  We report a direct measurement of the 0-1 R(0) vibrational transition
frequency in ground-state hydrogen deuteride (HD) using infrared-ultraviolet
double resonance spectroscopy in a molecular beam. Ground-state molecules are
vibrationally excited using a frequency comb referenced continuous-wave
infrared laser, and the excited molecules are detected via state-selective
ionization with a pulsed ultraviolet laser. We determine an absolute transition
frequency of 111 448 815 477(13) kHz. The 0.12 parts-per-billion (ppb)
uncertainty is limited primarily by the residual first-order Doppler shift.
