Picosecond delays of light emitted by streamer in atmospheric pressure air: analysis of N2(C3Πu) and N+2 (B2Σ+u ) emissions and fundamental streamer structure
Résumé
Both experimental and theoretical analysis of ultra-short phenomena occurring dur- ing the positive streamer propagation in atmospheric pressure air is presented. It is shown that as the streamer passes a spatial coordinate, emission maxima from radiative states with different excitation energies follow with different delays. Associating the po- sition of the streamer head with the maximum value of the self-enhanced electric field, a maximum delay of 220 ps was experimentally found for the peak emission of the second positive system of molecular nitrogen. On a later stage of streamer development this delay can reach as much as 400 ps. Different delays for first negative and second positive systems emission maxima are caused by differences in the dynamics of populating the radiative states, due to different excitation rates. It is shown that emission maxima delays linearly depend on the ratio of streamer radius and its velocity. This is found to be one of the fundamental features of the streamer structure and its use in streamer diagnostics is proposed. Moreover, radially-resolved spectra are synthetized for selected subsequent picosecond moments in order to visualize spectrometric fingerprints of radial structures of N2(C3Πu) and N+2 (B2Σ+u ) populations created by streamer-head electrons.
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