident, beam, it was necessary to accept the signal only in the vertical dimension of approximately one-quarter of the detector diameter. The full horizontal dimension was used to achieve the maximum wavelength coverage. Channels in the vertical direction were summed since we were not interested in spatial resolution but only
Volume 155,number 4,5
in using the vertical dimension of the detector to increase the totaI signal coIlected. This resulted in a total background noise of ~2 x 1OS5 cps/wavelength channel or an average of one count in each wavelength channel every 15 h. Fig. 2a shows the Stokes shifted hyper-Raman spectra from a cell containing C6H6 after 11 h of integration with the Mepsicron and the laser Qswitched at I kHz. Higher Q-switching rates led to greater thermal blooming in C6H6 as a result of the slight absorption at 1.064 pm, which is caused by the overtones of the C-H stretching modes [ 181. Even at 1 kHz, the laser beam was slightly defocused, thus reducing the hyper-Raman signal, which depends on the square of the incident laser intensity. The Stokes peaks in fig. 2a are labeled in accordance with Herzberg’s notation. The large peak at 790 cm-’ corresponds to the u4 mode (a,,), which is both IR and hyper-Raman active. The smaller peak at 400 cm-’ corresponds to vZo (e,,), which is solely hyper-Raman active. The problem of thermal blooming is much less
pronounced in C,D, since the wavelengths at which the C-D stretching mode overtones occur are longer than those of the C-H stretching mode overtones. With the laser Q-switched at 6 kHz, fig. 2b shows the Stokes hyper-Raman spectra from C,D, (99.5 at% D) after integrating the lower spectrum for 6 h and the upper spectrum for 10 h. The reduced thermal blooming allows a higher repetition rate and tighter focusing. Consequently, the signal-to-noise ratio of
6
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CHEMICAL PHYSICS LETTERS
1
(a)
the spectra ,is substantially improved. Comparing these spectra to the predicted modes in fig. 1, we observe vZOat 340 cm-’ and vq at 495 cm-‘, both appropriately down shifted from those of C,H,. The origin of the peak at 660 cm-’ (denoted by an asterisk) is unknown as it does not correspond to a hyper-Raman-allowed fundamental mode of C6D6. However, this peak has the proper energy to correspond to a hyper-Raman process involving the pll mode, which is Raman active only from the ground state. It is doubtful that the collisional or high incident intensity perturbation to the selection rules is sufficient to account for the relatively large magnitude of the peak. In fig. 2b, the peak at 8 10 cm-’ corresponds to v,~, which is hyper-Raman active, and/or vN4,which is both IR and hyper-Raman active. The peak at 950 cm-’ can be assigned to the us mode, which is hyper-Raman active only. We did not detect any signal corresponding to the v19 mode, the next hyper-Raman active mode (see fig, 1). Two closely spaced peaks are observed at 1290 and 1330 cm-‘. The origin of the former peak is unknown. The latter corresponds to the IR and hyper-Raman-active v,~ mode. The small peak at 1450 cm-’ probably corresponds to the v2 + v, combination mode which has been observed with IR spectroscopy and is both IR and hyper-Raman active. Fig. 3 shows the Stokes and anti-Stokes hyper-Raman spectra for C6D, in the low frequency shift region (0 to 600 cm - ’). These spectra were obtained
Anti-Stokes
0.20
I ma Hyper-Raman shift (mi’)
Fig. 2. Hyper-Raman spectra of C,H, integrated for 11h with a Nd: YAG laser Q-switched at 1kHz (a) and of C,D, integrated for 6 h in the lower spectrum and 10 h in the upper spectrum, both with the laser Q-switched at 6 kHz (b).
t---
Stokes
HypwRayleigh
400
I
i
i
i
4 \
Deuterated nenzeoe
200 Hyper-Ramnn
0
200 shift (cm-‘)
400
600
Fig. 3. Stokes and anti-Stokes hyper-Raman spectra of C,D, in the low-frequency shift region (0 to 600 cm-‘) integrated for 5 h with a Nd:YAG laser @switched at 6 kHz. The broad hyperRayleigh peak is observable at 0 cm-’ shift.
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with the laser Q-switched at 6 kHz and an integration time of 5 h. Hyper-Raman peaks associated with modes v4 and v20 are readily detected in both Stokes and anti-stokes spectra. The relative intensities between the Stokes and anti-Stokes peaks are in accord with a Bose-Einstein population distribution at 25 “C (slightly above the laboratory temperature). The broad hyper-Rayleigh peak, centered at the secondharmonic frequency (0 cm- ’), is the result of intermolecular interactions in liquid C6Ds which remove the inversion symmetry of an isolated C6Ds molecule and thereby allow some second-harmonic generation. The hyper-Rayleigh intensity from liquid CC& is several orders of magnitude greater than that of liquid CsDh because an isolated Ccl, molecule lacks inversion symmetry. Figs. 4a and 4b show the Stokes and anti-Stokes hyper-Raman spectra for CCL, measured at two different input intensities, where the enormous hyper-Rayleigh peak is not displayed. The three hyper-Raman active modes ( vl, v3, and vq) of CCL, are present in the Stokes and anti-Stokes spectra, All three of these modes are also Raman active. The anti-Stokes hyper-Raman process involves a three-photon transition starting from an excited vibrational mode ( vI, v3, or v,) and ending at the ground state. At lower incident laser intensity, the ratios of hyper-Raman intensity for the anti-Stokes and Stokes peaks correspond approximately to the Bose-Einstein population distribution at a slightly elevated temperature (see fig. 2a). However, at higher incident laser intensity (see fig. 4b), the antiStokes-to-Stokes intensity ratios (0.6, 0.3, and 0.35 for modes vl, v3, and Y,, respectively) far exceed those expected from the room temperature population distribution. The laser-dependent anti-Stokes-to-Stokes intensity ratio can be explained by considering the selective population associated with the SRS process. Since v,, vj, and vq are Raman active, the SRS process taking place in the IR wavelength region can populate these levels. During the input laser pulse, the selective pumping rate of the v, mode exceeds that of the v3 and v4 modes because the spontaneous Raman cross section of the v, mode is known to be larger than that of the v3 and v4 modes. In addition, the spontaneous Raman cross section of the v4 mode is larger than that of the v3 mode. Although the anti494
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Hyper-Raman shift (cm” ) Fig. 4. Stokes (top) and anti-Stokes (bottom) hyper-Raman spectra of Ccl4 at low (a) and high (b) incident laser intensity. The population distribution deduced from the relative anti-Stokes and Stokes intensities is in accord with a near room temperature Bose-Einstein distribution for the low incident laser intensity only.
