The potential of solid phase endoperoxides as a means to produce single-delta oxygen in the gas phase in concentrations useful to chemical oxygen-iodine lasers was investigated. The 1,4 - endoperoxide of ethyl 3- (4-methyl - 1-naphthyl) propanoate was deposited over an indium-oxide layer on a glass plate. Single-delta oxygen was released from the endoperoxide upon heating the organic film by means of an electrical discharge through the conductive indium oxide coating. The evolution of singlet-delta oxygen was determined by measuring the dimol emission signal at 634 nm. Comparison of the measured signal with an analytic model leads to two main conclusions: virtually all the oxygen being evolved is in the singlet-delta state and in the gas phase, and there is no significant quenching other than energy pooling on the time scale of the experiment (approximately 10 msec). The use of solid phase endoperoxide as a singlet-delta oxygen generator for an oxygen-iodine laser appears promising

Busch, G. E.

English

NASA Technical Reports Server

NASA Contractor Report 172364
,-. NASA-CR-172364
19840020830
SINGLETOXYGENGENERATORFORA SOLAR
POWEREDCHEMICALLYPUMPEDIODINE LASER
George E. Busch
KMSFUSION, INC.
Ann Arbor, Michigan 49106
Contract NAS1-17584
April1984 _I_ _'J.,-_,.u P_PVU_
::;; __,i1984
_.ANGL_EYRESEARCHCE_'_TER
LIBRARY, NASA
HA_._PTON,VlRGINI.a_
v
NZ A
NationalAeronauticsand
SpaceAdministration
Langley Research Center
Hampton,Virginia23665
https://ntrs.nasa.gov/search.jsp?R=19840020830 2020-03-20T22:40:54+00:00Z

PROJECTSUMMARY
Atomic iodine is used successfully as the active element in high power
lasers operating at 1.3 micron wavelengths. The iodine laser transition is
efficiently pumped by molecular oxygen in its singlet delta excited state.
The objective of this research was to investigate and evaluate a method for
generating singlet oxygen (02 ) gas directly from solid-phase material.
Thin films of a substituted naphthalene endoperoxide were pulse heated
and the release of 021A was monitored through its dimol emission at 634 nm.
The time profile of the dimol emission was successfully modeled with a simple
numerical code which also aided in determinations of absolute specific yield
of 02_A released. These studies demonstrated that 021A can be released into
the gas phase in high yields (~ 100%) upon thermolysis of the endoperoxide.
Furthermore, 021A can be quantitatively released from thin films of
endoperoxide in time scales as short as a millisecond.
In an integrated system, singlet oxygen generation is proposed as a
three-step process in which certain triplet sensitizers convert ground-state
oxygen to singlet oxygen by absorption of solar energy. The singlet oxygen
oxidizes an appropriate acceptor molecule to form an endoperoxide which serves
to trap and concentrate the singlet oxygen. For subsequent release of the
stored oxygen, the endoperoxide is thermally decomposed, generating 02_A in
the gas phase and leaving the original acceptor molecule for recycling. The
overall energy conversion efficiency from sunlight to laser light is
potentially greater than 10%. The results of the Phase I research program
demonstrate that step three of this three-step process can be very
efficienct. Steps one and two will be addressed in future studies.
I
PROJECTOBJECTIVES
The Phase I objectivewas to determinethe potentialof solid phase
endoperoxidesas a means to producesingletoxygen in the gas phase in
concentrationsusefulto chemicallasers. Questionsof whether quenching
could be held to acceptablelevelsand whethersufficient02_aevolution rates
could be achievedwere to be addressed, Perhapsthe full potentialof
endoperoxidesfor 02_a productionhas not yet been determined,but yields
approaching100% have been demonstratedfor thin films of solid
endoperoxide. This has been accomplishedon short time scales (~ 1 ms) and
thus many of the questionsabout problemsof quenchingand achievable
021A evolutionrates have been put to rest.
