Jeffrey L. Krause

Coherent electron transport through an azobenzene molecule: A light-driven molecular switch

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.

Optical control of molecular dynamics: Molecular cannons, reflectrons, and wave‐packet focusers

We consider the control of molecular dynamics using tailored light fields, based on a phase space theory of control [Y. J. Yan et al., J. Phys. Chem. 97, 2320 (1993)]. This theory enables us to calculate, in the weak field (one‐photon) limit, the globally optimal light field that produces the best overlap for a given phase space target. We present as an illustrative example the use of quantum control to overcome the natural tendency of quantum wave packets to delocalize on excited state potential energy curves. Three cases are studied: (i) a ‘‘molecular cannon’’ in which we focus an outgoing continuum wave packet of I2 in both position and momentum, (ii) a ‘‘reflectron’’ in which we focus an incoming bound wave packet of I2, and (iii) the focusing of a bound wave packet of Na2 at a turning point on the excited state potential using multiple light pulses to create a localized wave packet with zero momentum. For each case, we compute the globally optimal light field and also how well the wave packet produce...

Spectral quantization of transition state dynamics for the three-dimensional H+H2 reaction

Applying a spectral quantization method, we find the positions and widths of 32 transition state resonances in the three‐dimensional reaction H+H2 with J=0. The assignment of many of the resonances appears to follow asymmetric stretch and bend progressions for a linear triatomic molecule.

Dynamics of short-pulse excitation, ionization and harmonic conversion

In recent years there have been very significant advances in short pulse, high intensity laser technology. Lasers with pulse lengths of 0.1–1 ps and wavelengths from 0.2–1 μm can be focused to produce intensities from 1012 to above 1018 W/cm2. One major use of these systems has been for studies of the response of atoms and molecules to such intense, well characterized electromagnetic fields. Because these pulses are very short, neutral atoms can survive to experience intensities where theoretical treatments based on the traditional perturbation expansion of the wave function in terms of the field-free states will fail completely to describe the dynamics of the system. An explicit, non-perturbative time-dependent calculation is one approach which can represent these strong field effects.

The dynamics of proton transfer in H5O2

We perform high-level, quantum molecular dynamics simulations of proton transfer in the protonated water dimer, H5O2+. The electronic structure of the system is calculated concurrently with the nuclear motion using Born–Oppenheimer molecular dynamics plus density functional theory. Performing the calculations at finite (thermal) temperatures allows us to observe and quantify such effects as the broadening of the electronic density of states, the thermal splitting of degenerate states, the shift of the highest occupied molecular orbital, the thermal expansion of the dipole moment, and the thermal shift, coupling and broadening of the vibrational density of states. At two of the temperatures considered (225 K and 360 K), we find that H5O2+ exists in a dynamical equilibrium state in which the proton oscillates between two water molecules. The characteristic frequencies of the proton motion are very sensitive to temperature. At 40 K and 225 K, strong peaks are identified in the vibrational spectrum correspond...

Theoretical study of the interaction of molecular oxygen with a reduced TiO2 surface

Abstract We investigate the re-oxidation of the reduced TiO2 (1 1 0) surface by adsorption of molecular oxygen using quantum-chemical, ab-initio periodic Hartree–Fock calculations. The absorption sites considered are anion defects and cation Ti(5f) positions. The results show evidence of strong charge transfer from the defect surface to the O2 molecule. In agreement with experiment, we find that the most stable species is one in which molecular oxygen adsorbs at the defect sites as O2−. Adsorption of three O2 molecules per vacancy site is thermodynamically favored compared to one or two adsorbed molecules.

Adaptive Control of Pulse Phase In A Chirped Pulse Amplifier

Using experimental feedback, we demonstrate that a chirped-pulse amplifier can adaptively learn to compensate for the higher-order phase dispersion that is inherent in the amplification process. A genetic algorithm-based search routine is used to repetitively update the pulse phase in a programmable pulse stretcher during a plasma breakdown experiment to maximize the magnitude of spectral blueshift. Reductions in pulse duration from 37 to 30 fs and substantially better wing structure are typically obtained as a result of the optimization.

Quantum control of multidimensional systems: Implementation within the time‐dependent Hartree approximation

The exact formulation of quantum control is now well known and sufficiently general to describe multidimensional quantum systems. The implementation of this formalism relies on the solution of the time‐dependent Schrodinger equation (TDSE) of the system under study, and thus far has been limited for computational reasons to simple quantum systems of very small dimensionality. To study quantum control in larger systems, such as polyatomic molecules and condensed phases, we explore an implementation of the control formalism in which the TDSE is solved approximately using the time‐dependent Hartree (TDH) approximation. We demonstrate formally that the TDH approximation greatly simplifies the implementation of control in the weak response regime for multidimensional systems. We also present numerical examples to show that the TDH approximation for the weak response case is sufficiently accurate to predict the laser fields that best drive a quantum system to a desired goal at a desired time, in systems contain...

Periodic Hartree–Fock study of the adsorption of molecular oxygen on a reduced TiO2 (110) surface

We present a theoretical analysis of O2 adsorption on a reduced TiO2 (110) rutile surface, based on periodic ab initio Hartree–Fock calculations. Three different orientational approaches, three different spin symmetries, and two different adsorption sites are considered. We also consider the possibility that the surface can absorb more than one oxygen molecule. Positions of the surface ions, oxygen ions belonging to the third and fourth layers of the slab, and the bond lengths of the O2 and O2-substrate distances are optimized. Adsorption energies, admolecule-substrate bond lengths, spin densities and Mulliken charges are analyzed. The model is tested by comparing spin densities and relaxation parameters obtained for the reduced TiO2 (110) system to previous theoretical results. Finally, we discuss the relationship of our results to experimental observations of thermal desorption rates at low temperatures (100–600 K).

Transferring vibrational population between electronic states of diatomic molecules via light-induced-potential shaping

We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials (LIPs). Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses (intuitive or counter-intuitive) and on the sign of the detuning (below or above the first transition) four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of ...