MULTIPHOTON IONIZATION AND DISSOCIATION OF H2O

We investigate multiphoton ionization (MPI) of H2O at 532 nm excitatio n wavelength in an intensity range of 10(12)-10(13) W cm(-2). We find strong fragmentation of the molecule with formation of OH+, H+, O+, an d H-2(+) as charged dissociation products. Photoelectron spectra show that the charged products originate in photodissociation of H2O+. Ioni c dissociation starts after excitation of H2O+ to the (B) over tilde(2 )B(2) state and probably high vibrational levels of the bending mode i n (A) over tilde(2)A(1). Symmetric (C-2v) is significantly less probab le than asymmetric (C-s) dissociation. MPI of H2O mainly populates ion ic (X) over tilde (000) and (100) vibronic states below 5 x 10(12) W c m(-2) light intensity. At high intensity also (B) over tilde vibronic states seem to become directly populated with rising probability. All lowest-order MPI electron groups are accompanied by at least one above -threshold ionization group. A sensitive dependence of the photoelectr on spectra on light intensity may point to bond-angle softening of low -lying (X) over tilde vibronic states of H2O+ which is expected to be brought about by a favourable resonant one-photon coupling between the (X) over tilde and (A) over tilde electronic states.