The Emission Spectrum of Molecular Hydrogen in the Extreme Ultraviolet

Experimental procedure and results A new grazing incidence vacuum spectrograph containing a two meter grating, designed by Professor H. E. White and built in the shop of the University of California Physics Department, has been used to photograph more than 2200 lines in the emission spectrum of molecular hydrogen between 1375A and 810A. The known spectrum of this molecule has thus been extended about 200A toward shorter wave-length and more than 1400 new lines have been obtained. Identification of lines The $2p^{1}\ensuremath{\Sigma}\ensuremath{-}1s^{1}\ensuremath{\Sigma}(B\ensuremath{-}A)$ band system has been extended so that a number of bands ending on the three lowest vibrational levels of the normal electronic state are now included. A new analysis of the bands of the $2p^{1}\ensuremath{\Pi}_{\mathrm{cd}}\ensuremath{-}1s^{1}\ensuremath{\Sigma}$ system has also been made. Observed and calculated wave numbers are given for all identified lines. Except in the case of perturbed lines these two values agree always within the probable error of experiment (about 2 ${\mathrm{cm}}^{\ensuremath{-}1}$).The $1s^{1}\ensuremath{\Sigma}$ stateValues of the molecular constants for each observed vibrational level in the normal $1s^{1}\ensuremath{\Sigma}$ state have been calculated and tables of the rotational energy function, $F$, have been constructed. The new values of $B$ fall regularly upon a smooth ${B}_{v}:v$ curve, so that it is no longer necessary to assume that a perturbation exists in the $v=0$ vibrational level. This ${B}_{v}:v$ curve has, however, a positive curvature for low values of $v$ in contradistinction to the known negative curvature for higher values of $v$. The new $\ensuremath{\Delta}{G}_{v}:v$ curve is also found to be smooth and of the same general shape as the ${B}_{v}:v$ curve.The $2p^{1}\ensuremath{\Sigma}$ stateThe available data on the $2p^{1}\ensuremath{\Sigma}$ state from bands in the visible spectrum given by Richardson and Davidson have been combined with the ultraviolet data for calculation of the rotational constants. The value of ${B}_{0}$ thus obtained agrees with that given by these authors. The ${B}_{v}:v$ curve for this state is found to have a decided positive curvature for low values of $v$, the curvature becoming much less as $v$ increases. The new and independent values of $\ensuremath{\Delta}{G}_{v}$ agree to within 0.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ with those given by Richardson and Davidson. Tabulated values of the rotational energy function are given.The $2p^{1}\ensuremath{\Pi}_{\mathrm{cd}}$ stateBecause of the $\ensuremath{\Lambda}$-type doubling in this state we find two "effective" values of each of the rotational constants $B$, $D$ and $F$. The true value of $B$ is, in our case, the effective $B$ given by the $c$ component levels. The values of ${B}_{v}$ for these latter levels fall quite regularly on a smooth ${B}_{v}:v$ curve. On the other hand, the ${B}_{v}$ values derived from the $d$ component levels do not lie on any smooth curve. This is due to a number of perturbations in the $d$ component levels. Besides a number of ordinary rotational perturbations there is one apparent vibrational perturbation of 10.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the $v=3$ state. Values of the constants Values of ${B}_{e}$ for the $1s^{1}\ensuremath{\Sigma}(A)$, $2p^{1}\ensuremath{\Sigma}(B)$ and $2p^{1}\ensuremath{\Pi}_{\mathrm{cd}}(C)$ states are found to be 60.8715 (60.587), 19.987, and 31.288 (30.065) ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. Similarly the values of $\ensuremath{\Delta}{G}_{e}$ for these states are 4417.9 (4371.), 1358.33, and 2468 (2444) ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. The value of ${\ensuremath{\nu}}_{e}$ for the $2p^{1}\ensuremath{\Sigma}\ensuremath{-}1s^{1}\ensuremath{\Sigma}$ system is 91,708 ${\mathrm{cm}}^{\ensuremath{-}1}$ and the frequency of the 0-0 band is ${\ensuremath{\nu}}_{00}=90,206.1$ ${\mathrm{cm}}^{\ensuremath{-}1}$. Corresponding values for the $2p^{1}\ensuremath{\Pi}_{\mathrm{cd}}\ensuremath{-}1s^{1}\ensuremath{\Sigma}$ system are ${\ensuremath{\nu}}_{e}=100,049$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\nu}}_{00}=99,087.4$ ${\mathrm{cm}}^{\ensuremath{-}1}$. These new values differ appreciably in some cases from the best previous results (as given in parentheses above) and these changes are due to many new assignments and reassignments of lines as well as to the reinterpretation of the apparent perturbation in the $v=0$ level of the normal state.