Features of Molecular Structure Beneficial for Optical Pumping

TL;DR

This paper challenges traditional criteria for molecular optical pumping, showing that intervening electronic states can be beneficial and that transition dipole moments, not just Franck-Condon factors, determine effective state preparation, demonstrated through SiO$^+$.

Contribution

It introduces a revised perspective on molecular structure criteria for optical pumping, emphasizing the role of intervening states and transition dipole moments over FCFs, with a model for TDM approximation.

Findings

Intervening electronic states can facilitate parity flips in state preparation.

A simple model for transition dipole moments explains vibrational cooling in SiO$^+$.

Decay pathways can be optimized for efficient optical cycling.

Abstract

Fast and efficient state preparation of molecules can be accomplished by optical pumping. Molecular structure that most obviously facilitates cycling involves a strong electronic transition, with favorable vibrational branching (diagonal Franck-Condon factors, aka FCFs) and without any intervening electronic states. Here, we propose important adjustments to those criteria, based on our experience optically pumping SiO$^+$. Specifically, the preference for no intervening electronic states should be revised, and over-reliance on FCFs can miss important features. The intervening electronic state in SiO$^+$is actually found to be beneficial in ground rotational state preparation, by providing a pathway for population to undergo a parity flip. This contribution demonstrates the possibility that decay through intervening states may help state preparation of non-diagonal or polyatomic molecules. We also expand upon the definition of favorable branching. In SiO$^+$, we find that the off-diagonal FCFs fail to reflect the vibrational heating versus cooling rates. Since the branching rates are determined by transition dipole moments (TDMs) we introduce a simple model to approximate the TDMs for off-diagonal decays. We find that two terms, set primarily by the slope of the dipole moment function ($d\mu/dx$) and offset in equilibrium bond lengths ($\Delta x = r_e^g-r_e^e$), can add (subtract) to increase (decrease) the magnitude of a given TDM. Applying the model to SiO$^+$, we find there is a fortuitous cancellation, where decay leading to vibrational excitation is reduced, causing optical cycling to lead naturally to vibrational cooling.