Electron and photon induced dynamics of astrochemical molecules

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry

Abstract

Since the first molecules were identified in the interstellar medium 75 years ago,[1] our understanding of astrochemistry has increased significantly. However, there is still a long way to go before we are even close to achieving a full understanding of the complex environment within an interstellar gas cloud. Due to the impracticality of studying molecules in space, laboratory-based studies must be used to acquire the data required to develop and refine astrochemical models.[2] To model ionisation accurately, both total and partial ionisation cross sections (probabilities), as well as an understanding of the fragmentation dynamics, i.e. the fragment kinetic energy release and angular distribution, are required. Our laboratory is well placed to study both electron and photoionisation in detail.
The primary objective of this project will be the experimental and theoretical study of electron-induced and photon-induced processes in a variety of astrochemical molecules that are stable under terrestrial conditions. The results of these studies, along with those carried out by other members of the group, will be used to produce an online database of ionisation cross sections, branching ratios, and fragment kinetic energy release distributions for the molecules studied.
Total electron ionisation cross sections will be measured as a function of electron energy using a beam-gas instrument already operational within our laboratory. These cross sections quantify the probability of ionising the parent ions, but give no information on the fragment ions formed. Partial ionisation cross sections give the probability of forming a particular fragment ion, and can be measured in our laboratory as a function of electron energy using an electron-molecule crossed beam instrument equipped with velocity-map imaging detection. The partial ionisation cross-sections obtained in these measurements are only relative values, but can be converted to absolute values by normalising to the total ionisation cross section measured in the beam-gas instrument.
Before partial ionisation cross section and fragment kinetic energy release measurements can begin, some recommissioning work is needed on our electron-molecule crossed beam instrument. This will include optimising both the electron and molecular beams, as well as installing a Pixel Imaging Mass Spectrometry (PImMS) camera. The PImMS camera is a fast imaging sensor with a 12.5 ns time resolution which, when used in a time of flight experiment, allows all fragments to be imaged during each acquisition. This will significantly reduce acquisition time, and allow correlations between the motions of different ions formed in the same process to be detected. The ion imaging detector will be fitted with a new ultrafast scintillator to replace the existing P47 phosphor. The reduced decay time of the new scintillator will improve the mass resolution of the experiment.
Theoretical studies using Kim and Rudd's binary-encounter-Bethe (BEB) model,[3,4] used to predict total electron ionisation cross-sections, will also be carried out, with a particular focus initially on optimising a correction required for molecules containing third-row atoms. An attempt will also be made to extend the model to predict partial electron ionisation cross sections. Other models will also be considered, and the results compared with the BEB model. Our aim is to achieve good agreement between theory and experiment, so that we can predict ionisation cross sections of molecules which would be a challenge to study experimentally.
This project falls within the EPSRC chemical reaction dynamics and mechanisms, computational and theoretical chemistry, analytical science and plasma and lasers research areas.

Publications

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Heathcote D (2018) Total electron ionization cross-sections for neutral molecules relevant to astrochemistry in Journal of Physics B: Atomic, Molecular and Optical Physics

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Köckert H (2019) C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CFI. in Physical chemistry chemical physics : PCCP

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1810922 Studentship EP/N509711/1 01/10/2016 30/09/2019 David Heathcote
 
Description The work carried out as a result of this funding has generated a large amount of experimental data, and lead to the development of a number of analysis programmes which are now being used extensively within our research group. Data has been recorded both as a result of experiments in our laboratory on electron ionisation of a number of molecules relevant to astrochemistry, atmospheric chemistry and plasma etching, and at external laser facilities; our group has carried out experiments at the Central Laser Facility using the Artemis apparatus, and have been part of a number of multinational collaborations running experiments at the Deutsches Elektronen-Synchrotron (DESY) research centre, specifically using the Free electron laser in Hamburg (FLASH) beamline.

