- Email: [email protected]
A consideration of the ruby laser-induced decomposition of azoethane
Received 30 Novdmber 1971’
“_.
..:. .’ ‘I ,. .’ Ruby lzzr-induced decomposition of azoethane ‘mvg products different from the near W photodikciation. :....: Therefore, although in some experiments product yields varied quaiiratically wie la&r pow&i_, the principal diss&ia~ : ‘: -. tion was not via two-photon photoexci+tion. The two-photon photodissociation crass section is <. 10LT2 &n4 tic ;, ._: .. .,.. ; :
photon-’
,,
:,
:
molecule-‘.
In a recent study of the ruby laser phbt6lysis
of
azoethane in the gas phase, Speiser et al. [ 1] conclude from the quadratic dependence of nitrogen yields upon laser power that azoethane is decomposed by a two-photon photodissociation. We summarize in this
position
giving nitrogen
and ethyl
radicals
as primary :
products [4]. Both gaseous and liquid azoethane sainples were’ irradiated with a ruby laser Q-switched with either a :’ cryptocyarke
iqmethatiol
blekhabie
filter; or with a,
Kerr cell electro’~ptical shutter. The resonant cavity ;’ bear on the mechanism of the laser-induced decompowas formed by a square knife edge robf prism and a F’yrex optical flat. The averaged output power’was sition of ,azoethane in both the gas and liquid phases. From a combined spectroscopic and analytical 50 MW, and a pulse envelope half-width of 30 nsec -, point of view, relatively few systems should be more was obtained. For gas phase experiments the beam waf f%ussed by a 2.cm focal length lenk into a quartz cell ’ favorable for multiphoton photodissociation than azoethane. To obtain simultaneous absorption of two .’ containing less than.8 torr of koethke’k To irradiate liquid samples the laser beani tias partially fo’cuked,” .: photons in a system.transparent at the iaser’frequency, recollimated’to a 0;5,cm2 crossisectionaj area, and.then A&esystem should have a.state at twice.this frequency directed vertically through 0.25 qin of azqethane in a and nearby states which combine With both the ground ,.qua&cell. ‘:, and fmal s&es. These intermediate states may be of Following& phase kadiations.tIie &tire saniple: higher energy than the final state. If the molecule has a center of symmetry, parity niust be conserved in the. was analyzed. For li&d samples an,aliquot w& ‘taken: for’analysis by distiktidn~for 5 min fr6m.78,0 “C to’, ‘. two’-photon transition. For photochemical s,tudies, the final state should lead with high probabihty to -196. ‘C. Hydrocarbon products -coht@ni,ng iwoXio chemical reaction, yielding identifiable products which four carbon atonis were separated gas chrom&o&p& : .ically and monitored with, a flank ioniiafioc dekc,tcrl’citn be’sensitively analyzed. Azoethane fulfils these Fquirenients and is,, in addition; representative ofa ., :’ For these,specie&l0 12.mol&u!es could-be dktected class d,f,molectiles.of considerable pkotdchemical, instrume@ally. However, despite carefi?i@$c&&~l ’ .-’ .,’: .; .,,,,,, :,... :.‘-:’ -. ‘,inter?,!!. -T@ n j + absorption”$ *.lBg “t 3550 A ‘. : ‘-.,-. .. : ‘I:‘;, is weakly allowe& in the one-photon,spectrum‘due to 7 it pr&$uie~ of tioethane >_8 tk, a -. ‘~spa$?‘~$$ic$~ ... .vibronic coupling-[31 &id leads to molectilar decom; .. ai th$~fdcal poinijof the laser beam. ,,‘.:_.: -. ‘;‘.. ._.;. .,: I conimunication
results
of an earlier study
(21 W&ch
.; ‘,.’ ., ., .:: _, ._ .. ...’ .. : ,‘._’ ,_ .,, .; .,...(.,. : .:;’ ., , ; .., ..: ; : : : :.. 1 ‘..-,.-, .-. ; _..., ‘;, : ., ,,., ‘.
,. . .
.
._.
:
.‘., .: .,. .: .. ;. .. . . ., ,, .: ” ‘: .’ .::,,.:; .,.’ I,‘, .’ :.._. _.,:,., ._ --, .’ .:: ;., ‘-. .I,’ ‘,_, .: : ,I, ‘\,.’ .. ‘. ,_ ,.. . .,: . ,;, . - -.
