Strong-Field Molecular Ionization from Multiple Orbitals

We demonstrate strong-field ionization from multiple orbitals of excited-state uracil molecules. The molecules are excited to the first bright state by an ultrafast laser pulse in the deep ultraviolet and then ionized with a strong-field laser pulse in the near infrared during ultrafast relaxation back down to the ground state. We measure time- and angle-dependent ion yields for multiple fragments created by strong-field ionization, and interpret the temporally and angularly resolved yields via ab initio electronic structure calculations. We find that the angular distribution for the electron removed from the lowest unoccupied molecular orbital follows the symmetry of the molecular orbital, whereas ionization of the molecule by removing electrons from deeper bound orbitals is more complicated.

Popular Summary

When a molecule is exposed to a strong non-resonant laser field, the electric potential binding electrons to the molecule can be severely distorted, allowing some of the bound electrons to escape. This process, known as strong-field ionization has been the focus of many recent studies. It is a key component of attosecond (${10}^{-18}\sec $) science, and has enabled imaging of molecular orbitals with subangstrom spatial resolution, as well as tracking of the motion of both nuclei and electrons. In this paper that combines experimental and theoretical work, we address the qualitative differences between strong-field ionization of a polyatomic molecule through the removal of a weakly vs tightly bound electron. These differences are an important part of understanding the attosecond dynamics of multi-electron molecular systems and in imaging electrons beyond the most weakly bound one in such systems.

In the experiment we apply a commonly used pump-probe sequence of laser pulses to a dilute gas of uracil molecules. The first (pump) pulse brings the molecules into an excited electronic state, by removing an electron from an occupied orbital and transferring it to a previously unoccupied higher lying orbital. The second (probe) pulse ionizes the molecules and initiates dissociation in a fraction of them. We measure the yields of the molecular ions as a function of both the time delay between the two laser pulses and the angle between their polarization vectors. Interpreting the data with the aid of ab initio electronic-structure calculations, we demonstrate an interesting contrast: While the probability for the escape of an electron as a function of the angle between pump and probe polarization from the highest occupied orbital follows the symmetry of that orbital, the escape of an electron from a deeper bound orbital does not, since it involves the rearrangement of electrons among a number of orbitals due to electron correlation as well as the coupling between the resulting ionic states. This latter observation is new.

This work not only highlights the importance of electron correlation in strong-field ionization, but also extends the promise of observing, and perhaps even controlling, the dynamics of electronic wave packets launched by ionizing molecules to a coherent superposition of multiple ionic states.

Figure 1

Geometry of ground-state uracil and transition dipole moment for the ${\mathrm{S}}_0\to {\mathrm{S}}_2$ electronic transition.

Figure 2

Time-of-flight mass spectra of uracil for negative and positive time delays ($+/-200\mathrm{fs}$). Both spectra are normalized to the parent-ion yield.

Figure 3

Top panel: Pump-probe signals from 42, 41, and 28 amu fragments. The signal for each fragment is normalized. Bottom panel: Pump-probe ion yield vs pump polarization at 200 fs pump-probe delay. The azimuth angle corresponds to the relative polarization between the pump and the probe beams, while the radial distance is the ion count in arbitrary units.

Figure 4

Dominant electronic configuration and characters for relevant states of the neutral molecule and the ion. The participating orbitals are shown as well. The orbitals are shown for the molecule in the same orientation as shown in Fig. 1.

Figure 5

Pump-probe ion yield vs pump polarization and pump-probe delay. Top panel: For 42 amu. Bottom panel: For the parent ion. The azimuth angle corresponds to the relative polarization between the pump and the probe beams, while the radial distance is proportional to the ion yield in arbitrary units. Both the parent ion and 42 amu show a decay in the yield with increasing pump-probe delay.