Fundamental Vibration of Molecular Hydrogen

The fundamental ground tone vibration of H-2, HD, and D-2 is determined to an accuracy of 2 x 10(-4) cm(-1) from Doppler-free laser spectroscopy in the collisionless environment of a molecular beam. This rotationless vibrational splitting is derived from the combination difference between electronic excitation from the X-1 Sigma(+)(g), v = 0, and v = 1 levels to a common EF1 Sigma(+)(g), v = 0 level. Agreement within 1 sigma between the experimental result and a full ab initio calculation provides a stringent test of quantum electrodynamics in a chemically bound system.