The Role of “Rotational Freezing” in the Polarisation‐Dependent Two‐Photon Excitation of Triply Degenerate Vibrations in Spherical‐Top‐Molecules

Abstract

AbstractUnder certain circumstances at low rotational temperatures the multiphoton excitation probabilities of vibrational‐rotational levels in high‐symmetric molecules do not depend on the rotational quantum numbers J and M of the initially populated levels. Then the rotation of the molecule as a whole is “frozen”. This has the consequence that during the absorption process the photon spin is not transferred to the molecular rotation but to the molecular vibration. In more detail the two‐photon A1g(v = 0) → A1g(v = 2) transition in SF6 at 944.5 cm−1 is theoretically studied: If the “average lifetime” in the intermediate (v = 1)‐level is small compared with the effective molecular rotation period the considered transition is forbidden for circularly polarised light. Further the excitation process of the Eg(v = 2) and F2g(v = 2) vibrational levels is investigated; it is discussed under which conditions it is not necessary to consider the rotational degrees of freedom for a calculation of transition dipole moments.