Milan Šindelka

Strong impact of light-induced conical intersections on the spectrum of diatomic molecules

We show that dressing of diatomic molecules by running laser waves gives rise to conical intersections (CIs). Due to the presence of such CIs, the rovibronic molecular motions are strongly coupled. A pronounced impact of the CI on the spectrum of the Na2 molecule is demonstrated via numerical calculation for weak and moderate laser intensity, and an experiment is suggested on this basis. The position of the light-induced CI and the strength of its non-adiabatic couplings can be chosen by changing the frequency and intensity of the used running laser wave. This offers new possibilities of controlling the photo-induced rovibronic molecular dynamics.

Laser-induced conical intersections in molecular optical lattices

Conical intersections of potential energy surfaces widely appear in molecules which consist of more than two atoms. No conical intersections exist in the case of free diatomic molecules. We show here that diatomic molecules interacting with standing laser waves produce periodic arrays of conical intersections. At these laser-induced intersections the non-adiabatic effects are infinitely strong. The translational and rovibrational molecular motions become strongly coupled to each other and energy exchange between the various degrees of freedom influences the dynamics of the system. As an illustrative example, an effect of the laser-induced conical intersections on trapping of ultracold diatomic molecules by light is discussed.

Conical intersections induced by light: Berry phase and wavepacket dynamics

Conical intersections (CIs) play an important role in nonadiabatic molecular processes. Characterizing and localizing them is important for describing and controlling electronic energy flow in molecules. It is known that no CI appears in free diatomic systems. In earlier works (Moiseyev et al 2008 J. Phys. B.: At. Mol. Opt. Phys. 41 221001, Sindelka et al 2011 J. Phys. B.: At. Mol. Opt. Phys. 44 045603) it was pointed out that CIs can be formed both by standing and running laser waves even in diatomics. The energetic and internuclear positions of these CIs depend on the laser frequencies, while the strength of their nonadiabatic couplings can be modified by the field intensities. In this work, we calculate the topological or Berry phase of the light-induced CI in the Na2 molecule. The presence of this phase is a clear fingerprint of the laser-induced CI. In addition, we perform a detailed study of the wavepacket propagation and discuss effects which reflect the significant presence of the laser-induced CI.

Hund’s multiplicity rule: From atoms to quantum dots

In 1965, Davidson has shown that the textbook explanation for the Hunds multiplicity rule in atoms, based on the Pauli principle, is wrong. The reason for the failure of the textbook proof, as has been given later by others and as appears today in modern textbooks, it is based on the need to introduce angular electronic correlation into the calculations. Here, we investigate an applicability of this argumentation for helium and for the case of two-electron spherically symmetric rectangular quantum dots (QDs). We show that, for helium and also for the QD, the differences between the singlet and triplet excited states can be explored by calculations within the framework of the mean-field approximation, and, surprisingly, without the need of introducing the angular electronic correlation. Moreover, our calculations have shown that the triplet state of the QD is lower in energy than the corresponding singlet state due to lower electronic repulsion contribution, exactly as being assumed in the oldest explanation of the Hunds rule based on the Pauli principle.

The effect of polarization on the light-induced conical intersection phenomenon

It has already been shown that dressing of diatomic molecules by standing or running linear polarized laser waves gives rise to conical intersections (CIs). Due to the presence of such CIs, the rovibronic molecular motions are strongly coupled. Here we prove that only perfect linear polarized light induces CIs in diatomic molecules. However, the fingerprints of the light-induced conical intersection survive for an elliptical polarization but not for a circular polarization. Therefore, the effects of the light-induced conical intersection can be controlled by varying a physical parameter (polarization of the laser). Such a CI controllable physical parameter does not exist in field-free polyatomic molecules. An illustrative numerical example for a sodium dimer showing the dependence of the absorption spectrum of Na2 upon ellipticity is given.

Theory of diatomic molecules in an external electromagnetic field from first quantum mechanical principles

We study a general problem of the translational/rotational/vibrational/electronic dynamics of a diatomic molecule exposed to an interaction with an arbitrary external electromagnetic field. The theory developed in this paper is relevant to a variety of specific applications, such as alignment or orientation of molecules by lasers, trapping of ultracold molecules in optical traps, molecular optics and interferometry, rovibrational spectroscopy of molecules in the presence of intense laser light, or generation of high order harmonics from molecules. Starting from the first quantum mechanical principles, we derive an appropriate molecular Hamiltonian suitable for description of the center of mass, rotational, vibrational, and electronic molecular motions driven by the field within the electric dipole approximation. Consequently, the concept of the Born-Oppenheimer separation between the electronic and the nuclear degrees of freedom in the presence of an electromagnetic field is introduced. Special cases of the dc/ac-field limits are then discussed separately. Finally, we consider a perturbative regime of a weak dc/ac field, and obtain simple analytic formulas for the associated Born-Oppenheimer translational/rotational/vibrational molecular Hamiltonian.