26 research outputs found
Topical Review: Extracting Molecular Frame Photoionization Dynamics from Experimental Data
Methods for experimental reconstruction of molecular frame (MF)
photoionization dynamics, and related properties - specifically MF
photoelectron angular distributions (PADs) and continuum density matrices - are
outlined and discussed. General concepts are introduced for the non-expert
reader, and experimental and theoretical techniques are further outlined in
some depth. Particular focus is placed on a detailed example of numerical
reconstruction techniques for matrix-element retrieval from time-domain
experimental measurements making use of rotational-wavepackets (i.e. aligned
frame measurements) - the ``bootstrapping to the MF" methodology - and a
matrix-inversion technique for direct MF-PAD recovery. Ongoing resources for
interested researchers are also introduced, including sample data,
reconstruction codes (the \textit{Photoelectron Metrology Toolkit}, written in
python, and associated \textit{Quantum Metrology with Photoelectrons}
platform/ecosystem), and literature via online repositories; it is hoped these
resources will be of ongoing use to the community.Comment: 65 pages, 17 figures. HTML version with interactive figures on
Authorea:
https://www.authorea.com/users/71114/articles/447808-extracting-molecular-frame-photoionization-dynamics-from-experimental-data
Code and data archive on Figshare:
http://dx.doi.org/10.6084/m9.figshare.2029378
Laser-induced rotational dynamics as a route to molecular frame measurements
Doctor of PhilosophyDepartment of PhysicsVinod KumarappanIn general, molecules in the gas phase are free to rotate, and measurements made on such samples are averaged over a randomly oriented distribution of molecules. Any orientation dependent information is lost in such measurements. The goal of the work presented here is to a) mitigate or completely do away with orientational averaging, and b) make fully resolved orientation dependent measurements. In pursuance of similar goals, over the past 50 years chemists and physicists have developed techniques to align molecules, or to measure their orientation and tag other quantities of interest with the orientation. We focus on laser induced alignment of asymmetric top molecules.
The first major contribution of our work is the development of an effective method to align all molecular axes under field-free conditions. The method employs a sequence of nonresonant, impulsive laser pulses with varied ellipticities. The efficacy of the method is first demonstrated by solution of the time dependent Schr\"{o}dinger equation for iodobenzene, and then experimentally implemented to three dimensionally align 3,5 difluoroiodobenzene. Measurement from molecules aligned in this manner greatly reduces orientational averaging. The technique was developed via a thorough understanding and extensive computations of the dynamics of rotationally excited asymmetric top molecules.
The second, and perhaps more important, contribution of our work is the development of a new measurement technique to extract the complete orientation dependence of a variety of molecular processes initiated by ultrashort laser pulses. The technique involves pump-probe measurements of the process of interest from a rotational wavepacket generated by impulsive excitation of asymmetric top molecules. We apply it to make the first measurement of the single ionization probability of an asymmetric top molecule in a strong field as a function of all relevant alignment angles. The measurement and associated calculations help identify the orbital from which the electron is ionized. We expect that this technique will be widely applicable to ultrafast-laser driven processes in molecules and provide unique insight into molecular physics and chemistry
A Laboratory Frame Density Matrix for Ultrafast Quantum Molecular Dynamics
In most cases the ultrafast dynamics of resonantly excited molecules are
considered, and almost always computed in the molecular frame, while
experiments are carried out in the laboratory frame. Here we provide a
formalism in terms of a lab frame density matrix which connects quantum
dynamics in the molecular frame to those in the laboratory frame, providing a
transparent link between computation and measurement. The formalism reveals
that in any such experiment, the molecular frame dynamics vary for molecules in
different orientations and that certain coherences which are potentially
experimentally accessible are rejected by the orientation-averaged reduced
vibronic density matrix. Instead, Molecular Angular Distribution Moments
(MADMs) are introduced as a more accurate representation of experimentally
accessible information. Furthermore, the formalism provides a clear definition
of a molecular frame quantum tomography, and specifies the requirements to
perform such a measurement enabling the experimental imaging of molecular frame
vibronic dynamics. Successful completion of such a measurement fully
characterizes the molecular frame quantum dynamics for a molecule at any
orientation in the laboratory frame
An Investigation into Neuromorphic ICs using Memristor-CMOS Hybrid Circuits
The memristance of a memristor depends on the amount of charge flowing
through it and when current stops flowing through it, it remembers the state.
Thus, memristors are extremely suited for implementation of memory units.
Memristors find great application in neuromorphic circuits as it is possible to
couple memory and processing, compared to traditional Von-Neumann digital
architectures where memory and processing are separate. Neural networks have a
layered structure where information passes from one layer to another and each
of these layers have the possibility of a high degree of parallelism.
