Laser-excitation technique for the measurement of absolute transition probabilities of weak atomic lines

A new laser selective-excitation technique for the determination of absolute transition probabilities of weak allowed, intersystem, and forbidden atomic lines has been developed. It exploits the fact that the oscillator strength is proportional to the number of stimulated absorptions and emissions produced by a narrow-band laser pulse of known energy which is in resonance with an atomic transition. The method has been tested by studying the 3s²¹S₀-3s3p³P₁ᵒ intersystem transition in Mg I. Magnesium vapor was produced in a resistively heated oven and excited with a Nd:YAG laser-pumped dye laser. The lower-level population was monitored with the hook method while the equivalent width of the ³Pᵒ-3s4d³D multiple was used to determine the ³P₁ᵒ level population, which is equal to the number of stimulation absorptions. A laser-produced plasma was used as a background continum source. The laser energy was determined with a disk calorimeter. We show that multiphoton, collisional, and power-dependent processes and mode structure in the laser do not affect the measurement. Our result, A=(2.19±0.30)×10⁺² sec^-1, or f=(2.06±0.29)×10^-6, is in good agreement with theoretical and other experimental data. The technique could be applied to lines several orders of magnitude weaker than the MgI intersystem line.