Rotational predissociation of H2O(C 1B1) studied by multiphoton ionization spectroscopy in a supersonic free jet

The predissociation of cold water molecules (17–75 K) is investigated in a supersonic free jet after excitation to the C 1B1 state using simultaneous absorption of three laser photons in the range 369–374 nm. Absorption of a further photon in this wavelength range ionizes the water molecule (X 1A1 (0,0,0)→C 1B1 (0,0,0)→H2O+(X); (3+1)-multiphoton ionization spectrospcopy). The line half-widths and intensities in the rotationally resolved spectrum show a pronounced dependence on the parent rotational excitation in the C state. Supersonic cooling simplifies the spectra drastically and reduces overlap of lines, so that a considerable number of individual linewidths could be measured. The linewidth measurements and spectrum simulations show excitation of a-axis rotation to be dominant. a-axis parent molecular rotation couples the excited C 1B1 state to an unbound state of A1 electronic symmetry. The linewidth measurements point to a weaker Coriolis coupling between the C 1B1 state and a 1A2 state of H2O via rotation around the c axis. According to ab initio calculations, a water molecule in this 1A2 state dissociates essentially without barrier to O(1D)+H2. This process may explain production of nearly 10% O(1D) atoms in the wavelength range 123–125 nm.