Knowledge Bank

The state-to-state vibrational predissociation dynamics of the (H$2$O)$2$ dimer were studied by resonance-enhanced multiphoton ionization (REMPI) and velocity-map imaging (VMI) to obtain pair-correlated product energy distributions. The 2+1 REMPI spectra of the H$2$O photofragments were recorded via the ~{C}$1$B$1$ (000) $\leftarrow$ ~{X}$1$A$1$ (000 and 010) transition following a vibrational excitation of the dimer's bound-OH stretch fundamental. The fragment' center-of-mass translational energy (c.m. {\em E}) distributions were determined from VMI of selected rotational states of the detected H$2$O photofragments. The c.m. {\em E} distributions were then converted to pair-correlated H$2$O cofragment rotational level distributions. This is the first experiment in which H$2$O products with bend ($\nu 2$) excitation were observed by REMPI. The dissociation energy of the dimer was determined from the images with spectroscopic accuracy. The predissociation mechanism of (H$2$O)$2$ will be discussed and compared with the corresponding hydrogen bonded dimers of an acid (HCl-H$2$O) and a base (NH$3$-H$2$O).