Sub-nanosecond delays of light emitted by streamer in atmospheric   pressure air: analysis of N$_2$(C$^3\Pi_u$) and N$_2^+($B$^2\Sigma_u^{+}$)   emissions and fundamental streamer structure
  Both experimental and theoretical analysis of an {ultra-short} phenomena occurring during the positive streamer propagation in atmospheric pressure air is presented. {With tens-of-picoseconds and tens-of-microns precision, it is shown that when the streamer head passes a spatial coordinate, emission maxima from N$_2$ and N$_2^+$ radiative states follow with different delays. These different delays are caused by differences in the dynamics of populating the radiative states, due to different excitation and quenching rates.} {Associating the position of the streamer head with the maximum value of the self-enhanced electric field, a delay of 160$\,$ps was experimentally found for the peak emission of the first negative system of N$_2^+$.} {For the first time, a delay dilatation was observed experimentally on early-stage streamers and clarified theoretically.} {Thus, in the case of second positive system of N$_2$ the delay can reach as much as 400$\,$ps.} In contrast to the highly-nonlinear behaviour of streamer events, it is shown theoretically that emission maxima delays linearly depend on the ratio of the streamer radius and its velocity. This is found to be one of the fundamental streamer features and its use in streamer head diagnostics is proposed. Moreover, radially-resolved spectra are synthetized for selected subsequent picosecond moments in order to visualize spectrometric fingerprints of radial structures of N$_2$(C$^3\Pi_u$) and N$_2^+$(B$^2\Sigma_u^+$) populations created by streamer-head electrons. 
Tomas Hoder, Zdenek Bonaventura, Anne Bourdon (EM2C), Milan Simek
physics.plasm-ph