Angle dependence of dissociative tunneling ionization of NO in asymmetric two-color intense laser fields

Abstract

Dissociative tunneling ionization of nitric oxide (NO) in linearly polarized phase-locked two-color femtosecond intense laser fields (45 fs, $\ensuremath{\lambda}=800$ and 400 nm, total field intensity $I=1\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$) has been studied by three-dimensional ion momentum imaging. The ${\mathrm{N}}^{+}$ fragment produced by the dissociative ionization, $\mathrm{NO}\ensuremath{\rightarrow}\mathrm{N}{\mathrm{O}}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{N}}^{+}+\mathrm{O}+{e}^{\ensuremath{-}}$, exhibits a butterflylike momentum distribution peaked at finite angles with respect to the laser polarization direction. In addition, a clear dependence on the relative phase between the two laser fields is observed, showing that the tunneling ionization occurs efficiently when the electric field points from N to O. For the highest kinetic energy component, the observed orientation dependence is well explained with theoretical calculations by the weak-field asymptotic theory for the 2\ensuremath{\pi} highest occupied molecular orbital (HOMO). On the other hand, the peak angle shifts toward the laser polarization direction as the kinetic energy decreases, indicating that pathways other than direct ionization from the HOMO contribute to the dissociative ionization.