THE PURE ROTATION SPECTRUM OF MOLECULAR HYDROGEN

$^{1}$S.C. Back, J.H Lacy, and T.R., Geballe, Ap. J. (Letters) 234, L213 (1979). $^{2}$R.F. Knacke and E.T. Young, Ap. J. (Letters) 242, L183 (1980): also Ap. J. (Letters) 249, L65 (1981). $^{3}$D.E. Jennings and J.W. Brault, Ap. J. (Letters), to be published. $^{4}$S.L. Bragg, J.W. Brault, and W.B. Smith, to be published.Pure rotational quadrupole spectra of $H_{2}$ were first successfully observed in a source outside the solar system with the detection of emission from the $12.28 \mu m$ S(2) transition in the Orion $nebula^{1}$. More recently the high -J transitions S(8), S(9), S(12), S(13), S(14), and S(15) were also observed in $Orion.^{2}$ The pure rotation spectrum of molecular hydrogen has been recorded in the laboratory using Fourier transform and diode laser spectrometers. Frequencies and quadrupole strengths have been derived for the transitions S(2), S(3), S(4), and S(5) at 12.3, 9, 7, 8.0, and $6.9 \mu m$ wavelength, respectively. An immediate result of this study was a frequency measurement for the S(2) $transition^{3}$ originally detected in Orion. The frequency determined, $814.4250 cm^{-1}$ is different by $-0.027 cm^{-1}$ from the value used in those Orion observations. In addition, this analysis has shown that the usual set of rotational constants B, D, and H are not sufficient for characterizing the ground state rotational levels of $H_{2}$ up to $j=7$. The additional constant L is needed to fit the laboratory data (including S(0) and S(1) frequencies obtained from combination differences measured in the vibrational $v=0-1$ $band^{4}$). At least five rotational constants are needed to fit both the laboratory measurements and those high-J line frequencies observed in the Orion Molecular Cloud