Mode to mode energy transfer in OCS directly pumped by a CO2 laser

Abstract

Laser‐induced fluorescence has been observed from all the modes of OCS after pumping the 0→2ν₂ overtone transition with the P (22), 9.6 μ line of the Q‐switched CO₂ laser. In pure OCS approximately 145 collisions are required to equilibrate the laser pumped bending modes (ν₂=520 cm⁻¹) with the antisymmetric stretching mode (ν₃=2062 cm⁻¹), while approximately 915 collisions are required for ν₂–ν₁ (ν₁=859 cm⁻¹) equilibration. The overall equilibration of the vibrational degrees of freedom with the translational/rotational degrees of freedom (deactivation of ν₂) determined in these experiments corresponds to 7800 collisions for pure OCS, in excellent agreement with ultrasound results. The behavior of this relaxation process in OCS–rare gas mixtures suggests that very little vibrational energy is transferred directly into rotational degrees of freedom during deactivation of ν₂ as might be expected for a molecule like OCS with no light atoms such as H or D. Simple calculations of the V–T relaxation probability using a short range repulsive force model are in unexpectedly good agreement with the experimental data, particularly for pure OCS. On the other hand, a long range attractive potential, dipole–dipole calculation which emphasizes the large electrical anharmonicity of the OCS bending modes appears to be in good agreement with the experimentally observed ν₂–ν₃ V–V energy transfer probability.