Optical study of radicals (OH, O, H, N) in a needle-plate bi-directional pulsed corona discharge F. Liu, W. Wang, W. Zheng and Y. Wang Eur. Phys. J. D, 38 3 (2006) 515-522 Published online: 21 March 2006 DOI: 10.1140/epjd/e2006-00059-5