We present a first-principles study of ionization and high-order harmonic generation by benzene aligned in the polarization plane of a short circularly polarized laser pulse. Time-dependent density-functional theory within the adiabatic local-density approximation is employed to describe the 30 valence-electron dynamics in three dimensions. The multielectron approach enables us to study the effect of very strong laser fields, 10 14 -10 15 W cm Ϫ2 , where multiple ionization and high-order harmonic generation interplay. Large ionization currents are formed, causing ionization of 1-4 electron charges, while strong high-order harmonic generation is observed. The well-known recollision mechanism of high-order harmonic generation plays a part for moderate laser intensities but is fully suppressed for strong laser fields. The harmonic generation spectra are characterized by two distinguishable plateaus, where the structure of the first plateau is dominated by the 6k Ϯ1 (kϭ0,1,...) selection rule. The number of harmonics in the second plateau is insensitive to the duration of the pulse. The peaks appear in pairs or in threesomes, depending on the pulse duration.

Highly correlated internally contracted multireference configuration interaction wave functions are used to calculate the potential energy and spin-orbit coupling functions for the lowest electronic states of CO2+ dication. Using these functions, the positions and lifetimes of the corresponding vibronic states are evaluated by means of log-phase-amplitude, stabilization, and complex-scaling methods within the framework of a multichannel Schrodinger analysis. For the first time in the literature, the calculated lifetimes are in good agreement with the experiment, thereby proving the reliability of the predicted characteristics and adequacy of the used theory for a theoretical study of other molecular dications.

Molecular dynamics simulations have been employed for the study of photolysis of hydrogen bromide placed inside or on the surface of Ar 12 , Ar 54 , Ar 97 , and Ar 146 clusters, representing one to three icosahedral solvation shells. A large set of classical Wigner trajectories, which take into account the initial quantum rotational or librational delocalization of HBr, is generated and analyzed in terms of transient hydrogen populations inside the cluster and final kinetic energy distributions. The key result is that for fully solvated HBr the size effect on caging is dramatic in the studied range of cluster sizes, while it is only moderate for surface isomers. Simulations also demonstrate that caging can be efficiently turned off by a librational preexcitation of HBr on argon clusters. Calculations are compared to results of cluster experiments which have measured the kinetic energy distribution of hydrogens originating from HBr photolysis at 243 nm in or on argon clusters with an average size of 115 atoms, and a near-quantitative agreement is found.

We present model calculations of high-order harmonic generation in benzene, aligned in the polarization plane of circular polarized laser field. The resonance states of the system are obtained using complex scaling Floquet approach (i.e., within non-Hermitian quantum mechanics) combined with (t,t′) time propagation method. Our results show that the photo-induced dynamics of the model benzene molecule at the laser wavelength of 800 nm is dominated by a single long-lived resonance state up to the intensity of about 90 TW cm−2. The high-order harmonics emitted by the system obey the selection rules derived in [Phys. Rev. Lett. 80, 3743 (1998)] on the basis of the dynamical symmetry of the system, namely the emitted harmonics possess the frequencies (6±1)ω,(12±1)ω,…, where ω is the incident laser frequency. These symmetry-allowed harmonics are found to be the dominant ones in the spectrum also when the laser polarization deviates from the “ideal” circular one by about 5%. The nonlinear response of the model benzene molecule is found to originate mainly from the field-induced transitions between the bound states, in accordance with the earlier analytical theory. The cut-off position in the calculated high-order harmonic generation spectra depends linearly on the field strength in the studied intensity interval. Our numerical calculations reveal the enhancements of particular high-order harmonics in the plateau region of the spectrum at certain field intensities. We show that these enhancements occur under conditions of avoided crossing of two or several resonance quasi-energies in the complex energy plane.

A novel approach to the enhancement of photochemical reaction yield in hydrogen-containing clusters is outlined and applied to the process of the Cl 2 molecule formation from a UV-photolyzed Cl‚‚‚HCl species. The control mechanism consists of a far-IR preexcitation of the large-amplitude hydrogenic bending (librational) mode prior to the HCl photodissociation. Molecular dynamics simulations that properly sample the initial quantum state of the system show that this leads to more than a factor of 2 enhancement of the Cl 2 yield both in the parent and deuterated clusters.

We examine a complex-scaled configuration interaction [(CS)CI] for highly excited Feshbach resonances, where we study the 2s(2) resonance of helium as a test case. Sizable full-CI calculations are reduced by using a correctly defined minimum active space. We compare the convergence of the minimum active space for conventional Hartree-Fock (HF) orbitals obtained as solutions to Hermitian HF equations, to the convergence of minimum active space for complex orbitals obtained as solutions to complex-scaled HF equations. Ground-state optimized orbitals are compared to a simple modification of the HF method using the excited-state mean-field potential.

The Cl–HCl radical complex is investigated by a combination of accurate ab initio quantum chemical methods for the evaluation of the three lowest electronic potential energy surfaces and nonadiabatic couplings between them, and quantum evaluation of vibronic states using wave function propagation in imaginary time within a close coupling scheme. The sensitivity of the vibronic energies on the quality of the potential surfaces is clearly demonstrated. Moreover, it is shown that nonadiabatic couplings between the three lowest electronic states play an important role, especially for highly excited vibronic states. Since under experimental conditions the complex is prepared in a superposition of excited vibronic states close to the dissociation limit, the inclusion of nonadiabatic effects is crucial for a quantitative interpretation of future higher resolution spectroscopic experiments.

We present a non-Hermitian coupled-channel resonant diabatic approach to Rydberg protonium formation in the ultraslow rearrangement collision , where the kinetic coupling between the electronic and the nuclear motions is strong. We calculate complex diabatic potential energy curves and associated coupling matrix elements that vary smoothly with distance. The [Pn − e]− resonances split into two groups converging to the first and second hydrogen thresholds.