Saturated absorption spectroscopy of HD at 76 K

Precision spectroscopy of molecular hydrogen can be used to test the quantum electrodynamics theory and determine the proton-to-electron mass ratio in a four-body system. Saturated absorption spectroscopy of the R(1) line in the (2-0) band of HD is measured at 76 K. An asymmetric line profile is observed and analyzed, and the transition frequency is predicted to be 217 105 182 284 (11)stat(27)sys kHz. The value is nine times more accurate than our previous result obtained at room temperature and differs by 6.6 sigma from the recent result derived with a different line-shape model.