Yusong Liu, Spencer L. Horton, Jie Yang, J. Pedro F. Nunes, Xiaozhe Shen, Thomas J. A. Wolf, Ruaridh Forbes, Chuan Cheng, Bryan Moore, Martin Centurion, Kareem Hegazy, Renkai Li, Ming-Fu Lin, Albert Stolow, Paul Hockett, Tamás Rozgonyi, Philipp Marquetand, Xijie Wang, and Thomas Weinacht
Pump-probe measurements aim to capture the motion of electrons and nuclei on their natural timescales (femtoseconds to attoseconds) as chemical and physical transformations take place, effectively making “molecular movies” with short light pulses. However, the quantum dynamics of interest are filtered by the coordinate-dependent matrix elements of the chosen experimental observable. Thus, it is only through a combination of experimental measurements and theoretical calculations that one can gain insight into the internal dynamics. Here, we report on a combination of structural (relativistic ultrafast electron diffraction, or UED) and spectroscopic (time-resolved photoelectron spectroscopy, or TRPES) measurements to follow the coupled electronic and nuclear dynamics involved in the internal conversion and photodissociation of the polyatomic molecule, diiodomethane (CH2I2). While UED directly probes the 3D nuclear dynamics, TRPES only serves as an indirect probe of nuclear dynamics via Franck-Condon factors, but it is sensitive to electronic energies and configurations, via Koopmans’ correlations and photoelectron angular distributions. These two measurements are interpreted with trajectory surface hopping calculations, which are capable of simulating the observables for both measurements from the same dynamics calculations. The measurements highlight the nonlocal dynamics captured by different groups of trajectories in the calculations. For the first time, both UED and TRPES are combined with theory capable of calculating the observables in both cases, yielding a direct view of the structural and nonadiabatic dynamics involved.
Update Jan 2020: collected results are now online at ePSdata, which supersedes the previous OSF pages. This now includes DOIs for each dataset from Zenodo, and post-processing with the new python version of ePSproc.
An OSF project, collecting photoionization calculations (ePolyScat), and notes, is now available. This will be an ongoing resource for researchers in photoelectron spectroscopy, interferometry and related areas, and is part of our Open Science initiative.
Update Jan 2018 – a presentation covering this work was given at the PQE conference, video and slides are available online.
Update August 2017 – this article is now published in PRL, under the alternative title Molecular Frame Reconstruction Using Time-Domain Photoionization Interferometry.
Phys. Rev. Lett. 119, 083401 (2017), DOI: 10.1103/PhysRevLett.119.083401
(Feb 2017) New manuscript on the arxiv:
(Submitted on 29 Jan 2017)
Photoionization of molecular species is, essentially, a multi-path interferometer with both experimentally controllable and intrinsic molecular characteristics. In this work, XUV photoionization of impulsively aligned molecular targets (N2) is used to provide a time-domain route to “complete” photoionization experiments, in which the rotational wavepacket controls the geometric part of the photoionization interferometer. The data obtained is sufficient to determine the magnitudes and phases of the ionization matrix elements for all observed channels, and to reconstruct molecular frame interferograms from lab frame measurements. In principle this methodology provides a time-domain route to complete photoionization experiments, and the molecular frame, which is generally applicable to any molecule (no prerequisites), for all energies and ionization channels.
arxiv 1701.08432 (2017)
Supplementary material (theory, data and code) available at DOI: 10.6084/m9.figshare.4480349.
Our ongoing work on the calculation of time-dependent wavepackets and observables in photoionization is now collected in an OSF project (DOI: 10.17605/OSF.IO/RJMPD). Aspects of this work have previously been published, but much of the detail and methodology underlying the calculations has remained sitting on our computers. As part of our Open Science Initiative, we’re letting this data go free! Head over to the OSF project “Time-dependent Wavepackets and Photoionization – CS2” for more.
Figure shows TRPADs results (a) Calculated TRPADs (0.7eV) (b), (c) Comparison with expt. TRPADs (discrete times).
A brief glimpse at some recent work – scattering calculations for NO2. The figure shows the molecular-frame photoelectron flux for light-matter interaction of various geometries (linearly polarized light), hence scattering into different continua. The inset shows the ionizing orbital and molecular geometry.
This is just a snippet from an ongoing effort to explore quantum coherence in molecular ionization.
A snippet from some theory work in progress on time delays in molecular photoionization. The image below shows the energy and angle-resolved cross-section (surface topography) and Wigner delay (colour map) over a 40 eV range for CO. Unsurprisingly, for a molecular scatterer (albeit a simple heteronuclear diatomic) the map is quite complicated! Here the delays range from -200 to +200 attoseconds, and peak at the Carbon end of the molecule.
More on this soon… the paper is almost ready…
UPDATE Dec. 2015
Now on the arXiv:
Time Delay in Molecular Photoionization
P. Hockett, E. Frumker, D.M. Villeneuve, P.B. Corkum
arXiv 1512.03788, 2015
A snippet from today – playing around with continuum wavefunctions for photoionization & electron scattering. The image shows electron wavefunctions for scattering from CO at a few different electron energies (10, 12 and 14 eV).
A beautiful bit of quantum mechanics!
And hats off to R. Luchesse (Texas A&M) for distributing ePolyScat, which was used for the underlying computation.