Photofragment spectrum of bromine

We have measured the photofragment spectrum of Br2 at visible and near uv wavelengths using the absorption of a single photon to probe ungerade states and double photon absorption to probe gerade states invisible in the ordinary absorption spectrum. In these experiments, a pulsed polarized laser beam intersects a molecular beam in a vacuum chamber, and Br atom photodissociation fragments are detected with a mass spectrometer (i) as a function of the angle between the light polarization direction and the recoil path of the detected atoms, measuring the Ω quantum number of the excited molecular state, and (ii) as a function of time after the laser pulse, measuring the atomic states with which the molecular state correlates. The experimental results at 18 780, 21 450, and 28 810 cm−1 are in accord with the predictions of Mulliken that the visible absorption spectrum of Br2 is due to transitions to the A 1u(3Π), B 0+u(3Π), and 1u(1Π) states. We have also found double photon dissociation processes involving absorption of one 18 780 and one 9390 cm−1 photon, as well as such processes involving two 18 780 cm−1 photons. Calculations, following Mulliken, of the approximate vertical energies of the newly observed final dissociative states in these double photon processes are in accord with our experimental observations.