,_ ., :. ,. .,.. ..‘: :.;; _:. ,‘, ._.: -.a‘. .:, __-+, :: i _;_ ‘:__ .:,’ .-‘. Y:,,. -, :‘ : ‘_‘_‘ ,:I;, : : ,_.‘ : ‘ . ‘ .: ,, ,: ,.,‘,,...._,.;, :” ..,:: ::,, ., ,:
Volume 13, number 1
CHEMICALPHYSICSLE-i-TERS
1 .February 1972
Table 1 Typical azaethane decomp&tion products from different
ma.&-‘) uv
Roduct
Ethane Ethylene Propane
-
Propylene Butane
100
Fa& W
16 14
types of excitation butane’= 100
Pyrolysis at 280 “C
c60 64
23 10
42 13 100
6.6 2.5 100
in the gas phase. Expressed
rf discharge (Tesla coil)
Micrbwave discharge
1.50 230
1000 670
200 30 100
as r&&e
Laser
spark
420 70 100
yields with
Laser induced. decomposition
34 86
36 72
26 10 100
36 5 100
a) Our.ob&ved
pr oduct distribution is‘identical to that observed by Ausloos and Steacie [7]. with the full arc of a Hanovia SH medium pressure mercury lamp. Irradiated through an LIP window with the continuum resulting from a microwave discharge in argon at
‘b) had&cd
s,
of the starting material, residual concentrations of anticipated products from blank experiments fluctuated UP to 1013 molecules, thus limiting the sensitivity for detecting ph,otochemical decomposition to = 1013 ,events. : In early gas phase experiments, before the sensitivity of decomposition to ihe condition of the window surfaces was established [5] , we observed decomposition of azoethane using both normal burst and Q-switched laser pulses. Yields were high and variable, and the product distribution was distinctly different from t&t of the well characterized photodissociation of azoethane in the near ultraviolet.[4,6] . In particular, propant and propylene were found, which are typical of the decomposition of azoethane by many mechanisms other .than the near W photolysis. In table 1 relative product yields for, different modes of excitation of atioethtie in the gas phase are-compared with the pro-duct
distribution
from
these
laser-indueed
decorriposilions shown in the last column. The presence of propane and propyle& in the laser experiments, in contrast to their absence from the near W photolysis, is a strong indication ‘&at a two-photon photodecomposition’& 3ot the principal mechvlism of product ‘forma&i? ii the? experiments. We concluded that these high yield decdmpositions might have been surface induced. To’.test thii hypothesis, samples of gaseousazo&hane tit pressures of 6 torr were.irradiated with ten
.. :
:
‘.
150-200 torr.
induced by each laser pulse in the gas phase. From this value, we can obtain an upper limit of the two photon absorption cross section 8, as defined by Speiser et al. [II Y = a$&VI’A
t(h~)-~,
where our limit on the yield Y is as above; a (=0.753), a dimensionless constant to allow for the gaussian distribution of the light flux; the quantum yield [4,6] r$= 0.80; N is the number of molecules in the effective volume of azoethane exposed to the intense laser beam; I is the photon flux; Ar the laser pulse envelope half-width; h is Plank’s constant; and u the laser frequency, 4.3 X lOI set-’ . In these pho tolyses with the laser beam focussed, the photon flux increases rapidly with proximity to the focal point, whereas N in a given cross section perpendicular to the laser beam increases with distance from the focal point. An appropriate numerical integration was therefore made in estimating a maximum two-photon cross section. The upper limit of 6. then becomes 10~~~ cm4sec phbton-1 molecule-l, a factor of twenty he!ow the cross section reported by Speiser et al.i. In a series of liquid phase experiments in a cell that was not iigorously cleaned, a seemingly controllah!e $ The.vohime of azoethane
irradiated by the ruby laser in the experiments of ref. [I] is 60 ci$(Sp&ser, private cornmuniutiok). Llsing this volume, the‘two-photon cross set” tion Ff S+er et al. [l] becomes 2.3 X 10m5f cm4sec’ .. photon-’ mole$e-‘; ..
:,
,..
_/’
.’
