Excited water

At the end of the physical stage, which is within about 10 sec of the passage of the ionizing particle through the liquid, the track made by the particle contains H20", subexcitation electrons e , and electronically excited water molecules H2O in small clusters called spurs. From about 10 to 10 sec, the following processes are thought to occur and comprise the physicochemical stage [9,10] ... 

Very little is known about the decay channels for excited water molecules in the liquid phase. Consequently, they are generally assumed [9,10] to be essentially the same as those in the gas phase. 

The species so formed are likely to be separated in space and are more likely to react with solute than are radicals formed from excited water molecules. 

Fig. 38. Line excitation SEMI-RARE images of two-phase flow in a ceramic monolith rated at 200 cpsi. The signal intensity shows how far the initially excited water molecules have traveled in the period between line excitation and the image acquisition. The gas flow rate was (a) 0, (b) 100, (c) 200, and (d) iOOcin inin. Images acquired 78ms after r.f. excitation are shown. A 5mm-high slice of spins was initially excited along the direction of flow. The fleld-of-view is 50 mm (x) x 25 mm (z).

From the preceding discussion it may be inferred that in frozen aqueous solutions there are basically two different mechanisms by which H atoms can be formed (1) by reaction of the electron with acid anions according to Equation 4, and (2) by direct radiolysis of water molecules according to Equation 1. Earlier investigations of aqueous solutions at room temperature have also lead to the same conclusion (70). Although there are no experimental data from either the room temperature studies or from those on the frozen solutions, as to the actual nature of the second process, it is believed (70) that it is the dissociation of excited water molecules formed in the tracks of the fast electrons. 

With Na2S04, Na2HP04, NasP04, and Na2C08 as solutes where the mechanism of hydrogen atom formation and stabilization is different, the observed linear dependence of the H atom yields of the solute concentration is as expected on the basis of the proposed mechanism. Thus, since the probability of forming an H,OH radical pair in the hydration shell of the anion (—e.g., by dissociation of an excited water molecule) would be proportional to the anion concentration, and sigce the stabilization of the H atom is postulated as the result of the reaction of the OH radical with the anion in whose hydration shell it is formed, it follows that the yield should be proportional to the solute concentration. 

Kinetic Evidence that Excited Water is Precursor of Intraspur H2 in the Radiolysis of Water... 

Homogeneous kinetics is used instead of diffusion kinetics to express the dependence of intraspur GH, on solute concentration. The rate-determining step for H2 formation is not the combination of reducing species, but first-order disappearance of "excited water." Two physical models of "excited water" are considered. In one model, the HsO + OH radical pair is assumed to undergo geminate recombination in a first-order process with H3O combination to form H2 as a concomitant process. In this model, solute decreases GH, by reaction with HsO. In the other model, "excited water" yields freely diffusing H3O + OH radicals in a first-order process and solute decreases GH, by reaction with "excited water." The dependence of intraspur GH, on solute concentration indicates th,o = 10 9 — 10 10 sec. 

This effect of N08 ion is quantitatively consistent with a reaction mechanism (43) in which N08 interacts with an electronically excited water molecule before it undergoes collisional deactivation by a pseudo-unimolecular process (the NOs effect is temperature independent (45) and not proportional to T/tj (37)). Equation 1, according to this mechanism, yields a lifetime for H20 of 4 X 10 10 sec., based on a diffusion-controlled rate constant of 6 X 109 for reaction with N08 Dependence of Gh, on Solute Concentration. Another effect of NOa in aqueous solutions is a decrease in GH, with increase in N08 concentration (5, 25, 26, 38, 39). This decrease in Gh, is generally believed to result from reaction of N08 with reducing species before they combine to form H2. These effects of N08 on G(Ce+3) and Gh, raise the question as to whether or not they are both caused by reaction of N08 with the same intermediate. 

The applicability of homogeneous kinetics is attributed to first-order disappearance of H20, excited water, as the rate-determining step for H2 formation, instead of the combination of reducing species as commonly assumed when using the Samuel-Magee model. Two alternative physical models of H20 are proposed. In one, H20 is the HsO + OH radical pair which is assumed to undergo geminate recombination with... 

Then ionization which yields H30 + OH by the sequence of Reactions e, f, and g would result in formation of excited water by the reverse of Reaction h. Thus, the effect of both ionization and excitation would be formation of H20. ... 

