Light-Induced Temperature Jump Causes Power-Dependent Ultrafast Kinetics of Electrons Generated in Multiphoton Ionization of Liquid Water

Abstract

Picosecond geminate recombination kinetics for electrons generated by multiphoton ionization of liquid water become power dependent when the radiance of the excitation light is greater than 0.3-0.5 TW/cm 2 (the terawatt regime). To elucidate the mechanism of this power dependence, tri-400 nm photon ionization of water has been studied using pump-probe laser spectroscopy on the pico-and femtosecond time scales.We suggest that the observed kinetic transformations are caused by a rapid temperature jump in the sample. Such a jump is inherent to multiphoton ionization in the terawatt regime, when the absorption of the pump light along the optical path becomes very nonuniform. The heating of water is substantial (tens of o C) because the 3-photon quantum yield of the ionization is relatively low, ca. 0.42, and a large fraction of the excitation energy is released into the solvent bulk as heat. Evidence of the temperature jump is the observation of a red shift in the absorption spectrum of (thermalized) electron 2 and by characteristic "flattening" of the thermalization dynamics in the near IR. The temperature jump in the terawatt regime might be ubiquitous in multiphoton ionization in molecular liquids. The implications of these observations for femtosecond pulse radiolysis of water are discussed.

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

“…The matching irradiance dependencies of THz and NIR responses shown in Supplementary Figure 2 prove that the THz signal indeed originates from excess electrons for all pump-wavelengths. The 266 nm and 400 nm pump data follow the expected second-and thirdorder power laws [24,35], whereas the 800 nm pump signal depicts a threshold behavior with ≈1 TW/cm 2 as the onset of ionization. Such a threshold behavior indicates that a strong-field ionization rather than a multiphoton mechanism is behind the photo detachment with 800 nm photons, in accordance with the conclusions from Ref.…”

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

“…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. 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.…”

“…13 We also measure the kinetics of the solvated electron following two-photon ionization of deuterated water. Figure 3 compares a series of electron decay curves for H 2 O and D 2 O, excluding for clarity the traces that we obtain at an excitation energy of 10.6 eV.…”

“…It is also possible that the previous data for 8 eV excitation overestimate the survival probability, and therefore the ejection length, in D 2 O due to a contribution from threephoton absorption or from solvent heating by the pump pulse. 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.…”

Ultrafast pulse radiolysis using a terawatt laser wakefield accelerator

“…[5][6][7] Generation of such otherwise hard-to-access states, the unusual reaction regimes, and the inherently non-uniform deposition of the primary ionization/excitation events make it difficult if not impossible to simulate rapid radiolytic processes using ultrafast laser sources. [8][9][10][11][12][13][14] Studies of the fundamental processes of radiation induced reactions require an ultrafast source of ionizing radiation. Furthermore, this source must be accurately synchronized to an ultrafast laser that is used as a probe source for the detection of the short-lived species and their kinetics.…”