New laser system for measurements of dissociation rates of small molecules with picosecond temporal resolution

Abstract

A technique is presented for studying state-specific picosecond unimolecular reaction dynamics of small molecules in their ground electronic states, and hence their thermal unimolecular reactions. Formyl fluoride (HFCO) is chosen for its unique properties. HFCO dissociates over a barrier of 17,200 +/- 1,400 cm<SUP>-1</SUP> to form HF + CO. It has been shown that intramolecular vibrational energy redistribution (IVR) is far from complete at such energies. Thus substantial state- and mode-selectivity are exhibited in vibrational couplings and dissociation rates. Stimulated emission pumping (SEP) is used to selectivity populate single rovibrational levels in the electronic ground state. A third, variably delayed laser pulse, probes the depleting population of molecules from this level by laser induced fluorescence. The high temporal resolution of the dump and the probe laser pulses is realized by two distributed- feedback dye lasers amplified in (beta) -barium borate crystals serving as optical parametric amplifiers. The complete system is pumped by one active passive mode-locked Nd:YAG laser. The system delivers two independently tunable 16 ps laser pulses with a bandwidth of 1.48 cm<SUP>-1</SUP> and energies of up to 1 mJ.