DIODE LASER SPECTROSCOPY OF VAN DER WAALS COMPLEXES

Author Institution: Physical Chemistry Laboratory, Oxford University; Physical Chemistry Laboratory, Oxford University; Department of Chemistry, University of Oregon; Department of Chemistry, University of Rochester; Physical Chemistry Laboratory, Oxford UniversityA tunable diode laser has been used to directly observe infrared absorption of van der Waals complexes produced in a pulsed molecular beam source. Ar-OCS, Kr-OCS, and Ar-NNO have been studied in the 5 micron wavelength region with excellent signal to noise ratios and FWHM linewidths of 100 MHz. For Ar-OCS P. Q. and R branches of a perpendicular transition have been analyzed for both $K^{\prime\prime} = 0$ and $K^{\prime\prime} = 1$. Using the known ground state rotational constants, $A^{\prime} = 6690, B^{\prime} = 1510$, and $C = 1230$ MHz. The vibrational origin, $\nu_{o} = 2061.74 cm^{-1}$, is $0.46 cm^{-1}$ red shifted from the C=0 stretch of isolated OCS. The excited state geometry is virtually identical to the ground state structure. Preliminary analysis of similar data for Kr-OCS also indicates very little change in geometry, but the vibrational frequency shifts by $-0.9 cm^{-1}$ in this complex. In contrast, the vibrational frequency increases by $0.2 cm^{-1}$ in Ar-NNO and the distance from Ar to the NNO center of mass increases in the excited state. The observed linewidth arises from Doppler broadening in the uncollimated molecular beam. At this resolution there is no evidence for vibrational predissociation. Experimental details will be given and the results of more complete spectral analysis will be presented