DETAILS OF PHASE I RESEARCH
Release of 02_a from solid endoperoxidewas observedwith the experi-
mental apparatusshown in Figure 1, An endoperoxidecoated Nesatronplate
(PPG Industries)was mounted in full view of a photomultipliertube. An
interferencefilter with a 10 nm bandpasscentered near 634 nm spectrally
limitedthe light passingto the photomultiplierto only those wavelengthsin
the neighborhoodof the 02tA dimol emission.
The 1,4-endoperoxideof ethyl 3-(4-methyl-l-naphthyl)propanoate
was depositedon the indium oxide layer of a 5 cm square Nesatron glass plate
(see Figure 1) by spin coating. Typicalsurfacedensitiesof endoperoxide
2
ranged from 5 x 1015 to 4 x 1016molecules/cm2 dependingon the concentration
of the spin coat solution. Assuminga film mass densityof 1.2 g/cm3, film
thicknessesof 15 nm to 120 nm are calculated. Surface densitieswere
measured by redissolvinga depositedfilm in a known volume of solvent,
allowingthe solutionto sit at room temperatureovernightfor the conversion
of endoperoxideto naphthalene,and finallymeasuringthe opticalabsorption
of the solutionat 290 nm where naphthaleneabsorptionis prominent.
Singletdelta oxygenwas releasedfrom the endoperoxideupon heatingthe
organicfilm. The endoperoxidewas rapidlyheated by discharginga bank of
capacitors(100 to 400 microfarad,chargedto voltagesbetween400v and 660v)
throughthe conductiveindium oxide coatingof the Nesatronplate. Because
the endoperoxidefilm was thin and in intimatecontactwith the indium oxide
layer, the temperatureof the endoperoxidefilm trackedthe surface
temperatureof the Nesatronplate. In all cases, the indium oxide layer and
the depositedendoperoxidelayer were thin comparedto thermal diffusion
distancesin the glass substrateon a millisecondtime scale.
i For the case of a surfaceheat source deliveringenergy to the plate at
i a rate which is decayingexponentiallyin time, the time dependenceof
temperatureon the surfaceof the plate should followthe expression
E (D 3)I/2 1 e-(t/T)x
Ts (t) =g _ (_) i/2 Io (l - x)I/2 dx (1)
in which D is the thermal diffusivity of the glass substrate, K is the thermal
conductivity,E is the energy stored on the capacitorbank per unit area of
i the glass substrate,and • is the time constantfor energy discharge.
i Equation (1) is plottedin Figure2 for the time dependenceof the surface
temperatureof a Nesatronplate during the dischargeof a 200 pf capacitor
!
{ chargedto 600v.
Experimental Results
The dimol emissionsignal at 634 nm for a typical experimentis shown in
Figure 3, which displaysa scope trace of the photomultiplieroutput vs.
time. The time constantfor the detectionsystem is ~ 0.3 ms, while that for
3
the energydischargeis ~1.6 ms. Dimol emissionsignalswere surprisingly
reproduciblefrom sampleto sample implyinggood controlof the film thickness
with the spin coatingtechnique.
The decay of the dimol emission signal seen in Figure 3 was initially
thoughtto be due to quenchingreactions. However, it was soon realizedthat
diffusionat atmosphericpressureswould contributeto the apparentsignal
decay since dimol emission intensityis second order in 02_A concentration. A
computermodel was developedwhich simulatesthe emissionsignal quite well
and will be more fully discussedin the followingsection.
Since only 10% or less of the total light being emittedfrom the heated
endoperoxidesampleswas being transmittedby the interferencefilter,an
emission spectrumwas sought to verify that 02LAdimol emissionwas indeed
being observed,and not some accidentaloverlapwith emissionfrom unantici-
pated species.
Although signalto noise ratioswere good using the experimentalset-up
of Figure 1, emission is sufficientlyweak that attemptsto spectrallyresolve
the light requiredthe specialmeasuresshown in Figure 4. First, the direct
viewingof the plate done in Figure 1 was replacedby an integratingcell com-
prised of two plexiglassplates coated insidewith highly reflectiveBaS04. A
slit 1 mm wide by 1 cm high was cut in one face and this was imaged into the
input slit of an f/4 spectrograph. The output of the spectrographwas
directlycoupledto an image intensifier(gain 104- 10s) and the image
intensifieroutputwas coupledwith an f/1.5 lens to a silicon intensifier
target (SIT) vidicontube. The resultswere recordedon video tape and
subsequentlyfed to an image processorfor signal averaging.
