K. C. Kulander

Photodissociation of triatomic molecules: Application of the R‐matrix propagation methods to the calculation of bound‐free Franck–Condon factors

We present a method for calculating Franck–Condon factors for the photodissociation of triatomic molecules. We use the R‐matrix propagation techniques developed by Light and Walker for reactive scattering processes. The method is used to study the collinear dissociation of a model CO2 system. Franck–Condon overlaps are calculated for three different initial states and probabilities for populating particular vibrational levels of the CO fragments are obtained. We present results for a range of photon energies. We find that the partial cross sections exhibit a series of resonant or Fano line shapes. The structure in the cross sections depends sensitively on the initial vibrational state, but the peak values are not significantly effected.

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.

Spectra in the chaotic region: Methods for extracting dynamic information

Nonlinear dynamics is applied to chaotic unassignable atomic and molecular spectra with the aim of extracting detailed information about regular dynamic motions that exist over short intervals of time. It is shown how this motion can be extracted from high resolution spectra by doing low resolution studies or by Fourier transforming limited regions of the spectrum. These motions mimic those of periodic orbits (PO) and are inserts into the dominant chaotic motion. Considering these inserts and the PO as a dynamically decoupled region of space, resonant scattering theory and stabilization methods enable us to compute ladders of resonant states which interact with the chaotic quasicontinuum computed in principle from basis sets placed off the PO. The interaction of the resonances with the quasicontinuum explains the low resolution spectra seen in such experiments. It also allows one to associate low resolution features with a particular PO. The motion on the PO thereby supplies the molecular movements whose ...

Quasiclassical trajectory calculations and quantal wave packet calculations for vibrational energy transfer at energies above the dissociation threshold

Quantal wave packet calculations and quasiclassical trajectory calculations are reported for vibrational energy transfer and dissociation in collinear atom–diatom collisions. The system considered has the masses of H+H2 and is modelled with an extended LEPS potential energy surface. The conditions considered are initial vibrational states n1=0,1, and 4 and initial relative translational energies up to 12 eV for the wave packet calculations and up to 13 eV for the trajectory calculations. This is higher in energy than previous comparisons of quantal and trajectory calculations. The quantal transition probabilities show higher thresholds than the trajectory ones, and then they oscillate about the trajectory results. The first and second moments of the final vibrational action are similar for both kinds of calculation.

Time‐dependent calculations of molecular photodissociation resonances

The molecular photodissociation dynamics of a model, collinear CO2 system is investigated using the time‐dependent wave packet method. Resonance structures in the absorption line shape, found previously in time‐independent studies, are correlated to particular oscillatory motions in the dissociating wave packet dynamics. Dramatic changes in the absorption line shape are predicted for this system for short pulse excitation due to the removal of the effects of one class of resonances. Three different methods of solving the time‐dependent Schrodinger equation were tested and the optimal scheme was used in these calculations.

Intensity-dependent phase-matching effects in harmonic generation

In high-order harmonic generation by an intense laser, intrinsic phases can develop at the atomic level between the laser field and the individual emitted harmonics. Because intrinsic phases can vary rapidly with the laser intensity, they can strongly influence phase matching to the extent that the laser intensity varies within the generating medium. Previously reported measurements of broad far-field harmonic emission patterns as well as measured asymmetries in the emission with respect to the axial positioning of the medium in the focus can be explained by intrinsic phases. An experimental method for further study of intrinsic phases is proposed that involves harmonic generation in two counterpropagating laser beams. The periodic intensity modulation created by the two beams coupled with the intensity-dependent intrinsic phases allows harmonic light to propagate in directions with otherwise extremely poor phase-matching conditions.

Wave packet studies of predissociation in H3 Rydberg states

The predissociation of H3 Rydberg states by the two lowest electronic states of H3 is studied using time‐dependent wave packet techniques. The final vibrational state distribution function and branching ratios between two‐ and three‐body channels are calculated within a two‐dimensional approximation. The effect of initial vibrational excitation on these distributions is studied, and comparison is made to recent experiments.

Local and hyperspherical mode approximations to resonances in collinear atom–diatom reactions

A collinear, symmetric reactive scattering system which exhibits a highly oscillatory reaction probability as a function of collision energy has been studied. The A+BA model consists of two coupled Morse oscillators. The peaks in the reaction probability are due to resonances which have been analyzed using local and hyperspherical mode bound state expansions. In a small range of collision energy, resonances are found some of which are local and some hyperspherical in character. Resonance energies calculated using bound state techniques are compared to those from exact quantum mechanical scattering calculations.

Quantum‐mechanical calculations of the dissociation of H3 Rydberg states

We present three‐dimensional, time‐dependent quantum‐mechanical calculations of the dynamics of the dissociation of H3 Rydberg states at total energies up to 6 eV. The method used in this work employs a Chebychev propagator in time, and computes the kinetic‐energy operators in the discrete variable representation. We calculate the total dissociation cross section, as well as partial vibrational and rotational cross sections, and compare our results to previous two‐dimensional calculations and to experiment. The results display clear three‐dimensional effects, and indicate the importance of including both sheets of the H3 ground potential‐energy surface in the dynamics.

Spectra in the chaotic region: A quantum analysis of the photodissociation of H+3

A quantum theory of periodic orbit based resonances is presented and applied to the photodissociation of highly excited H+3. Ab initio stabilization computations are performed to show that periodic orbits are the origin of stable roots producing scars along the orbits in the system’s wave functions. Spacings and widths of the resonances are in satisfactory agreement with the experiment and verify the mechanism proposed by Gomez and Pollak. The validity and utility of the PO based resonance theory to study the dynamics of highly excited systems is demonstrated.A quantum theory of periodic orbit based resonances is presented and applied to the photodissociation of highly excited H+3. Ab initio stabilization computations are performed to show that periodic orbits are the origin of stable roots producing scars along the orbits in the system’s wave functions. Spacings and widths of the resonances are in satisfactory agreement with the experiment and verify the mechanism proposed by Gomez and Pollak. The validity and utility of the PO based resonance theory to study the dynamics of highly excited systems is demonstrated.