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The Role of “Rotational Freezing” in the Polarisation‐Dependent Two‐Photon Excitation of Triply Degenerate Vibrations in Spherical‐Top‐Molecules
Author(s) -
Ritze H.H.
Publication year - 1986
Publication title -
annalen der physik
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.009
H-Index - 68
eISSN - 1521-3889
pISSN - 0003-3804
DOI - 10.1002/andp.19864980616
Subject(s) - excitation , degenerate energy levels , atomic physics , dipole , physics , rotational transition , molecule , rotation (mathematics) , rotational–vibrational spectroscopy , rotational partition function , photon , molecular physics , quantum number , excited state , angular momentum , optics , quantum mechanics , geometry , mathematics
Under 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 A 1 g ( v = 0) → A 1 g ( v = 2) transition in SF 6 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 E g ( v = 2) and F 2 g ( 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.