Frontiers in Chemical Physics & Analysis
Rigoberto Hernandez
"Shake, rattle and rip: The far-from-equilibrium dynamics of colloids"
School of Chemistry & Biochemistry
Georgia Institute of Technology
Friday, November 20th, 2009
EMSL/Auditorium
1:30PM
Colloidal dispersions have been useful for many applications particularly because of the balance of microscopic heterogeneity and the wealth of tunable properties they exhibit at macroscopic length scales. One possible additional tunable parameter to control structure and function of colloidal formulations would arise from the effects of dynamic (or time-dependent) microscopic structure changes. We will discuss on-going theoretical and computational work which suggests that such nonequilibrium conditions can indeed give rise to novel molecular motion within colloidal suspensions. Specifically, suspensions of driven colloids—with a shape that changes in size or orientation—have been described using molecular dynamics simulations, coarse-grained particle dynamics simulations, and stochastic models. The nonequilibrium dynamics within these driven colloidal suspensions are not presently well understood because they involve several disparate time and length scales, but are increasingly being explored experimentally using optical microscopy of various nanoparticle suspensions. Examples of such materials include core-shell hydrogels and lyotropic liquids with magnetically alignable mesogens.
We have shown that in some cases the motion (such diffusion or molecular reorganization) arises as the projection of a simple model of a chemical system bilinearly coupled to a harmonic bath with a time-dependent coupling. Moreover, the stochastic model can be used to surmise the diffusion of a tagged particle in a colloidal suspension which swells or shrinks with time. The nonequilibrium stochastic model has also allowed us to define an equilibrium temperature for a particle coupled to multiple time-dependent unequal-temperature baths. It has been verified using a particle-based model. The structure of these materials also plays an important role in the correlations of the dynamics of the probe even when the structure is static as in the so-called Lorentz models. Recent results of the dynamics both within static sheared fluids will also be presented.
Stephen Leone
X-Ray Probing of Atomic and Molecular Dynamics in the Attosecond Limit
University of California, Berkeley
Lawrence Berkeley National Laboratory
Monday, November 9, 2009
EMSL/Auditorium
2:00PM
Pulses of soft x-rays produced by high harmonic generation of intense laser pulses are used to probe atomic and molecular processes through core level spectroscopy, revealing ionization, alignment, bond breaking, and intramolecular dynamics. The high harmonic process is also a key to create isolated attosecond pulses to study the timescales of electronic dynamics. With a new method of ultrafast transient absorption, the high harmonics are spectrally resolved after the sample to reveal the spectral transitions at various short delay times, into the attosecond limit. The methods are applied to alignment dynamics in high field ionization, molecular dissociative ionization, and wave packet dynamics among vibrational and electronic states.
Todd Martinez
Dr. Todd Martinez
Friday, October 30, 2009
EMSL/Auditorium
2:30PM
Photochemistry and Mechanochemistry from First Principles Dynamics
Dr. Rich Saykally
Select Adsorption of Ions to Aqueous Interfaces
College of Chemistry
University of California, Berkeley
Wednesday, May 13, 2009
EMSL/Auditorium
10:00AM
Dr. David Chandler
Combustion Research Facility, Sandia National Laboratories, Livermore
The Quest for Ultracold Molecules
Dr. David Chandler
Friday, March 20, 2009
EMSL Auditorium
11:00AM
The cooling of atoms to ultracold temperatures, where the quantum nature of the gas dominate their interactions, has resulted in spectacular discoveries, such as the realization and study of new states of matter like Bose Einstein Condensates, degenerate Fermi gasses, and Bardeen, Cooper Schrieffer fluids. In addition to forming new states of matter, cooling atoms to microKelvin temperatures, and below, has enabled ultra-high resolution spectroscopy studies and the extraction of information about the collisional dynamics of atoms and their interactions.
All of these areas of research have molecular analogs. The added complexity found in molecules offers the possibility of rich areas of investigation in spectroscopy, collision dynamics, etc. However, the field of ultracold molecule studies had remained generally unexplored due to the complexity of making ultracold samples.
In this seminar, Dr. Chandler will discuss the state of the field for making samples of ultracold molecules and show the recent progress in the technique of Kinematic Cooling. This technique uses a single collision with an appropriate atom to remove the translational motion from a molecule, thereby cooling it. Connection to other areas of physical chemistry, such as the near threshold photodissociation of van der Waals molecules, will also be discussed.
An interview with Dr. Carl Lineberger, University of Colorado, speaks to the role of chemical physics in society. Dr. Lineberger asserts that chemical physics was the discipline that first provided a molecular basis for all kinds of physical properties. He also promotes the use of interdisciplinary teams to solve complex problems.
W. Carl Lineberger
Prof. Dr. W. Carl Lineberger University of Colorado
Time resolved photoelectron spectroscopy of partially solvated anions
Friday, January 9, 2009
EMSL Auditorium
1:00PM
Ultrafast pump-probe studies of recombination in partially solvated, size-selected
dihalide cluster anions show long time coherent motions and the resulting non-statistical energy
flow in the cluster. For photodissociated I2 -(CO2)n, we observe a new type of recombination: a solvent asymmetry-driven energy transfer process without a condensed phase counterpart.
Very short recombination times are observed (~10 ps) with the chromophore only partially
solvated, and the recombination time steadily decreases with additional solvation. Theoretical
models point to the central role of the solvent electric field in the recombination process, but
suggest electron transfer processes that cannot be tested with a homonuclear dihalide
chromophore. To further test these concepts, we investigate the time-resolved recombination of
photodissociated IBr -(CO2)n clusters following excitation to the dissociative IBr - A´2II1/2 state of
the chromophore. In complete contrast to previous studies involving solvated I2 -, the observed
recombination times for IBr -(CO2)n increase dramatically with increasing cluster size. The basis
for this dramatic difference gives increased credence to the utility of a "solvent coordinate"
description of geminate recombination. Preliminary experiments utilizing time-resolved
photoelectron spectroscopy of the photodissociated anion show directly the solvent-driven
electron transfer, and permit the development of dynamical models that show the role of the
solvent in assisting long-range electron transfer.