Water 2-photon ionization

Nikogosyan DN, Oraevsky AA, Rupasov VI. (1983) Two-photon ionization and dissociation of liquid water by powerful laser UV radiation. Chem Phys 77 131-143. 

Fig. 1. Spectral evolution of the hot s-like state of hydrated electron generated in two 6.2 eV photon ionization of light water. The arrows indicate the trends observed in the direction of longer delay times of the probe pulses. Panel (a) demonstrates the evolution between 500 fs and 1.2 ps, showing considerable blue shift and fast decay of the IR features. Panel (b) shows the slow relaxation regime that is observed after 1.2 ps (note the logarithmic scale). In this regime, the band maximum is locked within 20 meV and the spectral evolution is due to relatively slow, isotope sensitive narrowing of the spectral envelope on the red side of the spectrum. This narrowing is likely to be caused by vibrational relaxation of the hot s-like state. See Ref 28 for more detail.

Since in densely populated industrial areas air and water pollution has become a serious problem, the study of pollutants and their reactions with natural components of our environment is urgently needed [15.75]. Various techniques of laser spectroscopy have been successfully employed in atmospheric and environmental research direct absorption measurements, laser-induced fluorescence techniques, photoacoustic detection, spontaneous Raman scattering and CARS (Chap. 8), resonant two-photon ionization, and many more of the sensitive detection techniques discussed in Chap. 6 can be applied to various environmental problems. This section illustrates the potential of laser spectroscopy in this field by some examples. 

We shall initially summarize the evidence proving that absorption of one laser photon by water induced ionization to hydrogen and hydroxyl ions. 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

FIGURE 4.2 Ionization efficiency as a function of photon energy in the gas phase of water. Data from Haddad and Samson (1986), with permission of Am. Inst. Phys. ... 

Another situation in which an already well-studied proton transfer reaction serves as a probe of a physical phenomenon has been suggested by Knight, Goodall and Greenhow (43, 44). They ionized water with single photons of Nd glass laser infrared radiation and measured an ion recombination rate constant for the reaction... 

Experimental confirmation of the order of MO energies for the water molecule is given by its photoelectron spectrum. Figure 5.13 shows the helium-line photoelectron spectrum of the water molecule. There are three ionizations at 1216, 1322 and 1660 kJ mol1. A fourth ionization at 3107 kJ mol-1 has been measured by using suitable X-ray photons instead of the helium emission. That there are the four ionization energies is consistent with expectations from the MO levels for a bent C molecule (see Figure 5.12). 

The photoelectron retains most of the energy of the incident photon and itself produces further ionizations. It produces many more H20 +, /e" pairs in losing its energy. The excess of energy possessed by the electrons so produced is used in such further ionizations until the electrons become thermalized, i.e. they have translational energies typical of the temperature of the bulk medium. Both types of ion become hydrated by interaction with the water solvent ... 

Apparently, it is this type of preionization that explains the appearance of solvated electrons in water at photon energy ph = 6.5 eV observed in Refs. 192 and 193. Thus, besides the ionization with the potential Ic given by formula (5.10), in polar media with high probability of rapid solvation of an ionized electron there may also occur preionization with solvation of the ejected electron with the potential h + K-... 

Although most experiments that use ionizing radiation have been carried out with y-rays and high-energy particles, notably electrons, one has also to keep in mind that photons that exceed 7 eV are capable of ionizing water. The photo-... 

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