Photoionization Yield vs Energy in H₂O and D₂O

Abstract

A simple conductivity jump method was used to measure the escaped solvated electron yield following twophoton excitation of water with Raman-shifted light from an amplified mode-locked Nd:YAG laser. Between 7.8 and 9.3 eV, the quantum efficiency for the escape yields changes from 1.9% to 22%, with an almost exponential dependence on the excitation energy. Quantum efficiency in D 2 O is smaller and resembles the H 2 O behavior at 0.35 eV lower energy. The quantum yield measured for one-photon excitation near the water absorption edge at 6.4 eV is a surprisingly large 1.3%. We propose that the mechanism for low energy photoionizaton of water is best described as a dissociative proton-coupled electron transfer to a preexisting trap.

Excitation-energy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3to12.4eV

“…Isotopic substitution potentially provides additional insight into the ionization mechanism by revealing the relative influence of nuclear and electronic dynamics in the ionization process, but only a handful of previous studies examine the isotope dependence of ionization in liquid water. 6,[21][22][23][24][25] We discuss the implications of our results for several possible ionization mechanisms in the context of recent liquid jet photoelectron experiments 26 and cluster ionization studies 2,3 that provide a framework for describing the electronic structure of water.…”

“…In the first case, it is the spatial distribution of trap states that determines the electron ejection length and is therefore independent of the excitation energy. 22 An increase in the number of available trap states with excitation energy explains the exponential increase in the ionization yield at low excitation energies in this picture. 22 On the other hand, the constant ejection length in this range may also be the result of the à and B excited states of water having the same excited molecular orbital.…”

“…Pumping the sample with high power laser pulses in order to obtain good signal levels often produces these unwanted nonlinear effects, 13,34 especially at low excitation energies where the ionization yield is very small ͑Ͻ5%͒. 22 Nonlinear effects may have a more pronounced influence on the signal in D 2 O, because the ionization yield is smaller in that liquid than in H 2 O, 22 but the 300 nm excitation pulses in our experiment are too weak for us to test this hypothesis for an excitation energy of 8.3 eV. Nonetheless, we have made a consistent measurement over a wide range of energies in order to compare the isotope dependence, giving us a more complete data set than was previously available.…”

“…13 Other experiments at excitation energies up to 10 eV support the observation that the ejection length increases rapidly above 9 -9.5 eV, 6,14-21 although there are substantial difficulties in comparing previous results due to different methods of reporting recombination yields and the use of different fitting procedures. Further complicating that comparison is the fact that only a few authors specifically examine the energy dependence of the ionization mechanism over a broad range of energies, 6,8,12,14,15,22 and that, with the exception of a recent study at 12.4 eV by one of our groups, 21 there are no reliable data for excitation energies above 10 eV.…”

Direct observation of the collapse of the delocalized excess electron in water

“…Femtosecond pump-probe experiments have been used to unravel details of the equilibration processes following the trapping by the solvent [5][6][7][8][9][10][11][12]. In addition, picosecond measurements on the recombination kinetics of e − aq have described the ejected electron's migration lengths from its geminate partners [13][14][15][16][17][18][19][20][21][22]. These studies evoke a picture of an excess electron that is at first delocalized in the conduction band and then localizes to the hydrated electron e − aq within about a picosecond.…”