Electronic spectroscopy of toluene in helium nanodroplets : evidence for a long-lived excited state.

Optical excitation of toluene to the S[subscript 1] electronic state in helium nanodroplets is found to alter the rate of production of the fragment ions C[subscript 7]H[subscript 7][superscript +] and C[subscript 5]H[subscript 5][superscript +] when the droplets are subjected to subsequent electron ionization. The optical excitation process reduces the abundance of C[subscript 7]H[subscript 7][superscript +] ions delivered into the gas phase, whereas C[subscript 5]H[subscript 5][superscript +] ions become more abundant beyond a minimum droplet size. This process contrasts with normal optical depletion spectroscopy, where the optical absorption of a molecular dopant in a helium nanodroplet shrinks the helium droplet, and thus, the electron impact cross-sections because of dissipation of the absorbed energy by evaporative loss of helium atoms. The observations here are interpreted in terms of formation of an excited state in the neutral molecule, which survives for several hundred μs. This long-lived excited state, which is assumed to be the lowest triplet electronic state, shows different cross-sections for production of C[subscript 7]H[subscript 7][superscript +] and C[subscript 5]H[subscript 5][superscript +] relative to the S[subscript 0] state.