Creative Research Enterprises Presentation

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A COPY OF CREATIVE RESEARCH ENTERPRISES
PRESENTATTION bySheo S. Prasad

• At the
• FALL 2001 AGU MEETING

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TABLE OF CONTENTS

• ABSTRACT
• BACKGROUND
• RECENT DEVELOPMENTS
• EVEN MORE RECENT DEVELOPMENTS
• (since the abstract was submitted)
• SUMMARY CONCLUSIONS
• URGENT TASKS AHED

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ABSTRACT

• O3(X1A1) with v high enough to produce N2O is
  not generated in O(3P), O2 recombination
• The possibility that some form of excited O3,
  resulting either directly or indirectly from
  O(1D), O2 recombination, produced N2O in
  Zipf-Prasad experiment cannot be ruled out at
  least for now. More experiments are needed.
• Pending those experiments, from an analysis of
  four experiments it is concluded that N2O is a
  product in the reactions of electronically
  excited O3(1B2) and O3(2 1A1) with N2 and in the
  photolysis of O3?N2 dimer

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IMPORTANCE OF N2O - O3 CONNECTION

• O3 N2O are two very important constituents of
  the atmosphere. Both are greenhouse gases and O3
  shields the biosphere from harmful UV-A and UV-B.
  There is also a destructive relationship between
  the two since N2O is the dominant in situ
  stratospheric source of NO.
• While the sources of the two gases are currently
  thought to be quite different, analyses of
  several experiments have suggested that some
  types of excited O3 might form N2O.
• An O3-N2O connection can be potentially important
  due to the stated destructive relationship.

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EVOLUTION OF N2O-O3 CONNECTIONHas twists
turns very familiar in path to progress!

• Prasad,1981 Electronically excited metastable
  triplet O3 was thought to be a possible source of
  N2O based on experiments that appeared to support
  formation of triplet O3 in O, O2 recombination
• Prasad,1994 10 experiments further support N2O
  formation via some type of excited O3. But, by
  this time the prospect of metastable excited
  triplet was lost. So, excited O3(X1A1, very high
  v) from O, O2 recombination was proposed as N2O
  source
• Zipf Prasad, 1998 O3(X1A1, very high v)
  appeared to explain the high yield of N2O in
  Zipf-Prasad experiment
• 2001 Role of O3(X1A1) eliminated (Estupinan).
  But, electronically excited O3 may be forming N2O
  after all ! (Prasad)

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THE RECENT DEVELOPMENTESTUPINANS NON-DETECTION
OF N2O IN N2/O2/O3 PHOTOLYSIS AT 532 NM

• Estupinans non-detection in this experiment does
  not mean that N2O is not produced by O3(X1A1,
  sufficiently high v). Instead, it means that the
  high vibration needed to produce N2O is not
  generated when O3(X) from O(3P) , O2
  recombination.
• Thus, the origin and identity of the species that
  produced N2O in Zipf-Prasad experiment remains an
  important chemical physics and atmospheric
  chemistry problem to be solved by more
  experiments. At least for now, that species may
  still be some form of excited O3 resulting either
  directly or indirectly from O(1D), O2
  recombination.

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MORE RECENT DEVELOPMENTAN ANALYSIS OF THE
SECOND SET OF ESTUPINANS EXPERIMENT AND THREE
OTHER PREVIOUS EXPERIMENTS

• Pending experiments needed to better understand
  Zipf-Prasads, the rest of the presentation will
  dwell on an analysis of experiments by
• Estupinan on N2/O2/O3 photolysis at 266 nm
• Gaedtke et al on on N2/O2/O3 photolysis at 254 nm
• Kajimoto Cvetanovic on N2/O2/O3 photolysis at
  254 nm and at N2 pressures from 27 to 113
  atmospheres
• DeMore Raper in liquid phase and 200 to 350 nm
• using a model of N2O quantum yield that explains
  the yield observed in all these experiments
  encompassing a very wide range of pressures and
  radiation wavelength and relative N2, O2 O3
  amount

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GAEDTKE ET AL EXPERIMENT SUGGEST N2O FORMATION
FROM PROCESS OTHER THAN O(1D), N2 ASSOCIATION

• Gaedtke et al (1973) found N2O formation in
  photolysis of N2/O2/O3 mixture at 254 nm and 1
  atm pressure and determined 2.7x10-36 as the the
  rate coefficient for the O(1D) N2 M -gt N2O
  M, if the observed N2O is attributed to that
  reaction.
• Later work by Kajimoto Cvetanovic (1975)
  suggested a much smaller (by factor of 7.7) rate
  coefficient (or, 3.5x10-37) and this smaller
  coefficient is currently recommended by NASA
  Panel.
• In retrospect, Gaedtke et al experiment imply
  that at lower pressures N2O may form more
  efficiently by process (es) other than O(1D)N2

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EFFICIENT FORMATION OF N2O BY PROCESSES OTHER
THAN O(1D), N2 ASSOCIATION IS CONFIRMED BY
ESTUPINAN EXPERIMENT

• 28 year later, Estupinan et al found a linear
  variation of the quantum yield ?N2O in 100 to
  1000 torr pressure range, ??N2O/?PN2 2.1x10-6
  when PN2 is in atm
• If the observed N2O is attributed to O(1D), N2
  association (as Estu-pinan et al did), the rate
  coefficient for the association reaction at 1 atm
  again turns out to be too large by a factor of
  almost 8 compared to current NASA Panel
  recommendation based on Kajimoto Cvetanovic
  experiment.
• Thus, it is urgent to search for the process(es)
  that could produce N2O more efficiently than the
  O(1D), N2 association at ?1 atm.

