Enhanced ionization of an inner orbital of I$_2$ by strong laser fields*

Using pump-probe spectroscopy, strong-field enhanced-ionization is found in an inner orbital of I$_2$. A wavepacket is launched in the B state of I$_2$, whose valence orbitals are $\sigma^2_g$$\pi^4_u$$\pi^3_g$$\sigma^1_u$, and singly ionized to the I + I$^+$ dissociation channel. The ionization signal peaks at two different internuclear separations: 7.3 and 8.7 a.u. The latter shows enhanced ionization of the $\sigma_u$ state, which has been studied before with the I$_2^+$ signal. However, the peak at smaller R corresponds to enhanced ionization of the $\sigma_g$ state. The peak at 8.7 a.u. in the dissociating channel reveals that there could be strong mixing of different molecular orbitals when the two iodine atoms are pulled apart.