Simulation Studies of Recombination Kinetics and Spin Dynamics in Radiation Chemistry

339 Pages·2014·10.01 MB·English

by  Amit Agarwal (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Amit Agarwal
Simulation Studies
of Recombination
Kinetics and Spin
Dynamics in
Radiation Chemistry
Springer Theses
Recognizing Outstanding Ph.D. Research
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Amit Agarwal
Simulation Studies
of Recombination Kinetics
and Spin Dynamics
in Radiation Chemistry
Doctoral Thesis accepted by
University of Oxford, UK
123
Author Supervisor
Dr. AmitAgarwal Dr. Nicholas Green
Reactor Chemistryand Materials Physical andTheoretical Chemistry
National Nuclear Laboratory Laboratory
Oxon Universityof Oxford
UK Oxford
UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-06271-6 ISBN 978-3-319-06272-3 (eBook)
DOI 10.1007/978-3-319-06272-3
Springer ChamHeidelberg New YorkDordrecht London
LibraryofCongressControlNumber:2014936852
(cid:2)SpringerInternationalPublishingSwitzerland2014
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Parts of the thesis have been published in the following journal articles:
1. Agarwal, Amit; Green, Nicholas. ‘‘Photodissociation of H2O2: singlet or tri-
pletprecursor?’’ Phys. Chem. Chem. Phys. (publication pending).
2. Agarwal, Amit; Green, Nicholas. ‘‘Competition between geminate ion recom-
binationandscavenginginlow-permittivitysolvents’’Phys.Chem.Chem.Phys.
(publication pending).
3. Agarwal, Amit; Green, Nicholas. ‘‘Simulating the exchange interaction in the
IRTframework:Slicepackage’’Phys.Chem.Chem.Phys.(publicationpending).
4. Agarwal, Amit; Green, Nicholas. ‘‘Reactive products in the IRT algorithm’’
Phys. Chem. Chem. Phys. (publication pending).
v
To my parents, Raj and Neeru
Supervisor’s Foreword
Radiation damage is important in a wide range of areas, from medicine and
biology to material science, and has obvious applications in nuclear power gen-
eration, radiation detection and quantification, waste treatment, polymer curing,
and nuclear medicine. Radiation chemistry is concerned with the molecular
understandingofthephysicalandchemicaltransformationsthattakeplaceduring
the course of radiation damage. Because the damage is highly localized along
radiation tracks, and microscopically clustered within these tracks, there are
challenging theoretical problems. The scope of radiation chemistry encompasses
the physical interactions between ionizing radiation and the molecule (femtosec-
ond timescale), the dynamics of molecular fragmentation and solvation (sub
picosecond timescale) and the fast combination of radical fragments within the
track (picosecond–nanosecond timescale), as well as the subsequent radical
chemistry. Radiation chemistry uses a combination of theory and computer
modeling, together with available experimental results, to interpret the chemical
nature ofradiationdamage intermsoftheunderlyingphysicochemicalprocesses,
and to elucidate the fundamental physics from the earliest chemical observations.
Amit Agarwal’s thesis reports a substantial contribution to the microscopic
simulation of radiation chemical reactions, extending the models in several areas,
including scavenging, spin, and relaxation effects. This thesis has made advances
indevelopingboththeMonteCarloRandomFlightsandtheIndependentReaction
Times (IRT) simulation tools. Particular highlights are the extension of these
methodstoincludeboththespin-exchangeinteractionandspinrelaxation,bothof
which are influential in radiolytic systems where many reactions are spin-con-
trolled.Inaddition,thestudyhasdiscoveredanovelcorrelationofthescavenging
rateontherecombinationtimeinlowpermittivitysolvents.Thisisafundamental
breakdownoftheassumptionsunderlyingthetheoryofdiffusionkinetics,butcan
still be accommodated in the IRT method, demonstrating the power of this
unconventional approach.
In conclusion, Amit Agarwal’s work allows one to model complex radiation
track structures and spur reactions with an explicit treatment for spin dynamics,
withoutcompromisingcomputationalresources.Thepredictivecapabilitiesgained
ix
x Supervisor’sForeword
by such anunderstanding have important applications inradiation protection, and
in the various areas of technology where radiation is used, such as the nuclear
industry, medicine, sterilization, food treatment, polymer curing, the preparation
of nano-colloids, power generation, and nuclear waste disposal.
Oxford, March 2014 Dr. Nicholas Green
Preface
Radiation chemistry is concerned with understanding the chemical kinetics fol-
lowing the application of ionising radiation. There are two main methods for
modeling recombination and spin dynamics in radiation chemical systems: The
Monte Carlo random flights algorithm, in which the trajectories of the diffusing
species are followed explicitly and the Independent Reaction Times (IRT) algo-
rithm, where reaction times are sampled from appropriate marginal distribution
functions. This thesis reports developments to both methods, and applies them to
better understand experimental findings, particularly spin relaxation effects.
Chapter 4 introduces current simulation techniques and presents newly devel-
opedalgorithmsandsimulationprograms(namelyHybridandSlice)formodeling
spatially dependent spin effects. A new analytical approximation for accurately
treating ion-pair recombination in low-permittivity solvents is also presented in
this chapter.
Chapter 5 explores the photodissociation of H O , where there is some con-
2 2
troversy in the literature on the spin state of the precursor. This chapter explores
the possibility of reproducing the observed spin polarization phase using the
Radical Pair Mechanism.
Chapter6presentstwonewalgorithmsfortreatingreactiveproductsintheIRT
framework. These have been tested for two chemical systems: (i) photodissocia-
tion of H2O2 where the (cid:2)OH are scavengeable; (ii) water photolysis which pro-
duces Hþ; (cid:2)OH and e(cid:3). In the latter case a careful handling of three body
aq
correlations is required.
Chapter 7 presents simulation results, which suggest a strong correlation
between scavenging and ion recombination in low permittivity solvents (a fun-
damental breakdown of the assumptions underlying the theory of diffusion
kinetics). A path decomposition method has been devised that allows IRT simu-
lations to be corrected for this effect.
Chapter 8 presents evidence for spin-entanglement and cross-recombination to
actasanextra sourceofspinrelaxation forion-recombinationinlowpermittivity
solvents.Itishypothesizedthatthiseffectcontributestotheanomalousrelaxation
times observed for certain cyclic hydrocarbons.
xi