Using the technique of frequency-domain interferometry, we
demonstrate a new way of studying laser-induced breakdown at the
surface of dielectric materials. A theoretical model based on
electron production by multiphoton ionisation, inverse bremsstrahlung
heating, and collisional ionisation is in quantitative agreement with
both the detailed time variation of the dielectric constant and the
pulse width variation of the fluence threshold. From the complex
reflection coefficient measured with the two probe pulse
polarisations in quadrature, we deduce the time variation of the dielectric constant
of silica during breakdown.