Experimental evidence for selection rules in multiphoton double ionization of helium and neon

We report on the observation of a multipeak structure in the correlated two-electron energy distribution from strong-field double ionization of helium using laser pulses with a wavelength of 394 nm and an intensity of 3 x 10(14) W/cm(2). For selected regions of electron emission angles the peaks emerging at energies corresponding to an odd number of absorbed photons are suppressed. We interpret this as direct results of quantum-mechanical selection rules. For neon these features occur for even photon numbers. By that we attribute this effect to the parity of the continuum wave function. A comparison with analogous data for single-photon double ionization is given.