High Resolution Stimulated Raman Spectroscopy From Collisionally Populated States After Optical Pumping: The 3(2)2(2) And (2)+2(4)+(5)2(4)+(5)Q Branches Of (C2h2)-C-12 And The 3(2)2(2)Q Branch Of (C2d2)-C-12.

In this work, we demonstrate the combined use of a pump-probe Raman-Raman technique and the naturally occurring process of vibration-to-vibration (V-V) collisional relaxation to populate and obtain high-resolution Raman spectra of vibrationally excited states that cannot be easily accessed by other means. First, a stimulated Raman pump stage is used to promote population to a v(i)=1 excited state. This is followed by a time interval whose length is chosen so that, at the working pressure, significant V-V collisional energy transfer takes place and the quanta of vibrational excitation initially placed in v(i)=1 naturally reach other excited states. Finally, after this delay, the probe stage is used to obtain high-resolution Raman spectra of hot bands departing from these collisionally populated states. The technique has been demonstrated on two isotopologues of acetylene, (C2H2)-C-12 and (C2D2)-C-12, and has allowed the observation of transitions departing from the 2(2) ( sigma g+) and 2(4)+(5) ((I)(u)) collisionally populated states. Accurate values of the spectroscopic parameters for the v(2)=3 vibrationally excited states for both isotopologues were obtained, for the first time, from the least-squares analysis of the assigned transitions. In addition, the observation of Q branch lines of v(2)=1, v(4)=2, v(5)=1 ((I)(u)) v(4)=2, v(5)=1 ((I)(u)) band for (C2H2)-C-12 allowed the spectroscopic characterization of the upper state. Copyright (c) 2016 John Wiley & Sons, Ltd.