Production and unimolecular decay rate of rotationally selected polyatomic molecular ions

Abstract

We present the first example of unimolecular decay rates of a polyatomic system in which reactions are rotational state selected. Internal energy specification, including the selection of the J-rotational angular momentum quantum number, is achieved via a two laser pump–pump experiment. The first laser pumps selected rotational features in the 601 transition of the neutral molecule and creates vibrationally and rotationally state selected ions following 1+1 two-photon absorption. A second laser further excites the state selected ions to an energy of 5.3 eV which is above the dissociation threshold for four competing decay channels of the benzene cation. Slow reactive decay of these ions is observed in a reflectron time-of-flight mass spectrometer and the total decay rate constant k(E,J) is measured as a function of J while keeping the energy of the second laser constant. The results are compared with calculations based on a modified RRKM model that includes consideration of the J dependence of the decay rate.