A spectrum recordedwith the set-upof Figure 4 is displayedin Figure
5. Two peaks are shown: one at 634 nm correspondingto dimol emissionto the
ground vibrationalstate of each molecule,and one at 700 nm for emissionto
the state with one moleculevibrationallyexcited.
The peaks of Figure 5 are rotationallybroadenedsignificantlybeyond
the 20 nm resolutionof the instrumentation.The positionsof the calibration
4
lines in the upper panel of Figure 5 were obtainedby recordinga 633 nm HeNe
laser;lineat spectrometersettings:592, 633, and 700 nm. No evaluationhas
been made as to whetherthe magnitudeof broadeningof the spectrallines at
634 and 700 nm is reasonablefor the temperaturerange 300-500° K. The possi-
bility of contributionsof uv light from higherdiffractionorders of the
gratingwas eliminatedby using a 570 nm cutoff filter. The spectrum in
Figure 5 is also in qualitativeagreementwith the 634 nm interferencefilter
experimentsusing the configurationin Figure 1, in which it was observed that
the 10 nm bandpass,59% peak transmissionfilter reducedsignalsby about a
factor of 10-12 from experimentswith the filter removed.
Model Calculations
The decay of the dimol emission signal observedin Figure 3 is primarily
due to diffusion. The intensityof emissionseen by the photmultiplertube is
proportionalto the concentrationof 02_a squaredand integratedover the
distance from the generatingsurface,
I a _o C2(x, t) dx
At times large comparedwith the duration over which 02_A is released into the
gas phase,the 021A concentrationcan be approximatedby
e-X2/4Dt)/(D_ t) &/2C (x,t) : (Po
the concentrationexpected at time t, a distancex from the generating
surface. Po is the number of singletdelta oxygen moleculesper unit area
instantaneouslyreleasedat the generatingsurfaceat time t = O. Performing
the integralover space gives a dimol emissionsignal,
2 _/2
I _Po /(2 _ Dt) ,
which diminisheswith the square root of time. Here D is the diffusionco-
efficientof 02_A in air (consideredthe same as that for ground state
oxygen).
This analysis is not expectedto accuratelycalculatethe dimol emission
signal at times comparableto the 02LA releasetime. It also fails to account
for the energy pooling reactionwhich can be importantat early times when
the 021A densityis large. The energy pooling reactionmay also reduce the
number of 02LAmolecules (Po)which diffuseaway and contributeto the signal
at later times. Because of these considerationswe developeda computer code
which more accuratelymodels the dimol emission signal. The one dimensional
diffusionequation,
2
BC_ D BC 2@ t -----_-Kc + F (x, t)@x
was expressedas a differenceequation and solved numericallyfor c(x,t). The
second term on the right describesthe energy pooling reaction;the third term
is an 021A generationfunction. A simple Arrheniusform for 021A release
F(x,t) = A e-Ea/RT (EP(t))
from the endoperoxidefilm into the gas phase at X = 0 was used with the pre-
exponentialand activationenergy parametersestimatedfrom thermogravimetric
data. EP(t) is the densityof endoperoxide(molecules/cm2) in the thin film.
F(x,t) is a functionof endoperoxidefilm temperature(whichitself is a func-
tion of time). Finally,the numericalsolutionfor c(x,t) was squaredand
integratedover x to give a number proportionalto the dimol emission signal.
A comparisonbetweencalculatedsignal and experimentaldata is shown in
Figure 6;
A final step was taken to determinethe absolutemagnitudeof the cal-
culated signal. The computercalculationswere found to be a sensitive
function of temperature,which is determinedby the energy dischargedthrough
the circuit,the diffusionparameters,and the parameterschosen for the
Arrhenius02_A releaserate. The latter quantitieswere obtained from
thermogravimetricanalysisand can not be said to be well determined.
