Quantum yields, 4 for the ionization of water to hydrogen and hydroxyl ions, present as transients in excess of their equilibrium concentration levels following irradiation by a pulse derived from a Q-switched laser, have been measured as a function of temperature and excitation wavelength. Irradiation wavelengths used were the ruby and neodymium laser values 0.694 and 1.06 pn, and stinulated Raman scattering shifts of these to 0.975 and 1.41 pm. (b (298 K) increased from 9 x lo-' to 9 x with decrease of h from 1.41 to 0.694 pm. The reaction occurred after absorption of a single photon into the excited 0-H stretching vibrational levels : the energy of the laser photon E was in a91 cases in excess of the activation energy for reaction, which is shown to equal A H 0 for the self dissociation of water. The expression 4 = &(l -A H e / E ) s is based upon competition between reaction and relaxation, and correlates all the data with s = 8.4f0.2 and 4o = (1.7f.O.3)~ lo-".Values of s and & are interpreted with a hydrogen-bonded species (H20)1 as the reactive entity.The ionization of water is the first reported example of an i.r. photochemical reaction in the liquid phase.
An experimental arrangement suitable for the undergraduate laboratory is described that permits the quantitative study, at several different levels of sophistication, of forced, damped, simple harmonic motion in a mechanical system. The system comprises a spring supporting a ball that oscillates in an oil dashpot. The chief experimental innovation is an optical transducer that allows the oscillations to be observed on an oscilloscope screen. Values of oil viscosities deduced from the measured damping factors reveal large discrepancies that are resolved by a fuller consideration of the fluid dynamics of the damping media. Transverse viscous waves are launched that enhance the effective velocity sheer. The increase in effective inertia of the ball resulting from the fluid displaced is also readily observed. These effects are easily allowed for by the application of simple correction factors, thus allowing observation of all the major resonance phenomena in a quantitative manner.
The ability of the spherical step potential well to model the Ramsauer–Townsend effect, the near disappearance of low-energy electron scattering in the noble gases argon, krypton, and xenon, is discussed. It is shown that conditions can be found for which the cross section vanishes as the energy approaches zero, but that the noble gas phenomenon, where the cross sections show sharp minima at finite energies, cannot be modeled. The low-energy s-wave cross section can be made to vanish, but the p-wave cross section rises too rapidly with energy to permit a minimum in the total cross section. In addition it is shown that the condition for the vanishing of the s-wave cross section at zero or low energy also implies the occurrence of a d-wave resonance at zero or low energy. In the noble gases, the polarization term appears to be the essential feature of the atomic potential which gives rise to the effect.
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