**Research Activity: Photochemistry and Radiation Research**

Division: Chemical Sciences, Geosciences, and Biosciences

Primary Contact: Mary E. Gress (<mary.gress@science.doe.gov>;
301-903-5827)

Team Leader: Eric A. Rohlfing

Division Director: Walter J. Stevens

**Portfolio Description:**

The activity supports fundamental molecular level research on
interactions of radiation with matter in the condensed phase.

The photochemistry research effort emphasizes fundamental processes
aimed at the capture and conversion of solar energy. Biomimetic models
(photochemical and photoelectrochemical) seek to mimic the key aspects
of photosynthesis -- antenna, reaction center, catalytic cycles, and
product separation. The research encompasses organic and inorganic
photochemistry, photoinduced electron and energy transfer,
photoelectrochemistry, biophysical aspects of photosynthesis, and
molecular assemblies for artificial photosynthesis.

The radiation sciences effort supports fundamental studies on chemical
effects produced by the absorption of energy from ionizing radiation.
The radiation chemistry research encompasses heavy ion radiolysis,
models for track structure and radiation damage, characterization of
reactive intermediates, radiation yields, and radiation-induced
chemistry at interfaces. Accelerator-based electron pulse radiolysis
methods are employed in studies of highly reactive transient
intermediates, and kinetics and mechanisms of chemical reactions in the
liquid phase and at liquid/solid interfaces.

**Unique Aspects:**

This activity is the dominant supporter (85%) of solar photochemistry in
the U.S. and the sole supporter of radiation chemistry. Specialized
electron pulse radiolysis facilities at Notre Dame, ANL, and BNL serve
the academic research community, industrial users, and other national
laboratories. A new laser-driven electron accelerator at BNL features a
5 picosecond pulse width and the capability for synchronized electron
pulse-laser pump-laser probe experiments.

**Relationship to Others:**

The solar photochemistry research effort interfaces with activities in
BES, including: Energy Biosciences activities in biochemical aspects of
photosynthesis; Chemical Physics in theoretical calculations of excited
states; Catalysis and Chemical Transformations on electron transfer
reactions in homogeneous and microheterogeneous solutions, and advanced
catalytic materials; and Materials Sciences fundamental photovoltaics
research. The research is relevant to EE activities in the Office of
Solar Energy Technologies on photovoltaics, and in the Office of
Hydrogen and Superconductivity Technologies on hydrogen production.

The radiation sciences activity interfaces in BES with Catalysis and
Chemical Transformations in reaction kinetics in homogeneous solutions,
and Materials Sciences in radiolytic damage to glasses and
radiation-induced corrosion of structural materials. There are also
important interfaces with EM activities in waste remediation and NE
activities on nuclear reactors, and nuclear processing and storage.
Radiolytic processes in solution, particularly heavy ion radiolysis, are
of interest to the NIH regarding radiation damage to biological systems
in medical diagnosis and therapy.

**Significant Accomplishments:**

Stratospheric ozone depletion by chlorofluorocarbons was predicted by F.
Sherwood Rowland of UC, Irvine, in 1974. Professor Rowland's research,
solely supported by this activity, involved the chemistry of "hot"
chlorine atoms produced by nuclear recoil and complementary photolytic
reactions. Rowland was awarded the Nobel Prize in 1995. Radiotracers for
nuclear medicine were pioneered by Alfred Wolf at BNL. The "special
pair" model for electron donor chlorophyll molecules in photosynthesis
was introduced by Joseph Katz and James Norris of ANL. Photosynthetic
molecular models for light to chemical energy conversion were developed
by Michael Wasielewski of ANL and by Professors Gust, Moore, and Moore
of Arizona State University. The "inverted region" in Marcus electron
transfer theory was verified in pulse radiolysis experiments by John
Miller at ANL.

**Mission Relevance:**

Solar photochemical energy conversion is a long-range option for meeting
the world's future energy needs. An alternative to solid-state
semiconductor photovoltaic cells, the attraction of solar photochemical
and photoelectrochemical conversion is that fuels, chemicals, and
electricity may be produced with minimal environmental pollution and
with closed renewable energy cycles. A strong interface with EE solar
conversion programs exists at NREL, involving shared research,
analytical and fabrication facilities, and a jointly shared project on
dye-sensitized solar cells.

Radiation chemistry methods are of importance in solving problems in
environmental waste management and remediation, nuclear energy
production, and medical diagnosis and radiation therapy. Fundamental
studies on radiation-induced processes complement collocated NERI and
EMSP projects.

