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UV frequency metrology has been performed on the a3Pi - X1Sigma+ (0,0) bandof various isotopologues of CO using a frequency-quadrupled injection-seedednarrow-band pulsed Titanium:Sapphire laser referenced to a frequency comblaser. 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 thea3Pi state are determined with an accuracy of 500 kHz. From these measurements,in combination with previously published radiofrequency and microwave data, anew set of molecular constants is obtained that describes the level structureof the a3Pi state of 12C16O and 13C16O with improved accuracy. Transitions inthe different isotopologues are well reproduced by scaling the molecularconstants of 12C16O via the common mass-scaling rules. Only the value of theband origin could not be scaled, indicative of a breakdown of theBorn-Oppenheimer approximation. Our analysis confirms the extreme sensitivityof two-photon microwave transitions between nearly-degenerate rotational levelsof different Omega-manifolds for probing a possible variation of theproton-to-electron mass ratio, \mu=m_p/m_e, on a laboratory time scale.

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