HD as a Probe for Detecting Mass Variation on a Cosmological Time Scale

The strong electronic absorption systems of the $B{}^1\Sigma _u^{+}-X{}^1\Sigma _g^{+}$ Lyman and the $C{}^1\Pi _u-X{}^1\Sigma _g^{+}$ Werner bands can be used to probe possible mass-variation effects on a cosmological time scale from spectra observed at high redshift, not only in ${\mathrm{H}}_2$ but also in the second most abundant hydrogen isotopomer HD. High resolution laboratory determination of the most prominent HD lines at extreme ultraviolet wavelengths is performed at an accuracy of $\Delta \lambda /\lambda \sim 5\times {10}^{-8}$, forming a database for comparison with astrophysical data. Sensitivity coefficients $K_i=d\ln {\lambda }_i/d\ln \mu$ are determined for HD from quantum ab initio calculations as a function of the proton-electron mass ratio $\mu$. Strategies to deduce possible effects beyond first-order baryon/lepton mass ratio deviations are discussed.

Figure 1

Recording of the W0R1 line of HD (upper) with étalon markers (lower) and an $I_2$-saturation spectrum (middle) for calibration. The line marked with an asterisk is the $t$-hyperfine component of the $B\mathrm{\text{−}}X(11,2)$ $P(52)$ rotational line in $I_2$ at $16546.92662{\mathrm{cm}}^{-1}$ used as an absolute reference.

Figure 2

Calculation of transition wavelengths as a function of $\mu$ for two lines in HD. It illustrates that some lines undergo an upward shift and others a downward shift upon a variation of $\mu$.