**Scientific Challenges:**

In solar photoconversion, knowledge gained in charge separation and
long-distance electron transfer needs to be applied in a meaningful way
to activation of small molecules (CO~2~, N~2~, and H~2~O) via
photocatalytic cycles. Experimental and theoretical studies on
photosynthetic pigment-protein antenna complexes should lead to advances
in design of efficient and robust artificial light-collecting molecular
assemblies. Computational chemistry methods incorporating recent
advances in calculation of excited states should be developed and
applied in design of photocatalysts and molecular dynamics simulations
in artificial photosynthesis. There are also challenges in fundamental
understanding of photoconversion processes -- energy transfer and the
generation, separation, and recombination of charge carriers -- in
organic-based molecular semiconductors, which could lead to a new type
of inexpensive and flexible solar cell. Fundamental studies on
photochemical reaction pathways offer opportunities for less energy
intensive and more environmentally benign processing of specialty
chemicals and high volume industrial intermediates.

A recent workshop "Research Needs and Opportunities in Radiation
Chemistry" has identified new directions, connections, and impacts of
radiation chemistry. A common theme is the need to explore radiolytic
processes that occur across solid-liquid and solid-gas interfaces, where
surface chemistry can be activated and changed by radiolysis.
Solid-liquid interfaces abound in nuclear reactors and high level
radioactive wastes. Colloidal particles participate in gas production,
gas retention, and in organic degradation of high level wastes. In
regard to environmental remediation, radiation chemistry is one of the
most promising advanced oxidation processes for degradation of organic
pollutants. A more fundamental understanding of radiolytic reactions in
heterogeneous media is needed in order to predict and control radiation
chemical transformations in complex environmental systems. A proposed
subpicosecond laser photocathode electron accelerator at ANL would
enable investigation of the primary events in radiation chemistry, now
virtually unknown except for theoretical models, wherein fundamental
processes link physics to the chemistry of radiolysis.

**Funding Summary:**

**Dollars in Thousands**

**[FY 2002]{.underline}** **[FY 2003 Request]{.underline}** **[FY 2004
Request]{.underline}**

26,096 29,032 28,973

> **[Performer]{.underline} [Funding Percentage]{.underline}**
>
> DOE Laboratories 55.0%
>
> Universities 42.0%
>
> Other 3.0%

These are percentages of the operating research expenditures in this
area; they do not contain laboratory capital equipment, infrastructure,
or other non-operating components.

The program provides funding for 53 university grants supporting about
50 students, 55 postdoctoral research associates, and partially
supporting about 60 faculty. There are nine programs at DOE laboratories
supporting

51 senior staff and 44 graduate students and postdocs. Programs at the
laboratories are multi-investigator efforts on problems that require
extensive participation by experienced scientists. In photochemistry,
major research

groups are supported in inorganic photochemistry and electron transfer
at BNL; in photoelectrochemistry at NREL and NDRL; and in photosynthesis
at Ames, ANL, and LBNL. Many of the research efforts at the DOE
laboratories involve strong collaborative interfaces with university and
industrial communities. The radiation chemistry effort is

centered at specialized electron pulse radiolysis facilities at Notre
Dame, ANL, and BNL. The Notre Dame Radiation Laboratory serves as the
primary radiation research user facility, hosting approximately 40
users/year from academia and industry.

**Projected Evolution:**

In solar photochemistry, artificial photosynthetic systems on the
nanoscale will be explored that combine photosensitization,
energy/electron transfer to a final charge-separated state stabilized by
novel microheterogeneous environments, and photocatalytic half-cycles
for activation of small molecules. Photoinduced charge transfer at the
semiconductor/liquid interface will be explored in novel single molecule
spectroscopic studies at nanocrystal junctions. Nanometer-spaced
electrodes will be employed in studies of electron tunneling through
solvents and of bridging "molecular insulators" and "molecular wires."
Electronic communication among semiconductor quantum dots will be
investigated in nanostructured assemblies comprised of arrays of quantum
dots coupled to proteins or carbon nanotubes. Photoconversion processes
in molecular semiconductors will be explored in new, self-organized
liquid crystalline films. Studies on photophysical aspects of
photosynthesis relevant to artificial systems will exploit
state-of-the-art single molecule spectroscopy, near-field fluorescence
microscopy, and ultrafast absorption spectroscopic methods. Theory and
modeling will assume a far more significant role in the program, with
the ability to calculate excited states and with new methods and
computer capacity to treat complex systems.

Radiation chemistry methods will be applied in studies of
radiation-induced chemistry at interfaces, radiolytic intermediates in
supercritical fluids, mechanistic analysis of electron transfer, and
characterization of excited states in dual pulse radiolysis/photolysis
experiments. Effects of ionizing radiation in macromolecules rather than
ensembles of molecules will be explored in nanobubbles by atomic force
microscopy. ANL will soon demonstrate the feasibility of a tabletop,
terawatt laser/electron and X-ray generator for subpicosecond pulse
radiolysis and ultrafast X-ray studies. Radiation chemistry groups at
ANL, BNL, and NDRL will forge stronger links to environmental management
and nuclear energy programs.

November 13, 2023