CMOS-Memristor based neural network accelerators provide a method of speeding
up neural networks by making use of this parallelism and analog computation. In
this project we have conducted an initial investigation into the current state
of the art implementation of memristor based programming circuits. Various
memristor programming circuits and basic neuromorphic circuits have been
simulated. The next phase of our project revolved around designing basic
building blocks which can be used to design neural networks. A memristor bridge
based synaptic weighting block, a operational transconductor based summing
block were initially designed. We then designed activation function blocks
which are used to introduce controlled non-linearity. Blocks for a basic
rectified linear unit and a novel implementation for tan-hyperbolic function
have been proposed. An artificial neural network has been designed using these
blocks to validate and test their performance. We have also used these
fundamental blocks to design basic layers of Convolutional Neural Networks.
Convolutional Neural Networks are heavily used in image processing
applications. The core convolutional block has been designed and it has been
used as an image processing kernel to test its performance.Comment: Bachelor's thesi
Ultrafast Field-Resolved Nonlinear Optical Spectroscopy in the Molecular Frame
We resolve the real-time electric field of a femtosecond third-order
nonlinear optical signal in the molecular frame. The electric field emitted by
the induced third-order polarization from impulsively pre-aligned gas-phase
molecules at room temperature, in a degenerate four-wave mixing (DFWM) scheme,
is measured using a spectral interferometry technique. We show that by
measuring both the amplitude and phase of the emitted femtosecond pulse,
information related to electronic symmetries can be accessed. The nonlinear
signal is measured around a rotational revival to extract its molecular-frame
angle dependence from pump-probe time delay scans. By comparing these
measurements for two linear molecules, carbon dioxide (CO2) and Nitrogen (N2),
we show that the measured second-order phase parameter (temporal chirp) of the
signal is sensitive to the valence electronic symmetry of the molecules,
whereas the amplitude of the signal does not show such sensitivity. We compare
these measurements to theoretical calculations of the chirp observable in the
molecular frame. This work is an important step towards using field-resolved
nonlinear optical measurements to study ultrafast dynamics in electronically
excited molecules.Comment: 10 pages, 3 figures and 1 supplemental documen
N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation
Citation: Troß, J., Ren, X., Makhija, V., Mondal, S., Kumarappan, V., & Trallero-Herrero, C. A. (2017). N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation. Physical Review A - Atomic, Molecular, and Optical Physics, 95(3). doi:10.1103/PhysRevA.95.033419We measure multi-orbital contributions to high harmonic generation from aligned nitrogen. We show that the change in revival structure in the cutoff harmonics has a counterpart in the angular distribution when a lower-lying orbital contributes to the harmonic yield. This angular distribution is directly observed in the laboratory without any further deconvolution. Because of the high degree of alignment we are able to distinguish angular contributions of the highest occupied molecular orbital 1 (HOMO-1) orbital from angle-dependent spectroscopic features of the HOMO. In particular, we are able to make a direct comparison with the cross section of the HOMO-1 orbital in the extreme ultraviolet region. © 2017 American Physical Society
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 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 CFI 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
Measuring the Angle-Dependent Photoionization Cross Section of Nitrogen using High-Harmonic Generation
We exploit the relationship between high harmonic generation (HHG) and the molecular photorecombination dipole to extract the molecular-frame differential photoionization cross section (PICS) in the extreme ultraviolet (XUV) for molecular nitrogen. A shape resonance and a Cooper-type minimum are reflected in the pump-probe time delay measurements of different harmonic orders, where high-order rotational revivals are observed in N₂. We observe the energy- and angle-dependent Cooper minimum and shape resonance directly in the laboratory-frame HHG yield by achieving a high degree of alignment, [SEE FORMULA IN ABSTRACT cos2 θ] 0.8. The interplay between PICS and rotational revivals is confirmed by simulations using the quantitative rescattering theory. Our method of extracting molecular-frame structural information points the way to similar measurements in more complex molecules
Bayesian inferencing and deterministic anisotropy for the retrieval of the molecular geometry in gas-phase diffraction experiments
Currently, our general approach to retrieve the molecular geometry from
ultrafast gas-phase diffraction heavily relies on complex geometric simulations
to make conclusive interpretations. In this manuscript, we develop a broadly
applicable ultrafast gas-phase diffraction method that approximates the
molecular frame geometry distribution using Bayesian
Inferencing. This method does not require complex molecular dynamics simulation
and can identify the unique molecular structure. We demonstrate this method's
viability by retrieving the ground state geometry distribution
for both simulated stretched NO and measured ground
state NO. Due to our statistical interpretation, we retrieve a
coordinate-space resolution on the order of 100~fm, depending on signal
quality, an improvement of order 100 compared to commonly used Fourier
transform based methods. By directly measuring the width of
, this is generally only accessible through simulation,
we open ultrafast gas-phase diffraction capabilities to measurements beyond
current analysis approaches. Our method also leverages deterministic ensemble
anisotropy; this provides an explicit dependence on the molecular frame angles.
This method's ability to retrieve the unique molecular structure with high
resolution, and without complex simulations, provides the potential to
effectively turn gas-phase ultrafast diffraction into a discovery oriented
technique, one that probes systems that are prohibitively difficult to
simulate.Comment: 16 pages, 8 figures, 2 tables. Please find the analysis code and
templates for new molecules at https://github.com/khegazy/BIG