Ionization suppression of ${\mathrm{Cl}}_2$ molecules in intense laser fields

The strong field ionization of ${\mathrm{Cl}}_2$ molecules is investigated by using an ultrashort pulse $\mathrm{Ti}$:sapphire laser. A spatial imaging technique is used in such measurements to reduce the effect of spatial integration. ${\mathrm{Cl}}_2$ shows strong ionization suppression as do other diatomic molecules having valence orbitals with antibonding symmetry $\left({\mathrm{O}}_2{\mathrm{,S}}_2\right)$ when compared with the field ionization of atoms with nearly identical ionization potential. A more general molecular tunneling ionization model is proposed, and the calculations are in reasonable agreement with the measurements. Our results support that antibonding leads to ionization suppression, a trend that only ${\mathrm{F}}_2$ goes against and that needs to be further investigated.

Figure 1

Schematic setup of the imaging spectrometer. A side cut of the SIMION three-dimensional model showing the electrodes, the equipotential lines, and parts of the paths of three different groups of ions created along the beam propagation axis and imaged on a position sensitive detector (PSD) are illustrated.

Figure 2

Typical detection images representing the target area. The $z$ axis is the laser beam axis while the ion extraction axis points towards the page. The ${\mathrm{Xe}}^{+}$ and ${\mathrm{Cl}}_2^{+}$ signals were separated in the time-of-flight spectrum by software gates. The laser pulse energy was $51\mu \mathrm{J}$ while the mixed-gas pressure was kept at $3\times {10}^{-7}\text{Torr}$.

Figure 3

Single ionization yields for $\text{Xe}$ and ${\text{Cl}}_2$ for linearly polarized laser pulses obtained using the imaging technique. The data correspond to a laser pulse energy of $51\mu \mathrm{J}$. Calculations based on the MO-PPT model are shown by curves. The dotted line indicates the ionization probability of a virtual $\mathrm{Xe}$ atom with $\mathrm{IP}\mathrm{\text{of}}11.48\mathrm{eV}$, i.e., an IP identical to that of ${\mathrm{Cl}}_2$. Thus, the strong ionization suppression of ${\mathrm{Cl}}_2$ becomes evident.