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 yiel ds vibrationally and partly rotationally resolved spectra of polyatomi c molecular ions and of cluster ions when the resulting threshold ions are measured as a function of the excitation energy. The field ionize d threshold ions are monitored and separated from the non-energy-selec ted ions in a reflecting field mass spectrometer with high mass resolu tion. The decay of the molecular or cluster ion core is monitored by t he appearance of threshold ions at the daughter ion mass as a function of the selectively excited vibrational state. In this way, upper limi ts for dissociation thresholds of neutral and ionized dimers are obtai ned which are smaller than recent theoretical values from the literatu re. The appearance of daughter fragment ions after delayed pulsed-fiel d ionization indicates that high Rydberg orbits are not destroyed by t he dissociation of the core. Possible applications of our technique fo r the production of state-selected ions are discussed.