Pulse duration and collision effects in the multiphoton dissociation of HDCO

Measurements of multiphoton dissociation (MPD) of HDCO gas irradiated by intense CO₂ laser pulses varying in shape and having a duration from 150 ns to ～ 1 μs are reported. It is found that for the same laser pulse energy the short pulses dissociated about an order of magnitude more molecules than the long pulses. Taking into consideration the collision-induced MPD of HDCO previously found, it is shown that to a reasonably good approximation, all the present MPD data for the different pulse durations can be represented on a single, smooth and sharply rising curve of yield versus intensity. This finding is in contrast to the flat intensity response at constant pulse energy found recently for collisionless MPD in SF₆.