Laser intensity determination using nonadiabatic tunneling ionization of atoms in close-to-circularly polarized laser fields.

We conceive an improved procedure to determine the laser intensity with the momentum distributions from nonadiabatic tunneling ionization of atoms in the close-to-circularly polarized laser fields. The measurements for several noble gas atoms are in accordance with the semiclassical calculations, where the nonadiabatic effect and the influence of Coulomb potential are included. Furthermore, the high-order above-threshold ionization spectrum in linearly polarized laser fields for Ar is measured and compared with the numerical calculation of the time-dependent Schrödinger equation in the single-active-electron approximation to test the accuracy of the calibrated laser intensity.