Photodissociation Electronic Spectra of Cold Protonated Quinoline and Isoquinoline in the Gas Phase.

Photofragmentation electronic spectra of isolated single-isomeric N-protonated quinoline (quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of ∼40 K using a mass-selective, 10 cm spectral resolution, photodissociation spectrometer. Additionally, ab initio adiabatic transition energies calculated using the RI-ADC(2) method have been employed to assist in the assignment of the spectra. Three electronic transitions having ππ* character were clearly evidenced for both protonated ions within the UV and deep-UV spectral ranges. The corresponding spectra at room temperature were previously reported by Hansen et al., together with TD-DFT calculations and a careful analysis of the possible fragmentation mechanisms. This information will be complemented in the present study by appending better resolved spectra, characteristic of cold ions, in which well-defined vibrational progressions associated with the S ← S and S ← S transitions exhibit clear 0-0 bands at hν = 27868 and 42230 cm, for protonated quinoline, and at hν = 28043 and 41988 cm, for protonated isoquinoline. Active vibrations in the spectra were assigned with the help of calculated normal modes, looking very similar to those of the structurally related protonated naphthalene. Finally, we have observed that the bandwidths associated with the deep-UV S ← S transition denote a lifetime for the S excited state in the subpicosecond time scale, in contrast with that of S.