  The ejection of material during laser ablation gives rise to the development
of discontinuities in the ambient gas. Several of these discontinuities are
observed and characterized, including externally and internally propagating
shock waves, contact surface, and the ionization front. Qualitative
experimental observations and analysis of these discontinuities is presented.
Results from shadowgraphy enabled determination of an irradiance threshold
between two different ablation mechanisms, and determination of several stages
of plasma plume evolution. Consideration of the refractive index as a dynamic
sum of the contributions from gas and electrons led to separate identification
of ionization front from the contact surface. Furthermore, ionization front was
observed to lead the shock wave at the earlier stage of the ablation.
