Abstract:Resonance Raman intensities of perdeuterated cis-stilbene were measured using excitation at 266 nm. For the six analysed bands at 1612, 1564, 1537, 1372, 1325 and 958 cm −1 , no large changes in intensity distribution relative to the parent compound were found in accordance with the scrambling coefficients showing that the corresponding modes n 7 , n 8 , n 9 , n 10 , n 11 and n 15 of d 12 -cis-stilbene have all of the Raman activity. The S 1 equilibrium geometry and vibrational force field for cis-stilbene wer… Show more
“…This frequency is in good agreement with the frequency of a spontaneous Raman band of S 1 cis -stilbene observed in the frequency domain (∼229 cm -1 ) , 2 (a) Time-resolved absorption signal of cis -stilbene in cyclohexane (5 × 10 -2 mol dm -3 ) . The pump and probe wavelengths are 315 and 660 nm, respectively.…”
Section: Resultssupporting
confidence: 83%
“…The frequency of the nuclear wavepacket motion observed in the ultrafast transient absorption measurement agreed very well with the frequency of the most prominent band (229 cm -1 ) in the resonance Raman spectra of S 1 cis -stilbene reported by Matousek et al , This vibration was assigned to a totally symmetric vibration that consists of CC torsion, C-phenyl torsion and C-phenyl in-plane bending motions. , A corresponding oscillation was also observed in a femtosecond time-resolved photoionization study in the gas phase by Fuss et al Thus, among all vibrational modes of S 1 cis -stilbene, this ∼220 cm -1 mode appeared with the highest intensity in all the reported spectroscopic experiments. Therefore, it is desirable to clarify the situation that makes the ∼220 cm -1 mode carry significant intensity in these spectroscopic experiments.…”
We carried out wavelength-dispersed time-resolved absorption measurements of cis-stilbene to investigate the mechanism of the appearance of the approximately 220 cm-1 oscillation, which has been assigned to the nuclear wavepacket motion of the S1 state. The observed oscillatory pattern showed almost same amplitude and phase across the absorption peak at 645 nm, which indicates that the modulation of the transition intensity gives rise to the quantum beat. We also carried out a semiquantitative numerical simulation of the time-resolved absorption spectra based on the effective linear response theory, in which we newly incorporated the Herzberg-Teller coupling model by introducing a coordinate-dependence of the transition moment. The results of these experiments and simulation clearly showed that the intensity of the quantum beat arises from a significant coordinate dependence of the Sn<--S1 transition moment, i.e., non-Condon effect. It was concluded that the vibronic coupling of the Sn state with other electronic states is so large that the Herzberg-Teller coupling predominantly contributes to the intensity of the quantum beat of the totally symmetric approximately 220 cm-1 vibration. The present work suggests a general importance of the non-Condon effect in spectroscopy involving highly excited electronic states.
“…This frequency is in good agreement with the frequency of a spontaneous Raman band of S 1 cis -stilbene observed in the frequency domain (∼229 cm -1 ) , 2 (a) Time-resolved absorption signal of cis -stilbene in cyclohexane (5 × 10 -2 mol dm -3 ) . The pump and probe wavelengths are 315 and 660 nm, respectively.…”
Section: Resultssupporting
confidence: 83%
“…The frequency of the nuclear wavepacket motion observed in the ultrafast transient absorption measurement agreed very well with the frequency of the most prominent band (229 cm -1 ) in the resonance Raman spectra of S 1 cis -stilbene reported by Matousek et al , This vibration was assigned to a totally symmetric vibration that consists of CC torsion, C-phenyl torsion and C-phenyl in-plane bending motions. , A corresponding oscillation was also observed in a femtosecond time-resolved photoionization study in the gas phase by Fuss et al Thus, among all vibrational modes of S 1 cis -stilbene, this ∼220 cm -1 mode appeared with the highest intensity in all the reported spectroscopic experiments. Therefore, it is desirable to clarify the situation that makes the ∼220 cm -1 mode carry significant intensity in these spectroscopic experiments.…”
We carried out wavelength-dispersed time-resolved absorption measurements of cis-stilbene to investigate the mechanism of the appearance of the approximately 220 cm-1 oscillation, which has been assigned to the nuclear wavepacket motion of the S1 state. The observed oscillatory pattern showed almost same amplitude and phase across the absorption peak at 645 nm, which indicates that the modulation of the transition intensity gives rise to the quantum beat. We also carried out a semiquantitative numerical simulation of the time-resolved absorption spectra based on the effective linear response theory, in which we newly incorporated the Herzberg-Teller coupling model by introducing a coordinate-dependence of the transition moment. The results of these experiments and simulation clearly showed that the intensity of the quantum beat arises from a significant coordinate dependence of the Sn<--S1 transition moment, i.e., non-Condon effect. It was concluded that the vibronic coupling of the Sn state with other electronic states is so large that the Herzberg-Teller coupling predominantly contributes to the intensity of the quantum beat of the totally symmetric approximately 220 cm-1 vibration. The present work suggests a general importance of the non-Condon effect in spectroscopy involving highly excited electronic states.
“…Even if we assume that our ground-state normal mode assignments are all correct, which is unlikely, and apply reasonable constraints on the possible excited-state geometry changes (i.e., assuming that the directions of the bond length changes are what would be expected based on a more zwitterionic excited state), tens of thousands of sign combinations remain possible. Frequencies and intensities of isotopic derivatives would be helpful in both solidifying the ground-state vibrational assignments and defining the signs of the deltas, but a very large number of isotopomers would be needed to achieve anything close to a unique solution. − …”
Resonance Raman spectra and cross sections of a "push-pull" chromophore containing a julolidine donor and a thiobarbituric acid acceptor have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong visible charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the ∼30 Raman-active vibrations as well as the solvent reorganization energies. Several vibrational modes undergo significant (5-15 cm -1 ) frequency changes as the solvent is varied, signaling solvent polarity effects on the ground-state electronic structure. The excited-state geometry changes are solvent dependent for some vibrational modes but not for others. The total vibrational reorganization energy decreases, and the solvent reorganization energy increases with increasing solvent polarity in all solvents except the one protic solvent, methanol, which is anomalous in both respects. Tentative assignments are made for the ground-state vibrational modes by comparison of the Raman frequencies and infrared frequencies and intensities with those calculated using density functional theory, as well as by comparison with model compounds. The results are discussed within the context of the two-state valence-bond model for the electronic properties of conjugated push-pull chromophores.
We present the picosecond time-resolved resonance Raman (ps-TR 3 ) spectra of the first excited singlet state (S 1 ) of cis-stilbene (d 0 /, fully deuterated cis-stilbene (d 12 ) and a,a -d 2 -cis-stilbene (d 2 ) measured in hexane at pump and probe wavelengths 267 and 630 nm, respectively. The main observation is that in the lower wavenumber region the spectra are dominated by a second-order overtone progression originating from a 229 cm −1 fundamental band for the S 1 d 0 cis-stilbene. The observed low-wavenumber modes of the excited cis-stilbene are substantially more resonantly enhanced than the modes around 1600 cm −1 .
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