  The response time of the electron to light in photoemission is difficult to
define and measure. Tunneling ionization of atoms, a strong-laser-induced
photoemission process, provides a semiclassical case for visiting the problem.
Here, we show that the response time can be determined at the boundary between
quantum and classic. Specifically, tunneling is instantaneous but a finite
response time (about 100 attoseconds) is needed for the state of the tunneling
electron to evolve into the ionized state around tunnel exit. This time can be
well described with a compact expression related to some basic laser and atomic
parameters. Moreover, it can be directly mapped to and easily decoded from
photoelectron momentum with a simple mapping, allowing an unambiguous
measurement. These results shed light on definition and measurement of the
response time of photoemission.
