Theoretical/computational studies of molecular photochemistry and laser control
The interaction of light (naturally occurring or from lasers) with molecules can induce a variety of dynamical processes including electronic excitation, structural change, dissociation, and vibration. Recently, it has become possible to tailor laser fields to actively manipulate these dynamical processes. We look to develop and utilize both electronic structure and dynamical methods to understand molecular photochemistry and to manipulate molecular processes.
Optimal control theory for laser control:
The use of tailored laser pulses to control chemical processes has received much attention recently, in part due to the rapid development of experimental pulse shaping techniques. We are using optimal control theory (OCT) to determine the tailored laser pulses needed to achieve control. In order to treat laser control for polyatomic molecules with more than a few (3-6) degrees of freedom, we are developing the use of the Multi-Configurational-Time-Dependent-Hartree (MCTDH) method for solving the time-dependent Schrödinger equation of the involved states in combination with OCT. In related work, we are examining the use of genetic algorithms to determine the optimal laser fields for control. The initial focus is on the study of molecular quantum computing where vibrational states represent the quantum bits (qubits) and tailored laser fields are used for implementing quantum gate operations.
Figure 1. XFROG of a control pulse showing frequency components as a function of time
Molecular photodissociation dynamics:
Molecular photofragmentation often involves multiple electronically excited states and non-adiabatic transitions between these states may occur as the molecule dissociates. Our understanding of the dissociation dynamics requires both high-level theoretical calculations and the measurement of a variety of observables, particularly angular momentum distributions. We are interested in determining the complete angular momentum distributions and vector correlation coefficients (alignment and orientation) for atomic fragments resulting from molecular photofragmentation. We are also interested in developing general purpose direct dynamics (ab initio molecular dynamics) software for the study of molecular photodissociation/photochemistry of polyatomic molecules for which quantum dynamics calculations are not feasible.
Photochemistry of fluorescent proteins:
Fluorescent proteins derived from Aequorea victoria jellyfish green fluorescent protein (GFP) are widely used in cell and molecular biology as fluorescent labels and reporter molecules. Protein engineering allows creating a number of fluorescent proteins with various photophysical properties by changing the structure of the chromophore and/or protein environment around the chromophore. We are interested in computational modeling of the photochemical processes in fluorescent proteins to gain mechanistic insights necessary to improve and extend their utility in molecular biology.
R.R. Zaari and A. Brown, "Effect of diatomic molecular properties on binary laser pulse optimizations of quantum gate operations," J. Chem. Phys., 135 044317 (2011) (7 pages).
S.Y.Y. Wong, D. Benoit, M. Lewerenz, A. Brown and P.-N. Roy, "Determination of molecular vibrational energies using the ab initio semiclassical initial value representation: Application to formaldehyde," J. Chem. Phys., 134 094110 (2011) (10 pages).
R.R. Zaari and A. Brown, "Quantum gate operations using mid-infrared binary shaped pulses on the rovibrational states of carbon monoxide," J. Chem. Phys., 132 014307 (2010) (9 pages).
M. Schroeder and A. Brown, "Generalized filtering of fields in optimal control theory: Application to symmetry filtering for quantum gate operations," New J. Phys. 11 105031 (2009). (13 pages).
M. Schroeder and A. Brown, "Realization of the CNOT quantum gate operation in 6D ammonia using the OCT-MCTDH approach," J. Chem. Phys., 131, 034101 (2009). (10 pages)
J. A. Key, S. Koh, Q. K. Timerghazin, A. Brown, and C.W. Cairo, "Photophysical characterization of triazole substituted coumarin fluorophores," Dyes and Pigments, 82, 196-203 (2009).
Q. K. Timerghazin, H.J. Carlson, C. Liang, R.E. Campbell, and A. Brown, Computational prediction of absorbance maxima for a structurally diverse series of engineered green fluorescent protein chromophores, J. Phys. Chem. B, 112, 2533-2541 (2008).