STRUCTURE AND PHOTODISSOCIATION SPECTRA OF MIXED ETHENE ACETONE CLUSTERS

Infrared photodissociation spectra of the mixed complexes C2H4-CH3COCH 3 and C2H4-(CH3COCH3)2 have been observed after size selecting them by scattering from a helium beam combined with mass spectrometric detect ion. The excitation of the nu7-out of plane symmetric wagging mode of ethene near its gas phase frequency at 949.3 cm-1 with a CW-CO2 laser leads to a characteristic depletion of the cluster beam. The dissociat ion spectrum of the 1:1 complex can be explained by two peaks at 950.8 and 961.6 cm-1. Calculations of minimum energy configurations and ban d shifts based on an empirical site-site potential show that these fre quencies can be attributed to the absorption of two different isomers. They correspond to the two different binding patterns of the H atoms of ethene to the 0 atom of acetone and those of acetone to the C-C gro up of ethene, respectively. For the 1:2 complex, a large peak at 958.5 cm-1 and a smaller one around 940.5 cm-1 are found which can be expla ined in a similar manner by several isomers found in the structure cal culations.