Direct determination of state-to-state rotational energy transfer rate constants via a Raman-Raman double resonance technique: ortho-acetylene in v2=1 at 155 K

Abstract

A new technique for the direct determination of state-to-state rotational energy transfer rate constants in the gas phase is presented. It is based on two sequential stimulated Raman processes: the first one prepares the sample in a single rotational state of an excited vibrational level, and the second one, using the high resolution quasi-continuous stimulated Raman-loss technique, monitors the transfer of population to other rotational states of the same vibrational level as a function of the delay between the pump and the probe stages. The technique is applied to the odd-J rotational states of v(2)=1 acetylene at 155 K. The experimental layout, data acquisition, retrieval procedures, and numerical treatment are described. The quantity and quality of the data are high enough to allow a direct determination of the state-to-state rate constant matrix from a fit of the experimental data, with the only conditions of detailed balance and of a closed number of states. The matrix obtained from this direct fit is also compared with those obtained using some common fitting and scaling laws.

Communication: Observation of homonuclear propensity in collisional relaxation of the 13C12CD2 (v2 = 1) isotopologue of acetylene by stimulated Raman spectroscopy

“…The experimental setup used in this work is based on an apparatus originally used for high resolution Raman spectroscopy of vibrationally excited states, [13][14][15] that later evolved into a more elaborate setup for the determination of stateto-state rotational energy transfer rate coefficients. 16 For the observations described in this communication a simplified version of the latter setup has been used. The experiment follows a similar scheme to the ones used in the works of Orr (Ref.…”

“…We will just provide a brief overview of the technique and refer the reader to Ref. 16 for a much more detailed description of the experimental apparatus.…”

“…The pump and probe stages of the experiment are similar to those described in our latest setup, 16 with one fixed frequency beam (in the pump stage it is a Nd:YAG laser and in the probe stage a frequency-locked Ar + ion laser, both represented as upward arrows in Fig. 1) and one tunable beam (the downward arrows, provided in both stages by pulsed amplification of tunable cw seeds in pulsed dye amplifiers, in turn pumped by Nd:YAG lasers).…”

Rotational and vibrational energy transfer in vibrationally excited acetylene at energies near 6560 cm−1

“…Three commonly used fitting laws have been examined. 6,11,35,55,56 The first of these, the exponential gap law (EGL), defines endothermic transfer events by the expression…”

“…Laser induced fluorescence (LIF) from theà −X transition has been commonly used as the means to follow the populations in the vibrationally excited states. More recently, Doménech et al 6,7 have demonstrated the use of stimulated Raman scattering for this purpose.…”