Metastable Hydrogen Molecules

The existence of a metastable electronic state of ${\mathrm{H}}_{2}$ has been established by means of the molecular beam magnetic resonance technique. Observations have been made of the radio-frequency Zeeman spectrum of natural and para-hydrogen. At a magnetic field of about 5 gauss the strongest line corresponds to a $g$ value of 0.83. The appearance potential for this line is 11.9\ifmmode\pm\else\textpm\fi{}0.25 ev. Observation of the quadratic Zeeman effect made it possible to estimate the fine structure of the $N=2$ level of para-hydrogen. The observed intervals are: $f(J=2\ensuremath{\leftrightarrow}J=3)=5730\ifmmode\pm\else\textpm\fi{}200$ Mc/sec, $f(J=2\ensuremath{\leftrightarrow}J=1)=5000\ifmmode\pm\else\textpm\fi{}200$ Mc/sec. This represents the first complete resolution of the fine structure of a rotational level of ${\mathrm{H}}_{2}$. Knowledge of the electronic structure of ${\mathrm{H}}_{2}$, agreement of the appearance potential with spectroscopic data, agreement of the fine structure with partially resolved optical spectra, and good agreement of the Zeeman spectra with theory lead to the conclusion that the metastable state must be ${\mathrm{H}}_{2}$, $c(1{\ensuremath{\sigma}}_{g}, 1{\ensuremath{\pi}}_{u})^{3}\ensuremath{\Pi}_{u}$.