A spectroscopic investigation of the nonrigid–rigid transition of (benzene)13 during free jet expansion

We have traced the evolution of the benzene-h6 transition in (C6H6)(C6D6)12 as a function of time (distance from the nozzle) in a supersonic expansion. Taking advantage of the special properties of the isotopically substituted (benzene)13 resonant two-photon ionization spectrum, we present the first experimental evidence for a nonrigid–rigid transition in a single-size molecular cluster. Initially, under the conditions of our free-jet expansion, (benzene)13 condenses in a nonrigid form, then solidifies into a well-defined structural configuration.