Results are presented related to the use of two-photon dissociation of HgBr2 molecular vapor as a non-linear fast-opening passive mode locker of a long-pulse XeCl laser. A dye solution, with or without scatterers, is also examined for comparison. Pulses with durations below our resolution limit of 400 ps and with a modulation better than 90% are reported. The low threshold, the non-destruction and the broadband absorption causing photodissociation make this molecule useful for laser pulse compression and decoupling for many lasers in the ultraviolet region.