The document summarizes an experiment investigating how photon energy affects isotopic ratios during ozone formation. Oxygen gas was exposed to light from mercury, hydrogen, and planned krypton lamps at different energies, and the resulting ozone was analyzed. Preliminary results found enrichment of 17O and 18O isotopes consistent with literature. Reducing a step in the Chapman cycle reaction had no effect. Future experiments are planned to gain more insight, such as using higher energy krypton light and temperatures, to test the resonance effect hypothesis. The anomalous mass-independent isotope effect seen in ozone formation still lacks a full explanation after decades of research.

1. Conclusion
Deborah Gardner, Subrata Chakraborty, Mark Thiemens Stable Isotopes Laboratory, Department of Chemistry and Biochemistry, University of California, San Diego
Introduction Experimental Questions
• Does the energy level of incoming photons affect the isotopic ratio of exotic oxygen
during ozone formation?
• Does reducing Chapman Cycle equation (4) have any effect on enrichment?
The first step of ozone formation process is the dissociation of an oxygen molecule to form atomic
oxygen. Based on the incoming photon energy, the product atomic oxygen will be at different energy
states (figure 2). Because the formation of ozone can run backward (dissociation of ozone to O2 and
O), reducing this reaction by means of trapping the ozone in metastable state, may result in significant
increases in slope.
The origin of the isotope effect found in ozone formation is not fully understood, as mentioned
earlier; the study of energy dependency may add an important piece to this puzzle.
Controlled variables
• Starting isotopic
ratio
• Flux of photons
• Reaction Time
• Pressure (±10 Torr)
Independent variable
• Energy of photons
• Energy state at
collection
Dependent variable
• Product enrichment
Methods & Procedure
Results
The oxygen isotopic ratio (slope of lines in figure 5) in product oxygen using both the mercury and
hydrogen lamps is enriched in both 17O and 18O. The enrichments using the Hg lamp (~110-120 ‰) in
18O is consistent with literature data (Thiemens 1983). The enrichment trend is mass-independent in
all studies as the slope value varies from 0.78-0.83 (mass-dependent slope is 0.52). There is no
significant change in slope when Chapman Cycle equation 4 is reduced.
More experiments are required to gain impactful insight. Plans to continue using a krypton lamp and
at high temperatures are planned. If the resonance effect hypothesis is correct, differences between
collection (stable vs. metastable) should disappear as all molecules will begin the reaction at a higher
energy state from the beginning.
Although these and future findings have no practical application, they can be used as proxies to
compare with theoretical solutions that can provide direction for future research aimed at resolving
the isotope effect in ozone formation in whole.
References
Babikov, Dmitri, Brian K Kendrick, Robert B Walker, Reinhard Schinke, and Russel T Pack. "Quantum origin of an anomalous isotope
effect in ozone formation." Chemical Physics Letter 372 (2003): 686-691.
Babikov, Dmitri, Brian K Kendrick, Robert B Walker, Russel T Pack, Paul Fleurat-Lesard, and Reinhard Schinke. "Formation of ozone:
Metastable states and anomalous isotope effect." Journal of Chemical Physics 119, no. 5 (August 2003): 2577-2589.
Gao, Yi Qin, and Marcus, Rudolph A. "Strange and Unconvential Isotope Effects in Ozone Formation." Science 293 (July 2001): 259-263.
Thiemens, Mark H and Heidenreich III, John E. "A non-mass-dependent isotope effect in the production of ozone from molecular
oxygen." Journal of Chemical Physics 78 (1983): 892-895.
Heidenreich III, John E, and Mark H Thiemens. "A non-mass-dependent oxygen isotope effect in the production of ozone from
molecular oxygen: the role of molecular symmetry in isotope chemistry." Journal of Chemical Physics 84, no. 4 (February 1986):
2129-2136.
Ivanov, Mikhail V, and Dmitri Babikov. "Efficient quantum-classical method for computing thermal rate constant of recombination:
Application to ozone formation." Journal of Chemical Physics 136, no. 184304 (May 2012): 1-17.
Ivanov, Mikhail V, and Dmitri Babikov. "On molecular origin of mass-independent fractionation of oxygen isotopes in the ozone forming
recombination reaction." Edited by Mark H Thiemens. Proceedings of the National Academy of Science (PNAS) 110, no. 44 (October
2013): 17708-17713.
Ivanov, Mikhail V, and Dmitri Babikov. "On stabilization of scattering resonances in recombination reaction that forms ozone." J.
Chemical Physics 144 (April 2016): 154301.
Marcus, Rudolph A. "Theory of mass-independent fractionation of isotopes, phase space accessibility, and a role of isotopic symmetry."
Edited by Mark H Thiemens. Proceedings of the National Academy of Sciences (PNAS) 110, no. 44 (October 2013): 17703-17707.
Opthos Instruments, Inc. Vacuum Ultraviolet Sources. n.d. http://www.e-opthos.com/sources.htm (accessed May 20, 2016).
