  Laser-induced electron tunneling underlies numerous emerging spectroscopic
techniques to probe attosecond electron dynamics in atoms and molecules. The
improvement of those techniques requires an accurate knowledge of the exit
momentum for the tunneling wave packet. Here we demonstrate a photoelectron
interferometric scheme to probe the electron momentum longitudinal to the
tunnel direction at the tunnel exit by measuring the photoelectron holographic
pattern in an orthogonally polarized two-color laser pulse. In this scheme, we
use a perturbative 400-nm laser field to modulate the photoelectron holographic
fringes generated by a strong 800-nm pulse. The fringe shift offers a direct
experimental access to the intermediate canonical momentum of the rescattering
electron, allowing us to reconstruct the momentum offset at the tunnel exit
with high accuracy. Our result unambiguously proves the existence of nonzero
initial longitudinal momentum at the tunnel exit and provides fundamental
insights into the non-quasi-static nature of the strong-field tunneling.
