Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A02: Velocity Map Imaging IFocus Recordings Available
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Sponsoring Units: DCP Chair: Authur Suits, University of Missouri Room: McCormick Place W-175C |
Monday, March 14, 2022 8:00AM - 8:36AM |
A02.00001: Velocity Map Imaging at molecular ice surfaces Invited Speaker: David H Parker
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Monday, March 14, 2022 8:36AM - 9:12AM |
A02.00002: Probing the Kinetics and Dynamics at Surfaces using Ion Imaging methods Invited Speaker: Theofanis Kitsopoulos I will introduce the use of slice imaging to measure catalytic rates for site-specific elementary reactions thus offering remarkable opportunities to advance our fundamental understanding of heterogeneous catalysis. We call this approach Velocity Resolved Kinetics (VRK). This strategy makes the formidable task of describing site-specific chemical reaction mechanisms and elementary rates in heterogeneous catalysis facile, while its necessity we justified on the prototypical CO oxidation reaction on Pt. I will show examples of reactions involving atoms H, O, N, and how to measure important factors that influence the kinetics of elementary reactions at surfaces, e.g. the chemical nature of the catalyst and the geometry of the active sit. VRK is a "bottom-up" approach to catalysis, i.e., building and understanding complex heterogeneous chemical catalysis, from the site-specific kinetics of the elementary building block reactions. Our measurements, serve for benchmarking first principles calculations of reaction rates in surface chemistry. Our methodology measures the kinetics in the microsecond to millisecond regime with temperatures in the 100 to 1000 K range, i.e, conditions more relevant to industrial conditions. |
Monday, March 14, 2022 9:12AM - 9:48AM |
A02.00003: Velocity Map Imaging of Weakly- or Quasi-bound Autodetached Electrons in Transient Complex Anions Invited Speaker: Caroline Jarrold Some classes of reaction complexes, such as hydrogen abstraction reaction complexes, possess transient dipole moments sufficiently large to support a dipole bound electron, forming a weakly-bound transient anion. An example is the abstraction of hydrogen from an alkene by the hydroxyl radical. We describe how we have prepared these neutral reaction complexes by photodetachment of the stable ion-molecule complex formed between the hydroxide anion and alkenes. Detaching near threshold leads to the creation of a transient dipole-bound anion state of the complex as H-abstraction proceeds. Using velocity map imaging of the electrons that are ejected along the reaction path, discrete kinetic energies of the electrons encode the vibrational states populated in the final radical products. Velocity map imaging therefore is a powerful tool for studying these transient complexes. |
Monday, March 14, 2022 9:48AM - 10:24AM |
A02.00004: Artifical Neural Networks for Processing Velocity Map Images Invited Speaker: Dave Townsend We present the first demonstration of artificial neural networks (ANNs) for the removal of Poissonian noise in velocity map imaging (VMI) measurements with very low overall counts. The approach is successfully applied to both simulated and real experimental data relating to the detection of photoions/photoelectrons in unimolecular photochemical dynamics studies. Our results reveal an excellent level of performance, with the ANNs transforming images that are unusable for any form of quantitative analysis into statistically reliable data with an impressive similarity to benchmark references. We also then present a second ANN strategy that we refer to as Arbitrary Image Reinflation (AIR), developed for reconstructing 3D photoproduct distributions from a single 2D projection. The AIR approach is demonstrated for distributions that possess cylindrical symmetry about an axis parallel to the imaging plane and, importantly, those that also do not. Given the widespread use of VMI methods within the chemical dynamics community, we anticipate that the use of ANNs for data processing has significant potential impact – particularly, for example, when working in the limit of very low absorption/photoionization cross-sections, or when attempting to reliably extract subtle image features originating from phenomena such as photofragment vector correlations or photoelectron circular/elliptical dichroism. |
Monday, March 14, 2022 10:24AM - 10:36AM |
A02.00005: Absolute-phase-resolved strong-field ionization Yasashri R Ranathunga, Duke A Debrah, Gabriel A Stewart, Suk Kyoung Lee, Wen Li Many important physical processes such as non-linear optics and coherent control are highly sensitive to the absolute carrier-envelop-phase (CEP) of driving ultrashort laser pulses. A significant amount of previous theory work has been carried out to study the effect of the absolute CEP on strong field ionization and related phenomenon such as high harmonic generaton (HHG) and nonsequential double ionization (NSDI). This makes the measurement of absolute CEP immensely important in attosecond and high field physics. Even though relative CEPs can be measured with a few existing methods, the estimate of the absolute CEP has not been straightforward and has always required theoretical inputs. Recently, we have developed an in-situ method for measuring the absolute CEP of elliptical polarized few-cycle pulse without assistance of theoretical modelings[1]. Here we will show that the absolute CEP of linear polarized light can also be measured with a similar method. This capability enables the measurement of absolute-phase-resolved strong field ionzaiton for the first time.. We are able to compare the experimental results directly with those obtained with numerical solutions of time-dependent Schrodinger equations (TDSE). Preliminary results suggest the TDSE method might have issues in modeling strong field multi-electron dynamics, which have been routinely carried out to help understand the dynamics or calibrate CEP measurement. This failure could be due to the employed single active electron approximation and warrants further investigation. |
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