OVERTONE STATE-SELECTIVE ISOMERIZATION BY A SERIES OF PICOSECOND INFRARED-LASER PULSES - MODEL SIMULATIONS FOR BE2H3D-(C2-EPSILON-]C3-EPSILON)

The vibrational state-selective isomerization of Be2H3D- in the electr onic ground state by a series of picosecond infrared laser pulses is s imulated, using a one-dimensional model in the framework of the Born-O ppenheimer and semiclassical dipole approximations. Three pulses pumpi ng the sequential overtone transitions serve to excite the anion from its vibrational ground state, representing the stable C2upsilon config uration, to a delocalized vibrational state with energy close to the p otential energy barrier. These three pump-pulses are followed by a dum p-pulse which induces the overtone transition from the delocalized sta te to a vibrationally-excited state of the slightly less stable isomer of Be2H3D- with C2upsilon symmetry. The overall reaction probability for optimal laser pulses with sin2-shapes is about 95%. The model is b ased on ab initio calculations of the potential energy surface and the dipole function for the electronic ground state of Be2H3D- at the MP4 /6-31++G level, with corresponding vibrational energies and dipole tr ansition matrix elements. The laser-stimulated dynamics of the ultrafa st state-selective isomerization is described by a representative, tim e-dependent wave packet which is driven by the laser pulses, according to the time-dependent Schrodinger equation, equivalent to a set of li near differential equations for the time-dependent amplitudes of vibra tional eigenstates which constitute the wave packet. This set of diffe rential equations is solved by using both standard numerical technique s and an efficient quasiresonant smoothing algorithm.