Research in the Stephenson Lab
Absorption
of light by molecules to generate electronically excited species is a crucial
step in many chemical reactions.
The experiments carried out in my laboratory are designed to address a
number of fundamental issues in our understanding of the photochemistry and
photophysics of small molecules.
Our current focus is on trying to understand the energy transfer
processes that occur when atoms and molecules collide. Specifically, we have examined the
electronic and vibrational excitation and relaxation that accompanies the
collision of a rare gas atom and a halogen molecule excited to an ion-pair
electronic state. Our first target
in these investigations has been I2 colliding with He, Ar and I2(X). Our two-color laser-induced fluorescence
technique is quite general, however, and we plan thorough investigations of
energy transfer pathways in species such as Br2, ICl and IBr. The questions that we hope to answer in
these experiments include: are there preferences (“propensity rules”)
that govern the amount of vibrational energy transfer when the electronic state
changes? What is the role of
angular momentum conservation in vibrational energy transfer in the ion-pair
states of the halogens? How does
changing the mass of the rare gas atom affect the dynamics? What is the competition between
electronic, vibrational and rotational energy transfer? In addition, under the right conditions
reactions can occur between rare gas atoms and halogen molecules excited to the
ion-pair states. Competition
between reactive collisions and energy transfer encounters should provide a
rich and important opportunity to investigate photochemistry on a fundamental
level.
In
our experiments to date, we excite I2 to the E(0g+)
ion-pair electronic state using two-color laser excitation, with the
well-characterized B electronic state serving as an intermediate. By judicious choice of the transitions
used, we can assure single rotational level excitation in the E state. We then record the wavelength-resolved
emission spectrum of I2 as a function of the pressure of the
collision partner. Changes in the
emission spectrum are assigned to collision-induced energy transfer and are
analyzed to extract energy transfer pathways and probabilities.
In
the figure at right, a portion of the I2 emission spectrum is
shown. The top panel is the
spectrum recorded with only 40 milliTorr of I2 present in the cell;
the lower panels show the spectra that result when 2000 milliTorr of He and Ar
are added. In all three cases the
I2 molecules are initially prepared in the ground vibrational level
of the E ion-pair state. The emission features appearing at 332-338 nm and
344-350 nm are due to the E ® A and E ® B" transitions, respectively. The additional emission features, 300 - 328 nm and 338 - 344 nm are found to be pressure
dependent. The low wavelength
transitions are assigned to emission from the D electronic state, while the emission
with a peak around 342 nm is due to the b and D' electronic
states. The observation of
emission from the D state when only low pressure I2 is present
indicates that E ® D electronic energy transfer occurs in I2(E)
+ I2(X) collisions with a relatively large cross section (18 Å2).
E ® D transfer also occurs in I2(E)
+ He, Ar collisions, though with significantly lower cross sections. Finally, collisions of I2(E)
with rare gas atoms also populates the b and D' states,
while this pathway is absent in the case of collisions with I2(X).
Our
fits to these spectra indicate that a number of vibrational levels in the D, b and D' states are populated; broad distributions of rotational states
are observed as well. In general,
the distribution of vibrational populations in the D state is insensitive to
whether I2 collides with He, Ar or I2(X). The vibrational distributions observed in the b and D' states are somewhat different from the D state, though once
again, He and Ar collisions produce roughly similar distributions. The relative intensities observed in
the spectrum indicate that collisions with He are more likely to result in
population in the D state than is the case with Ar, while the b and D' states are favored in Ar collisions. These data provide a sensitive test of the existing theories
of electronic energy transfer, and point to the need for more detailed
theoretical and computational investigations addressing the importance of, for
example, Franck-Condon and energy gap effects.
We
also have the opportunity to examine the collision-induced vibrational and
rotational relaxation that occurs within the E state. In this case, we find that collisions with Ar produce a
distribution of rotational states that is consistent with the power gap scaling
laws that have been used for a variety of systems. When the collision partner is He, however, the power gap
relationships fail to reproduce the experimental data, perhaps reflecting the
extreme constraints imposed by the conservation of angular momentum when the
collision partner is very light.
Our experiments to date have focused on low, medium and high levels of
initial rotational excitation (J = 23, 55 and 98). For J = 23 and 55, the effects of collision-induced vibrational excitation of the I2 are also
apparent. Our immediate goals are
to extend the work to include heavier collision partners (i.e., Xe) and to explore the dynamics when I2
is initially excited to higher vibrational states in the E electronic state.
Publications
describing this work:
C.J. Fecko, M.A. Freedman and T.A. Stephenson,
“Collision-Induced Energy Transfer from
v = 0 of the E(0g+) Ion-Pair
State in I2: Collisions with I2(X)”, J. Chem.
Phys., 115, 4132 (2001).
C.J. Fecko, M.A. Freedman and T.A. Stephenson,
“Collision-Induced Energy Transfer from
v = 0 of the E(0g+) Ion-Pair State
in I2: Collisions with He and Ar”, J. Chem. Phys., 116, 1361
(2002).