Energy Selection in Nonadiabatic Transitions.

In this work we investigate whether and how a molecule undergoing a nonadiabatic transition can show different energy mean values and distributions in the two electronic states that are populated. We analyze three models, of which models I and II mimick the limiting cases of almost adiabatic and almost diabatic regimes, respectively, and are solvable by first-order perturbation theory. Model III represents realistically the photodissociation of a diatomic molecule and is treated numerically. The three models provide a consistent picture of the energy selection effect. For a typical avoided crossing, the wavepacket component that undegoes the transition between the two adiabatic states has a larger mean value of energy than the other component, both for upward and for downward transitions. The analysis of model II shows that the Landau-Zener rule can be deduced in a fully quantum mechanical way. We believe that the energy selection effect can be observed experimentally in the photodissociation of diatomic molecules. The effect should be particularly relevant for wavepackets endowed with a broad energy spectrum, as the result of excitation with ultrashort light pulses.

Photo-orientation of axial molecules.

We examine the photo-orientation of molecules in a linearly polarized field and the ensuing optical anisotropy of a sample. We propose a theoretical model that considers both photoinduced reorientation and rotational diffusion, for the case of linear or axial molecules not interacting among them, as in dilute solutions in viscous media. We perform numerical simulations to highlight the dependence on the parameters of the molecular reorientation processes, on the intensity of the exciting light, and on the use of cross polarized pulses. As a realistic example we simulate the photo-orientation of azobenzene in ethylene glycol.

Photo-orientation of molecules on a surface.

We propose a simple mathematical model for the photo-orientation of molecules adsorbed on a surface and with one rotational degree of freedom. We analyze the general properties of the kinetics and we illustrate them by numerical simulations. We provide some useful relationships between the parameters characterizing the process and the anisotropy of the sample.

Rotational averaging and optimization of laser-induced population transfer in molecules.

The dynamics of a molecule subject to a short laser pulse is investigated, with focus on the averaging over initial rotational states and on the optimization of laser parameters for the efficient population transfer between vibrational and electronic states. A relation is established between final-state populations obtained with a fixed orientation and those based on a full treatment of the rotational degrees of freedom. In the short-pulse approximation, rotational averaging amounts to integrating the fixed molecule results over all orientations. The theory is applied to a variety of model systems and verified with numerical calculations using Gaussian pulses. We calculate target state populations with three procedures, optimizing the laser pulse for a fixed orientation without orientational averaging, averaging without changing the laser parameters, and reoptimizing the parameters after averaging. The analysis of the two-level system provides a reference for the order of magnitude of the effects of averaging. The three-level system brings out the relevant role of the geometry of polarization vectors and transition dipoles. The multiphoton excitation of a Morse oscillator shows the importance of taking into account the dependence of resonance frequencies on the laser intensity. Within a proton transfer model we discuss the results obtained with and without chirping and we show that "optimizing after averaging" can be as effective as choosing a more refined pulse shape.

Alignment of molecules in pulsed resonant laser fields.

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.