VIBRONIC STRUCTURE OF THE CYCLOPENTADIENYL RADICAL AND ITS NONRIGID VAN-DER-WAALS CLUSTER WITH NITROGEN

Fluorescence excitation and two color mass resolved excitation spectro scopy are employed to study the D-1((2)A''(2))<--D-0((2)E''(1)) vibron ic transitions of the cyclopentadienyl radical (cpd) and its van der W aals cluster with nitrogen. The radical is created by photolysis of th e cyclopentadiene dimer and cooled by expansion from a supersonic nozz le. The cpd(N-2)(1) cluster is generated in this cooling process. Mass resolved excitation spectra of cpd are obtained for the first 1200 cm (-1) of the D-1<--D-0 transition. The excitation spectrum of cpd(N-2)( 1) shows a complicated structure far the origin transition. With the a pplication of hole burning spectroscopy, we are able to assign all the cluster transitions to a single isomer. The features are assigned to a 55 cm(-1) out-of-plane van der Waals mode stretch and contortional ( rotational) motions of the N-2 molecule with respect to the cpd radica l. Empirical potential energy calculations are used to predict the pro perties of this cluster and yield the following results: (1) the N-2 m olecular axis is perpendicular to the cpd fivefold axis and parallel t o the plane of the cpd ring with the two molecular centers of mass lyi ng on the fivefold ring axis; (2) the binding energy of cpd(N-2)(1) is 434 cm(-1); and (3) the rotational motion of the N-2 molecule is esse ntially unhindered about the cpd fivefold axis. The molecular symmetry group D-5h(MS) is applied to the nonrigid cluster and optical selecti on rules exclude even<->odd transitions (Delta n=0, +/-2, +/-4,... all owed) between the different contortional levels. Tentative assignments are given to the observed contortional features based on these consid erations. The barrier to internal rotation is also small in the excite d state. The results for the cpd(N-2)(1) van der Waals cluster are com pared to those for the benzene (N-2)(1) and benzyl radical (N-2)(1) cl usters. (C) 1995 American Institute of Physics.