Therefore,the absolutescale for our calculatedsignalwas chosen using the
followingprocedure. An endoperoxidesamplewas depositedon the Nesatron
substrateand pulse heated three times. The dimol emission signal was
recordedfor each releaseof 02_A. The energy of discharge (the same for all
three experiments)was consideredan adjustableparameterin the modeling
calculationsand was chosen in such a manner that the calculatedpeak signal
6
decreasedfrom one pulsed dischargeto the next in the same proportionsas the
measured signal peak. This procedureis equivalentto compensatingfor poorly
determinedA and Ea in the Arrheniusexpressionby changingthe temperature
- and allowing the discharge energy parameter to carry the experimentally
determined information. Table I shows this comparison. The fitting criteria
are satisfied with a discharge energy of 1.68 J/cm 2, a number which is less
than 10%different from the value suggested by the apparent capacitor voltage
and the measured effective area of the Nesatron plate. The fit is sensitive
to 0.01J/cm 2 change.
Along with the forced agreement of the proportionate fall off in peak
dimol emission signal comes not only a remarkably good fit of the time profileL_
of the emission signal (Figure 6) but good agreement with the a_solute
numerical values of the emission signal (Table I). The latter ii_plies that
close to 100% of the released oxygen is in the singlet delta state.
The close agreement between the calculated signal time profile and the
measured signal also implies that in fact 021A is released into the gas phase
(It would be difficult to model the signal decay curves if it is assumed that
021A remains in and emits from the solid phase.) Separate experiments were
performed in which a confining glass plate was introduced in front of the
sample surface. If 021A is emitted from the gas phase, we would expect to see
a change in the signal once the gap between the sample and confining plate is
reduced to values approximately equal to a diffusion distance (for times less
than I0 ms). Indeed we see a large increase in signal for small gap distances
(about 0.2 mm).
The excellent agreement between model and experiment, considering the
simplicity of the model, leads to several conclusions: 1) the principle
features of the experiment seem to be contained within the model, 2) there is
no significant quenching other than energy pooling on the time scale of the
experiment, and 3) virtually all of the oxygen being evolved is in the singlet
delta state and is, in fact, released into the gas phase. Any alternative
processes would be likely to lead to reduced signal levels and faster decays
than those observed.
Table 1. Peak emissionsignalsfor a repetitivelyheated sample.
Signal (volts)
Expt. # Model Measured
1 .11 .11
2 .046 .046
3 .019 .028
8
Light Tight
. Ag Painted Cu Electrode Enclosure
\
Interference
Filter
f
Spin-coated
Substmte
5cm x 5cm x 0.31cm \
Glass \
Substrate
7cm >
GlassCover \
In 0, Plate __Conductive
Coating Diffusion
Volume
Extended Red S-20
2" Photomultiplier
Removable F/5
Aperture
Ag Painted Cu Electrode
Figure 1. Expedmental Apparatus Used for Recent KMS EndoperoxideStudies.
g
5O0
350
300
250
0.000 0.005 0.010 0.015 0.020
Tlme (seconds)
Figure 2. Surfacetemperatureof glasssubstrate.
I0
A• !
m
m
O_
m(n
Time (2 ms/div)
Figure 3L Time-resolved Photomultiplier Signal. Interference Filter centered
at 636.5 nm with Bandpass of 11 nm FWHM. Dimol Emission from
spin-coated Endoperoxide film pulse-heated with 30J.
11
• . .- .
Integrating
Cell
F/3.5 F/4.2, 200mm focal
Neutral Imaging length Spectrograph
Density Filters Lens | SIT Camera
\
,peroxide J I
He - Ne Sample Image Light
Alignment Intemlfler ShleldLaser FastShutter
f ,H
I
Monitor Video Cassette Recorder
Figure A. Experlmental Apparatus for Spectmlly Resolved Llght Emlsslon from
Pulse-Heated Endoperoxlde.
12
•..°
632 700 761
" Reference Spectrum(nm)
-- Vidicon Field
Signal AveragedVidicon Field
-_ Intensity
Figure 5. Spectrally-resolved Emission from Spin-coated Endoperoxide film
pulse-heated with 44J.
