  The balance of the linear photon momentum in multiphoton ionization is
studied experimentally. In the experiment argon and neon atoms are singly
ionized by circularly polarized laser pulses with a wavelength of 800 nm and
1400 nm in the intensity range of 10^{14} - 10^{15} W/cm^2. The photoelectrons
are measured using velocity map imaging. We find that the photoelectrons carry
linear momentum corresponding to the photons absorbed above the field free
ionization threshold. Our finding has implications for concurrent models of the
generation of terahertz radiation in filaments.
