My research focuses on
theoretical aspects of chemical dynamics, the branch of chemistry that
describes the nature of physical and chemical processes that underline
the
progress of chemical reactions of chemical reactions with the aim to
achieve
understanding of such processes and the ability to predict their course
of
evolution. In particular, my studies deal with chemical processes
involving
interactions between light and matter, chemical reactions in condensed
phases and
at interfaces and transport phenomena in complex systems. Following is
a
summary of my main studies and achievements:
(b) Molecular
dynamics in condensed phases.A
molecular level description of chemical reactions in condensed phases
is
intrinsically difficult because of the inherent complexity of the
underlying mechanism.
This
complexity stems from the many-body nature of the process and from the
fact
that the relevant motions occur on a wide range of timescales. The goal
of
basic research in this area is to explain and predict rates, directions
and
mechanisms of chemical reactions in solid and liquid hosts and to
clarify the
essential roles played by the host transport properties on one hand and
by the
molecular dynamics on the other. Our group has made basic studies that
made a
substantial progress toward achieving this goal. In particular we have
pioneered the development of the non-Markovian theory of activated rate
processes in condensed phases[74, 80, 84, 86, 89, 104, 111, 114] that
focuses on the essential role of the
relative molecular and environmental timescales in determining the
course and
rate of chemical reactions, and have made fundamental studies of
solvation
dynamics in polar solvents and its role in controlling processes
involving
charge transfer in condensed phases. The latter effort has focused both
on
quantum and classical aspects of solvation dynamics[155, 157, 163, 169,
181, 187] and also lead to
series of fundamental studies (in collaboration with U. Landman and R.
B. Barnett) on the
spectroscopy and dynamics of electrons in aqueous environments, in
particular
in water clusters.[134, 137, 143, 144, 146, 148]
(c) Ionic
transport in complex
environments.Ionic transport in condensed phases is a
fundamental process that governs properties of electrolyte solutions
and
solids. Such processes are fundamental to the behavior of
electrochemical
systems, and understanding the way they occur in complex environments
is a
prerequisite for the current research and development of new types of
technologically important ionic conductors such as solid and polymer
electrolytes. Solvation dynamics of ions in simple and complex
dielectric
fluids is but one example of my work on ionic transport. Our most
significant
contributions in this area are his studies of ionic transport in solid
and
polymeric ionic conductors, in which me and my coworkers have outlined
the
factors that determine to conduction properties of these systems. In
particular, with Druger and Ratner, I have developed the dynamic
percolation
theory[88, 97] for ionic transport in polymer ionic conductors, which
has
become an
important reference point in this field and has since proved to be very
useful
for transport phenomena in other systems characterized by dynamic
disorder.[129, 145, 173, 223]
This interest has also lead to recent important contributions to the
understanding of ion transport through membrane channels.[201, 211, 244]
(d) Optical properties and photochemistry of adsorbed molecules.The special properties of molecular interactions at interfacial environments govern the course of important classes of chemical processes such as heterogeneous catalysis (the process in which the rate of a chemical reaction is enhances at the interface), electrochemical processes and corrosion. My work on the electromagnetic response of adsorbed molecules on dielectric surfaces has led me to develop, with Professor Gersten, the electromagnetic theory of surface enhanced Raman scattering (SERS) from such molecules[61, 68, 83, 99, 153] and to make the theoretical prediction (later confirmed experimentally) that other photophysical and photochemical processes involving molecules adsorbed on rough dielectric surfaces are similarly affected. For example, our theoretical prediction[92,101] that electronic energy transfer can be considerably enhanced on rough metal surfaces and near metal particles was subsequently confirmed (L. M. Folan et al, Chem. Phys. Letters 118, 322(1985) and an impressive recent demonstration by P. Andrew and W. L. Barnes, Science, 306, 1002 (2004)), and the prediction that plasmon resonance enhancement is particularly strong in specific configurations such as the spacings between metal particles[70] and in small cavitis on metal surfaces[95a], has become the basis for explaining the observation of superstrong SERS signals at particular sites observed by single-molecule spectroscopy. This also implies that photochemical reactions may be effectively catalyzed on such surfaces, a theoretical idea[69] that was later patented with L. Brus. In conceptually related theoretical studies I have investigated the chemical and photochemical properties of electrons and molecules adsorbed on or solvated in dielectric particles and clusters.[116, 121, 127, 131, 140, 144] Of particular interest in these latter system is the system size dependence of the observed property as the system evolves from a small cluster to a macroscopic bulk system.[116]
(e) Electron
transport
through molecular layers and wires.The possibility that
molecules and small molecular assemblies can replace conventional
conductors
and semiconductors in nano-scale electronic devices has become a
subject of
intense discussion. The new fundamental issues associated with such
systems -
the electronic structure, charge transfer properties, energy transfer
and
relaxation and capacitive properties, to name just a few, of molecules
connecting to conducting leads, present new theoretical and
experimental
challenges. Past efforts in our group is focused on developing
theoretical and
numerical tools for studying electron transmission through such
interfaces.
