Correlation-Driven Transient Hole Dynamics Resolved in Space and Time in the Isopropanol Molecule

The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or hole) dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump–x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first tentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.

Popular Summary

One of the most fundamental problems in the study of how matter responds to light is the electronic response of a quantum system to an impulse. This can trigger oscillations in the charge density, leading to transfer of charge across molecular bonds, which, in turn, couples to other degrees of motion (e.g., chemical change) in the system. This charge transfer is central to a deeper understanding of a broad range of applications such as mitigating radiation damage, controlling photochemistry, and optimizing photovoltaics. Here, we combine recent theoretical tools with recent developments in x-ray laser sources to demonstrate a new experimental method for directly measuring ultrafast electronic dynamics.

We use femtosecond pulses from an x-ray free-electron laser to carry out measurements of sudden photoionization in a small molecule, isopropanol. The photoionization leaves behind a positively charged hole in the molecular orbital, whose dynamics we observe and then interpret using our advanced theoretical approach. Removal of an electron from different molecular orbitals produces distinct types of temporal evolution, which we observe in experiments and reproduce with our theoretical calculations.

The measurement method we develop opens the door for probing ultrafast electronic dynamics in larger molecules and more complex materials.

Figure 1

Photoelectron spectrum of isopropanol, recorded using XUV synchrotron radiation at 90 eV (blue line). Gray vertical lines mark the spectral intensity calculated using ADC(2)x, colored lines are contributions to the spectral intensity of different orbitals $8a$ (purple) to $5a$ (green). Black dashed line denotes the double-ionization potential of isopropanol at 28 eV, measured at EPFL using spectrally filtered HHG in a photoelectron-photoion coincidence set up. Inset: Ball and stick diagram of the isopropanol gauche conformer. Our resonant x-ray probe interrogates the local electron density at the oxygen site (red). Abbreviations: DIP—double ionization potential.

Figure 2

(a) Experimental setup with two color pulses (pump central photon 502 eV, probe central photon energy 514 eV) from the fresh slice mode with delay controlled by chicane, the interaction point equipped with a hemispherical electron analyzer and a downstream x-ray spectrometer to determine the pulse spectra and relative energies on every shot. (b) Pump step opens a number of valence ionization channels including creation of inner valence holes (IVH). The delayed probe pulse can strongly interact via a O $1a\to \mathrm{I}\mathrm{V}\mathrm{H}$ transition (a channel only open if inner valence ionization has occurred). Following this there can be Auger-Meitner decay back to the O $1a$ hole with the emission of Auger-Meitner electrons of characteristic energy that can be detected.

Figure 3

(a) Probe central photon energy dependence of residual signal at delay $\sim 0\mathrm{fs}$. (b) Time dependence of residual resonant high kinetic energy electron signal at higher probe photon energy, about expected $1a\to 7a$ transition (blue), and (c) time dependence of residual resonant high-energy electron yield at lower probe photon energy about expected $1a\to 6a$ transition (red). Fits to an exponentially modified Gaussian functional form are shown in black and the extracted decay time is given in each panel.

Figure 4

Time dependence of excess Auger-Meitner electrons for a resonant probe, compared to a nonresonant probe. The signal is integrated over the region of peak flux, between binding energies of 17 and 25 eV, revealing a time-dependent resonant Auger-Meitner signal. A fitted exponential decay convolved with a Gaussian measurement response function is fitted by bootstrapping across time and energy bins (see the Appendix pp1) and is shown in black. This resonance, which maps the $6a$ hole population, reveals a lifetime limited by the measurement response function of $1\pm 2\mathrm{fs}$.

Figure 5

Hole survival probability of different hole states arising from removal of an electron from (neutral) inner valence orbitals $8a$ (purple, outermost) to $5a$ (green, innermost) after sudden ionization of isopropanol. The colored survival probability curves correspond to a mixture ($1:1$) of the two lowest conformers of the molecule. The black curves correspond to the averaged dynamics over 500 geometries at a temperature of 300 K. The Hartree-Fock orbitals of the neutral are shown in insets for the gauche conformer.

Figure 6

Transient high kinetic energy ${\mathrm{CH}}_3^{+}$ ion yield, attributed to the decay of the intermediate cationic channel in isopropanol using an XUV pump at 20 eV and 650 as duration and a 4 fs IR probe to induce transitions to the second ionization continuum.