Atomemissionsspektroskopie mit laseranregung an graphit unter erhöhtem Druck—I. Plasmaeigenschaften

The paper describes the vaporization of graphite, the plasma form, the plasma properties and the interaction of laser radiation with the plasma for when the beam of a free running laser is focused on graphite in air at atmospheric pressure and in He, O₂, or Ar at pressures in a range from (1–40) × 10⁵ Pa.The power densities of the focused laser radiation (focal distances of condenser lens 150 to 50 mm) reach 10⁷$\text{W}\text{cm}{}_2$. The extent of vaporization depends on the pulse energy (under 40 × 10⁵ Pa not unambiguously, however) and the ambient pressure.The plasma is observed with normal, framing (to 10⁷ frames per second), and streak photography (to 25 $\text{mm}\text{μs}$). The plasma form depends, among other things, on the ambient gas, the power density of the laser pulse on the sample, and the ambient pressure. Increasing the pressure in the range from 25 × 10⁵ to 40 × 10⁵ Pa modifies the plasma form from a “plume” to a “fungus” with toroidal head. The average velocity of the plasma particles in air is about 3 × 10⁵$\text{cm}\text{s}$, which is 10 times higher than the velocity of the luminous plasma front. Under increased pressure a strong interaction between the laser radiation and the plasma takes place resulting in absorption by inverse bremsstrahlung (>50% in 40 × 10⁵ Pa He). In this way the luminosity of the plasma is enhanced by a factor of 100 (delay to the start of the laser pulse about 50 μs). This absorption of the laser beam by inverse bremsstrahlung becomes the dominating mechanism of excitation of the plasma and causes a strong temperature gradient from the point of absorption to the fringe.