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

Abstract

UV frequency metrology has been performed on the a 3 Π -X 1 Σ + (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 (δν/ν = 3 · 10 −9 ), while the energy differences between rotational levels in the a 3 Π 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 a 3 Π state of 12 C 16 O and 13 C 16 O with improved accuracy. Transitions in the different isotopologues are well reproduced by scaling the molecular constants of 12 C 16 O 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 Ω-manifolds for probing a possible variation of the proton-to-electron mass ratio, µ = mp/me, on a laboratory time scale.

Perspective: Tipping the scales: Search for drifting constants from molecular spectra

“…Crosses, also shown in the figure, indicate sensitivity coefficients that were calculated using a full molecular Hamiltonian. 63 From the figure, it can be seen that resonances occur near A/B ∼ 25 which is close to the A/B values for the 12 13 C 16 O have a sensitivity that is almost 500 times that of a pure rotational transition. An experiment to measure these transitions in a double-resonance molecular beam machine using a two-photon microwave absorption is currently under construction in our laboratory.…”

“…The crosses indicate sensitivity coefficients that were calculated while taking higher-order contributions to the Hamiltonian into account. 62,63 FIG. 4.…”

Ramsey-type microwave spectroscopy on CO (a3Π)

“…Later, the a 3 Π state was studied using radio frequency [22][23][24], microwave [25][26][27], infrared [28,29], optical [30] and UV spectroscopy [31]. We have recently measured selected transitions in the CO a 3 Π -X 1 Σ + (0-0) band using a narrow-band UV laser [20] resulting in a set of molecular constants that describes the level structure of the a 3 Π state with an absolute accuracy of 5 MHz with respect to the ground state and a relative accuracy of 500 kHz within the a 3 Π state. Fig.…”

“…After passing a 1 mm skimmer, the molecular beam is crossed at right angles with a UV laser beam that excites the molecules from the X 1 Σ + ground state to the a 3 Π state. Details of the laser system are described elsewhere [20,33,34]. Briefly, a Ti:sapphire oscillator is locked to a CW Ti:sapphire ring laser and pumped at 10 Hz with a frequency doubled Nd:YAG-laser.…”

“…Recently, it was noted that the two-photon transition between the J = 8, Ω = 0 and the J = 6, Ω = 1 levels in the a 3 Π state is 300 times more sensitive to a possible variation of the proton to electron mass ratio (µ) than purely rotational transitions [19,20]. We plan to measure these transitions with high precision.…”