Mechanism of gas breakdown by lasers

When intense laser pulses are focused down to yield radiation intensities of about 6 × 10 11 w cm -2 , it is observed that air at atmospheric pressure breaks down with spark emission. It is proposed that the breakdown results from cascade or avalanche ionization, the initial electron being produced by an unspecified multi-photon transition. A free electron absorbs energy from the radiation field by making a free-free transition in the field of an atom (the system of electron plus atom constituting a negative ion). This process (bremsstrahlung absorption) is the quantum-mechanical description of the classical process considered to explain microwave breakdown-absorption of energy Δ W at electron-atom collisions due to a change of phase of the oscillatory electron velocity relative to the oscillatory electric field. It is wrong to regard bremsstrahlung absorption as a separate process from microwave breakdown process. Also the classical theory does not fail when Δ W < hv , because the absorption of a photon cannot be measured in a time less than the reciprocal of the transition probability for the free-free absorption, and during this time many electron-atom collisions may occur. For the case of atomic hydrogen gas the cross section for free-free absorption in the negative ion is known, and on the grounds that the classical rate of absorption of energy should equal the quantum mechanical rate of absorption, an equivalent electron-atom cross section of 7.4 × 10 -16 cm 2 is derived. After an ionization a secondary electron gains sufficient energy in the above radiation field to cause further ionization in a time of 0.73 × 10 -10 sec, with losses being neglected. After about 43 generations of secondary electrons (for which a time of 3 × 10 -9 sec is required) the gas in the focal volume (4 × 10 -6 cm 3 ) is fully ionized. Only when the gas is 100% ionized will the plasma strongly absorb the laser pulse by free-free transitions in the neutral atom system. Finally, the neglect of losses due to diffusion and recombination is discussed, and it is shown that the atom or positive ion gas will come to thermal equilibrium with the electron gas only after the cascade ionization is complete. A final peak gas temperature of the order of 10 7 °K should be reached.