Lifetime for Excited Water. tHio s.h8o, where fe.Hto, is the rate constant reaction of solute with excited water (reaction of solute with H30 in one model and with H20 in the other model), is a constant which can be derived from results summarized by Equations 1-14 as discussed below and which is independent of any constant errors in absolute dosimetry. Let Gh,o denote the yield of H20 which disappears intraspur by a first-order process with resultant H2, H202, and H20 formation. Let a denote the number of H2 molecules formed for each H20 which disappears intraspur. 

The water radical cation, produced in reaction (1), is a very strong acid and immediately loses a proton to neighboring water molecules thereby forming OH [reaction (3)]. The electron becomes hydrated by water [reaction (4), for the scavenging of presolvated (Laenen et al. 2000) electrons see, e.g., Pimblott and LaVerne (1998) Pastina et al. (1999) Ballarini et al. (2000) for typical reactions of eaq, see Chap. 4], Electronically excited water can decompose into -OH and 11- [reaction (5)]. As a consequence, three kinds of free radicals are formed side by side in the spurs, OH, eaq , and H . To match the charge of the electrons, an equivalent amount of ED are also present. 

Vander Wal, R.L., Scott, J.L., and Crim, F.F. (1991). State resolved photodissociation of vibrationally excited water Rotations, stretching vibrations, and relative cross sections, J. Chem. Phys. 94, 1859-1867. 

Weide, K., Hennig, S., and Schinke, R. (1989). Photodissociation of vibrationally excited water in the first absorption band, J. Chem. Phys. 91, 7630-7637. 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

Hydroxyl radicals are also available by decomposition of excited water molecules. 

The jump-return or 1, 1 method is a very simple and elegant solution because rather than destroying the water signal it simply does not excite water in the first place. We saw in Chapter 8, Figure 8.19 that a null in excitation occurs at the center of the spectral window, and this can be adjusted to put the water peak exactly on-resonance. A jump-return NOESY spectrum of a small protein will be shown later in this chapter. Jump-return and some more complicated variations ( 1,1 - echo and binomial ) are not applicable to all experiments, however, and require some careful tuning and adjustment to work well. They also distort the peak intensities throughout the spectrum and greatly reduce the intensities near the water resonance. 

In the calculations we used an approximate relationship, Ph2o 3Ph2I between the formation probability of excited water molecules and molecular hydrogen [4, 48]. 

As it can be seen in Figure 3.2, five potential energy surfaces are accessible to reactants but the mainly contribution comes from the lowest X A PES, which correlates with the X A ground state of the H2O molecule. In this PES the title reaction proceeds without energy barrier from reactants to products through a highly excited water molecule. 

High energy / emitters, particularly 32P, can be counted directly in aqueous solutions without recourse to stintillation cocktails, since electrons with energies > 0.5 MeV excite water molecules to emit blue light which can be observed by the photomultiplier. This, so-called Cerenkov counting method, has an efficiency of about 40 %. 

At the time, it was known that water could be decomposed by heat or by UV irradiation. However, irradiation by X-rays seemed to show no decomposition of very pure water. This led Hugo Fricke to conclude that radiation created two forms of excited water, which could react with additives in the system or decay back to normal water. Today we certainly know that radiation does decompose water. 

It is also probable that free radicals are formed from excited water-molecules outside the ionization spurs. These would contribute to the net formation of hydrogen and hydroxyl free radicals. Therefore, although important refinements have been added, the fundamental description of the action of ionizing radiations on water remains as summarized by Allen. ... 

An alternative method for producing molecular hydrogen and hydrogen peroxide, proposed by Johnson and Weiss, is based on the direct interaction of excited water-molecules. 

The reaction of H atoms with vibrationaUy excited water moiecuies... 

Inversion of water-flanking signals during shaped pulse presaturation Multiplet-selective excitation, band-selective excitation, water suppression Separation of resonances on the basis of electrophoretic migration rates. 

These experiments gave very different results than those for photodissociation of water in the A state. Photodissociation via the B state is an example of predissociation and the observed internal energy distribution of the OH is found to peak near 20kcal mol-1 of rotational energy [22], Ab initio studies have found that the bound B state possesses a deep potential well associated with a linear HOH minimum [23-26], Excitation from the well-known bent ground state to the linear B state imparts large amounts of bending excitation into the electronically excited water molecule. The... 

According to the molecular beam experiments on the reaction of vibrationally excited water with H atoms [6-10],... 

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