Multiple ionization of atomic and molecular iodine in strong laser fields

We compare the multiple ionization of atomic and molecular iodine using 100-fs laser pulses at 610 nm over a wide range of laser intensities (2×${10}^{12}$ W/${\mathrm{cm}}^2$-3×${10}^{15}$ W/${\mathrm{cm}}^2$). We show that the elongation of the molecular ion to a critical distance drastically enhances the multiple ionization probability of the molecule compared to that of the atom. In particular, the yield of the ten-times-ionized molecule is higher than that of the three-times-ionized atom around ${10}^{14}$ W/${\mathrm{cm}}^2$. This observation supports recent theoretical work on the role of the electron localization in intense-field molecular ionization. Around ${10}^{15}$ W/${\mathrm{cm}}^2$, the quintuple ionization of atoms is observed, whereas fast ${\mathrm{I}}^{2+}$ fragments arising from the ${\mathrm{I}}_2$ Coulomb explosion are not further ionized, even with 300- or 500-fs laser pulses. This result supports the concept of molecule stabilization in strong laser fields.