Experimental evidence for electron repulsion in multiphoton double ionization

We have measured the coincident momenta of both electrons emitted in argon double ionization at 780 nm and 4.7 x 10(14) W cm(-2). For electrons which are emitted with very small momentum transverse to the electric field, we find that the two electrons have highly unequal momentum. This is in contrast to the situation for larger transverse momenta for which most electrons are found with similar momentum. We interpret this observation as a manifestation of electron repulsion. We also identify contributions from electron impact excitation.