13
0.02
0.00_
0.000 0.005 0.010 0.015 0.020
Tlme (seconds)
Figure 6. dimol emissionslgnal.l-]- experlmentaldata, solldcurve-comput.ermodel.
14
TECHNICALFEASIBILITY
The demonstrationthat 02_A can be efficientlyreleasedfrom thin films
of solid endoperoxidesis an importantstep in the developmentof a solar
pumped iodine laser. A prioriwe were hopefulthat 02Zawould be able to
diffusethrough100 A films without significantquenching. The achievementof
high yields from 1000 A films is very encouraging,and the possibilitynow
exists that even substantiallythickerfilms could be used. This would
ultimatelytranslateinto simplersystem designsby relaxationof high surface
area requirements.
A number of technicalquestionsstill need to be addressedbefore the
feasibilityof this approachto solar pumpingof an iodine laser can be said
to be demonstrated. Some questionsstill remainabout 02zA releasefrom
endoperoxides;for example,can the high efficienciesand rapid releasebe
maintainedfor materialswith lower vapor pressuresthan those tested to date,
so that no buffer gas would be requiredto preventsublimationof endoperoxide
upon heating. Of course, steps one and two Of an integratedlaser system
remainto be investigated;namely,original02_a generationby solid phase
triplet sensitizersand transferof 02_ato the acceptormaterial for
formationof endoperoxide. The high degree of success in this Phase I
feasibilitystudy indicatesthat further researchin these remainingareas is
warranted.
15
1. ReportNo. 2. GovernmentAcceaionNo. 3. Recipient'$CatalogNo.
NASACR-172364
4. Title and Subtitle 5. Report Date
SingletOxygen Generatorfor a Solar Powered April 1984
ChemicallyPumped Iodine Laser e.Performln,jOrganization(',ode
U-1462
7. Author(s) 8. Performing Organization Report No.
George E.-Busch
10. Work Unit No.
9. PerformingOrganizationNameandAddress
KMS Fusion, Inc.
3621 South State Road, P.O. Box !567 11.co,t,,ctorG,antNo.
Ann Arbor, Michigan48106 NAS1-17584
13. Type of Report and Period Covered
12. SponsoringAgency Name and Addreu
NationalAeronauticsand Space Administration Contractor.Reporl;
Washington,D. C. 20546 14.Sl:)onsoringA ency Code
15. Suppietnentiry Notes
Langley Technical Monitor: Dr. EdmundO. Conway
Final Report
16. A_a_
An experimental study was conducted to determine the potential of solid
phase endoDeroxides as a means to produce singlet-delta oxygen in the gas phase
in concentrations useful to chemical oxygen-iodine lasers. The 1,4 - endoperoxide
of ethyl 3- (4-methyl - 1-naphthyl) propanoate was deposited over an indium-oxide
layer on a glass nlate. Sin-let-deltaoxygen was releasedfrom the endoperoxide
upon heatingthe orqanicf$l_.by means of an electricaldischargethroughthe
conductiveindium oxide coating. The evolutionof singlet-deltaoxygenwas
determinedbv measurinqthe dimol emission signalat 634 nm.
Comparisonof the measured signalwith an analyticmodel leads to two main
conclusions: 1) virtuallyall the oxygen being evolvedis in the singlet-delta
state and in the gas phase, and 2) there is no significantquenchingother than
energy poolingon the time scale of the experiment(-10 msec).
The use of solid-phaseendoperoxideas a singlet-deltaoxygen generatorfor
an oxygen-iodinelaser appearspromising.
17. KW Wor_ (Suggut_ by Author(s)) 15. Disui_ti_ Statement
SingletOxygen
Iodine Laser Unclassified-unlimited
Endoperoxide Subject Category20
19. Sacurity ClamJf. (of this report) 20. Security Clasdf. (ef this liqle) 21. No. of Pages 22. Price
Unclassified Unclassified 16 A02
.-=o_ ForsalebytheNationalTechnicalInformationService,Springfield.Virginia2216t

u
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Singlet oxygen generator for a solar powered chemically pumped iodine laser

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