Infrared Photodissociation of Van der Waals Molecules

Infrared photodissociation of van der Waals molecules is investigated using low power cw infrared lasers. Information gained concerns the dynamics of vibrational predissociation and the van der Waals interactions. Clusters formed in supersonic molecular beams are irradiated for approximately 0.5 msec and the fraction of clusters remaining intact is measured as a function of laser wavelength and power. Detailed homogeneous and inhomogeneous line shape models are presented and used to analyze the results. Effects such as fluence broadening and orientational inhomogeneity are described. Van der Waals molecules studied include dimeric clusters of ethylene with rare gases, hydrogen halides and non-hydrogen bonding polyatomic molecules. Homogeneous widths of clusters excited near the ν7 frequency of free ethylene correspond to lifetimes ranging from 0. 44 psec for (C2H4)2 to greater than 10 psec for Ne•C2H4. These are attributed to vibrational predissociation constrained by conservation of angular momentum. Other conceivable broadening mechanisms are discussed. Spectra obtained by exciting the ν7 mode in different types of ethylene clusters are quite dissimilar. Lineshape analysis indicates that the ν7 transition in (C2H4)2 occurs as a hybrid band. The same transition in (C2H4·HF occurs as a perpendicular band. The rare gas-ethylene clusters are less rigid than the others and excitation of hindered internal rotation of C2H4 accompanies the ν7 absorption. All of the ethylene clusters exhibit blue shifts and intensity enhancement, as compared to ν7 absorption by free ethylene, which are attributed largely to electrostatic interactions.