a copy of creative research enterprises’ presentattion by sheo s. prasad

12 Dec 2001 Creative Research Enterprises’ Presentationat AGU 2001 Fall Meeting

Slide 1

A COPY OF CREATIVE RESEARCH ENTERPRISES’

PRESENTATTION

by

Sheo S. Prasad

At the

FALL 2001 AGU MEETING


Slide 2

TABLE OF CONTENTS

• ABSTRACT

• BACKGROUND

• RECENT DEVELOPMENTS

• EVEN MORE RECENT DEVELOPMENTS(since the abstract was submitted)

• SUMMARY & CONCLUSIONS

• URGENT TASKS AHED


Slide 3

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 O3N2 dimer


Slide 4

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.


Slide 5

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)


Slide 6

THE RECENT DEVELOPMENT:ESTUPINAN’S NON-DETECTION OF N2O IN

N2/O2/O3 PHOTOLYSIS AT 532 NM

• Estupinan’s 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.


Slide 7

MORE RECENT DEVELOPMENT:AN ANALYSIS OF THE SECOND SET OF

ESTUPINAN’S EXPERIMENT AND THREE OTHER PREVIOUS EXPERIMENTS

Pending experiments needed to better understand Zipf-Prasad’s, 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


Slide 8

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 -> 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


Slide 9

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.


Slide 10

PRODUCTION FROM ELECTRONICALLY EXCITED O3 IS A LOGICAL CHOICE

• None of the other species expected in N2/O2/O3 photolysis (like O2(1g) ,

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 -> N2O# + O2*# (where superscripts * and #

represent, respectively, electronic and combined vibrational and translational excitations) may be the logical process.


Slide 11

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


Slide 12

RATE CONSTANT FOR O3(1B2) + N2 -> N2O# + O2*#

DERIVED FROM ESTUPINAN’S 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 Estupinan’s N2O & lifetime = 10 fs, k = 8x10-12cm3s-1


Slide 13

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 Estupinan’s 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).


Slide 14

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 & Cvetanovic’s model of N2O from O(1D), N2 association [p2 variation]

• Contribution of O3N2 + hv N2O+ O2 that represents the photolysis of O3 component of the O3N2 and the O(1D), N2 association inside the dimer. [Also, p2 variation]

• Details are in a preprint available for distribution to those interested.


Slide 15

THE N2O FOR THE O3(1B2) COMPONENT FROM KAJIMOTO & CVETANOVIC AND N2O FROM ESTUPINAN DATA AGREE EXCELLENTLY

Pressure (atm)

20 40 60 80 100 120

N2O

Yield

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014observed yield

O3(1B2) + N2 part (p

1 variation) O3-N2 dimer part (p2 variation)

O(1D) + N2 + M part (p2 variation)total modeled yield


Slide 16

THE N2O FROM O3 EXCITED BY HUGGINS BAND AND DIMER EFFECT ARE ALSO

EXPERIMENTAL REALITIES

Wavelength

240 260 280 300 320 340 360

N2O yield

0.000

0.005

0.010

0.015

0.020

0.025Observed N2O Yield with error bars

O3(1B2) part O3-N2 dimer part

O(1D) + N2 part Sum of all partspotential yield from O3(

1B2)over

the entire Hartley-Huggins band


Slide 17

THE ATMOSPHERIC PRODUCTION OF N2O FROM O3 EXCITED BY HUGGINS BANDS MAY BE

SIGNIFICANT

N2O Production rate (cm-3 s-1)

0 50 100 150 200 250

Altitude (km

)

0

10

20

30

AB

O3 Concentration (cm-3)

1e+9 1e+10 1e+11 1e+12 1e+13

n(O3)A & B representtwo different ways of diurnal averaging.

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


Slide 18

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.


Slide 19

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 ZP’s lamp, and the latter with O3/air ratio tending to zero.


Slide 20

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.

a copy of creative research enterprises’ presentattion by sheo s. prasad

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