The optical breakdown induced in air at atmospheric pressure by Nd:YAG Q-switched laser pulses is studied in terms of the spectral features of the emitted radiation in the wavelength range 180–850 nm during the first 200 ns after the laser pulse onset. During the plasma build up, radiation emission features intense, broadband, and structureless ultraviolet–visible spectra before the appearence of atomic lines on the microsecond scale. Also, the emitting plasma kernel, imaged during the buildup and decay stages in the early tens of nanoseconds, turns out to have a size of ∼0.3 mm and a volume of ∼0.02 mm3. The coupling of direct emission data and broadband absorption measurements allowed us to retrieve peak values of electron temperature above 100,000 K and of an optical depth of the order of unity, under the assumptions of local thermodynamic equilibrium and a homogeneous kernel. The simultaneous occurrence of such temporal, spatial, and spectral features of the plasma kernel suggests its exploitation as a pulsed, bright, and broadband ultraviolet–visible light source.
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