We report here the direct measurements of electron momentum distributions for ethylene using the ($e$,$2e$) reaction at different impact energies from 400 to 2400 eV. The “turn up” effects in the ($e$,$2e$) cross sections of the $1b_{3g}$ orbital compared with the plane-wave impulse approximation calculations were observed at low and high momentum regions, and such discrepancies become smaller with the increase of the impact electron energies. It is suggested that the observed discrepancies are due to the distorted-wave effects in molecules, while appropriate theoretical calculations using distorted waves in molecules could not be achieved until now.

• Received 27 October 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.163201