Picosecond resonance Raman studies of vibrational cooling of electronically excited trans-stilbene in alcohols and alkanes

Abstract

The vibrational cooling of electronically excited trans-stilbene has been studied in ethanol, hexanol, hexane, and decane by pump-probe resonance Raman ( R R ) spectroscopy employing pulses of 1.5-2.0 ps with 15 cm-I spectral width. Excitation 3200 cm-' above the electronic origin creates a transient rise in the stilbene vibrational temperature of approximately 150 K. The subsequent intermolecular transfer of this excess energy to the surrounding solvent leads to time-dependent changes in the peak position and bandwidth of the ethyleneic band a t -1565 cm-1. In agreement with previous studies, we find that the lineshape of this mode is nearly Lorentzian a t all delay times and that the width decays exponentially with increasing delay time. The peak position, however, shifts to higher frequency only after a 3-5-ps induction period. The vibrational cooling kinetics, as measured by the dynamics of the bandwidth, d o not depend on solvent, though the dephasing time of the ethylenic mode is faster in alcohols than in alkanes. We interpret these results as arising from coupling of the ethylenic motion to two or more low frequency modes that can exchange energy with the solvent via collisions.W e discuss the role of solvent-solute interactions and solvent thermal conduction in the cooling process.

Time-Resolved Raman Studies of Photoionization of Aromatic Compounds in Polar Solvents: Picosecond Relaxation Dynamics of Aromatic Cation Radicals

“…22,26,33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] In this section, we first focus on the temporal changes in the center wavenumbers of the cation Raman bands. The slow increases in the cation Raman intensities are discussed in the next subsection.…”

“…33,46,47,49,56 For studying intermolecular vibrational relaxation in solutions, analyses of the changes in center wavenumbers and bandwidths of picosecond time-resolved Raman bands have been recognized as one of the useful method. 22,26,33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] Iwata and Hamaguchi have analyzed the high-wavenumber shifts and the band narrowings of the picosecond time-resolved Raman bands of trans-stilbene in the S 1 state. 38,39 The time dependence of the peak position of the olefinic CdC stretching band was found to be a good indicator of the vibrational temperature change in the solute.…”

“…33 The high-wavenumber shifts and the band narrowings of the vibrational bands, which occur on a picosecond time scale, have been observed in other vibrationally hot polyatomic neutral molecules. 26,35,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] All the dynamics have been considered to be associated with intermolecular vibrational relaxation of hot neutral solutes toward thermal equilibria with solvents.…”

“…58 However, the time scale of the intermolecular vibrational relaxation of the cation radicals (17 ps in acetonitrile and 13 ps in ethyl acetate in Figure 7) is roughly the same as that of the intermolecular vibrational relaxation of the polyatomic neutral molecules reported so far. 33,35,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][59][60][61][62] This suggests that the solute-solvent interaction has only a minor effect on the vibrational relaxation observed as the shifts of picosecond vibrational bands. This behavior can be explained by a very rapid cooling model.…”

Picosecond transient resonance Raman study on the excited‐state conformational dynamics of a highly ruffled nickel porphyrin

“…Picosecond transient/time-resolved Raman spectroscopy has proved to be an ideal tool for studying the molecular photodynamics involving structural and conformational changes such as photoisomerization, 1 -3 photodecomposition, 4,5 and vibrational relaxation 6,7 of the excited states of molecules since it provides a wealth of information on molecular structure and conformation. Recent advances in the time-resolved picosecond Raman measurement technique with the advent of the Nd:YAG-pumped Ti:sapphire regenerative amplifier laser system have made more practical time-resolved resonance Raman spectroscopy, which requires stable picosecond pulses with a high temporal and spectral resolution and a high repetition rate.…”