Tunneling ionization of atoms is the basis of many phenomena and techniques, which requires people of a comprehensive understanding of this crucial physical process. Recent experiments have proved the existence of the nonzero initial longitudinal momentum spread at the tunnel exit. However, the initial longitudinal velocity was usually set to zero in the adiabatic regime. In this article, we numerically study the mapping of the initial longitudinal velocity to the spiderlike photoelectron momentum distributions in hydrogen atom by deploying the semiclassical rescattering model and the time-dependent Schrödinger equation. Nonzero longitudinal initial velocity, either an offset or an offset distribution, is considered in the semiclassical rescattering model. Longitudinal and transverse cut-plots of the photoelectron momentum distributions are employed for discussion. The final longitudinal momentum of the electron is found to be sensitive to the initial longitudinal velocity, which offers us a method of determining the information of the initial longitudinal velocity from a photoelectron momentum distribution according to this linear relationship. We unveil that either an offset or an offset distribution for the initial longitudinal velocity can perfectly reproduce the same spiderlike PMDs. The semiclassical results are backed by the full quantum simulation. It is anticipated that more precise investigation is mandatory in deepening the knowledge of the initial longitudinal velocity in strong field ionization of atoms.
