Laser mass-spectrometric studies of optical damage in caf₂

Resonance-ionization mass spectrometry (RIMS) has been used as a diagnostic for interrogating optical damage events. RIMS involves multistep laser photoionization, generally through real intermediate states, followed by mass separation and detection. Uncoated CaF2 substrates were irradiated by pulses from a Nd:YAG laser operating at ω, ω3, and ω4. Interrogation of the ablated material revealed the presence of Ca atoms and CaF radicals after the onset of breakdown. Velocity distributions of these species have been measured. At an ablation wavelength of 1.06 μm (ω), we observe a thermal (830-K) Maxwell-Boltzmann distribution. At shorter wavelengths, deviations from purely Maxwellian distributions are observed. In addition, the multiphoton ionization excitation spectrum of CaF (F²À ← X²±Σ+) has been recorded as a function of ablation wavelength, and from this qualitative estimates of internal excitation are made. The results are discussed in terms of concepts developing in the study of laser damage and ablation.