Fast-beam photodissociation imaging of molecular ions

Abstract

A novel arrangement to study photofragmentation of molecular ions using fast beam (keV) imaging techniques combined with fast ion-beam pulsing and trapping has been realized. The setup was transferred to the new free-electron laser in Hamburg (FLASH) for the first user experiments. With this apparatus experiments were conducted on the two rare-gas dimer ions Ne2+ and Ar2+ utilizing a pulsed Nd:YAG laser at 2.33 and 4.66eV photon energy and on HeH+ using the free electron laser at 38.8 eV. Photofragmentation of Ar2+ was measured at 266nm, where the C2Sigma+g repulsive curve dominates. The angular fragment distribution was mainly parallel to the laser polarization, as expected, but a sizable transverse contribution increasing at energy releases corresponding to vibrationally excited target ions was found. Although spin-orbit coupling in Ar2+ remains as the most plausible explanation, the observed size of the effect is much larger than expected using theoretical predictions. Photodissociation of the Ne2+ was measured at 532nm where the A2Piu curve is dominant and the C2Sigma+g curve becomes important for higher vibrational target excitation. Also here, a mainly parallel angular charactistic was observed plus a perpendicular contribution which remained high throughout the observed kinetic energy release regime. The photofragmentation studies of HeH+ with the free electron laser at 32nm wavelength, revealed in the kinetic energy release spectrum, a contribution of 50% from highly excited He0 fragments, indicating the important contribution of many high-lying repulsive curves. The angular distributions showed a dominant contribution of Sigma->Pi transitions, underlining the large importance of this symmetry among the excited curves contributing to photodissociation. The cross sections for both symmetries were measured with a total of (1.4+-0.7)*10-18 cm2.