2005 University of South Florida, Ph.D. in Chemistry, with Professor Brian Space
2000 University of South Florida, B.A. in Chemistry
Liquid interfaces are ubiquitous and important in chemistry and the environment; aqueous interfaces are especially important. Thus, with the advent of interface-specific nonlinear optical spectroscopies, most importantly sum-frequency generation spectroscopy (SFG), such interfaces have been intensely studied, both theoretically and experimentally. SFG spectroscopy is a powerful experimental method for probing the structure and dynamics of interfaces. The measurement produces a signal that is roughly analogous to a traditional infrared lineshape but differs in that only molecules that reside at the interface contribute to the measurement. The resulting lineshapes are also more complicated than traditional infrared measurements and, in principle, contain detailed information about the interfacial environment. This complexity makes the ability to theoretically reproduce and interpret the spectral signature critical to exploiting the promise of the spectroscopy.
My research focuses on theoretically modeling fundamentally and technologically important interfaces. The proposed studies build on my previous work in this area that represented one of the first successful attempts at theoretically describing SFG signals from liquid interfaces. First, classical molecular dynamics (MD) methods (solving Newton's equations for the molecular system) are used to describe the dynamics of an interface (a snapshot of an MD model of the air/water interface is shown in Figure 1 along with a cartoon of the SFG experimental setup). Two complementary theoretical approaches - time correlation function (TCF) and instantaneous normal mode (INM) methods - then use the MD trajectories as input to both calculate the SFG spectrum and to ascertain the molecular origin of the SFG signal. The goal is to directly link the experimental spectroscopy to the underlying structure and dynamics of the interfaces.
Our approach is to compare TCF spectra with experiment to establish our molecular dynamics (MD) methods can reliably describe the system of interest. We then employ INM methods to analyze the molecular and dynamical basis for the observed spectroscopy. We have been able to elucidate, on a molecularly detailed basis, a number of interfacial line shapes, most notably the origin of several intermolecular modes in the SFG spectra for the water/vapor interface.
We find the resonance at 875 cm-1 is due to a wagging mode localized on a single water molecule and is highlighted in green in Figure 2. We also uncovered the origin of other intermolecular modes at 95 cm-1 for the SSP and PPP spectra, and at 225 cm-1 for the SPS spectra. These resonances are due to hindered translations perpendicular to the interface for the SSP and PPP spectra (highlighted in yellow), and translations parallel to the interface for the SPS spectra (highlighted in black).
The SSP, PPP, and SPS are various polarization conditions for SFG spectroscopy. These species represent well-defined populations of water molecules at aqueous interfaces previously unknown. The well-known free O-H mode is highlighted in blue in Figure 2. Both approaches lead to signals in excellent agreement with experimental measurements. Our work demonstrates how TCF and INM methods can be used in a complementary fashion to describe interfacial vibrational spectroscopy. The success of both approaches suggests that theory can play a crucial role in interpreting SFG spectroscopy at more complex interfaces.
I would like to proceed to examine, along with undergraduate students, surfactants at environmentally important interfaces. Surfactants are used to modify chemical and physical properties of interfaces. When surfactants adsorb at a water surface the surface properties are significantly altered. An important question that has been asked is how the surfactant alters the hydrogen bonding of water at these surfaces. Experimental studies of surfactants at interfaces can be greatly aided by theoretical studies. The experimental literature contains many hypotheses as to the structural implications of the data, but most remain unconfirmed and theoretical investigations will serve to resolve this uncertainty.
Theoretical Modeling of Interface Specific Vibrational Spectroscopy: Methods and Applications to Aqueous Interfaces by Angela Perry, Christine Neipert, Preston Moore and Brian Space, Chem. Rev. 106 (4) 1234-1258 (2006)
A Theoretical Description of the Polarization Dependence of the Sum Frequency Generation Spectroscopy of the Water/Vapor Interface, by Angela Perry, Christine Neipert, Christina Kasprzyk Ridley, Tony Green, Brian Space and Preston Moore, J. Chem. Phys. 123 144705-144716 (2005)
Identification of a Wagging Vibrational Mode of Water Molecules at the Water/Vapor Interface by Angela Perry, Christine Neipert, Christina Ridley, Preston Moore and Brian Space, Phys. Rev. E. 71 050601(1) - 050601(4) (2005)
A Time Correlation Function Theory of Two Dimensional Infrared Spectroscopy with Applications to Liquid Water, by Russell DeVane, Christina Ridley, Angela Perry, Christine Neipert, T. Keyes and Brian Space, J. Chem. Phys. 121 3688-3701 (2004)
A Combined Time Correlation Function and Instantaneous Normal Mode Study of the Sum Frequency Generation Spectroscopy of the Water/Vapor Interface, by Angela Perry, Heather Ahlborn, Brian Space, and Preston Moore, J. Chem. Phys. 118 8411-8419 (2003)