Vibronic coupling limits the use of high-lying electronic states in complex molecules for laser cooling

TL;DR

This study reveals that vibronic coupling in complex molecules causes additional decay pathways, limiting the effectiveness of high-lying electronic states for laser cooling, and suggests focusing on the lowest excited state for better cooling prospects.

Contribution

First detailed spectroscopic analysis of the C state in CaOPh and SrOPh, demonstrating vibronic mixing effects that impact laser cooling strategies in large molecules.

Findings

Vibronic coupling enables additional decay pathways in the C state.

Non-adiabatic coupling strength estimated at 0.1 cm-1.

Only the lowest electronic excited state should be considered for laser cooling.

Abstract

Laser cooling of large, complex molecules is a long-standing goal, instrumental for enabling new quantum technology and precision measurements. A primary consideration for the feasibility of laser cooling, which determines the efficiency and technical requirements of the process, is the number of excited-state decay pathways leading to vibrational excitations. Therefore, the assessment of the laser-cooling potential of a molecule begins with estimate of the vibrational branching ratios of the first few electronic excited states theoretically to find the optimum cooling scheme. Such calculations, typically done within the BO and harmonic approximations, have suggested that one leading candidate for large, polyatomic molecule laser cooling, alkaline earth phenoxides, can most efficiently be laser-cooled via the third electronically excited C state. Here, we report the first detailed spectroscopic characterization of the C state in CaOPh and SrOPh. We find that nonadiabatic couplings between the A, B, and C states lead to substantial mixing, giving rise to vibronic states that enable additional decay pathways. Based on the intensity ratio of these extra decay channels, we estimate a non-adiabatic coupling strength of 0.1 cm-1. While this coupling strength is small, the large density of vibrational states available at photonic energy scales in a polyatomic molecule leads to significant mixing. Thus, this result is expected to be general for large molecules and implies that only the lowest electronic excited state should be considered when judging the suitability of a molecule for laser cooling.