A New Sub-Doppler Fluorescence Imaging Method in Studying Laser Ablation of B Atoms at 248 nm

A new sub-Doppler fluorescence imaging method has been applied to study the laser ablation of B atoms at 248 nm with a power density of ∼1.7 × 108 W/cm2. Two-dimensional velocity distributions of the laserablated B( P°1/2,3/2) atoms are measured. The angular distributions of the ablated B atoms are velocitydependent; the higher the speed of the B atom is, the more centralized the distribution of the forward peaking will be, indicating that the ablation plume undergoes an unsteady adiabatic expansion. The speed distributions of the B atoms are well fitted to the shifted Maxwellian functions and are found to be bimodal, including a fast component with a temperature of 1.8 × 104 K topping out at 5.8 eV and a slow component of 3.1 × 103 K reaching a maximum at 2.8 eV. While a plasma reaction is responsible for the fast component, the slow one results from photochemical processes.