We evaluate particle production in highly asymmetric head-on collisions of
lasers pulses due to non-perturbative coherent action of many photons. We
obtain the yield of electron-positron pairs, which is controlled by the photon
content of the weaker pulse, and show that the wavelength of the weaker pulse
and the momentum asymmetry determine laboratory energy of the produced
particles.Comment: 4 pages, 1 figur