Hyperfine structure in a vibrational quadrupole transition of ortho-H2

A Lamb dip spectroscopic measurement of the Q(1) transition in the (2-0) overtone band of ortho-H-2 is performed with the ultra-sensitive NICE-OHMS intracavity absorption technique at 1238 nm. For the first time individual hyperfine components of a vibrational transition in H2 are resolved, displaying a pattern of 4 overlapping components with singly isolated components at the red and blue side. Weak components at a line strength of 4.4 x 10(-28) cm/molecule could be observed as a Lamb dip. Analysis yields a hyperfineless purely rovibrational transition frequency for the Q(1) (2-0) line in H(2 )at unprecedented accuracy for any rovibrational transition in a molecular hydrogen isotopologue. Its frequency is 242,091,630,140 (9) kHz under the assumption that only a blue recoil component is observed. A 1.7 sigma deviation from the most advanced quantum electrodynamics calculations is obtained, forming a challenge for theory. [GRAPHICS]