Triple-Resonance Spectroscopy Reveals the Excitation Spectrum of Very Cold, Isomer-Specific Protonated Naphthalene

The excitation spectrum of very cold, isomerically pure protonated naphthalene is obtained by resonant dissociation spectroscopy. The cations are prepared by a pair of laser pulses which ionize 1-hydronaphthyl radicals at threshold, thereby creating only cations of a particular isomer in the vibrational ground state. Due to the small bandwidth of the first excitation laser, only the lowest rotational states are selected. The cold cation spectrum reveals a rich structure, some of which had previously been attributed to hot bands. However, the nature of the prepared cations is such that no hot bands appear, and the structure is assigned to a' modes, and a' combinations of a '' modes in the C-s point group. The 12 excited-state frequencies extracted agree well with those calculated previously at the RI-CC2 level. [Alata, I.; Dedonder, C.; Broquier, M.; Marceca, E.; Jouvet, C. J. Am. Chem. Soc. 2010, 132, 17483-17489.]