BOND SELECTIVE INFRARED MULTIPHOTON EXCITATION AND DISSOCIATION LINEAR MONODEUTERATED ACETYLENE

Quantum mechanical simulations of vibrational excitation of monodeuter ated linear acetylene (HCCD) with linearly polarized, frequency-swept, intense but nonionizing infrared laser pulses are performed. The aim is selective dissociation of either H or D atoms by optimal shaping of the laser pulses. We use a discrete variable representation and a com pact (<400 states) bright-state expansion to represent the wave functi on during and after the pulse. Wave packet propagations in the bright- state expansion are at least an order of magnitude faster than discret e variable representation wave packet propagations. This enables optim al-control calculations to find the best parameters for the laser puls es. The dynamics of CH-bond breaking with infrared pulses are very dif ferent from the dynamics of CD-bond breaking. This is a direct consequ ence of CH being the highest-frequency mode in the molecule. Selective CH-bond breaking is possible with two synchronized pulses, the first being quasi-resonant with the Delta upsilon=1 transitions in the CH st retch between upsilon=0 and upsilon=8, and the second being quasireson ant with Delta upsilon=2 transitions at higher upsilon. H-atom yields as high as 7.7%, with H to D yield ratio as high as 2.1, are demonstra ted. Selective CD-bond breaking is possible using a single, subpicosec ond, frequency-swept pulse. D-atom yields as high as 3%, or D to H ato m yield ratios as high as 3.9, are calculated. (C) 1996 American Insti tute of Physics.