Stokes-to-Stokes intensity ratio will also depend on the vibrational relaxation time of the particular modes and saturation effects in the stimulated Raman pumping process, the ratios are in qualitative agreement with those expected from the relative R.aman cross sections of the modes. At high input intensity, we did not observe additional hyper-Raman peaks in the anti-Stokes or Stokes spectra as a result of transitions from the selectively populated mode3 to levels other than the ground state. Determination of numerical values for the various hyper-polarizability tensor elements from the detected hyper-Raman signal intensities is limited by the lack of polarization analysis and the degree of accuracy to which we can estimate the laser intensity within the cell when thermal blooming is present. We
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assumed that the interaction volume is a cylinder with a diameter of 200 pm and a height of 3 mm and that the laser intensity is 1 GW/cmZ. The hyper-Raman scattering cross section, i.e. the number of hyper-Raman scattered photons per incident photon per molecule, is estimated to be 1O-36 and 10M3’for the Y, and vzo modes in C6D6, respectively. In conclusion, the hyper-Raman spectra of C,H, and C6Ds were readily observable when the combination of a Q-switched mode-locked laser and a gated Mepsicron was used. A much better signal-to-noise ratio in the hyper-Raman Stokes and anti-Stokes spectra was obtainable from C,D, because thermal b@oming in C6D6 is a less serious problem than in C&I,. The solely hyper-Raman active modes ( vzo, v,) and the IR and hyper-Raman active Ul d ? and modes (u,, u14, and Y,~) have been identified in the C6D6 hyper-Raman spectra. The intensity ratio of the anti-Stokes to the Stokes peaks in Ccl, hyperRaman spectra at higher input intensity was noted to be larger than the ratio at lower input intensity. This, increase above the room-temperature BoseEinstein distribution is a result of selective laser-induced population (via the SRS process) of the higher vibrational levels which are both Raman and hyperRaman active for noncentrosymmetric molecules such as Ccl,. No such input intensity dependence was noted for C6D6 because the vibrational levels that can be selectively populated by the SRS process are mutually exclusive to those vibrational levels that are involved in the hyper-Raman transitions for centrosymmetric molecules such as C6D6 and C6H6. We gratefully acknowledge the partial support of this research by the US Department of Energy, En-
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ergy Conversion and Utilization Technologies gram (Contract No. DE AS04-87AL42879).
Pro-
References [ I] J.C. Decius and J.E. Rauch, Ohio State Symposium
on Molecular Structure and Spectroscopy, Paper 48 (June 1959). [2] R.W. Terhune, P.D. Maker and C.M. Savage, Phys. Rev. Letters 14 (1965) 681. [3] M.A. Washington, A.Z. Genack, H.Z. Cummins and R.H. Bruce, Phys. Rev. B 15 (1977) 2145. [4] L.D. Ziegler and J.L. Roebber, J. Chem. Phys. 136 (1987) 371. [5]D.V. Murphy, KU. von Raben, R.K. Chang and P.B. Dorain, Chem. Phys. Letters 85 (1982) 43. [6] J.T. Golab, J.R. Sprague, K.T. Carron, G.C. Schatz and R.P. Van Duyne, .I. Chem. Phys. 88 (1988) 7942. [7] CM. Savage and P.D. Maker, Appl. Opt. 10 (1971) 965. [8] M.J. French and D.A. Long, J. Raman Spectry. 3 (1975) 391. [9] J.H. Verdieck, S.H. Peterson, C.M. Savage and P.D. Maker, Chem. Phys. Letters 7 ( 1970) 2 19. [lo] W.J. Schmid and H.W. Schriitter, Chem. Phys. Letters 45 (1977) 502. [ll]H.Vogt,Phys.Rev.B36(1987)5001. [12] S.J. Cyvin, J.E. Rauch and J.C. Decius, J. Chem. Phys. 43 (1965) 4083. [ 131 J.H. Christie and D.J. Lockwood, J. Chem. Phys. 54 (1971) 1141. [ 141 D.L. Andrews and T. Thirunamachandran, J. Chem. Phys. 68 (197&j 2941. [ 15 ] G. Henberg, Molecular spectra and molecular structure, Vol. 2. Infrared and Raman spectra of polyatomic molecules (Van Nostrand Reinhold, New York, 1945) p. 118. [ 161 P.D. Maker, 4th International Raman Conference, Brunswick, Maine (August 1974). [ 171 W.P. Acker, B. Yip, D.H. Leach and R.K. Chang, J. Appl. Phys. 64 (1988) 2263. [ 18 ] K.V. Reddy, D.F. Heller and M.J. Berry, J. Chem. Phys. 76 (1982) 2814.
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