Fate of excited states in jet-cooled aromatic molecules: Bifurcating pathways and very long lived species from the S-1 excitation of phenylacetylene and benzonitrile

摘要

Pump-probe delayed ionization studies on phenylacetylene and benzonitrile in a supersonic beam reveal the production of a low-ionization-potential (similar to 5.7 eV) species lasting more than hundreds of microseconds after excitation to the S I state. Excitation of the molecules was done with a frequency-doubled, Fourier transform-limited, pulse-amplified cw laser, and the rotationally resolved structure of the S-1-S-0 transition ensures that the excited molecules are monomers. Excited-state photoelectron spectroscopy shows that the long-lived species are formed during the light pulse but not by transfer from the fluorescing S, population after the pulse, even though the S, spectral signature is present in the long-lived action spectrum. This behavior differs greatly from that found in benzene and with most commonly held pictures of radiationless transitions in large molecules.