M.L. Niu, F. Ramirez, E.J. Salumbides, W. Ubachs
High-precision two-photon Doppler-free frequency measurements have been performed on the CO A1Π - X1Σ+ fourth-positive system (2,0), (3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for rotational quantum numbers up to J = 5, have been determined at an accuracy of 1.6 × 10-3 cm-1, using advanced techniques of two-color 2  +  1' resonance-enhanced multi-photon ionization, Sagnac interferometry, frequency-chirp analysis on the laser pulses, and correction for AC-Stark shifts. The accurate transition frequencies of the CO A1Π - X1Σ+ system are of relevance for comparison with astronomical data in the search for possible drifts of fundamental constants in the early universe. The present accuracies in laboratory wavelengths of Δλ/λ = 2 × 10-8 may be considered exact for the purpose of such comparisons. © 2015 AIP Publishing LLC.
Department of Physics and Astronomy, LaserLaB, VU University, De Boelelaan 1081, HV Amsterdam, 1081, Netherlands; Department of Physics, University of San Carlos, Cebu City, 6000, Philippines
