Infrared photodissociation and cluster-specific detection of internally cold (C2H4) van der Waals complexes

Infrared photodissociation spectroscopy of selected, internally cold, weakly bound van der Waals ethylene clusters was performed. A steady molecular beam of C2H4 clusters is crossed by the IR radiation of a pulser CO2 laser. Subsequent scattering by a secondary He-beam disperses the cluster beam and allows unequivocal detection of selected cluster species which is not disturbed by fragmentation processes in an ionizer. The dependence of IR photon absorption on cluster size was investigated in the region of the nu-7 vibration of C2H4; predissociation spectra were recorded for (C2H4)n complexes with n=2,3,4, and 5. The absorption profiles are structureless and all have, regardless of cluster size, the same width and the peak at the same position. From the dependence of the dissociation yield on the laser fluence, transition probabilites were determined for n=2,3, and 4.