Infrared absorption spectra are reported for rare-gas-carbon dioxide van der Waals complexes in the region of the v3 fundamental vibrational transition of C 0 2 using a pulsed molecular beam and a diode laser. The observed linewidths of 90 MHz (f.w.h.m.) allowed the determination of rotational constants for the ground and first excited vibrational states along with the frequencies of the band origins and centrifugal distortion constants which were constrained to be equal in both vibrational states. Effective structures of the complexes have been determined.

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We present fluorescence excitation and one-and two-color resonant two-photon ionization spectra of 3-phenyl-1-propionic acid (PPA), its complexes with water, PPA(H 2 O) n , where n ) 1-3, and its p-hydroxy derivative, 3-(4-hydroxyphenyl)-1-propionic acid (HPPA), at the vibronic and partially resolved rovibronic level. Different conformations of the alkyl side chain in the bare molecule produce two spectral bands in the 0 0 0 region for PPA and three bands for HPPA. Analysis of the rovibronic band contours of these features is consistent with two conformational structures, referred to as anti and gauche. The anti structure is characterized by an extended side chain and a predominantly b-type band contour. The gauche structure is characterized by a side chain folded back toward the phenyl ring and a hybrid band contour. Analysis of this contour shows that there is a significant rotation of the transition moment within the molecular frame. Rovibronic band contour analysis of the 1:1 water complex of PPA yields a structure consistent with the acid acting as a proton donor. The rotational band contour of the 1:2 water complex is consistent with a cyclic structure in which each water is involved in two hydrogen bonds.

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