,:
Volume
13, humber-1
CHEMiCAL
PHYSIdS
LETTERS
1 Februd ..
1972
-
atmosphere’of 0,. Irradiation of a sample d&led into this clean cell gave a reduction’in product of over ” an order of magnitude. Only very small amountsof. ‘. product with the see distribution asndted above could be ob&ined after repeated 1Cng’: it was concluded that the products observed in the$e experiments. resulted from surface contamination, not fromhomogeneous photodissociation, and therefore.in’the liquid phase an upper limit of = 1013 reactions per
laser pulse could be’attributed to two-photon dissociation. Although laser decompositions may be induced in azoethane, we have been unable to detect two-photon
photodecomposition at the 1012-1013 event level.
I
I
I
2
IO
20
LASER
POWER
Fig. 1 . Dependence of propane yield from the laser-induced decomposition of liquid azoethane on laser power (arbitrary units). Slope = 2.
decomposition was observed. Ethane, ethylene, propane, propylene and butane were produced with relative yields of 32:70:24:5: 100. Yields of all products monitored were second order in light intensity as exemplified by propane in fig. 1. Examination of the sample celt revealed that the surface in contact the
with
liqtiid developed a well defined gray, hazy pattern
after several laser pulses. To test the effect of this surface phenomenon on the product yield, an experiment was performed in which the laser was held at constant power and the liquid samples were irradiated with successively larger numbers of pu!ses, varying from one to six. It was found that the product yield was greatest for the first few pulses and subsequent irradiation gave very little additional product. Further evidence that the products observed in this study resulted from surface effects tias obtained by thorough cleaning of the sample cell by repeated flaming, both under vacuum and .wheri filled with ari
Quantitative observations beloti this level are experimentally difficult because even where suff$iently sensitive detection is available, chemitial purity places a restriction on minimum detectable amounts of product. Should an appropriately clean @tern be available, at these low product levels further complications will be introduced by high energy electromagnetic radiation and charged particles emitted from the container surface [S ] . The failure to observe photo-. dissociation products from two-photon absorption.to the 1B, state both in gas and liquid phases may be thought surprising in light of the initial qualitative assessment that azoethane would be a favorable model system for such processes. However, semi-quantitative calculations of product yields, made’under the assumption that two-photon absorption in azoethane occurs via an intermediate state [ 181, indicate that the experimental sensitivity at the laser powers used in this early work is just at the threshold for detection. Finally, caution must be exercised in evaluating evidence for two-photon
photochemical
processes.
Ascribing two-photon mechanisms to systems in which second-order dependence of product yields upon incident extremely
photon
flux has not been denionstrated
is
tenuous,. Even
when second-order dependence of products upon light intensity is observed, the possibility of surface effects must be borne in mind, as the present examp1.e clearly demonstra!& ;,
Some of this work was performed at Princeton .. University with the p’atial support of the U.S. Office -of Naval Retiarch. ‘, .,’
‘,
.’ “i?l M.B:‘Rbbin. :
,‘. -.
R;R.-Hart (1967) :564.
Soc.89
:,
‘.:..:
;:
.,
.;
and N=A_ Kukbler
:
J- A&. Chem.
‘. -
[4] h. Cerfontain ahd-Kcd. Kutkke, Can. J.&em. 36 (1958) 344. [Sl .D.&. Rousseau; GB.._I$r&i’and W.E. Falconer,:J. Appl. ‘. phys. 39 (1968) ,333@. ISI W-C. Woishawand’O& Rice, J. Chem. Phys. 46 (1967) : 2021. .. ‘. I::. (71 P. Ausloos and E-W-R. Sfeacie; Bull. Sm. Chim. (1954)87. [81R. PanteU,. F. Pradere, J. Hanus, M. Schott and H. Purhoff, J. Chem. Phys 46 (1967) 3507.
:
“_.
..:. .’ ‘I ,. .’ Ruby lzzr-induced decomposition of azoethane ‘mvg products different from the near W photodikciation. :....: Therefore, although in some experiments product yields varied quaiiratically wie la&r pow&i_, the principal diss&ia~ : ‘: -. tion was not via two-photon photoexci+tion. The two-photon photodissociation crass section is <. 10LT2 &n4 tic ;, ._: .. .,.. ; :
photon-’
,,
:,
:
molecule-‘.
