Rotational level spacings in HD from vibrational saturation spectroscopy

The R(1), R(3), and P(3) rovibrational transitions in the (2-0) overtone band of the HD molecule are measured in Doppler-free saturation using the technique of NICE-OHMS spectroscopy. For the P(3) line, hitherto not observed in saturation, we report a frequency of 203 821 936 805 (60) kHz. The dispersive line shapes observed in the three spectra show strong correlations, allowing for extraction of accurate information on rotational level spacings. This leads to level spacings of ??(J=3)???(J=1) = 13 283 245 098 (30) kHz in the v = 0 ground state and A(J=4)???(J=2) = 16 882 368 179 (20) kHz in the v = 2 excited vibration in HD. These results show that experimental values for the rotational spacings are consistently larger than those obtained with advanced ab initio theoretical calculations at 1.5o-, where the uncertainty is determined by theory. The same holds for the vibrational transitions where systematic deviations of 1.7???1.9a are consistently found for the five lines accurately measured in the (2-0) band.