UV-frequency metrology on CO (a3Π): Isotope effects and sensitivity to a variation of the proton-to-electron mass ratio

UV-frequency metrology has been performed on the a(3)Pi - X-1 Sigma(+) (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 (d upsilon/upsilon = 3 x 10(-9)), while the energy differences between rotational levels in the a(3)Pi state are determined with an accuracy of 500 kHz. From these measurements, in combination with previously published radio-frequency and microwave data, a set of molecular constants is obtained that describes the level structure of the a(3)Pi state of (CO)-C-12-O-16 and (CO)-C-13-O-16 with improved accuracy. Transitions in the different isotopologues are well reproduced by scaling the molecular constants of (CO)-C-12-O-16 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.