Mass selected pulsed field threshold ionization and coherent ion dip spectroscopy of van der Waal's complexes

摘要

Abstract: Mass selected pulsed field threshold ionization of optically excited high Rydberg states is a new method for the production of state-selected molecular and cluster ions. Exclusive detection of state-selected threshold ions is achieved by the kinetic energy analysis of ions in a reflectron mass spectrometer. This leads to vibrationally resolved spectra of molecular ions, like benzene, xylene, carbazole, etc. and ionic complexes of these molecules with noble gas atoms. The detected threshold cluster ions are state-selected with different internal energies depending on the excited vibrational state. Above a certain vibrational excess energy their decay is observed by the disappearance of vibrational peaks in the threshold ion spectrum at the parent mass and the simultaneous appearance of threshold daughter ions. Upper and lower bounds for the dissociation energies of the neutral and ionic complexes are deduced and compared with recent theoretical results. The coherent interaction of two narrowband Fourier- transform limited nanosecond laser pulses with gas phase molecular systems leads to the new technique of coherent ion dip spectroscopy (CIS) for rotationally resolved spectroscopy of polyatomic molecules and clusters. It is based on coherent effects with a special time sequence of the two pulses and yields a population dynamic in a three level system which is different from that of incoherent excitation experiments: At resonance, for different time sequences of the two pulses no or a complete population transfer from an initial to a final state is achieved and nearly 100% deep ion dips are observed in spectroscopic investigations. !23