Structure And Dynamics Of Molecular Clusters Via High Resolution IR Absorption Spectroscopy

The combination of high resolution (<10-3 cm-1) cw tunable difference frequency generation (2.2-4.2 μm) with high sensitivity (10-6/√Hz) long path length absorption methods in pulsed slit supersonic jets has permitted spectroscopic investigation of many weakly bound molecular complexes. Discussion will focus on three complementary areas of experimentation. 1) Cluster formation in the molecular beam is probed via sub-Doppler, velocity resolved absorption profiles of monomer species. Spatially dependent beam clustering is strongly manifested through loss of monomer absorption intensity at line center. 2) IR spectra of simple van der Waals molecules such as ArHF are obtained in the υ1 HF stretching region. Information on all modes in the complex is extracted. 3) IR spectra of hydrogen bonded complexes such as HFCO2 are observed which exhibit large changes in average molecular geometry as a function of vibrational state. Surprisingly low intermolecular bending frequencies are evidenced in the spectra via hot bands, and provide dynamical information on coupled vibrational-rotational motion in floppy molecular systems.