Laser-induced breakdown of helium

A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Torr (0.24-2.27 MPa) by ruby laser radiation of pulse-length 40 ns. It is shown that, when the variation of electron energy gain in elastic collisions is taken into account, the agreement between computed and experimental breakdown threshold intensities is improved by a factor of 10 compared with calculations in which only the mean energy gain is computed. The agreement is also improved significantly by incorporating the collisional and multiphoton ionisation of excited helium atoms.