Direct evidence of the generality of charge-asymmetric dissociation of molecular iodine ionized by strong laser fields

We present ion time-of-flight data showing strong-field ionization of iodine molecules up to I-2(13+) using 33-fsec laser pulses. Every even molecular charge state shows an charge asymmetric dissociation channel (I-2(2n+) --> I(n+1+) + I(n-1+)) with a significant branching ratio (15-30 %) in addition to the symmetric dissociation channel (I-2(2n+) --> In+ + In+) showing the general nature of this process. Furthermore, all charge states are formed at internuclear separations less than the critical separation for electron localization and enhanced ionization (R-c). One channel, in particular, I-2(10+) -->I6+ + I4+, represents a direct electronic excitation by nonresonant strong-field interaction to a state 20.9 eV above the ground state (I-2(10+) --> I5+ + I5+). We show that the interaction of the dipole moment of the asymmetric charge distribution with the external field can account for this energy and the excitation mostly likely involves charge-transfer states. These results establish strong-field ionization of molecules as an efficient method for populating highly excited electronic states. [S1050-2947(98)01612-6].