Knowledge Bank

Laser optogalvanic (LOG) spectra of bromine vapors (50--200 mTorr pressure) in an rf discharge $(\sim 30MHz)$ have been studied by means of a pulsed, flash-lamp pumped, tunable dye laser. Signals were normalized on a pulse-by-pulse basis before averaging and the effective duty-cycle was forced towards unity in order to obtain smooth spectra even at low pulse-repetition rates. Contrary to the usual arrangement, the pick-up coil was placed outside the rf excitation electrodes and excellent LOG signals were obtained when a transverse optical excitation was imposed in the region between the pick-up coil and the rf electrode. The LOG signal consists of two components: a fast component $(\sim 1\mu s)$ that is atomic in origin and is almost synchronous with the excitation laser pulse and a slow component $(50-100\mu s)$ which is molecular in origin. The spectrum of the fast component, excited with $14900-17000cm-1$ laser radiation, shows atomic (ionic as well as neutral) transitions originating from various excited states; that of the slow component, similarly excited, is the normal $B¯\leftarrow X¯$ bromine molecular spectrum. The nature and origin of the two distinct components in the LOG signal will be discussed and some spectroscopic analysis will be presented.