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

Polished silica (synthetic SiO2 glass) samples were ablated to vacuum by 26 6 nm laser pulses at a flounce of 110 J/cm(2). The concentration of ground level atomic silicon in the plume was measured as a function of time, by la ser-induced fluorescence (LIF), at distances of 5-7 mm from the ablated poi nt. The angular velocity distribution was found to be highly asymmetric wit h respect to surface normal, as the LIF signal dropped abruptly at angles a pproaching the laser direction. The symmetric morphology of the craters tha t 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 phot ochemical channel is suggested to be dominant in this case. Doppler broaden ing, calculated from the lateral velocity distribution, was found to be con sistent with the broadening of the silicon absorption spectrum. (C) 2001 Am erican Institute of Physics.