A recent study of the wavelength dependence of tunnel ionization of O2 in strong laser fields confirms that interference between the electron wave packets ionized from different cores, which is not included in the MO-ADK theory, actually plays an important role.Although experimental investigations of tunnel ionization of molecules are frequently carried out based on high-order harmonic generation or above-threshold ionization techniques, they usually lack the capability to directly access the lower-lying orbitals of molecules due to the difficulties in distinguishing the contributions to the total ionization rate from the individual orbitals.

Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) have shown that the fluorescence emission from molecules provides an efficient way to investigate the alignment-dependent ionization from the lower lying orbitals. As an all optical means concerning only the wavelengths in the visible and near-ultraviolet regions,the technique is simple. [PHYSICAL REVIEW LETTERS.2013,111,133001]

In principle, this technique can be applied to a wide range of molecules (e.g., N₂, O₂, HCl, H₂O, etc.), as far as their fluorescence emissions are in the suitable spectral ranges and detectable by the available spectrometers. Moreover, it is noteworthy that though the influence of anisotropy on fluorescence radiation itself has been successfully minimized with a back-detection scheme in our experiment for a linear molecule CO₂, it might still add some extra complexities and difficulties for applying this method to more complex molecules, e.g., nonlinear molecules, which remains an interesting topic for further investigation. Because of the exceptionally rich physics in the ionization process of molecules, the unique capability of directly probing the ionization from the lower-lying orbitals in an angular-resolvable manner will provide us not only the potential of retrieval of geometrical information of lower-lying orbitals, but also an ideal test bed for existing theoretical models.