Tunnel ionization of diatomic atmospheric gases (N2, O2) by laser radiation

We present the results of calculation of the rates of tunnel ionization of N-2 and O-2 molecules from the ground and excited vibrational states by linearly polarized laser radiation. These data are necessary for the theoretical description of filamentation. For N-2, we consider ionization from the 3 sigma(g) and 1 pi(u) shells, which give comparable contributions. For O-2, we consider ionization from both the ground X-3 Sigma(-)(g) and excited a(1)Delta(g) electronic terms. We demonstrate that for N-2, the ionization rate is maximum when the electric field vector of laser radiation is directed along the molecular axis (for 3 sigma(g) shell) or perpendicular to it (for 1 pi(u) shell), whereas for the O-2 molecule, for both electronic states considered, the rates have maxima when the angle between the electric field and molecular axis is equal to 45 degrees.