Internuclear Separation Dependent Ionization of the Valence Orbitals of I2 by Strong Laser Fields

Using a pump-dump-probe technique and Fourier-transform spectroscopy, we study the internuclear separation R dependence and relative strength of the ionization rates of the pi and sigma electrons of I-2, whose valence orbitals are sigma(2)(g)pi(4)(u)pi(4)(g)sigma(0)(u). We find that ionization of the highest occupied molecular orbital (HOMO)-2 (sigma(g)) has a strong dependence on R while the HOMO and HOMO-1 do not. Surprisingly, the ionization rate of the HOMO-2 exceeds the combined ionization rate of the less bound orbitals and this branching ratio increases with R. Since our technique produces target molecules that are highly aligned with the laser polarization, the sigma orbitals will be preferentially ionized and undergo enhanced ionization at larger R compared to the pi orbitals. Nevertheless, it is highly unusual that an inner orbital provides the dominant strong field ionization pathway in a small molecule.