Two-photon excitations of the water molecule studied by time-resolved ion momentum spectroscopy

Photoexcitation and photodissociation dynamics of the water molecule have been extensively studied both experimentally and theoretically, strongly motivated by atmospheric and biological importance of this system. While most of these studies focused on the dynamics induced by a single vacuum-ultraviolet photon, the same range of excited states can be populated by two-photon absorption. Since the latter process can be efficiently driven by wavelength-tunable femtosecond ultraviolet (UV) sources, it significantly facilitates time-domain measurements. In this work, we explore the dynamics induced by two-photon excitation of water in a femtosecond UV pump / near-infrared (NIR) probe experiment. Here, the NIR probe pulse ionizes the UV-excited neutral molecule, and the time evolution of the system is mapped by measuring the yields and kinetic energy distributions of the resulting ionic fragments as a function of UV-NIR delay. We analyze the wavelength dependence of two-photon excitations in 240-250 nm range and compare the results for H₂O and D₂O molecules.