We present the development of nanoscale pH sensors based on functionalized silver nanoparticles and surface-enhanced Raman scattering (SERS). The SERS spectrum from individual silver nanoparticle (50-80 nm in diameter) clusters functionalized with 4-mercaptobenzoic acid shows a characteristic response to the pH of the surrounding solution and is sensitive to pH changes in the range of 6-8. Measurements from nanoparticles incorporated in living Chinese hamster ovary cells demonstrate that the nanoparticle sensors retain their robust signal and sensitivity to pH when incorporated into a cell.

The production of OH and HO(2) in Cl-initiated oxidation of cyclohexane has been measured using pulsed-laser photolytic initiation and continuous-laser absorption detection. The experimental data are modeled by master equation calculations that employ new G2(MP2)-like ab initio characterizations of important stationary points on the cyclo-C(6)H(11)O(2) surface. These ab initio calculations are a substantial expansion on previously published characterizations, including explicit consideration of conformational changes (chair-boat, axial-equatorial) and torsional potentials. The rate constants for the decomposition and ring-opening of cyclohexyl radical are also computed with ab initio based transition state theory calculations. Comparison of kinetic simulations based on the master equation results with the present experimental data and with literature determinations of branching fractions suggests adjustment of several transition state energies below their ab initio values. Simulations with the adjusted values agree well with the body of experimental data. The results once again emphasize the importance of both direct and indirect components of the kinetics for the production of both HO(2) and OH in radical + O(2) reactions.

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The OH concentration in the Cl-initiated oxidation of cyclohexane has been measured between 6.5-20.3 bar and in the 586-828 K temperature range by a pulsed-laser photolytic initiation-laser-induced fluorescence method. The experimental OH profiles are modeled by using a master-equation-based kinetic model as well as a comprehensive literature mechanism. Below approximately 700 K OH formation takes place on two distinct time-scales, one on the order of microseconds and the other over milliseconds. Detailed modeling demonstrates that "formally direct" chemical activation pathways are responsible for the OH formation on short timescales. These results establish that formally direct pathways are surprisingly important even for relatively large molecules at the pressures of practical combustors. It is also shown that remaining discrepancies between model and experiment are attributable to low-temperature chain branching from the addition of the "second oxygen" to hydroperoxycyclohexyl radicals.

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O(2) large densities of ozone (O(3)) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O(2). The process is initiated in some undefined manner, possibly through weak two-photon O(2) dissociation, which results in a small amount of O(3) being generated. As soon as any O(3) is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O(2) (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O(3) molecule destroyed. Recombination then converts these atoms to O(3), and thus O(3) production in the system is autocatalytic. A deficiency exists in current models of O(3) photochemistry in the upper stratosphere and mesosphere, in that more O(3) iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O(2) vibrational distributions resulting from O(3) photodissociation and about relaxation rates of vibrationally excited O(2) a case can be made for the importance of incuding this mechanism in the models.

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S(1D) atoms have been produced by photodissociation of OCS at 248 nm and resonance-enhanced photoionization has been used to follow their decay. Rate coefficients for removing S(1D) have been determined for C2H4, C2H6, H2, CH4, N2O, OCS, CO2, N2, O2, and Ar at 300 K and upper limits determined for SF6, Ne, and He. (45±10)% of the collisions of S(1D) with OCS produce S(3P) atoms.