A time constant of 1.8 fs in the dissociation of water excited at 162 nm

Probing the first excited-state of H2O, HDO and D2O by ionization at 810 nm reveals in the parent-ion yields time constants of 1.8, 2.1 and 2.5 fs, respectively, during which the molecule leaves the Franck–Condon region, stretching the bonds of by about 0.25 Å. The OH+ signal rises slightly more slowly (1.8 + 1.7 fs), because only then is the dissociation energy of the parent ion overcome. The subsequent decay (3.3 fs) is caused by the decreasing ionization probability. The detection of such short times is intimately connected with the sensitivity of the probe technique to geometrical changes in the sub-Ångström range.