Investigation of tunneling processes in laser-induced ionization of argon

A method of determining the laser intensity necessary for ionization of atomic states is described. Results are presented for ionization of argon states I–V at 10−4 Torr using a short (25 psec)-pulse neodymium laser at intensities up to 4×1016 W cm−2. The results agree with computer calculations of the Keldysh tunneling model in the regime γ⩽1 and with results previously obtained in helium. The electron energy spectra technique employed further verifies the existence of the ponderomotive force.