Multiple-photon dynamics in molecules: A new approach to high resolution spectroscopy of highly excited vibrational states

Multi-photon transitions with two simultaneously interacting IR laser fields lead to final excited states with frequencies n nu n(1) nu(1) + n(2) nu(2), with n the total number of photons absorbed and (n, n(1), n(2)) = (2, 1, 1), (3, 2, 1), (4, 1, 3), etc. The nature of the actual transition is determined by shift measurements, where the lasers are frequency-tuned by delta nu(i) in opposite directions keeping the sum frequency, n nu, resonant with the molecular transition. This technique opens a new spectral range for multi-photon transitions and a unique identification of the observed features. For n(1) and n(2) both positive the excitation will lead to a ''normal'' up-up multi-photon transition. Many three- and four-photon transitions in the nu(3) vibrational ladder of SF6 could be resolved with a resolution of 1 MHz, as well as four new two-photon transitions. As long as n(1) + n(2) greater than or equal to 0, one of the two n(i) may be negative resulting in an, e.g., up-down excitation pathway with its particular selection rules. The up-down excitations are demonstrated both for one- and two-photon transitions using the frequency shift technique. The different possible excitation schemes which meet the resonance condition for these transitions lead to interference effects and local couplings to highly excited states. Changes in resonance frequency for a one-photon transition (n = 1), due to these effects, are demonstrated. Evidently, the radiative coupling of participating levels to high-lying or quasi-continuum states may drastically change for different delta nu(i) leading both to ac Stark shift and transition probability variations. (C) 1996 Academic Press, Inc.