Attosecond delays between dissociative and non-dissociative ionization of polyatomic molecules

Abstract

The interplay between electronic and nuclear motions in molecules is a central concept in molecular science. To what extent it influences attosecond photoionization delays is an important, still unresolved question. Here, we apply attosecond electron-ion coincidence spectroscopy and advanced calculations that include both electronic and nuclear motions to study the photoionization dynamics of CH4 and CD4 molecules. These molecules are known to feature some of the fastest nuclear dynamics following photoionization. Remarkably, we find no measurable delay between the photoionization of CH4 and CD4, neither experimentally nor theoretically. However, we measure and calculate delays of up to 20 as between the dissociative and non-dissociative photoionization of the highest-occupied molecular orbitals of both molecules. Experiment and theory are in quantitative agreement. These results show that, in the absence of resonances, even the fastest nuclear motion does not substantially influence photoionization delays, but identify a previously unknown signature of nuclear motion in dissociative-ionization channels. These findings have important consequences for the design and interpretation of attosecond chronoscopy in molecules, clusters, and liquids.

Probing Photoionization Dynamics in Acetylene with Angle-Resolved Attosecond Interferometry

“…This provides additional proof that for molecular photoionization the interference between different partial waves is not adequate enough to explain the angular dependency of the observed delays. As shown in the case of polycyclic aromatic hydrocarbons, 22 depending on whether the ionized system is two-or, three-dimensional, the photoionization time delay can change dramatically. It stems from the delocalization of the hole created at the instant of ionization, affecting the dispersion of the emitted photoelectron wave packet in the residual molecular potential.…”

“…To provide a qualitative reasoning behind the observed trend in acetylene, first, we note that following ionization from the 1π u orbital the molecular ion retains its linear geometry. 44 Following the same procedure applied in the case of polycyclic aromatic hydrocarbons, 22 we assume that after removal of an electron from the outermost molecular orbital, the created hole remains delocalized across the entire length of the molecule. Due to the lack of any spherical symmetry, unlike the case of atomic photoionization, the photoelectrons emitted along the C �C bond will experience a different short-range potential compared to those emitted perpendicular to the bond.…”

“…The amplitude and phase of oscillations at 2ω 0 for the mixture, extracted from the RABBITT measurements, are shown as the blue curve in both Figure 2a,b. Using the methodology described elsewhere, 22 the individual measurements for the two targets were rescaled in time assuming the oscillations observed in the gas mixture to be a linear combination of those measured separately for Ar and C 2 H 2 . The reconstructed amplitude and phase of oscillations (black dashed−dotted curve in Figure 2a,b) show excellent agreement with those measured for the mixture, proving the robustness of the technique.…”

“…25 Despite being a well-developed method, it is still challenging to implement it, especially for molecular systems, since ionization by an attosecond pulse train (APT) representing a harmonic comb in the frequency domain can lead to spectral congestion in the measured photoelectron spectra. 26 Nevertheless, the RABBITT technique is now getting applied to more and more complex molecular systems, such as clusters, 21 polycyclic aromatic hydrocarbons, 22 etc. to understand the physical origin of time delays following ionization by an extreme ultraviolet (XUV) pulse.…”

“…With its first experimental realization in 2010, photoionization time delays in ultrafast light–matter interaction have now become an indispensable tool in studying the photoionization dynamics in various systems, ranging from atoms , and molecules − in the gas phase to solids , and liquids . The measured photoionization time delays were found to be sensitive toward a variety of physical properties such as Fano resonance, , shape resonance, − chirality, collective excitation, effect of the chemical environment, , shape and symmetry of the system, and mass of the target, , just to name a few. One of the interferometric techniques giving access to these time delays is known as the reconstruction of attosecond beating by interference of two-photon transitions, or, RABBITT .…”