UV frequency metrology on CO (a3Pi); isotope effects and sensitivity to a variation of the proton-to-electron mass ratio

TL;DR

This study performs high-precision UV frequency measurements on CO isotopologues to refine molecular constants and explore the potential of microwave transitions for detecting variations in the proton-to-electron mass ratio.

Contribution

It provides improved molecular constants for CO isotopologues and demonstrates the sensitivity of microwave transitions to fundamental constant variations.

Findings

Achieved 5 MHz accuracy in band origin measurement.

Refined molecular constants for CO isotopologues.

Confirmed the potential of microwave transitions to probe fundamental constant variations.

Abstract

UV frequency metrology has been performed on the a3Pi - X1Sigma+ (0,0) band of various isotopologues of CO using a frequency-quadrupled injection-seeded narrow-band pulsed Titanium:Sapphire laser referenced to a frequency comb laser. The band origin is determined with an accuracy of 5 MHz (delta \nu / \nu = 3 * 10^-9), while the energy differences between rotational levels in the a3Pi state are determined with an accuracy of 500 kHz. From these measurements, in combination with previously published radiofrequency and microwave data, a new set of molecular constants is obtained that describes the level structure of the a3Pi state of 12C16O and 13C16O with improved accuracy. Transitions in the different isotopologues are well reproduced by scaling the molecular constants of 12C16O via the common mass-scaling rules. Only the value of the band origin could not be scaled, indicative of a breakdown of the Born-Oppenheimer approximation. Our analysis confirms the extreme sensitivity of two-photon microwave transitions between nearly-degenerate rotational levels of different Omega-manifolds for probing a possible variation of the proton-to-electron mass ratio, \mu=m_p/m_e, on a laboratory time scale.