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PRODUCTION FROM ELECTRONICALLY EXCITED O3 IS A
LOGICAL CHOICE

• None of the other species expected in N2/O2/O3
  photolysis (like O2(1?g) , O2(b 1?)) has enough
  energy to produce N2O
• In principle photolysis of O3 with Hartley band
  photons can produce vibrationally excited O2 with
  vibrational energy needed to possibly generate
  N2O. However, at 266 nm this too is not possible.
• O3(X,1B1) with high v attainable in O, O2
  recombination has already been eliminated
• Thus, O3(1B2) N2 -gt N2O O2 (where
  superscripts and represent, respectively,
  electronic and combined vibrational and
  translational excitations) may be the logical
  process.

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THE FACT THAT ELECTRONICALLY EXCITED O3 HAS
LIFETIME OF MOSTLY FEMTOSECONDS SHOULD NOT CAUSE
MUCH CONCERN

• Possibly, when a N2 comes so close to a O3(1B2)
  that it might react then the close proximity
  perturbs the dissociation dynamics to an extent
  that there is time to form the transition state.
• The "net" reaction may involve a hitherto
  unrecognized, electronically excited, O3 with
  shallow minimum in its potential energy surface
  into which a fraction of O3(1B2) may change by
  curve crossing
• Also, after all short lived O2(B3 ?) with
  lifetime of ps or less is known to react

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RATE CONSTANT FOR O3(1B2) N2 -gt N2O O2
DERIVED FROM ESTUPINANS ?N2O IS VERY REASONABLE

• At any point in the irradiated region the very
  small but finite number density of O3(1B2) that
  have not as yet lost their identity is
• n(O3(1B2)) J n(O3) / kdiss kdiss
  (lifetime)-1
• The corresponding quantum yield is (k/kdiss
  )n(N2) where k is the rate constant of the title
  reaction
• With Estupinans ?N2O lifetime 10 fs, k
  8x10-12cm3s-1

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IF O3(1B2) FORMS N2O, THENTHE FOLLOWING SHOULD
ALSO HOLD

• There should be considerable N2O formation when
  O3 is excited to the secondary minima of the 2
  1A1 or to the quasi-bound portion of the 1B2
  potential energy surface that are responsible for
  the Huggins bands (despite the negligible yield
  of O(1D) in this region).
• The ?N2O derived in from Estupinans data should
  be consistent with that from the high pressure
  data of Kajimoto and Cvetanovic.
• From a reinterpretation of Kajimoto Cvetanovic
  and DeMore Raper experiments, using a more
  complete model of ?N2O, both constrains are found
  to be fulfilled (as will be now explained).

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THE MODEL OF ?N2O USED TO REINTERPRET KAJIMOTO
CVETANOVIC AND DEMORE RAPER DATA HAS FOLLOWING
FEATURES

• Quantum yield from O3(1B2) and O3(2 1A1) Linear
  in pressure p
• All elements of Kajimoto Cvetanovics model of
  ?N2O from O(1D), N2 association p2 variation
• Contribution of O3?N2 hv ? N2O O2 that
  represents the photolysis of O3 component of the
  O3?N2 and the O(1D), N2 association inside the
  dimer. Also, p2 variation
• Details are in a preprint available for
  distribution to those interested.

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THE ?N2O FOR THE O3(1B2) COMPONENT FROM KAJIMOTO
CVETANOVIC AND ?N2O FROM ESTUPINAN DATA AGREE
EXCELLENTLY
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THE ?N2O FROM O3 EXCITED BY HUGGINS BAND AND
DIMER EFFECT ARE ALSO EXPERIMENTAL REALITIES
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THE ATMOSPHERIC PRODUCTION OF N2O FROM O3
EXCITED BY HUGGINS BANDS MAY BE SIGNIFICANT

• Significance
• Direct way of producing mass-independent heavy
  O-atom enrichment in N2O
• Total atmospheric production (2-3)x108 N2O cm-2
  s-1 is substantial

A B represent two different ways of diurnal
averaging.
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SUMMARY CONCLUDING REMARKS

• (1) N2O is a quite possible and atmospherically
  significant product when O3 is excited by
  Huggins band (310-340 nm) photons in air
• (2) Since this production may occur in the
  stratosphere, missing sinks of N2O are implied,
  if the possibility in (1) is upheld by
  experiments
• (3) O3(X1A1) with so high v that they might
  produce N2O are not generated in O(3P), O2
  recombination.But, this does not preclude its
  formation in other ways such as fluorescence from
  O3(1B2)
• (4) The identification of the species
  responsible for N2O observed by Zipf-Prasad is an
  important chemical physics and atmospheric
  chemistry problem. For now at least, that species
  may still be some form of excited O3 resulting
  either directly or indirectly from O(1D), O2
  recombination.

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EXPERIMENTAL RESEARCH TASKS THAT NEED URGENT
ATTENTION

• Since DeMore Raper experiment was done in
  condensed phase, it is most important to study
  the production of N2O with high spectral
  resolution when gas phase mixtures of air and O3
  are irradiated by Huggins band photons at various
  atmospherically significant temperatures and
  pressures, simultaneously examining the isotopic
  composition of the product N2O
• It is also important to repeat Zipf-Prasad (ZP)
  and Estupinan experiments the former with
  spectrally finely resolved light source spanning
  the range of wavelengths covered by ZPs lamp,
  and the latter with O3/air ratio tending to zero.

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EXPERIMENTAL RESEARCH TASKS (Cont.)

• The fact that Estupinan et al experiments done
  with the marvels of modern laboratory techniques
  gave the same answer as was obtained 28 years ago
  with much simpler techniques available at that
  time shows that
• the needed set of experiments can be done with
  relatively simple techniques available at even
  moderately equipped laboratories.
• It is therefore hoped that this presentation will
  enthuse many others to experimentally check the
  interpretations presented here.