Self‐Cooling of a Gas, Absorbing Laser Radiation, Due to the Intensification of Photoabsorption Convection

The effect of decrease in the temperature of an absorbing gas in a volume occupied by the beam of a continuously operated laser with decrease in the beam diameter has been investigated experimentally for the case of constant radiation power. Based on theoretical evaluations it has been inferred that the intensification of photoabsorption convection is the reason for the temperature decrease. It has been shown that an optimum diameter of the beam exists for which (for a given radiation power) the temperature of the absorbing gas attains its maximum (accordingly the maximum is attained by the yield of the reaction of thermal decomposition of the gas in the case of its pyrolysis). When the diameter is optimum the heat loss from the beam volume by heat conduction is equal to the heat loss by photoabsorption convection. It has been inferred that in constructing an adequate theoretical model of laser‐induced chemical vapor deposition it is necessary to take into account the convection.