Non-adiabatic transitions in a diatomic molecule under a bichromatic laser field

Edgar Barriga Department of Physics, Faculty of Sciences, University of Chile, Santiago, Chile.

Edgar Barriga Aguirre (a), Luis E. F. Foà Torres (b), Carlos Cárdenas Valencia (a)

(a) Department of Physics, Faculty of Sciences, University of Chile, Santiago, Chile.

(b) Department of Physics, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago, Chile.

Degeneracies in molecular systems have been studied extensively in the last decades. This kind of singularity has profound impacts on the dynamics since non-adiabatic effects arise due to the coupling between the electronic and nuclear motion [1]. For a N-state system the non-crossing rule requires a (N-1)(N+2)/2 conditions over internal coordinates to achieve a degeneracy point in configuration space. Despite this seemingly strong restriction conical intersections of more than two states have shown to be not just a curiosity but the rule [2]. The existence of degeneracies is not restricted to the field-free case. It has been shown that they can be created by external means [3,4].

In this work we show that by using a bichromatic radiation field a three-state degeneracy can be created in diatomic molecules. This situation is forbidden for field-free diatomic molecules due to the non-crossing rule. The threefold degeneracy comes along with novels kind of two-state intersections (also forbidden for field-free diatomic molecules). We study the effects of the polarization of the lasers in these different induced intersections between the potential energy surfaces. The system chosen is the Cs$_2$ molecule.

The time-dependent Hamiltonian is treated with the Many Mode Floquet Theory (MMFT) [5]. We use the MMFT formalism to calculate the long-time averaged transition probabilities. We found a non-trivial dependence of the relative phase in this observable. Our results establish a way that can be implemented experimentally to control the non-adiabatic dynamics in molecular systems through the relative phase.

[1] X. Zhu and D. R. Yarkony, Molecular Physics 114, 1983–2013 (2016).

[2] L. Shen, B. Xie, Z. Li, L. Liu, G. Cui, and W.-H. Fang, The Journal of Physical Chemistry Letters 11, 8490–8501 (2020).

[3] B. Garraway and S. Stenholm, Optics communications 83, 349–357 (1991).

[4] N. Moiseyev, M. Šindelka, and L. S. Cederbaum, Journal of Physics B: Atomic, Molecular and Optical Physics 41, 221001 (2008).

[5] T.-S. Ho, S.-I. Chu, and J. V. Tietz, Chemical Physics Letters 96, 464–471 (1983).