Influence of molecular ion reactions on optical breakdown in gases

A new mechanism is proposed for optical breakdown in gases, based on molecular ion reactions. It is shown that a molecular ion breakdown mechanism must play the dominant role at high gas pressures. A simplified model is used to obtain an expression for the threshold power density$\omega_\Pi$. Experiments are discussed in which low-threshold gas breakdown was observed. Optical breakdown in heated air is considered; this is distinguished by anomalously low values of $\omega_\Pi\thicksim100-1000$ kW/cm${}^2$ and is due to ionization-thermal explosion of the air in the laser radiation field.