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373 (May 2015): 1-12.
Isotope Effect in Ozone Formation: Assessing the Relationship
Between Photon Energy and Stabilization (A33E-0293)
• Verify all lines have
been evacuated
• Insert 200-800 Torr (as
applicable) of Ultra
High Purity Oxygen
(OX UHP 200) gas into
reaction chamber
Preparation
• Energize appropriate
resonance lamp with
microwave generator
• Allow to react for 3
hours
Photolysis
• Ozone collects in the
collection chambers
using vacuum flasks of
liquid nitrogen
• Tfreeze,O3 = -192.2°C
• Tfreeze,O2 = -218.8°C
• Tboil,N2 = -195.8°C
• Diatomic oxygen gas
vacuumed out
Separation
• Ozone gas is isolated in
lower chambers to
ensure breakdown back
to diatomic oxygen
before final collection
• Former ozone then
collected in sample
tube
• Analyzed using mass
spectrometer
Collection
Mercury (Hg) lamp: 184.9nm (6.7eV) and 253.7nm (4.9eV)
Krypton (Kr) lamp: 116.5nm (10.6eV) and 123.6nm (11.01eV)
Hydrogen (H) lamp: 121.6nm (10.2eV) (Lyman-α )
(Opthos Instruments, Inc.)
Figure 2 (left): Potential energy
diagram of molecular oxygen
(Starikovskiy 2015). In order for the
dissociation of oxygen (equation (1)
above) to occur from zero point
energy (solid black line with saddle
point at (0.12 nm, 0 eV)), the
molecule must gain enough energy
(red arrow indicates jump to higher
energy state) to exit the saddle
through one of the channels (purple,
blue, and orange arrows).
Figure 5: Diagram of relevant portions of experimental setup.
In addition to wavelength variation, the starting pressure was varied (200-800 Torr) to determine any
dependence on starting pressure. There was found to be none, only absolute δ17O and δ18O values
differed.
Figure 5: Collection data showing enrichments of minor isotopes (δx = 1000*(xO/16O)) of oxygen after
ozone formation. Terrestrial water and rock fractionation is represented by the ‘MDF’ line, X (1) is
enrichments without reducing CC eqn (4), and X (2) is enrichments while reducing CC eqn (4).
Acknowledgments
I extend my gratitude to those who helped complete this research: Haiyang Kehoe, Aubriana Morris,
and Christopher Immekus, as well as my mentors, Subrata and Mark.
SlopeHg,1: 0.83, R2
Hg,1: 0.61
SlopeHg,2: 0.89, R2
Hg,2: 0.93
SlopeH2,1: 0.76, R2
H2,1: 0.90
SlopeH2,2: 0.78, R2
H2,2: 0.85
SlopeMDF: 0.52 (Thiemens 1983)
Figure 3 (right): Diagram of recombination
of oxygen to produce 16O18O18O (Babikov
2003). Equation (2) of the Chapman Cycle
(CC) describes the recombination process,
where O3 is first formed with excess energy
(metastable). In order to stabilize, the
molecule must transfer the excess energy
to a quencher (M in CC eqn (3)). The
diagram to the right demonstrates how
heavier oxygen molecules (18O18O vs.
16O18O in this example) begin the process of
recombination with the advantage of lower
zero point energy. The energy level of the
newly formed O3 must be low enough to
rest in the saddle point (yellow) and avoid
dissociating back to O2 and O (CC eqn (4)).
Figure 4 (left): Visualization of the different
calculated stability resonances (sharp spikes)
for two different isotopologues of ozone
(16O18O 18O and 16O 16O18O) (Babikov 2003).
The resonance hypothesis relates molecular
lifetime to the energy state. The energy states
with the longest lifetimes are considered
resonances. Consistent with the idea of
advantage due to weight, the heavier
combination (16O18O 18O) has 3 resonances
greater than the other (16O 16O18O), consistent
with isotope effect enrichment. In 30 years of
research since the discovery of the isotope
effect in ozone, there have been many
theories (Heidenreich 1986, Gao 2001,
Babikov 2003, Ivanov 2012 2013 & 2016,
Marcus 2013) that explain portions of this
behavior, resonance effect being one, but not
one resolves it in its entirety.
Figure 1 (below). The formation of ozone from
molecular oxygen is governed by the set of
chemical equations known as the Chapman Cycle:
(1) 𝑂2 + ℎ𝑣 → 𝑂 + 𝑂
(2) 𝑂2 + 𝑂 → 𝑂3
∗
(3) 𝑂3
∗
+ 𝑀 → 𝑂3 + 𝑀
(4) 𝑂3 +ℎ𝑣 → 𝑂2 +𝑂
Oxygen has 3 isotopes (same atomic number,
differing number of neutrons), 16O, 17O, and 18O,
and in terrestrial water and rocks, fractionation
occurs mass-dependently. The governance of
mass is untrue in ozone (O3) formation, and was
found to be surprisingly mass-independent in
1983 (Thiemens).