High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process. Transitions from v2=1 in 12C2H2 and 13C2H2

The Raman spectra of the Q branches of the ν1+ν2−ν2 band of 12C2H2 and of the ν1+ν2−ν2 and 2ν2+2ν5−ν2 bands of 13C2H2 were recorded at high resolution in the gaseous phase. To obtain the spectra a two-step pump/probe technique was used. In the first step, molecules were pumped from the ground to the v2=1 excited state by means of a double pulsed stimulated Raman process. Then, spectra corresponding to the transitions from this state to higher ones are recorded by means of a high resolution quasicontinuous stimulated Raman process. The analysis of the experimental data, taking into account the fourth-order anharmonic interaction coupling the excited states for the 13C isotopomer, provided unperturbed values for the rotational and distortion constants of the excited states and for the band origins. Accurate values for the x120 anharmonicity constant have been calculated directly from the energies of the v1=v2=1 states for the first time.