Ultrafast Electronic Relaxation Dynamics of Ionized Liquid Water*

The ionization of liquid water serves as the principal trigger for a myriad of phenomena that are relevant to radiation chemistry and radiation biology. The earliest events that follow the ionization of liquid water, however, remain relatively unknown. We have embarked on a series of studies to investigate the ultrafast dynamics of intense laser-ionized liquid water. Optical pump-probe spectroscopy employing few-cycle pulses in the visible (500--700 nm) and short-wave infrared (0.9--1.7 $\mu$m) is used to reveal the fate of the electron that is initially injected into the conduction band by ionization. These experiments yield the lifetime of the conduction-band electron and the timescale for vibrational cooling of the {\it s} electron. Remarkably, our results suggest that the relaxation of the conduction band electron to the hydrated {\it s} electron proceeds via an intermediate state --- possibly the elusive {\it p} state electron --- whose lifetime is found to be 63 $\pm$ 3 fs (94 $\pm$ 8 fs) in H$_{2}$O (D$_{2}$O). These results provide a comprehensive view of the electronic relaxation dynamics of ionized liquid water.