Numerical simulation of nonadiabatic electron excitation in the strong-field regime. 3. Polyacene neutrals and cations.

The electron optical response for a series of linear polyacenes and their molecular ions (mono and dications) in strong laser fields was studied using time-dependent Hartree-Fock theory. The interactions of benzene, naphthalene, anthracene, and tetracene with pulsed fields at a frequency of 1.55 eV and intensities of 8.77 x 10(13), 3.07 x 10(13), 1.23 x 10(13), and 2.75 x 10(12) W/cm2, respectively, were calculated using the 6-31G(d,p) basis set. Nonadiabatic processes, including nonadiabatic time evolution of the dipole moment, Löwden charges, and occupation numbers, were studied. The nonadiabatic response increased with the length of the molecule and was greatest for the molecular monocations. The only exception was tetracene, in which the very strong response of the dication was due to a near resonance with the applied field. The intensity and frequency dependence of the dipole moment response for the monocations of naphthalene and anthracene was also calculated. As the intensity increased, the population of higher-energy excited-states increased, and as the frequency increased, the excitation volume increased in good agreement with the Dykhne approximation.

Rapid proton transfer mediated by a strong laser field.

Kinetic energy distributions of ejected from a polyatomic molecule, anthraquinone, subjected to 60 fs, 800 nm laser pulses of intensity between 0.2 and 4.0 x 10(14) W x cm(-2), reveal field-driven restructuring of the molecule prior to Coulomb explosion. Calculations demonstrate fast intramolecular proton migration into a field-dressed metastable potential energy minimum. The proton migration occurs in the direction perpendicular to the polarization of the laser field. Rapid field-mediated isomerization is an important new phenomenon in coupling of polyatomic molecules with intense lasers.

A numerical simulation of nonadiabatic electron excitation in the strong field regime: Linear polyenes.

Time-dependent Hartree-Fock theory has been used to study of the electronic optical response of a series of linear polyenes in strong laser fields. Ethylene, butadiene, and hexatriene have been calculated with 6-31G(d,p) in the presence of a field corresponding to 8.75 x 10(13) W/cm2 and 760 nm. Time evolution of the electron population indicates not only the pi electrons, but also lower lying valence electrons are involved in electronic response. When the field is aligned with the long axis of the molecule, Löwdin population analysis shows large charges at each end of the molecule. For ethylene, the instantaneous dipole moment followed the field adiabatically, but for hexatriene, nonadiabatic effects were very pronounced. For constant intensity, the nonadiabatic effects in the charge distribution, instantaneous dipole, and orbital populations increased nonlinearly with the length of the polyene. These calculations elucidate the mechanism of the strong field nonadiabatic electron excitation of polyatomic molecules leading to their eventual ionization and fragmentation. The described computational methods are a viable tool for studying the complex processes in multielectron atomic and molecular systems in strong laser fields.

Numerical simulation of nonadiabatic electron excitation in the strong field regime. 2. Linear polyene cations.

Time-dependent Hartree-Fock theory has been used to study the electronic optical response of a series of linear polyene cations (+1 and +2) in strong laser fields. The interaction of ethylene, butadiene, and hexatriene, with pulsed and CW fields corresponding to 8.75 x 10(13) W/cm(2) and 760 nm, have been calculated using the 6-31G(d,p) basis set. Nonadiabatic processes including nonlinear response of the dipole moment to the field and non-resonant energy deposition into excited states were more pronounced for the monocations in comparison with dications. For a given charge state and geometry, the nonadiabatic effects in the charge distribution and instantaneous dipole increased with the length of the polyene. For pulsed fields, the instantaneous dipole continued to oscillate after the field returned to zero and corresponded to a non-resonant electronic excitation involving primarily the lowest electronic transition. For a given molecule and fixed charge state, the degree of nonadiabatic coupling and excitation was greater for geometries with lower excitation energies.

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.

Coulomb explosion of large polyatomic molecules assisted by nonadiabatic charge localization.

The electron-nuclear dynamics of the Coulomb explosion of a large polyatomic molecule, anthracene, is probed using kinetic energy distributions of produced H+ ions. The kinetic energy release of ejected protons exceeds 30 eV for anthracene exposed to 10(14) W cm(-2), 800 nm pulses of 60 fs duration. We propose a strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the charge localization lasts many laser periods and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of the H+ kinetic energy on the laser intensity. Dissociative ionization of a polyatomic molecule enabled by long-lived charge localization is a new type of electron-nuclear dynamics and is essential for understanding the pathways of molecular or ionic fragmentation in strong fields.