Comparison of the strong field ionization of N$_2$ and F$_2$: A TDDFT study

We compare strong field ionization probabilities of N$_2$ and F$_2$ molecules using time-dependent density functional theory (TDDFT) calculations. Accurate nuclear potentials and ground vibrational wave functions are incorporated into our study. For both molecules, the effect of molecular vibration is small, while that of the molecular orientation is significant. When compared to the ionization probability of a molecule at the equilibrium geometry, we estimate the effect of the ground state vibration to be within 3\% for N$_2$ and within 6\% for F$_2$, in the intensity range from 1 to $5\times 10^{14}$ W/cm$^2$. The molecular orientation dependent ionization probabilities for both molecules at various intensities are presented. They are strongly dependent on the laser intensity, and the anisotropy diminishes when the laser intensity is high. For laser intensities of 1.6 and 2.2 $\times 10^{14}$ W/cm$^2$ we find ionization probability ratios of 5.9 and 4.3, respectively, for the parallel versus perpendicular orientation of N$_2$. This is reasonably consistent with experimental measurements [1,2] For randomly oriented molecules, the ratio of the probabilities for N$_2$ and F$_2$ increases from about 1 at $10^{14}$ W/cm$^2$ to 2 at $4\times 10^{14}$ W/cm$^2$, which agrees well with experimental results [3]. [1] I. V. Litvinyuk {\it et al.}, Phys. Rev. Lett. {\bf{90}}, 233003 (2003). [2] D. Pavicic {\it et al.}, Phys. Rev. Lett. {\bf{98}}, 243001 (2007). [3] M. J. DeWitt {\it et al.}, Phys. Rev. Lett. {\bf{87}}, 153001 (2001).