Abstract
Breakdown in the rare gases caused by picosecond laser pulses from a Nd:glass laser is investigated. Threshold intensity for breakdown is measured as a function of pressure and measured values compared with those expected from theory. It is shown that an implicit assumption has been made in past theoretical treatments which may not apply on picosecond time-scales. Under certain conditions, this can lead to a multi-photon breakdown process which is density dependent. The experimental results are explained on this basis.