Abstract

The spectrum of boron hydride, as excited in the cathode of a Sch\"uler lamp, consists of at least three systems, $^{1}\ensuremath{\Pi}\ensuremath{\rightarrow}^{1}\ensuremath{\Sigma}$ of BH, $^{3}\ensuremath{\Sigma}\ensuremath{\rightarrow}^{3}\ensuremath{\Pi}$ of BH, and a new system, $^{2}\ensuremath{\Pi}\ensuremath{\rightarrow}^{2}\ensuremath{\Sigma}$, due to B${\mathrm{H}}^{+}$.BH, $^{1}\ensuremath{\Pi}\ensuremath{\rightarrow}^{1}\ensuremath{\Sigma}$.---This system has been extended to include the (2,2) band. One is thereby enabled to refine the rotational and vibrational constants of the system and, from a study of dissociation by rotation, to fix the heat of dissociation (${D}_{e}$) of the normal state at 3.60\ifmmode\pm\else\textpm\fi{}0.05 ev.BH, $^{3}\ensuremath{\Sigma}\ensuremath{\rightarrow}^{3}\ensuremath{\Pi}$, (0,0) head at $\ensuremath{\lambda}3694$.---The fine multiplet structure of this system has been resolved and the transition, previously reported to be $^{3}\ensuremath{\Pi}\ensuremath{\rightarrow}^{3}\ensuremath{\Sigma}$, established as $^{3}\ensuremath{\Sigma}\ensuremath{\rightarrow}^{3}\ensuremath{\Pi}$. The $^{3}\mathrm{II}$ state is regular and near case $b$ ($\frac{A}{B}=+0.47$).B${\mathrm{H}}^{+}$, $^{2}\ensuremath{\Pi}\ensuremath{\rightarrow}^{2}\ensuremath{\Sigma}$, (0,0) head at $\ensuremath{\lambda}3766$.---This system is the analog of the green bands of the isoelectronic molecule BeH. Application of the Hill and Van Vleck theory to the $^{2}\mathrm{II}$ state does not yield an $\frac{A}{B}$ which is constant with increasing $J$. Refinement of the theory to include the rotational $D$ term, along the lines suggested by Gilbert for $^{3}\mathrm{II}$ states, does not greatly improve the situation. The remaining discrepancy is probably due to unobservable $^{2}\ensuremath{\Sigma}$ doubling. The $\ensuremath{\Lambda}$ type doubling has been evaluated and compared with theory with results very like those in BeH