Based on the first-order perturbation theory, we show that the 8 wave function of a photoelectron is a wave packet with the same 9 width as the incident light pulse. Photoelectron detection measure-10 ments revealed that the widths of signal pulses were much shorter 11 than the light pulse and independent of the origin (photoemission or 12 other noises), which is an experimental observation of the wave 13 function collapse. Signal pulses of photoelectrons were distributed 14 along the time axis

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... within the same width as the light pulse, con-15 sistent with the interpretation of a wave function as a probability 16 distribution.