  The fundamental ground tone vibration of H2, HD, and D2 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^1Sg+, v=0 and v=1 levels to a common EF^1g+, 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.
