Novel Information about Vibrational Relaxation in Liquids Using Time-Resolved Stokes Probing after Picosecond IR Excitation

Abstract

The use of Stokes difference spectra for picosecond studies of vibrational relaxation dynamics in condensed matter is introduced and compared to the established techniques of measuring anti-Stokes scattering or IR transmission changes. A simple quantum mechanical description reveals the principal similarities and differences of the different probing processes for the case of moderately anharmonic oscillators, e.g., liquids with only relatively weak interactions of the sample molecules. The outcome is illustrated by novel experiments on neat liquid chloroform, whose relaxation is known quite well. It is shown that transient Stokes data allow additional information, particularly about the behavior of low-lying vibrational levels and the type of energy transfer during relaxation, to be obtained.

Vibrational relaxation of CH2I2 in solution: Excitation level dependence

“…This picture recovers the limiting mechanisms of state-specific IVR, where only a few states are in the first tier, and of statistical IVR, where a large number of states compose the first tier. The vibrational relaxation process is even more complicated in solution, [27][28][29][30][31][32][33][34][35][36][37][38][39][40] where intermolecular energy transfer ͑IET͒ to the solvent is important. IET depends on the internal vibrational modes of the solute, as well as the surrounding environment, since some modes transfer energy to the solvent more effectively than others.…”

Relaxation of the CH2 Stretching Modes of Liquid Dihalomethanes~!2008-06-13~!2008-11-11~!2008-11-26~!

“…Accordingly, 6 population is monitored at 2810 and 2997 cm -1 , whereas at 2884 and 2927 cm -1 the delayed maximum of the signals clearly indicates 1 dynamics. Without any doubt we observe the transition 6 2 6 at 2997 cm -1 . The other 6 type dynamics at 2810 cm -1 thus belongs to the 6 1 + 6 transition.…”

“…The scattered Raman photons are collected and spectrally analyzed with a highly sensitive CCD camera behind a monochromator. Details of the setup have been described previously [3,6].…”

“…In transient experiments vibrational population can be observed with Anti-Stokes photons directly at the fundamental frequency, while IR probing has to be done at a hot band frequency in order to avoid ground state depletion being mixed into the signal [6]. The spectral positions of hot bands can, under normal circumstances, be derived from the overtone spectral region.…”

“…Due to the wellseparated vibrational resonances of weakly interacting systems like, e.g., methane derivates, ultrafast pump-probe techniques based on infrared absorption and / or Raman scattering often have the potential to provide a comprehensive picture of individual relaxation channels and the corresponding rates in these systems, when both temporal and spectral information are analyzed [1][2][3][4][5]. The usually complementary nature of Raman and infrared activity of molecular vibrations necessitates to conduct investigations of this type with a combination of both experimental techniques [6]. Such a combined approach provides additional information: anti-Stokes Raman data allow direct access to vibrational excess population, the knowledge about which can then be used to assign excited state transitions seen in the transient IR spectra.…”