IR SPECTROSCOPY OF SIZE-SELECTED CLUSTERS AT 3-MU-M WAVELENGTH

An IR injection-seeded optical parametric oscillator (OPO) with narrow bandwidth (Gamma less than or equal to 0.25 cm(-1)) and high energy ( E greater than or equal to 4 mJ/pulse) output between 2.5 and 4 mu m h as been developed for cluster spectroscopy in molecular beams. The osc illator contains a LiNbO3 crystal is pumped by the 1,064 mu m fundamen tal of a Q-switched Nd:YAG laser. The IR seed radiation is obtained by difference frequency mixing the output of a tunable dye laser with th e second harmonic of the same Nd:YAG laser in a LiIO3 crystal. After c haracterization, the OPO has been used to measure absorption spectra o f small hydrogen-bonded clusters (HF)(n) and (H2O)(n) where n ranges f or 2 to 8. The various spectral features could be assigned to specific cluster sizes by dispersing the cluster beam with a secondary rare ga s beam and by measuring the laser-induced depletion as a function of t he scattering angle.