个人简介

B.S., 1979, University of Minnesota Ph.D., 1985, University of California at Berkeley Postdoctoral Fellow, 1985–1987, Stanford University Camille and Henry Dreyfus Distinguished Young Faculty Award, 1987. National Science Foundation Presidential Young Investigator, 1990. Alfred P. Sloan Foundation Research Fellow, 1992. Fellow of the American Physical Society, 2000. USC Educational Fund Research Award, 2007. Chemist of the Year, South Carolina Section of the ACS, 2014.

研究领域

Physical

Just as the static properties of macroscopic materials are determined by structure on Angstrom to nanometer length scales, the macroscopic, dynamic properties are determined by motion on the femtosecond to nanosecond time scales. For example, single-collision times in solids and liquids are near 0.1 ps; sound waves transmit conformational changes across a 10 nm diameter protein in 10 ps; solar energy captured by a semiconductor nanoparticle persists no longer than a few 10’s of ns. If we want to understand the macroscopic behavior of materials in terms of molecular properties, experiments on these ultrafast time scales are required. Although these times are natural for molecular processes, they can only be measured by using ultrafast laser technology. In our laboratory, pulses with durations as short as 50 fs and with peak intensities in the 10 gigawatt range are used to investigate these processes. These pulses are only 15 microns in physical length. They consist of 15–20 optical cycles if they are in the visible and only a few cycles if they are in the infrared. Using these pulses, we observe events as fast as the breaking of a chemical bond or of a single collision in solution. In addition to ultrafast lasers, our experiments need new methods to measure molecular motion using only pulses of light. We are actively developing complex sequences of pulses to measure new properties. These “multidimensional” spectroscopies exploit both the high intensity electric fields in the laser pulses and the fact that materials retain memory of the phase of their excitation during these short times. We have recently demonstrated that a sequence of six pulses (called a MUPPETS experiment) can measure the electronic-relaxation rate of specific subpopulations within a heterogeneous sample. A number of other new, but related, measurements have been predicted and are being brought into the laboratory to address problems of current interest.Examples include: Energy Pathways in Nanostructures. Semiconducting nanostructures offer great promise for harvesting solar energy, but no two nanostructures are ever identical at the atomic level. Lattice defects, variation in surface structure and misplaced passivating molecules are common and can greatly affect energy flow, but are difficult to detect in structural measurements. MUPPETS offers a means to measure the different fates of energy in different particles within a real sample. Peptide Dynamics. In some circumstances, proteins do not have a well defined structure: before folding, after denaturation, in “intrinsically disordered” proteins, and in short peptides. The speed of conformational changes makes it difficult to characterize either the range of conformations or the rate of interconversion between them. Ultrafast multidimensional experiments coupled with FRET (fluorescence resonance energy transfer) is a new approach that will be used to make these measurements. Glass Dynamics. At temperatures very near the glass transition, there is strong evidence that the liquid breaks up into microscopic regions of differing viscosity. Polarization-resolved MUPPETS measurements offer a route to trace this heterogeneity to its origins at higher temperatures and shorter relaxation times. Our research draws on chemistry, laser physics, statistical mechanics, spectroscopy, and biology to answer broad issues in chemistry, physics, and materials science. Students with a background in any of these field can contribute to our efforts. Growth into unfamiliar areas is expected and is facilitated by interactions with students and postdocs from a variety of fields. New methods using the coordinated action of six, ultrafast laser pulses can separate the dynamics of different molecular subensembles within a heterogeneous sample. The pulses (green) have passed through the lens on the left and are being focusing onto a common point in the sample (green cuvette) on the right.

近期论文

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H. Wu, and M. A. Berg, Two-Dimensional Anisotropy Measurements Show Local Heterogeneity in a Polymer Melt, J. Phys. Chem. Lett. 5, 2608 (2014). K. Sahu, H. Wu, and M. A. Berg, Rate Dispersion in the Biexciton Decay of CdSe/ZnS Nanoparticles from Multiple Population-Period Transient Spectroscopy, J. Am. Chem. Soc. 135, 1002 (2013). JACS Spotlight: Biexciton Decay Measurements Challenge Assumptions. M. A. Berg, Multidimensional Incoherent Time-Resolved Spectroscopy and Complex Kinetics, Adv. Chem. Phys. 150, 1 (2012). K. Sahu, S. J. Kern, and M. A. Berg, Heterogeneity of Reaction Rates in an Ionic Liquid: Quantitative Results from 2D-MUPPETS, J. Phys. Chem. A 115, 7984 (2011). S. J. Kern, K. Sahu, and M. A. Berg, Heterogeneity of the Electron-Trapping Kinetics in CdSe Nanoparticles, Nano Lett. 11, 3498 (2011). M. A. Berg, Hilbert-Space Treatment of Incoherent, Time-Resolved Spectroscopy. I. Formalism, a Tensorial Classification of High-Order Orientational Gratings and Generalized MUPPETS ""Echoes"", J. Chem. Phys. 132, 144105 (2010); II. Pathway Description of Optical Multiple Population-Period Transient Spectroscopy, ibid., 144106 (2010). S. Sen, D. Andreatta, S. Y. Ponomarev, D. L. Beveridge, and M. A. Berg, Dynamics of Water and Ions Near DNA: Comparison of Simulation to Time-Resolved Stokes-Shift Experiments, J. Am. Chem. Soc. 131, 1724 (2009). M. A. Berg, R. S. Coleman, and C. J. Murphy, Nanoscale Structure and Dynamics of DNA, Phys. Chem. Chem. Phys. 10, 1229 (2008). E. van Veldhoven, C. Khurmi, X. Zhang, and M. A. Berg, Time-Resolved Optical Spectroscopy with Multiple Population Dimensions: A General Method of Resolving Dynamic Heterogeneity, ChemPhysChem 8, 1761 (2007). S. Nath, D. C. Urbanek, S. J. Kern, and M. A. Berg, High-Resolution Raman Spectra with Femtosecond Pulses: An Example of Combined Time- and Frequency-Domain Spectroscopy, Phys. Rev. Lett. 97, 267401 (2006).