High resolution stimulated Raman spectroscopy from collisionally populated states after optical pumping: the 3ν 2 ←2ν 2 and ν 2 +2ν 4 +ν 5 ←2ν 4 +ν 5 Q branches of 12 C 2 H 2 and the 3ν 2 ←2ν 2 Q branch of 12 C 2 D 2 .

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, 12 C 2 H 2 and 12 C 2 D 2 , and has allowed the observation of transitions departing from the 2 ν 2 ( Σ g + ) and 2 ν 4  +  ν 5 ( I Π 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 Π u ) ← v 4  = 2, v 5  = 1 ( I Π u ) band for 12 C 2 H 2 allowed the spectroscopic characterization of the upper state. Copyright © 2016 John Wiley & Sons, Ltd.