Photofragmentation of hydrated iron ions Fe(HO) () at 532, 355 and 266 nm

Photofragmentation of Fe(H₂O)n ⁺ clusters (n =1-9) is investigated at three different wavelengths, 532, 355 and 266 nm. Two fragmentation pathways are observed depending essentially on the photon energy, but also on the parent size n. The fragmentation products belong to two ion families, Fe(H₂O)m ⁺ and FeOH(H₂O)m ⁺ , which correspond to dehydration and intracluster dehydrogenation reactions respectively. The ion yields are studied as a function of the laser fluence in order to determine the number of photons implied in the photofragmentation process. This allows us to estimate that the D(H₂O)_n-1Fe^+-(H₂O)] bond energy is ranging between 0.44 eV and 0.55 eV for .Photon absorption cross sections are also derived from the fluence experiments, and two different behaviors are observed: i) At 355 nm, far away from any transition, progressive solvation of the metal ion results in an increasing absorption cross section from n =2 to n =9. This can be attributed to a forbidden transition of bare , which becomes progressively allowed because of the interaction with more and more water ligands. ii) At 266 nm, close to several allowed transitions of bare , a distinct maximum is observed for the absorption of ion. It may be attributed to a change in the spin multiplicity when switching from and on one hand to Fe(H₂O) on the other. Received: 11 November 1997 / Revised: 18 February 1998 / Accepted: 22 April 1998