Mechanisms of electron injection into laser wakefields by a weak counter-propagating pulseZ. M. Sheng, W. M. Wang, R. Trines, P. Norreys, M. Chen and J. ZhangEur. Phys. J. Special Topics, 175 (2009) 49-55DOI: https://doi.org/10.1140/epjst/e2009-01116-5