A
recent series of papers that focus on the electron transmission
properties of
water[186, 189, 191, 193, 205, 212, 229] arguably the most important
electron transmitting medium, has
elucidated the effect of the interfacial structure on the transport and
has demonstrated a new resonance effect that explains the observed high
efficiency
of electron transmission through this medium.[206] More recent work has
focused on the conduction properties of single molecules and of
molecular layers,[213] the relation between molecular conduction and
molecular electron transfer,[215, 237, 233, 235, 236] the effect of
thermal relaxation and heating in current carrying junctions,[194,
207-209, 214, 224, 225] specific effects of electron-phonon
interactions such as the dependence of the lineshapes of inelastic
tunneling features on junction parameters[248, 249] and the emergence
of
non-linear transport behaviors such as multistability, hysteresis and
negative differential resistance.[254] Most recently we have
invetsigated heat conduction and heat rectification in molecular
junctions[243, 250, 256, 260] and have predicted some novel radiation
field effects in such systems,[230, 234, 257, 259] setting new
challenging goals to experimentalists.
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1. Molecular energy transfer |
2. Molecular dynamics in condensed phases |
3. Ionic transport |
4. Optical properties of adsorbed molecules |
5. Electron transport |
Others |
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1-20, 22, 25, 26, 28, 31-35, 39-43, 45-52,
55-58, 62, 63, 66, 78, 81, 102, 103, 125, 126, 151,162, 176, 177, 179,
195, 196, CB2, CB5 |
72, 74, 80, 84, 86, 89, 90, 93, 94, 96, 104,
111, 114, 119- 121, 126-128, 130, 131, 133, 134, 139, 140, 143-145,
147-150, 153-155, 157, 158, 160, 161, 163, 165, 167-170, 172-174, 176,
179, 181-185, 187, 190, 196, 197, 204, 210, CB10-15, CB22, CB23 |
59, 60, 64, 71, 76, 82, 87, 88, 97-99,
107-110, 115, 122, 123, 129, 132, 135, 136, 138, 139, 141, 145, 147,
152, 166, 167, 172, 173, 178, 180, 184, 199-202, 211, 219, 222, 223,
232, 244, 255, CB16, CB16, CB18, CB19, CB20, CB21 |
42, 61, 65, 68-70, 73, 75, 77, 79, 83, 85,
91, 92, 95a, 100, 101, 103, 106, 112, 113, 116, 118, 120, 121, 124,
127, 131, 133,134,137,140-144, 149, 153, 155, 156, 158, 159, 164, 171,
187, 182, CB4, CB8, CB9, CB10, CB11, CB12 |
118, 146, 148, 162, 174, 186, 189, 191-194,
198, 203, 205-209, 212-215, 217, 218, 224-230, 233-243, 246-7, 248-254,
256- 260, CB7,
CB17, CB24, CB25 |
21, 24, 27, 29, 30, 36-38, 44, 53, 54, 67,
105, 188, 216, 220, 221, 231, 245, CB1, CB3, CB6 |
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Representative
papers (italics - reviews): |
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186, 189, 191, 205,
206,207, 213,
214, 215, 218,
226,240,
243 ,248,
249 |
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