162 research outputs found
General phenomenology of ionization from aligned molecular ensembles
Single and multi-photon ionization of aligned molecular ensembles is
examined, with a particular focus on the link between the molecular axis
distribution and observable in various angle-integrated and angle-resolved
measurements. To maintain generality the problem is treated geometrically, with
the aligned ensemble cast in terms of axis distribution moments, and the
response of observables in terms of couplings to these moments. Within this
formalism the angular momentum coupling is treated analytically, allowing for
general characteristics - independent of the details of the ionization dynamics
of a specific molecule - to be determined. Limiting cases are explored in order
to provide a phenomenology which should be readily applicable to a range of
experimental measurements, and illustrate how observables can be sensitive to
fine details of the alignment, i.e. higher-order moments of the axis
distribution, which are often neglected in experimental studies. We hope that
this detailed and comprehensive treatment will bridge the gap between existing
theoretical and experimental works, and provide both quantitative physical
insights and a useful general phenomenology for researchers working with
aligned molecular ensembles.Comment: 23 pages, 14 figures, 2 table
Angle-resolved RABBIT: theory and numerics
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
Coherent Control of Photoelectron Wavepacket Angular Interferograms
Coherent control over photoelectron wavepackets, via the use of
polarization-shaped laser pulses, can be understood as a time and
polarization-multiplexed process. In this work, we investigate this
multiplexing via computation of the observable photoelectron angular
interferograms resulting from multi-photon atomic ionization with
polarization-shaped laser pulses. We consider the polarization sensitivity of
both the instantaneous and cumulative continuum wavefunction; the nature of the
coherent control over the resultant photoelectron interferogram is thus
explored in detail. Based on this understanding, the use of coherent control
with polarization-shaped pulses as a methodology for a highly multiplexed
coherent quantum metrology is also investigated, and defined in terms of the
information content of the observable.Comment: 9 pages, 5 figure
Probing Ultrafast Dynamics with Time-resolved Multi-dimensional Coincidence Imaging: Butadiene
Time-resolved coincidence imaging of photoelectrons and photoions represents
the most complete experimental measurement of ultrafast excited state dynamics,
a multi-dimensional measurement for a multi-dimensional problem. Here we
present the experimental data from recent coincidence imaging experiments,
undertaken with the aim of gaining insight into the complex ultrafast
excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm
light. We discuss photoion and photoelectron mappings of increasing
dimensionality, and focus particularly on the time-resolved photoelectron
angular distributions (TRPADs), expected to be a sensitive probe of the
electronic evolution of the excited state and to provide significant
information beyond the time-resolved photoelectron spectrum (TRPES). Complex
temporal behaviour is observed in the TRPADs, revealing their sensitivity to
the dynamics while also emphasising the difficulty of interpretation of these
complex observables. From the experimental data some details of the wavepacket
dynamics are discerned relatively directly, and we make some tentative
comparisons with existing ab initio calculations in order to gain deeper
insight into the experimental measurements; finally, we sketch out some
considerations for taking this comparison further in order to bridge the gap
between experiment and theory.Comment: 18 pages, 10 figures. Pre-print of JMO submissio
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
Photoionization dynamics of polyatomic molecules
The work presented in this thesis was carried out with the ultimate aim of learning about the photoionization dynamics of polyatomic molecules. This is a complex problem; in order to obtain sufficient experimental data to shed light on the dynamics careful measurement of photoelectron angular distributions (PADs) is required. Ideally these measurements are rotationally-resolved, and the angular distributions measured correspond to the formation of the molecular ion in a single rotational state. The ionization event, in the dipole approximation, can be completely described by the dipole matrix elements. If sufficient experimental data to determine the radial components of the matrix elements and associated phases, the dynamical parameters, can be obtained the photoionization experiment may be said to be complete. Analysis of such experiments requires that the initial state of the molecular system is also known, to this end resonance-enhanced multi-photon ionization (REMPI) schemes can be used in order to populate a single quantum state prior to ionization. The experiments presented here follow this methodology, with various REMPI schemes used to prepare (pump) and ionize (probe) the molecule under study, and the velocity-map imaging (VMI) technique used to (simultaneously) record the photoelectron spectra and angular distributions.
Two molecules have been studied experimentally, acetylene (C2H2) and ammonia (NH3). In both cases dynamical parameters pertaining to the formation of specific states (vibronic or vibrational) of the molecular ion have been determined from experimental data. Additionally, in the ammonia work, rotationally-resolved photoelectron images were obtained
Time-Resolved Photoelectron Spectra of CS_2: Dynamics at Conical Intersections
We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame
photoelectron angular distributions around conical intersections in CS_2. The technique employs
wave packet densities obtained with the multiple spawning method in conjunction with geometry- and
energy-dependent photoionization matrix elements. The robust agreement of these results with measured
molecular-frame photoelectron angular distributions for CS_2 demonstrates that this technique can
successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around
conical intersections in polyatomic molecules
Photoionization dynamics of polyatomic molecules
The work presented in this thesis was carried out with the ultimate aim of learning about the photoionization dynamics of polyatomic molecules. This is a complex problem; in order to obtain sufficient experimental data to shed light on the dynamics careful measurement of photoelectron angular distributions (PADs) is required. Ideally these measurements are rotationally-resolved, and the angular distributions measured correspond to the formation of the molecular ion in a single rotational state. The ionization event, in the dipole approximation, can be completely described by the dipole matrix elements. If sufficient experimental data to determine the radial components of the matrix elements and associated phases, the dynamical parameters, can be obtained the photoionization experiment may be said to be complete. Analysis of such experiments requires that the initial state of the molecular system is also known, to this end resonance-enhanced multi-photon ionization (REMPI) schemes can be used in order to populate a single quantum state prior to ionization. The experiments presented here follow this methodology, with various REMPI schemes used to prepare (pump) and ionize (probe) the molecule under study, and the velocity-map imaging (VMI) technique used to (simultaneously) record the photoelectron spectra and angular distributions.
Two molecules have been studied experimentally, acetylene (C2H2) and ammonia (NH3). In both cases dynamical parameters pertaining to the formation of specific states (vibronic or vibrational) of the molecular ion have been determined from experimental data. Additionally, in the ammonia work, rotationally-resolved photoelectron images were obtained
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
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