Infrared−Infrared Double Resonance Spectroscopy of the Isomers of Acetylene−HCN and Cyanoacetylene−HCN in Helium Nanodroplets.

Infrared−infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene−HCN and cyanoacetylene−HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene−HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene−HCN isomer. However, population transfer is not observed for the cyanoacetylene−HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement. [ABSTRACT FROM AUTHOR]