Experimental Evidence for Selection Rules in Multiphoton Double Ionization of Helium

We report on the observation of phase space modulations in the correlated electron emission after strong field double ionization of helium using laser pulses with a wavelength of 394~nm and an intensity of $3\cdot10^{14}$W/cm$^2$. Those modulations are identified as direct results of quantum mechanical selection rules predicted by many theoretical calculations. They only occur for an odd number of absorbed photons. By that we attribute this effect to the parity of the continuum wave function.