Multiphoton Ionization of Liquid Water with 3.0−5.0 eV Photons

Abstract

We report a picosecond laser study of the transient absorption of hydrated electrons generated by the 3-5 eV multiphoton ionization of liquid water. The geminate kinetics indicate that e aqis produced by at least three different mechanisms over this energy range. Power dependence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.47 eV, consistent with two-or three-photon excitation of the A ˜(1 B 1 ) lowest excited state. For (three-photon) excitation in the range 3.02-3.47 eV very little (e15%) geminate recombination is observed while for the (two-photon) excitation at shorter wavelengths significant recombination (g55%) is observed. In the region of 3.85-4.54 eV, photonenergy-independent kinetics indicate that e aqis produced via two-photon excitation of the A ˜state followed by an ionization process in which the electrons do not obtain any excess kinetic energy. For photon energies in the range of 4.75-5.05 eV, the escape fraction increases slightly, consistent with two-photon excitation of higher energy states. Simulation with a diffusion model shows that the electron is ejected at least 25 Å farther into the bulk for the 3.02-3.47 eV photon energies relative to two-photon ionization in the 3.67-5.0 eV range. We conclude that the larger distances result from a (3 + 1)-photon resonance-enhanced multiphoton ionization (REMPI) process, made possible by visible/near-UV absorption of the water excited states. Possible mechanisms of the water ionization are discussed. A new mechanism is proposed to explain the production of solvated electrons from excitation of the A ˜(1 B 1 ) state of liquid water, well below the Born-Oppenheimer ionization threshold. On the basis of the gas phase properties of this state, we assume a direct dissociation to give OH radical and H atoms, with the excess energy almost entirely transferred to kinetic energy of the H atoms: H 2 O* f OH + H(hot). It is proposed that the hot H atoms immediately react with an adjacent water molecule to form hydronium ion and a solvated electron, in a process analogous to the thermal reaction of H atoms with water at elevated temperatures: H(hot) + H 2 O f H 3 O + + e aq -.

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

“…The appropriate form of the distribution function depends on the ionization mechanism and is generally assumed to be either a Gaussian or an exponential function. Similar to previous reports, [10][11][12]19,20 we find that using either of these distribution functions for the initial ejection length fits the data equally well at all excitation energies. Additionally, the wide range of energies in our study allows us to test systematically how the choice of distribution function affects the values of ͗r 0 ͘ that come from fitting the data.…”

“…Work from a decade ago, using two-photon excitation to study total ionization energies in the range of 7.3-10.1 eV, showed that the average ejection length of electrons remains roughly constant below a threshold of about 9 -9.5 eV, but increases significantly above that energy. 12 That paper also reports multiphoton ionization above 10.1 eV, via a proposed 3 + 1 multiphoton mechanism, but more recent work shows that the proposed 3 + 1 multiphoton ionization mechanism is not correct and that solvent heating by the high-intensity pump pulse influences the geminate kinetics. 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.…”

“…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.…”

Revealing the ultrafast process behind the photoreduction of graphene oxide

“…2a,b are consistent with this model. The long-lived component of pure water corresponds to a residual DA/A 0 of B1.5 Â 10 À 3 after 1 ns and is assigned to a fully thermalized species of solvated electrons [34][35][36] . The increased residual DA/A 0 of GO-containing samples, when compared with pure water, results from the additional absorption of newly formed rGO.…”

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.…”