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 I₂¹³⁺ using 33-fsec laser pulses. Every even molecular charge state shows an charge asymmetric dissociation channel (I₂²ⁿ⁺→I⁽ⁿ⁺¹⁾⁺+I^(n-1)+) with a significant branching ratio (15--30 %) in addition to the symmetric dissociation channel (I₂²ⁿ⁺→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 (Rc). One channel, in particular, I₂¹⁰⁺→I⁶⁺+I⁴⁺, represents a direct electronic excitation by nonresonant strong-field interaction to a state 20.9 eV above the ground state (I₂¹⁰⁺→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.