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.
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.
UPDATE: 4th April 2017
The article is now published in the Journal of Chemical Physics, with an accompanying press release, The Inner Lives of Molecules, from AIP.
The full dataset and analysis scripts are now also available via OSF, DOI: 10.17605/OSF.IO/RRFK3.
Feb. 2017 – new article on the arXiv:
Time-resolved multi-mass ion imaging: femtosecond UV-VUV pump-probe spectroscopy with the PImMS camera
The Pixel-Imaging Mass Spectrometry (PImMS) camera allows for 3D charged particle imaging measurements, in which the particle time-of-flight is recorded along with (x,y) position. Coupling the PImMS camera to an ultrafast pump-probe velocity-map imaging spectroscopy apparatus therefore provides a route to time-resolved multi-mass ion imaging, with both high count rates and large dynamic range, thus allowing for rapid measurements of complex photofragmentation dynamics. Furthermore, the use of vacuum ultraviolet wavelengths for the probe pulse allows for an enhanced observation window for the study of excited state molecular dynamics in small polyatomic molecules having relatively high ionization potentials. Herein, preliminary time-resolved multi-mass imaging results from C2F3I photolysis are presented. The experiments utilized femtosecond UV and VUV (160.8~nm and 267~nm) pump and probe laser pulses in order to demonstrate and explore this new time-resolved experimental ion imaging configuration. The data indicates the depth and power of this measurement modality, with a range of photofragments readily observed, and many indications of complex underlying wavepacket dynamics on the excited state(s) prepared.
Now published in JCP:
The Journal of Chemical Physics 147, 013911 (2017);
A couple more from the Lytro lightfield camera… mouse around to move the images and change focus.
The Lytro digital camera introduces a Shack-Hartmann configuration into a digital SLR camera. Why? For “lightfield” (wavefront) imaging, allowing for depth information in the captured data. While this kind of thing has long been used for scientific instruments, in particular for laser beam measurements, the Lytro camera brings this capability (and the not insignificant post-processing know-how and hardware required) to photography in the visible. For rather more detailed information, check out the PhD thesis of Ren Ng, the founder of Lytro.
Here’s a demo image of our VIRP chamber, note that mousing around the image and clicking allows one to change the focus of the image, and the imaging plane. Mouse wheel to zoom. It’s going to be an excellent tool for scientific imaging!
* Banner image from Lytro.com.
Quantum imaging with undetected photons
Gabriela Barreto Lemos, Victoria Borish, Garrett D. Cole, Sven Ramelow, Radek Lapkiewicz & Anton Zeilinger
Interferometric imaging based on photon pairs, from the intro:
Information is central to quantum mechanics. In particular, quantum interference occurs only if there exists no information to distinguish between the superposed states. The mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference1, 2. Here we introduce and experimentally demonstrate a quantum imaging concept based on induced coherence without induced emission3, 4. Our experiment uses two separate down-conversion nonlinear crystals (numbered NL1 and NL2), each illuminated by the same pump laser, creating one pair of photons (denoted idler and signal). If the photon pair is created in NL1, one photon (the idler) passes through the object to be imaged and is overlapped with the idler amplitude created in NL2, its source thus being undefined. Interference of the signal amplitudes coming from the two crystals then reveals the image of the object. The photons that pass through the imaged object (idler photons from NL1) are never detected, while we obtain images exclusively with the signal photons (from NL1 and NL2), which do not interact with the object. Our experiment is fundamentally different from previous quantum imaging techniques, such as interaction-free imaging5 or ghost imaging6, 7, 8, 9, because now the photons used to illuminate the object do not have to be detected at all and no coincidence detection is necessary. This enables the probe wavelength to be chosen in a range for which suitable detectors are not available. To illustrate this, we show images of objects that are either opaque or invisible to the detected photons. Our experiment is a prototype in quantum information—knowledge can be extracted by, and about, a photon that is never detected.
Single-photon sensitive light-in-flight imaging
Time-resolved imaging with a 2D array of single-photon avalanche diodes. New from the Camera Culture group at MIT. A fairly trivial demo of a SPAD array, but makes for a visceral visual of what you can do.