Scientific Theme: Advanced Modeling and Observing Systems

Complementary Research: Innovative Research Program
Sunlight Initiated Chemistry by Low Energy Vibrational Overtone Excitation of Oxidized
Organics in the Atmosphere
Investigators: V. Vaida (CIRES, University of Colorado), R. Skodje (Department of Chemistry
and Biochemistry, University of Colorado)
Objectives. In a collaboration between experiment (Prof. V. Vaida) and theory (Prof. R. Skodje), we propose to
investigate a new paradigm for sunlight-initiated chemical reactions occurring by low energy by excitation of
vibrational overtones in the ground electronic state. The target for this study is oxidized atmospheric organics (acids,
alcohols). An important objective of this work is to investigate the role of water in catalyzing environmental
photoreactions. The contribution of such chemistry to photochemical processing of atmospheric aerosols, Arctic ice
and other environmental media will be investigated with expertise available in CIRES.
Background and Importance. The incoming beam of
photons from the Sun approximates the emission of a black
body at about 5800K with a maximum flux in the red. The
vast majority of photochemical reactions known in the
atmosphere involve a molecule‘s excited electronic states at
sufficiently high energy to rupture covalent bonds. This
process requires UV wavelengths >400nm. Some of us have
shown that photochemical reactions can occur in the ground
electronic state by absorption of radiation by vibrational
overtone transitions. The cross sections for such transitions
near chemical thresholds are very low, yet in the Earth‘s
atmosphere, excitation is effected near the solar
wavelengths maxima. Visible light, obviously abundant in the solar spectrum, is sufficiently energetic to excite high
overtone states of chromophores such as the hydroxyl-group (OH). If the overtone transition is sufficiently intense,
and if the vibrational energy is effectively transferred to the reaction coordinate, vibrational overtone absorption
may provide an important new mechanism for atmospheric chemistry. Previous studies by Vaida and coworkers
suggest that vibrational overtone absorption likely plays a role in a number of atmospheric reactions.
This work is important in establishing the fundamental foundations for a new paradigm for processing organics in
the atmosphere by sunlight. Environmental circumstances where such chemistry could be important are
photochemical processing of atmospheric aerosols on ice, especially Arctic ice, where observations suggest
additional mechanisms are needed to explain sunrise-promoted emissions from snowpacks.
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Innovative.
A new paradigm is proposed for
photochemistry in the atmosphere by solar
pumping of vibrational overtone transitions of
the ground electronic state. For organic acids
and alcohols under investigation, the excitedstate
photochemistry normally considered is
irrelevant since the electronic states absorb in
the UV at wavelengths filtered by stratospheric
ozone.
An interesting possibility is the lowering of the
transition states for reaction by water. If this
investigation determines the viability of such
water-catalyzed reactions, water becomes an
important environmental catalyst for
processing of oxidized organics not previously
considered.