Intense laser alignment in dissipative media as a route to solvent dynamics

Information content of high harmonics generated from aligned molecules

We derive an expression for the harmonic signal from nonadiabatically aligned molecules that accounts for both electronic and rotational motions. We identify a single approximation, which converts the expression into a physically transparent and computationally convenient form. Our analytical result gives explicitly the time dependence of the harmonic spectra, thus explaining the observations of a class of recent experiments. Moreover, it points to new opportunities for generating insights into the structure and dynamics of molecular systems through harmonic generation experiments from aligned molecules. This includes information regarding the rotational and electronic dynamics of isolated systems, as well as regarding the decoherence and relaxation in molecules subject to a dissipative environment.

Torsional control by intense pulses

We generalize the concepts of alignment and 3D alignment by moderately intense laser pulses to control both the overall rotations and the torsional motions of polyatomic molecules. Torsional control is applied to manipulate charge transfer events, hence introducing a potential route to light controlled molecular switches. Potential applications in areas such as molecular assembly, molecular spectroscopies, energy transfer, and molecule-based junctions are envisioned.

Optimal control of molecular alignment in dissipative media

We explore the controllability of nonadiabatic alignment in dissipative media, and the information content of control experiments regarding the bath properties and the bath system interactions. Our approach is based on a solution of the quantum Liouville equation within the multilevel Bloch formalism, assuming Markovian dynamics. We find that the time and energy characteristics of the laser fields that produce desired alignment characteristics at a predetermined instant respond in distinct manners to decoherence and to population relaxation, and are sensitive to both time scales. In particular, the time-evolving spectral composition of the optimal pulse mirrors the time-evolving rotational composition of the wave packet, and points to different mechanisms of rotational excitation in isolated systems, in systems subject to a decoherering bath, and in ones subject to a population relaxing bath.

Time-dependent alignment of molecules trapped in octahedral crystal fields

The hindered rotational states of molecules confined in crystal fields of octahedral symmetry, and their time-dependent alignment obtained by pulsed nonresonant laser fields, are studied computationally. The control over the molecular axis direction is discussed based on the evolution of the rotational wave packet generated in the cubic crystal-field potential. The alignment degree obtained in a cooperative case, where the alignment field is applied in a favorable crystal-field direction, or in a competitive direction, where the crystal field has a saddle point, is presented. The investigation is divided into two time regimes where the pulse duration is either ultrashort, leading to nonadiabatic dynamics, or long with respect to period of molecular libration, which leads to synchronous alignment due to nearly adiabatic following. The results are contrasted to existing gas phase studies. In particular, the irregularity of the crystal-field energies leads to persistent interference patterns in the alignment signals. The use of nonadiabatic alignment for interrogation of crystal-field energetics and the use of adiabatic alignment for directional control of molecular dynamics in solids are proposed as practical applications.

Dissipative dynamics of laser induced nonadiabatic molecular alignment

Nonadiabatic alignment induced by short, moderately intense laser pulses in molecules coupled to dissipative environments is studied within a nonperturbative density matrix theory. We focus primarily on exploring and extending a recently proposed approach [Phys. Rev. Lett. 95, 113001 (2005)], wherein nonadiabatic laser alignment is used as a coherence spectroscopy that probes the dissipative properties of the solvent. To that end we apply the method to several molecular collision systems that exhibit sufficiently varied behavior to represent a broad variety of chemical environments. These include molecules in low temperature gas jets, in room temperature gas cells, and in dense liquids. We examine also the possibility of prolonging the duration of the field free (post-pulse) alignment in dissipative media by a proper choice of the system parameters.