The dynamics of ethylene dimer infrared photodissociation in pulsed molecular beamsa)

Publisher Website
Google Scholar
Abstract
Molecular beam experiments have been carried out on the infrared photodissociation of the (C2H4)2, (C2D4)2, and (C2H4)⋅ (C2D4) van der Waals molecules. Measurements of the frequency and fluence dependences of the photodissociation cross sections were performed over the CO2 laser frequency range from 900 to 1100 cm−1, which covers the region of the ν7 out‐of‐plane vibration of C2H4 and the ν12 in‐plane vibration of C2D4. In addition, the monomer product speed and angle distributions were measured in order to obtain information on the final energy disposition in the products. Absorption in either vibrational mode induces dissociation of the cluster, but the linewidth associated with excitation of the ν7 mode of C2H4 is about four times larger than that associated with excitation of the ν12 mode of C2D4. Two‐laser ‘‘hole‐burning’’ experiments demonstrate the homogeneity of the ground‐state populations. The absorption intensities are not simply proportional to the number of molecules in the cluster capable of absorbing at a given frequency but instead appear to contain information on the structure of the dimer. The dissociation products are scattered isotropically in the center of mass system, with average recoil velocities which correspond to only a small fraction of the available energy appearing in translation. In the case of (C2H4)2, the lack of vibrational modes at the corresponding product internal energies implies that the remaining energy goes into product rotation. Based on the rather detailed spectroscopic and dynamical information provided by these experiments, and the assumption of a skewed parallel structure for the dimer, qualitative arguments are presented that the observed linewidths are determined by the rates of the mode‐specific anharmonic coupling of the monomer vibrations to the van der Waals coordinates, but not necessarily by the vibrational predissociation rates.