Photodissociation of state-selected cluster ions studied by mass-selective, pulsed-field threshold ionization spectroscopy

Delayed pulsed-field ionization of long-lived high Rydberg states yields vibrationally and partly rotationally resolved spectra of polyatomic molecular ions and of cluster ions when the resulting threshold ions are measured as a function of the excitation energy. The field ionized threshold ions are monitored and separated from the non-energy-selected ions in a reflecting field mass spectrometer with high mass resolution. The decay of the molecular or cluster ion core is monitored by the appearance of threshold ions at the daughter ion mass as a function of the selectively excited vibrational state. In this way, upper limits for dissociation thresholds of neutral and ionized dimers are obtained which are smaller than recent theoretical values from the literature. The appearance of daughter fragment ions after delayed pulsed-field ionization indicates that high Rydberg orbits are not destroyed by the dissociation of the core. Possible applications of our technique for the production of state-selected ions are discussed.