Multiphoton ionization and excitation of the water molecule studied by UV-pump / NIR-probe experiments

Ultraviolet (UV) excitation of the water molecule initiates rich structural dynamics, which are of interest for a broad range of biophysical and biochemical applications. Here we report on the experimental studies of femtosecond dynamics in D₂O triggered by the absorption of a few 243 nm photons. Specifically, at this wavelength, the absorption of two, three and four UV photons results in excitation of the neutral, ionization, and dissociative ionization, respectively. These dynamics are probed by a time-delayed 800-nm pulse, which can couple the intermediate states populated by the UV pump to a variety of singly, doubly and triply charged final states. By measuring the yields and kinetic energy distributions of the resulting ionic fragments as a function of UV-NIR delay, we disentangle different UV-induced pathways and characterize the translational energy of different dissociation channels. Triple coincidence measurement of the ions resulting from a three-body breakup induced by the probe allows us to characterize the ratio between two- and three-body dissociation upon UV excitation. We compare our measurements and molecular-dynamics simulations on highly excited neutral and low-lying cationic potential surfaces of the water molecule.