Production of cold bromine atoms at zero mean velocity by photodissociation

The production of a translationally cold (T <1 K) sample of bromine atoms with estimated densities of up to 10⁸ cm^-3 using photodissociation is presented. A molecular beam of Br₂ seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms^-1, corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source.