Absorption spectroscopy and photodissociation dynamics of small helium cluster ions

Abstract

The optical absorption of size-selected helium cluster ions was studied via photofragmentation spectroscopy. Absorption cross sections were measured for He+n (n=3, 4, 10, 21, and 30). A broad absorption peak was found, which for He+3 is centered at ≊5.3 eV, and which with increasing cluster size shifts slightly to the red. In addition, the kinetic energy release to the ionized and neutral photofragments was measured by a time-of-flight technique for cluster sizes between n=3 and 10. From the energy balance the total binding energy of the He+3 trimer ion was determined to be 2.6±0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes n=4–7. For n≳7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.