Abstract
Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C2D2, C2D4 and C2D6 reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.
Highlights
Strong-field ionization is a key topic in ultrafast science since it is an essential step in attosecond pulse generation[1,2,3,4,5], serves as a probe of electronic and nuclear dynamics[6,7,8,9,10,11,12,13] and is used to image molecular orbitals[14,15,16,17]
In this study we show that to correctly predict the angular character of strong-field ionization of small hydrocarbon molecules, formerly unoccupied molecular orbitals should be taken into account as well
We focus on a specific process initiated by intense few-cycle laser pulses in acetylene (C2D2), ethylene (C2D4), and ethane (C2D6), namely single ionization of the parent molecule followed by hydrogen elimination
Summary
Strong-field ionization is a key topic in ultrafast science since it is an essential step in attosecond pulse generation[1,2,3,4,5], serves as a probe of electronic and nuclear dynamics[6,7,8,9,10,11,12,13] and is used to image molecular orbitals[14,15,16,17]. Subsequent work has included examples of diatomics (CO, N2, HCl)[26, 56,57,58] where several orbitals participate in the tunneling process These studies and other recent efforts exploring strong-field molecular ionization of multi-electron systems[56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71] suggest that a full understanding of the ionization process and associated angular structure requires consideration of not just the highest occupied molecular orbital (HOMO) and the neighboring HOMO-1. We present a series of measurements that illustrate that these Rydberg contributions play an important role in the strong-field ionization of small hydrocarbon molecules starting from their electronic ground state
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