Our work in JCP has the (dubious?) honour of being selected as their Christmas card.
Photoionization is a complex quantum mechanical process, with a range of interfering channels playing a role in even the simplest case. For problems in quantum metrology and sensing, a detailed understanding of the process is desirable for accurate measurements; quantum control is also a possible outcome of such understanding. New research in this area will build on recent cutting-edge work at NRC (see below), which probed the fundamental quantum physics of photoionization in atoms and molecules, and metrology work which demonstrated the retrieval of electron wavefunctions via interferometric time-domain measurements.
Four areas of photoionization interferometry are the target of current research:
- Metrology and control with rotational wavepackets.
- Metrology and control with shaped laser pulses.
- Quantum dynamics probed via photoionization interferometry.
- Fundamental properties of photoion and photoelectron coherence.
Depending on interests and experience, project work will be in one (or more) of these areas.
An introduction to this topic, and recent work, can be found in Paul’s DAMOP 2017 talk Phase-sensitive Photoelectron Metrology (below), and via our blog.
Slides for Paul’s DAMOP talk are now available on figshare (DOI: 10.6084/m9.figshare.5049142).
Photoionization is an interferometric process, in which multiple paths can contribute to the final continuum photoelectron state. At the simplest level, interferences between different final angular momentum states are manifest in the energy and angle resolved photoelectron spectra: metrology schemes making use of these interferograms are thus phase-sensitive, and provide a powerful route to detailed understanding of photoionization . At a more complex level, such measurements can also provide a powerful probe for other processes of interest, for example: (a) dynamical process in time-resolved measurements, such as rotational, vibrational and electronic wavepackets, and (b) in order to understand and develop control schemes . In this talk recent work in this vein will be discussed, touching on “complete” photoionization studies of atoms and molecules with shaped laser pulses [1,2] and XUV , metrology schemes using Angle-Resolved RABBIT, and molecular photoionization dynamics in the time-domain (Wigner delays) .
 Hockett, P. et. al. (2015). Phys. Rev. A, 92, 13412.  Hockett, P. et. al. (2014). Phys. Rev. Lett., 112, 223001.  Marceau, C. et. al. (2017). Submitted. DOI: 10.6084/m9.figshare.4480349.  Hockett, P. et. al. (2016). J. Phys B, 49, 95602.
Update 29th June 2017 – a video of the talk is now also available.
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).
Our latest work with the PImMS camera, femtosecond VUV pulses, and velocity-map imaging, has been picked up for a press release by AIP.
The Inner Lives of Molecules
New method takes 3-D images of molecules in action
WASHINGTON, D.C., April 4, 2017 — Quantum mechanics rules. It dictates how particles and forces interact, and thus how atoms and molecules work — for example, what happens when a molecule goes from a higher-energy state to a lower-energy one. But beyond the simplest molecules, the details become very complex.
“Quantum mechanics describes how all this stuff works,” said Paul Hockett of the National Research Council of Canada. “But as soon as you go beyond the two-body problem, you can’t solve the equations.” So, physicists must rely on computer simulations and experiments.
Now, he and an international team of researchers from Canada, the U.K. and Germany have developed a new experimental technique to take 3-D images of molecules in action. This tool, he said, can help scientists better understand the quantum mechanics underlying bigger and more complex molecules.
The new method, described in The Journal of Chemical Physics, from AIP Publishing, combines two technologies. The first is a camera developed at Oxford University, called the Pixel-Imaging Mass Spectrometry (PImMS) camera. The second is a femtosecond vacuum ultraviolet light source built at the NRC femtolabs in Ottawa.
Mass spectrometry is a method used to identify unknown compounds and to probe the structure of molecules. In most types of mass spectrometry, a molecule is fragmented into atoms and smaller molecules that are then separated by molecular weight. In time-of-flight mass spectrometry, for example, an electric field accelerates the fragmented molecule. The speed of those fragments depends on their mass and charge, so to weigh them, you measure how long it takes for them to hit the detector.
Most conventional imaging detectors, however, can’t discern exactly when one particular particle hits. To measure timing, researchers must use methods that effectively act as shutters, which let particles through over a short time period. Knowing when the shutter is open gives the time-of-flight information. But this method can only measure particles of the same mass, corresponding to the short time the shutter is open.
The PImMS camera, on the other hand, can measure particles of multiple masses all at once. Each pixel of the camera’s detector can time when a particle strikes it. That timing information produces a three-dimensional map of the particles’ velocities, providing a detailed 3-D image of the fragmentation pattern of the molecule.
To probe molecules, the researchers used this camera with a femtosecond vacuum ultraviolet laser. A laser pulse excites the molecule into a higher-energy state, and just as the molecule starts its quantum mechanical evolution — after a few dozen femtoseconds –another pulse is fired. The molecule absorbs a single photon, a process that causes it to fall apart. The PImMS camera then snaps a 3-D picture of the molecular debris.
