The Band Spectrum of Caesium

Measurements of five distinct band systems of ${\mathrm{Cs}}_2$ have been made, in absorption with high and low dispersion, in magnetic rotation, and in fluorescence. Three of these systems, with maxima at 6250A, 7200A, and 7667A, have been reported before, but not analyzed. The fourth with a maximum at 4800A and the fifth, extending from 8950A to limit of sensitivity of the plate, are new. Quantum numbers have been assigned to some 218 bands in the system at 7667A. The upper and lower vibrational frequencies are 34.28 and 41.99 ${\mathrm{cm}}^{-1\mathrm{}}$, and an extrapolation yields 0.45 volt as the energy of dissociation of the ground state. This system, and the one beyond 8950A, apparently correspond to the green and red systems of sodium, respectively. The origin of the fifth system lies beyond the region of sensitivity of even the new Eastman $Q$ plates. Some regularities in the other systems have been found, but no complete analysis could be made, though a fluorescence spectrum was obtained in the 6250 system which showed the lower state to be the same as in the 7667A system. This 6250A system is extraordinarily complex and appears to contain some bands degraded to the red, some to the blue and some narrow regions of absorption bounded by edges on both sides. No magnetic rotation spectrum was found.