Communication: A vibrational study of propargyl cation using the vacuum ultraviolet laser velocity-map imaging photoelectron method

Abstract

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging photoelectron (VUV-VMI-PE) method, we have obtained a vibrationally resolved photoelectron spectrum of gaseous propargyl radical C 3H 3(X 2B 1) in the energy range of 0-4600 cm -1 above its ionization energy. The cold C 3H 3 radicals were produced from a supersonically cooled radical beam source based on 193 nm ArF photodissociation of C 3H 3Cl. The VUV-VMI-PE spectrum of C 3H 3 thus obtained reveals a Franck-Condon factor (FCF) pattern with a highly dominant origin band along with weak vibrational progressions associated with excitations of the C-C ν 5+(a 1) and CC ν 3+(a 1) symmetric stretching modes and the CCH ν 7+(b 1) out-of-plane bending mode of C 3H 3+(X 1A 1). The ν 5+(a 1) vibrational frequency of 1120 cm -1 determined in the present study is lower than the value deduced from the recent Ar-tagged infrared photodissociation study by 102 cm -1, confirming the highly accurate vibrational frequency predictions obtained by the most recent state-of-the-art ab initio quantum calculations. The observation of the FCF disallowed ν 7+(b 1) mode is indicative of vibronic interactions. The discrepancy observed between the FCF pattern determined in the present study and that predicted by a recent high-level quantum theoretical investigation can be taken as evidence that the potential energy surfaces used in the latter theoretical study are in need of improvement in order to provide a reliable FCF prediction for the C 3H 3C 3H 3+ photoionization system. © 2012 American Institute of Physics.