The Magnetic Rotation Spectrum and Heat of Dissociation of the Lithium Molecule

The magnetic rotation spectrum of the green, ${}_{}{}^1{}_{}{}^{}\Pi -{}_{}{}^1{}_{}{}^{}\Sigma$, band system of ${\mathrm{Li}}_2$ has been observed and used to locate bands with high quantum numbers and so to obtain an accurate value for the heat of dissociation of the normal ${\mathrm{Li}}_2$ molecule, yielding ${D_0}^{\prime \prime }=1.14\mathrm{}\pm 0.03$ volts. The spectrum is less simple than in ${\mathrm{Na}}_2$, for instance, in that we observe not only a strong line at the head of each band, but also a few lines of the $P$ and $R$, but not $Q$, branches. An improved method of plotting Franck-Condon diagrams is illustrated.