High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process. Transitions from v(2)=1 in (C2H2)-C-12 and (C2H2)-C-13

The Raman spectra of the Q branches of the nu(1) + nu(2)-nu(2) band of (C2H 2)-C-12 and of the nu(1) + nu(2)-nu(2) and 2 nu(2) + 2 nu(5)-nu(2) bands of (C2H2)-C-13 were recorded at high resolution in the gaseous phase. To obta in the spectra a two-step pump/probe technique was used. In the first step, molecules were pumped from the ground to the upsilon(2) = 1 excited state by means of a double pulsed stimulated Raman process. Then, spectra corresp onding to the transitions from this state to higher ones are recorded by me ans of a high resolution quasicontinuous stimulated Raman process. The anal ysis of the experimental data, taking into account the fourth-order anharmo nic interaction coupling the excited states for the C-13 isotopomer, provid ed unperturbed values for the rotational and distortion constants of the ex cited states and for the band origins. Accurate values for the x(12)(0) anh armonicity constant have been calculated directly from the energies of the upsilon(1) = upsilon(2) = 1 states for the first time. (C) 1999 American In stitute of Physics. [S0021-9606(99)00426-2].