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 ϩ ͑nϭ3, 4, 10, 21, and 30͒. A broad absorption peak was found, which for He 3 ϩ is centered at Ϸ5.3 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 nϭ3 and 10. From the energy balance the total binding energy of the He 3 ϩ trimer ion was determined to be 2.6Ϯ0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes nϭ4-7. For nϾ7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.

Some properties of the lower electronic states for nonlinear He3+ clusters

“…Thus, it can create a fast ionic and a fast neutral fragment, together with a slow neutral signal from the central helium atom as the system breaks up symmetrically along such a reaction coordinate, as seen in the experiments. 8 It therefore becomes of interest to further find out if other nuclear regions of the relevant PES still support this explanation and what is the possible role played by bent structures of the trimer ions in case they happen to be formed during the ionic cluster preparation after supersonic expansion.…”

“…Another, interesting experimental result showed that the product time-of-flight spectra of He 3 ϩ contain peaks related to one fast neutral He atom, to one fast ionic He ϩ and to a third slow neutral He atom. 8 This pattern was explained by our calculations when we examined the linear symmetric dissociative path, 9 and where we showed that the relevant excited electronic state, which can be reached via a dipoleallowed transition (1 2 ⌺ u ϩ ←X 2 ⌺ g ϩ ) yields a modified electronic density distribution where the positive charge now resides on one of the two wing He atoms, even for cluster geometries still within the Frank-Condon ͑FC͒ region of the Franck-Condon ground electronic configuration. Thus, it can create a fast ionic and a fast neutral fragment, together with a slow neutral signal from the central helium atom as the system breaks up symmetrically along such a reaction coordinate, as seen in the experiments.…”

“…9 Only the ground-state symmetry, 1 2 ⌺ ϩ , shows the existence of a basin where bound trimers clusters are located, while the next two electronic states are repulsive at all geometries within the relevant region of interaction, and therefore possible electronic excitations of the bound trimer must lead to its full dissociation when such states are reached in the excitation process, in complete analogy to what happens for the case of linear symmetric arrangements. 8,9 Along either of the shown coordinates, R 1 and R 2 , one can also follow adiabatically the ground electronic PES only and therefore reach the partial dissociative channel,…”

“…However, when excitation occurs to the next lowest-energy surface (2 2 ⌺ ϩ ), then the fragmentation process is dissociative for all linear arrangements, and the ensuing charge migration produces the various fragments, as indicated by experiments. 8 If we now consider a specific bent structure, i.e., the one given by a value of 60°for the Jacobi angular coordinate of the He 3 ϩ system ͑see Fig. 2͒, we can further analyze the possible effects of nonlinear configurations on the fragmentation process.…”

“…8 We have recently analyzed the nature of the electronic excitation process of the linear helium trimer, in a symmetric configuration of its ion, 9 and found that the behavior of the charge relocation after electronic excitation can explain the appearance of the various peaks observed in the time-of-flight experimental spectra 8 and can help us to understand the relative stability of the fragmentation species.…”