Two-photon absorption spectroscopy of mass-selected ions: N2O+ and CS+2

A technique for the spectroscopic characterization of mass-selected ions in ground and bound excited electronic states involving two-photon absorption and fragment ion detection is demonstrated. A triple quadrupole mass analyzer system is used. Collisionally relaxed ions (M+) from a high pressure source are mass selected, then excited sequentially by two laser colors (λ1,λ2) according to M+→λ1M+*→λ2M+**→F+i and fragment ions (F+i) are detected. Stable ionic states are characterized by scanning λ1 with constant λ2 and this is illustrated on the known transitions of N2O+ (Ã 2Σ+←X̃ 2Π) and of CS+2 (B̃ 2Σ+u←X̃ 2Πg). New spectroscopic information—vibrational frequencies and rotational constants—and dynamic parameters—lifetimes and fragmentation branching ratios—on N2O+(B̃ 2Σ+) and CS+2 (C̃ 2Σ+g) are obtained by scanning λ2 when λ1 is chosen to populate several or individual rotational levels of the intermediate state.