Velocity angular distribution of ground level atomic silicon in the plume of laser ablated silica

Abstract

Polished silica (synthetic SiO2 glass) samples were ablated to vacuum by 266 nm laser pulses at a flounce of 110 J/cm2. The concentration of ground level atomic silicon in the plume was measured as a function of time, by laser-induced fluorescence (LIF), at distances of 5–7 mm from the ablated point. The angular velocity distribution was found to be highly asymmetric with respect to surface normal, as the LIF signal dropped abruptly at angles approaching the laser direction. The symmetric morphology of the craters that were formed by ablation indicates that the above asymmetry is not likely to be attributed to the interaction of the laser or the ablated plume with the surface. Therefore, it is suggested that the silicon atoms, ejected in the direction of the incident laser beam, were preferentially ionized via a multiphoton off-resonance absorption process. The velocity of the silicon atoms was detected in the 1–13 km/s range. Since the velocity distribution for a thermal ablation process is expected to extend down to zero, the photochemical channel is suggested to be dominant in this case. Doppler broadening, calculated from the lateral velocity distribution, was found to be consistent with the broadening of the silicon absorption spectrum.

Advances in atomic emission, absorption and fluorescence spectrometry and related techniques

“…257 The distribution of ground state Si atoms in a plasma laser-ablated from polished silica has been measured by LIF as a function of time at a distance of 5-7 mm above the surface. 258 The angular distribution of atom velocities was highly asymmetric and the LIF signal dropped abruptly at angles approaching the laser direction. This effect was attributed to preferential ionization by multiphoton off-resonance absorption.…”