Silver monolayer‐protected clusters (MPCs) are an important new class of small metal nanoparticles with discrete sizes and unique properties that are eminently tunable; however, a fundamental understanding of the mechanisms of MPC formation is still lacking. Here, the basic mechanism by which silver‐glutathione MPCs form is established by using real‐time in situ optical measurements and ex situ solution‐phase analyses to track MPC populations in the reaction mixture. These measurements identify that MPCs grow systematically, increasing in size sequentially as they transform from one known species to another, in contrast to existing models. In the new sequential growth model of MPC formation, the relative stability of each species in the series results in thermodynamic preferences for certain species as well as kinetic barriers to transformations between stable sizes. This model is shown to correctly predict the outcome of silver MPC synthetic reactions. Simple analytic expressions and simulations of rate equations are used to further validate the model and study its nature. The sequential growth model provides insights into how reactions may be directed, based on the interplay between relative MPC stabilities and reaction kinetics, providing tools for the synthesis of particular MPCs in high yield.

The grating decomposition method: A new approach for understanding polarizationselective transient grating experiments. I. Theory J. Chem. Phys. 97, 69 (1992); 10.1063/1.463565 The grating decomposition method: A new approach for understanding polarizationselective transient grating experiments. II. Applications J. Chem. Phys. 97, 78 (1992); 10.1063/1.463525 Analysis of complex molecular dynamics in an organic liquid by polarization selective subpicosecond transient grating experimentsPolarization-selective transient grating experiments have been used to study the subnanosecond time scale dynamics of several sodium-seeded, premixed flames. Intensity gratings (in which both excitation beams are of the same polarization) were used to determine excited-state quenching collision rates, while polarization gratings (in which the excitation beams are cross polarized) were used to measure Na diffusion constants and the rates ofNa ground state magnetic sublevel population scattering collisions. In addition, the rates of scattering between the 3Pl/2 and 3P 312 excited state levels were measured using an excited state probing scheme.is constant across each fringe, whereas the polarization changes from rcp to linear at -45' to Jcp to linear at + 45'.

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Picosecond transient grating experiments were performed on a sodmm-seeded methane/air flame at very small fringe spacings (< I pm). At these fringe spacings, the grating signal decay occurs on a time scale fast compared to that of velocity changing collisions. The form of the decay is determined by the Maxwell-Boltsmann velocity distribution of the Na atoms. The data is tit with one adjustable parameter, the temperature. These experiments demonstrate a potentially useful approach for the non-invasive measurement of flame temperatures in small regions of a flame.

Picosecond four-wave mixing experiments have been used to study collisions in a Na-seeded, premixed, methaneair flame. Population gratings are used to measure Na excited-state quenching collision rates, while polarization gratings are used to measure Na ground-state hyperfine coherence randomizing collision rates and overall Na diffusion rates, even though these processes are slower than the excited-state quenching rate.The ability to probe noninvasively the local properties of flames (e.g., transport properties, species concentrations, temperature, reaction rates, and collision cross sections) over small (submillimeter) distance scales is essential to the understanding of combustion. The advent of lasers has allowed some of these processes to be studied, and techniques such as laserinduced fluorescence, Raman spectroscopy, and absorption measurements have become the mainstays of combustion research.' These experiments tend to be difficult to perform, however, owing to a combination of weak signals and a large amount of background noise (both from the flames and the laser beams themselves). For these reasons four-wave mixing (FWM) techniques have proved useful in combustion research; these techniques generally provide relatively strong signals that are spatially separated from the laser beams. Coherent anti-Stokes Raman spectroscopy 2 and other FWM techniques 3 have proved useful in obtaining information on steady-state flame properties such as temperature and concentration profiles. Four-, six-, and eight-wave mixing have also been used to study collision-enhanced spectra in sodium-seeded flames. 4 To study the fast-time-scale dynamics of flames (such as collisions, velocity distributions, and reactions), it is necessary to perform experiments with subnanosecond laser pulses. The picosecond transient grating 5 is a unique and powerful tool for flame research, and here we report the initial results of a study of a Na-seeded, premixed, methane-air flame.Previous research in our laboratory has demonstrated that the transient grating is an effective method for probing velocity distributions in low-pressure gases 6 (where low pressure signifies that there are no collisions over the experimental time and distance scales). In particular, experiments were performed on Na vapor in a heated cell. Population grating decays (in which all the laser beam polarizations are parallel to one another and perpendicular to the plane of the excitation beams) give the Fourier transform of the Na velocity distribution (which is known to be a Gaussian Maxwell-Boltzmann distribution in a low-pressure gas) as predicted by theory. Surprisingly, however, polarization grating decays (in which the excitation beams are cross polarized), although having an envelope corresponding to the population grating decays, show large beats at both the ground-state hyperfine level splitting (in all cases) and the excited-state hyperfine level splittings (when the same D line is excited and probed). A theory was developed that explains these beats qualitat...