By firing a laser pulse at later and later times at excited molecules, the researchers can use the PImMS camera to take snapshots of molecules at various stages while they fall into lower energy states. The result is a series of 3-D blow-by-blow images of a molecule changing states.
The researchers tested their approach on a molecule called C2F3I. Although a relatively small molecule, it fragmented into five different products in their experiments. The data and analysis software is available online as part of an open science initiative, and although the results are preliminary, Hockett said, the experiments demonstrate the power of this technique.
“It’s effectively an enabling technology to actually do these types of experiments at all,” Hockett said. It only takes a few hours to collect the kind of data that would take a few days using conventional methods, allowing for experiments with larger molecules that were previously impossible.
Then researchers can better answer questions like: How does quantum mechanics work in larger, more complex systems? How do excited molecules behave and how do they evolve?
“People have been trying to understand these things since the 1920s,” Hockett said. “It’s still a very open field of investigation, research, and debate because molecules are really complicated. We have to keep trying to understand them.”
The article, Time-resolved multi-mass ion imaging: femtosecond UV-VUV pump-probe spectroscopy with the PImMS camera, is now published in the Journal of Chemical Physics, and also available via the arXiv 1702.00744 and Authorea (original text), DOI: 10.22541/au.149030711.19068540.
The full dataset and analysis scripts are available via OSF, DOI: 10.17605/OSF.IO/RRFK3.
Angle-resolved (AR) RABBIT measurements offer a high information content measurement scheme, due to the presence of multiple, interfering, ionization channels combined with a phase-sensitive observable in the form of angle and time-resolved photoelectron interferograms. In order to explore the characteristics and potentials of AR-RABBIT, a perturbative 2-photon model is developed; based on this model, example AR-RABBIT results are computed for model and real systems, for a range of RABBIT schemes. These results indicate some of the phenomena to be expected in AR-RABBIT measurements, and suggest various applications of the technique in photoionization metrology.
Paul Hockett 2017 J. Phys. B: At. Mol. Opt. Phys. 50 154002
Pre-print available via Authorea, DOI: 10.22541/au.149037518.89916908.
See also the recent AR-RABBIT presentation for a brief intro to this topic.
The above image shows simulated velocity map images (left, middle) and angle and time-resolved measurements (right) for angle-resolved RABBIT measurements. In this type of measurement, XUV and IR pulses are combined, and create a set of 1 and 2-photon bands in the photoelectron spectrum. The presence of multiple interfering pathways to each final photoelectron band (energy) results in complex and information rich interferograms, with both angle and time-dependence.
A manuscript detailing this work is currently in preparation, and a recent presentation detailing some aspects of the work can be found on Figshare.
Update 24th March – new manuscript, Angle-resolved RABBIT: theory and numerics, pre-print available.
P. Darré, R. Baudoin, J.-T. Gomes, N. J. Scott, L. Delage, L. Grossard, J. Sturmann, C. Farrington, F. Reynaud, and T. A. Ten Brummelaar
Phys. Rev. Lett. 117, 233902 – Published 29 November 2016
The Astronomical Light Optical Hybrid Analysis project investigates the combined use of a telescope array interferometer and nonlinear optics to propose a new generation of instruments dedicated to high-resolution imaging for infrared astronomy. The nonlinear process of optical frequency conversion transfers the astronomical light to a shorter wavelength domain. Here, we report on the first fringes obtained on the sky with the prototype operated at 1.55 μm in the astronomical H band and implemented on the Center for High Angular Resolution Astronomy telescope array. This seminal result allows us to foresee a future extension to the challenging midinfrared spectral domain.
This is quite interesting as an application of photon up-conversion at low-light levels – in this case for interferometric IR telescope arrays. The demo in the paper doesn’t show any improvement on the existing configuration (i.e. no non-linear optical step), but in principle could: once one factors in not just lossy detection in the IR, but also lossy beam transport (in the conceptually similar VLTI system it’s about 10% efficient).
The header image shows fig. 1 from the paper.
New papers at Optics Letters & on the arxiv, looking at various aspects of nonclassical correlations in light-matter interactions:
Philip J. Bustard, Jennifer Erskine, Duncan G. England, Josh Nunn, Paul Hockett, Rune Lausten, Michael Spanner, and Benjamin J. Sussman
Optics Letters, Vol. 40, Issue 6, pp. 922-925 (2015)
P. Hockett, C. Lux, M. Wollenhaupt, T. Baumert
(Update – now published in PRA.)
P. Hockett, M. Wollenhaupt, C. Lux, T. Baumert
(Update – now published in PRA.)