Stanley M. Smith

A time-dependent Hartree–Fock approach for studying the electronic optical response of molecules in intense fields

For molecules in high intensity oscillating electric fields, the time-dependent Hartree-Fock (TDHF) method is used to simulate the behavior of the electronic density prior to ionization. Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density. A unitary transform approach is combined with the modified midpoint method to provide a stable and efficient algorithm to integrate these equations. The behavior of H2+ in an intense oscillating field computed using the TDHF method with a STO-3G basis set reproduces the analytic solution for the two-state coherent excitation model. For H2 with a 6-311++G(d,p) basis set, the TDHF results are nearly indistinguishable from calculations using the full time-dependent Schrödinger equation. In an oscillating field of 3.17 x 10(13) W cm(-2) and 456 nm, the molecular orbital energies, electron populations, and atomic charges of H2 follow the field adiabatically. As the field intensity is increased, the response becomes more complicated as a result of contributions from excited states. Simulations of N2 show even greater complexity, yet the average charge still follows the field adiabatically.

Electronic optical response of molecules in intense fields: Comparison of TD-HF, TD-CIS, and TD-CIS(D) approaches

Time-dependent Hartree-Fock (TD-HF) and time-dependent configuration interaction (TD-CI) methods with Gaussian basis sets have been compared in modeling the response of hydrogen molecule, butadiene, and hexatriene exposed to very short, intense laser pulses (760 nm, 3 cycles). After the electric field of the pulse returns to zero, the molecular dipole continues to oscillate due to the coherent superposition of excited states resulting from the nonadiabatic excitation caused by the pulse. The Fourier transform of this residual dipole gives a measure of the nonadiabatic excitation. For low fields, only the lowest excited states are populated, and TD-CI simulations using singly excited states with and without perturbative corrections for double excitations [TD-CIS(D) and TD-CIS, respectively] are generally in good agreement with the TD-HF simulations. At higher field strengths, higher states are populated and the methods begin to differ significantly if the coefficients of the excited states become larger than approximately 0.1. The response of individual excited states does not grow linearly with intensity because of excited state to excited state transitions. Beyond a threshold in the field strength, there is a rapid increase in the population of many higher excited states, possibly signaling an approach to ionization. However, without continuum functions, the present TD-HF and TD-CI calculations cannot model ionization directly. The TD-HF and TD-CIS simulations are in good accord because the excitation energies obtained by linear response TD-HF [also known as random phase approximation (RPA)] agree very well with those obtained from singly excited configuration interaction (CIS) calculations. Because CIS excitation energies with the perturbative doubles corrections [CIS(D)] are on average lower than the CIS excitation energies, the TD-CIS(D) response is generally stronger than TD-CIS.

Time-Dependent Density Functional Theory Calculations of Ehrenfest Dynamics of Laser Controlled Dissociation of NO + : Pulse Length and Sequential Multiple Single-Photon Processes

Intense laser field controlled dissociation reactions of the nitric oxide cation (NO(+)) are studied by ab initio Ehrenfest dynamics with time-dependent density functional theory. Intense electric fields with five different pulse lengths are compared, combined with potential energy surface and density of state analysis, to reveal the effect of pulse length on the control mechanism. Controllable dissociative charge states are observed, and the correlation between the laser pulse length and the probability of sequential multiple single-photon processes is presented. This work introduces a concept of using laser pulse length to control the sequential multiple single-photon process.

Numerical Bound State Electron Dynamics of Carbon Dioxide in the Strong-Field Regime

Time-dependent Hartree-Fock simulations for a linear triatomic molecule (CO(2)) interacting with a short IR (1.63 eV) three-cycle pulse reveal that the carrier-envelope shape and phase are the essential field parameters determining the bound state electron dynamics during and after the laser-molecule interaction. Analysis of the induced dipole oscillation reveals that the envelope shape (Gaussian or trapezoidal) controls the excited state population distribution. Varying the carrier envelope phase for each of the two pulse envelope shapes considerably changes the excited state populations. Increasing the electric field amplitude alters the relative populations of the excited states, generally exciting higher states. A windowed Fourier transform analysis of the dipole evolution during the laser pulse reveals the dynamics of state excitation and in particular state coupling as the laser intensity increases.

Demonstration of Tomographic Imaging of Chemical Species Using THz Time-Domain Absorption Spectroscopy

A technique has been developed to image chemical species using THz time-domain absorption spectroscopy. Preliminary demonstration on a jet flow of steam is reported.

Dimensionality reduction in nonlinear optical datasets via diffusion mapping: case study of short-pulse second harmonic generation

We have studied the application of the diffusion mapping technique to dimensionality reduction and clustering in multidimensional optical datasets. The combinational (input-output) data were obtained by sampling search spaces related to optimization of a nonlinear physical process, short-pulse second harmonic generation. The diffusion mapping technique hierarchically reduces the dimensionality of the data set and unifies the statistics of input (the pulse shape) and output (the integral output intensity) parameters. The information content of the emerging clustered pattern can be optimized by modifying the parameters of the mapping procedure. The low-dimensional pattern captures essential features of the nonlinear process, based on a finite sampling set. In particular, the apparently parabolic two-dimensional projection of this pattern exhibits regular evolution with the increase of higher-intensity data in the sampling set. The basic shape of the pattern and the evolution are relatively insensitive to the size of the sampling set, as well as to the details of the mapping procedure. Moreover, the experimental data sets and the sets produced numerically on the basis of a theoretical model are mapped into patterns of remarkable similarity (as quantified by the similarity of the related quadratic-form coefficients). The diffusion mapping method is robust and capable of predicting higher-intensity points from a set of low-intensity points. With these attractive features, diffusion mapping stands poised to become a helpful statistical tool for preprocessing analysis of vast and multidimensional combinational optical datasets.