Sean J. Kern

Heterogeneity of the Electron-Trapping Kinetics in CdSe Nanoparticles

The kinetics of electron trapping in CdSe nanoparticles are examined from 0.5 ps to 1.8 ns. The ensemble kinetics fit a slow power law, but two-dimensional measurements show that the decay of each nanoparticle is exponential. A model is proposed in which defect sites provide a gateway for surface trapping and are randomly distributed on the surface. The electric field from the particles dipole moment creates the observed heterogeneity in rates.

Heterogeneous Reaction Rates in an Ionic Liquid: Quantitative Results from Two-Dimensional Multiple Population-Period Transient Spectroscopy

The hypotheses that ionic liquids are structurally heterogeneous at the molecular level and, even further, that this heterogeneity can transfer to the rates of reactions run in ionic liquids is being actively debated. Here, this hypothesis is tested using multiple population-period transient spectroscopy (MUPPETS), an emerging type of multidimensional measurement that resolves the kinetics of subensembles within a heterogeneous sample. A previous MUPPETS study of the excited-state twisting and electronic relaxation of auramine indicated that an ionic-liquid solvent induces rate dispersion due to a combination of heterogeneous and homogeneous processes, but those data could not quantitatively separate these contributions [Khurmi, C.; Berg, M. A. J. Phys. Chem. Lett.2010, 1, 161]. New MUPPETS data that include phase resolution and subtraction of thermal gratings are presented here and are successfully modeled. The total range of reaction rates (10--90%) is a factor of 70. If the solvent effect is viewed as a set of local viscosities, the viscosity distribution is broad and highly asymmetric. However, if the solvent is viewed as changing a reaction barrier, the data correspond to a Gaussian distribution of barrier heights. The relaxation of each subensemble is nonexponential with an initial induction period, but the shape of the decay is invariant across the rate distribution. A small (2%), long-lived component is identified as a part of the homogeneous kinetic scheme and thus as a secondary channel for excited-state relaxation, not as an impurity or alternative ground-state form of auramine. On the basis of these results, we suggest that the primary cause of rate heterogeneity is a long-lived local electric field acting on the charge redistribution during the reaction.

Simultaneous time and frequency detection in femtosecond coherent Raman spectroscopy. II. Application to acetonitrile.

The preceding paper showed that, in principle, a high-resolution coherent Raman spectrum can be recovered using femtosecond probe pulses by combined detection in both time and frequency. This measurement is possible even when the pulses are too broad in frequency for conventional frequency-domain spectroscopy and too broad in time for conventional time-domain spectroscopy. In this paper, the method is tested on experimental coherent anti-stokes Raman spectroscopy data from acetonitrile. Compared to theoretical models, experimental data are complicated by noise and incomplete knowledge of the pulse structure. Despite these complications, most of the information in the Raman spectrum is recovered from the data: weak transitions are detected and natural-linewidth resolution is achieved across an 800 cm(-1) spectral range. However, circumstances in which experimental limitations result in missed features or ambiguities in the recovered spectrum are also identified. These results suggest where improvements in measurement and data analysis can be made.

Well-Resolved Coherent Raman Spectra from Femtosecond Pulses

Coherent anti-Stokes Raman spectra with resolution better than the inherent linewidths of organic liquids are measured with 50 fs pulses. Data is collected in both time and frequency and analyzed in a 2D frequency-frequency format.