Phase-sensitive Photoelectron Metrology (presentation at DAMOP 2017)

Phase-sensitive Photoelectron Metrology (presentation at DAMOP 2017)

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 [1]. 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 [1]. 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 [3], metrology schemes using Angle-Resolved RABBIT, and molecular photoionization dynamics in the time-domain (Wigner delays) [4].

[1] Hockett, P. et. al. (2015). Phys. Rev. A, 92, 13412. [2] Hockett, P. et. al. (2014). Phys. Rev. Lett., 112, 223001. [3] Marceau, C. et. al. (2017). Submitted. DOI: 10.6084/m9.figshare.4480349. [4] Hockett, P. et. al. (2016). J. Phys B, 49, 95602.

Update 29th June 2017 – a video of the talk is now also available.

Phase-sensitive Photoelectron Metrology – Dr. P. Hockett, presentation at DAMOP 2017 from femtolab.ca on Vimeo.

Time-dependent Wavepackets and Photoionization – CS2

Time-dependent Wavepackets and Photoionization – CS2

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).

Press Release: The Inner Lives of Molecules

Press Release: The Inner Lives of Molecules

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.”

Text reproduced from AIP.

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.

Reading today…

Reading today…

Nonlinear quantum optics mediated by Rydberg interactions

O Firstenberg, C S Adams and S Hofferberth

Published 30 June 2016© 2016 IOP Publishing Ltd
Journal of Physics B: Atomic, Molecular and Optical Physics, Volume 49, Number 15
Special Issue on Rydberg Atomic Physics

By mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons via electromagnetically induced transparency, it is now possible to realize a medium which exhibits a strong optical nonlinearity at the level of individual photons. We review the theoretical concepts and the experimental state-of-the-art of this exciting new field, and discuss first applications in the field of all-optical quantum information processing.

DOI: 10.1088/0953-4075/49/15/152003

Fascinating insight into the topic, which utilises the properties of Rydberg matter to enable traditional non-linear optics to cross over to the quantum regime. From the intro:

One remarkable success of advances in ultra-cold Rydberg physics is the realization of a medium with a large optical nonlinearity at the single photon level [1–3]. Highly excited Rydberg atoms bring something new to the history of optics as they enable quantum nonlinear media where photons are strongly interacting!

Recommended.

Angle-resolved RABBIT: theory and numerics

Angle-resolved RABBIT: theory and numerics

Update 28/06/17 – Now published in J. Phys. B, special issue on Correlations in Light-Matter Interactions.

New manuscript:

Angle-resolved RABBIT: theory and numerics

P. Hockett

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.

arXiv 1703.08586 (2017) 

See also the recent AR-RABBIT presentation for a brief intro to this topic.

Open Science Initiative

Open Science Initiative

Open science – the practice of making full research projects open and accessible, from inception to publication – is an increasingly important topic, and even appearing in the popular press, particularly with regard to transparency and reproducible in research… hence open science can be viewed as the opposite of bad science.

John Arnold Made a Fortune at Enron. Now He’s Declared War on Bad Science

Open science (along with the more general notion of open data) is also part of the Canadian Government’s Open Government action plan, which includes the statement that:

The Government of Canada will maximize access to federally-funded scientific research to encourage greater collaboration and engagement with the scientific community, the private sector, and the public.

 

As part of our work towards open science, our articles are increasingly available on open platforms (arXiv, Authorea). And, now, good things are happening with our data too. Thanks to the Open Science Foundation (OSF) and Figshare, it’s now easy to share data, code etc. and make it citable with a DOI.

Some of our recent open science data can be found at:

Time-dependent Wavepackets and Photoionization – CS2 (2013 – present)

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.

Time-resolved multi-mass ion imaging: femtosecond UV-VUV pump-probe spectroscopy with the PImMS camera (2017)

Bootstrapping to the Molecular Frame with Time-domain Photoionization Interferometry (2017)

Time Delay in Molecular Photoionization (2016)

Let your data be free!

Angle-resolved RABBIT: new work and presentation

Angle-resolved RABBIT: new work and presentation

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.

Monitoring Non-adiabatic Dynamics in CS2 with Time- and Energy-Resolved Photoelectron Spectra of Wavepackets

Monitoring Non-adiabatic Dynamics in CS2 with Time- and Energy-Resolved Photoelectron Spectra of Wavepackets

Feb. 2017 – New article in Chemical Physics Letters:

Monitoring Non-adiabatic Dynamics in CS2 with Time- and Energy-Resolved Photoelectron Spectra of Wavepackets

Kwanghsi Wang(a) Vincent McKoy(a)Paul Hockett(b)Albert Stolow(b, c, d),Michael S. Schuurman(b, d),

a A. A. Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA

b National Research Council Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada

c Department of Physics, University of Ottawa, ON K1N 6N5 Canada

d Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, ON, K1N 6N5, Canada

Highlights

• Time-resolved photoelectron angular distributions around conical intersections are studied.
• Ab initio multiple spawning method is applied to obtain wavepacket densities.
• Geometry and energy dependent photoelectron matrix elements are employed.
• Molecular and laboratory photoelectron angular distributions are used to illustrate the non-adiabatic dynamics.
• Photoelectron spectra are compared with measured values.

Abstract

We report results from a novel fully ab initio method for simulating the time-resolved photoelectron angular distributions around conical intersections in CS2. The technique employs wavepacket densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of the calculated molecular-frame photoelectron angular distributions with measured values for CS2 demonstrates that this approach can successfully illuminate, and disentangle, the underlying coupled nuclear and electronic dynamics around conical intersections in polyatomic molecules.

DOI: 10.1016/j.cplett.2017.02.014