Our experiments typically involve either electron bombardment or laser irradiation of the molecule under study. The radiation may ionise the molecule, in which case it experiences a force from an electrostatic lens within which the interaction takes place. This lens has potentials applied such that all ions with the same mass and velocity hit the same point on the detector; this is known as velocity-map imaging. The detector converts these ions into light, which we can then use a camera to record. We also record the time which the ions arrive at the detector, which is related to the mass over charge of the molecule. We can identify the molecules by the time it takes for them to reach the detector, we can identify the probability of forming each ion by the number of each ion arriving at the detector at a given time, and we can study the dissociation dynamics by interpreting the angular and radial distributions of the images we record. Other parameters are also recorded, but this is the general principle of the experiments. Recoil-frame covariance, a technique reliant on ultra-fast multi-mass imaging systems with data stored on a frame-by-frame basis, has been identified as a tool which allows a greater insight into the dissociation of multiply-charged ions, even when this is a relatively minor dissociation channel. Our preliminary investigations suggest this is a powerful tool in the analysis of our data, and have been and will continue to explore this over the coming months. Recoil-frame covariance defines one ion as a reference ion and a second as the ion of interest. Within each frame, each reference ion is rotated to lie along a reference direction, and the position of all the ions of interest are noted. This is repeated for all reference ions in the frame, and for each frame in the data acquisition. Ultimately, the technique shows the dissociation vector of the ion of interest relative to the reference ion, which can help unpick the dissociation mechanism of the molecule under study.

The main focus of this work has been on the interaction between an electron and a molecule at electron energies ranging from 50 - 100 eV. We have found that the relative proportion of ions being produced in each state remains largely unchanged over the electron energy range we have studied, leading us to postulate that for electron ionisation results in reaching some highly excited state which rapidly relaxes down to one of a set of lower-lying states, and the topology of this state then dictates the dissociation dynamics.

Finally, we have published a large number of total ionisation cross-sections, effectively the probability of ionising a molecule, calculated using the binary-encounter Bethe (BEB) model for a large number of molecules which have been detected in the interstellar medium.
Exploitation Route We have recorded a significant amount of data as a result of this funding, and the analysis of this data is an ongoing process. This will lead to a number of publications. Multiple data analysis scripts have been written in Python which are already being used by our group, and we plan to publish a number of these scripts for general use. We also play to purchase a new electron source, which is both capable of producing a higher current at low electron energies and able to produce electrons at high electron energies. Our current electron source suffers from a significant reduction in current on reduction of the electron energy which, coupled with the falling ionisation cross-section, leads to untenable signal-to-noise ratios. If we could study the processes at higher energy, we could achieve a greater insight into multiple ionisation processes.
Sectors Chemicals,Other

 
Title Total electron ionization cross-sections for neutral molecules relevant to astrochemistry 
Description This database contains total ionisation cross-sections (effectively the ionisation probability) calculated using the Binary-encounter Bethe (BEB) model for a large number of neutral molecules relevant to astrochemistry. Due to the extreme conditions, many exotic molecules found in the interstellar medium, and would react instantly if they were present on Earth. As such, theoretical models have an important role in providing cross-section data. Knowledge of total ionisation cross-sections, amongst other information, are required to produce comprehensive models of the electron ionisation processes, which would allow us to further our understanding of many systems, including but not limited to the interstellar medium. The BEB model was developed by Kim and Rudd (DOI: 10.1103/PhysRevA.50.3954), and the calculation method was developed by Bull et al. (DOI: 10.1021/jp210294p). The database is published in J/ Phys. B (DOI: 10.1088/1361-6455/aadd42) 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact In the production of this database key skills were developed, particularly in programming using Python and running calculations using Gaussian 09. 
 
Title Image analysis suite 
Description This software reads in image data recorded in imaging experiments, and can be used to symmetrise[1], invert the image with a variety of inversion methods provided by the PyAbel package[2], and gives the option to save the symmetrised image, the inverted image, and any/all of the radial, kinetic energy, momentum or velocity distribution of the inverted image. A graphical user interface has been built using the PySimpleGUI package. [1] Symmetrisation is used to increase the statistics where an axis of symmetry exists in the image by reflecting the image along an axis of symmetry. [2] In ion imaging, a two-dimensional projection of the scattering distribution is typically recorded. An inversion method can be used to recover the three-dimensional scattering distribution from the projection. 
Type Of Technology Software 
Year Produced 2019 
Impact As we have not yet published this software, there are no notable impacts beyond our research group. Within our research group, this software has streamlined image analysis, combining the vast majority of data processing steps required into a single package. This has allowed us to process our data more quickly, which in turn lets us focus on the analysis of our data. We hope to publish the software in the relatively near future, but there are a few modifications that need to be made to allow the software to perform in multiple environments without any backend modification before the software can be released.