Abstract

Multiphoton resonances play an important role in the multiple-photon excitation (MPE) of polyatomic molecules at low levels of vibrational excitation.^1,2 Methods of adiabatic inversion or excitation using properly shaped laser pulses may enable efficient highly selective excitation of vibration-rotation states connected with the ground vibrational state by multiphoton resonances.³ While selective excitation of rotational sublevels of low-lying vibrational states is of interest from a spectroscopic point of view, it is generally necessary to excite high vibrational levels to alter a molecule's chemical properties. Achieving mode- or bond-selective excitation, rather than heating, is a challenge that some believe to be insurmountable. Hose and Taylor⁴ pointed out that, although most highly excited vibrational eigenstates of the Henon-Heiles potential are ergodic (or, in a loose sense, chaotic), the small set of molecular eigenstates that correspond to classical quasiperiodic states has interesting and possibly exploitable properties. The quasiperiodic states tend to have a large overlap with one of the Born-Oppenheimer basis functions and substantially greater dipole strength than other states. This observation raises the intriguing possibility that it may be possible to achieve selective excitation of specific high vibrational states with simple properties in terms of normal-mode or local-mode eigenfunctions. This might be accomplished, for example, by properly designed successive steps of adiabatic excitation. Previous calculations a (1,N) system (consisting a single lower level coupled to an upper band of states) in which the rate of decay the dipole expectation value (or the overlap of the initial and final states) was shown to depend on the rate of rise of the laser pulse⁵ suggest that excitation of a multilevel system with successive adiabatic (slowly rising) laser pulses results in a state with a very long intramolecular relaxation (IMR) time.

© 1987 Optical Society of America