Frequency calibration of B 1Σu+-X 1Σg+ (6,0) Lyman transitions in H2 for comparison with quasar data

The Lyman and Werner spectroscopic transitions of molecular hydrogen, the most ubiquitous molecular spectral lines observed in the universe, provide a tool to probe a possible variation of the proton-to-electron mass ratio mu on a cosmological time scale. Such procedures require a database of zero-redshift, or laboratory-based wavelengths at the highest possible level of accuracy. Accurate transitions in the B (1)Sigma(+)(u)-X (1)Sigma(+)(g) (6,0) Lyman band of H2 are still missing in the set of laboratory data, due to previously encountered problems in generating the appropriate wavelengths using a narrow-band extreme ultraviolet laser source. Here frequency calibrations of the missing transitions are presented from a laser-based study at a (5 - 8) x 10(-8) accuracy level.