Calibration of the initial longitudinal momentum spread of tunneling ionization

Recently, the initial longitudinal momentum at the tunnel exit has attracted considerable interest in the strong-field community. We have measured the photoelectron momentum distribution of Kr atoms at an intensity of 1.2×10¹⁴0.28em0exW/cm² and the ellipticity-resolved angular distribution of Ne atoms at an intensity of 3.6×10¹⁴0.28em0exW/cm² (790-nm, 25-fs laser pulses). To shed more light on the debated issue, we have introduced an initial longitudinal momentum spread at the tunnel exit to compare with the measured photoelectron angular distributions using a dedicated semiclassical model. The simulation indicates that the value of this longitudinal momentum spread is in the range 0.0--0.1 a.u., which is less than the initial transverse momentum spread predicted by the tunneling theory.