In a recent study of the ruby laser phbt6lysis
of
azoethane in the gas phase, Speiser et al. [ 1] conclude from the quadratic dependence of nitrogen yields upon laser power that azoethane is decomposed by a two-photon photodissociation. We summarize in this
position
giving nitrogen
and ethyl
radicals
as primary :
products [4]. Both gaseous and liquid azoethane sainples were’ irradiated with a ruby laser Q-switched with either a :’ cryptocyarke
iqmethatiol
blekhabie
filter; or with a,
Kerr cell electro’~ptical shutter. The resonant cavity ;’ bear on the mechanism of the laser-induced decompowas formed by a square knife edge robf prism and a F’yrex optical flat. The averaged output power’was sition of ,azoethane in both the gas and liquid phases. From a combined spectroscopic and analytical 50 MW, and a pulse envelope half-width of 30 nsec -, point of view, relatively few systems should be more was obtained. For gas phase experiments the beam waf f%ussed by a 2.cm focal length lenk into a quartz cell ’ favorable for multiphoton photodissociation than azoethane. To obtain simultaneous absorption of two .’ containing less than.8 torr of koethke’k To irradiate liquid samples the laser beani tias partially fo’cuked,” .: photons in a system.transparent at the iaser’frequency, recollimated’to a 0;5,cm2 crossisectionaj area, and.then A&esystem should have a.state at twice.this frequency directed vertically through 0.25 qin of azqethane in a and nearby states which combine With both the ground ,.qua&cell. ‘:, and fmal s&es. These intermediate states may be of Following& phase kadiations.tIie &tire saniple: higher energy than the final state. If the molecule has a center of symmetry, parity niust be conserved in the. was analyzed. For li&d samples an,aliquot w& ‘taken: for’analysis by distiktidn~for 5 min fr6m.78,0 “C to’, ‘. two’-photon transition. For photochemical s,tudies, the final state should lead with high probabihty to -196. ‘C. Hydrocarbon products -coht@ni,ng iwoXio chemical reaction, yielding identifiable products which four carbon atonis were separated gas chrom&o&p& : .ically and monitored with, a flank ioniiafioc dekc,tcrl’citn be’sensitively analyzed. Azoethane fulfils these Fquirenients and is,, in addition; representative ofa ., :’ For these,specie&l0 12.mol&u!es could-be dktected class d,f,molectiles.of considerable pkotdchemical, instrume@ally. However, despite carefi?i@$c&&~l ’ .-’ .,’: .; .,,,,,, :,... :.‘-:’ -. ‘,inter?,!!. -T@ n j + absorption”$ *.lBg “t 3550 A ‘. : ‘-.,-. .. : ‘I:‘;, is weakly allowe& in the one-photon,spectrum‘due to 7 it pr&$uie~ of tioethane >_8 tk, a -. ‘~spa$?‘~$$ic$~ ... .vibronic coupling-[31 &id leads to molectilar decom; .. ai th$~fdcal poinijof the laser beam. ,,‘.:_.: -. ‘;‘.. ._.;. .,: I conimunication
results
of an earlier study
(21 W&ch
.; ‘,.’ ., ., .:: _, ._ .. ...’ .. : ,‘._’ ,_ .,, .; .,...(.,. : .:;’ ., , ; .., ..: ; : : : :.. 1 ‘..-,.-, .-. ; _..., ‘;, : ., ,,., ‘.
,. . .
.
._.
:
.‘., .: .,. .: .. ;. .. . . ., ,, .: ” ‘: .’ .::,,.:; .,.’ I,‘, .’ :.._. _.,:,., ._ --, .’ .:: ;., ‘-. .I,’ ‘,_, .: : ,I, ‘\,.’ .. ‘. ,_ ,.. . .,: . ,;, . - -.
,_ ., :. ,. .,.. ..‘: :.;; _:. ,‘, ._.: -.a‘. .:, __-+, :: i _;_ ‘:__ .:,’ .-‘. Y:,,. -, :‘ : ‘_‘_‘ ,:I;, : : ,_.‘ : ‘ . ‘ .: ,, ,: ,.,‘,,...._,.;, :” ..,:: ::,, ., ,:
Volume 13, number 1
CHEMICALPHYSICSLE-i-TERS
1 .February 1972
Table 1 Typical azaethane decomp&tion products from different
ma.&-‘) uv
Roduct
Ethane Ethylene Propane
-
Propylene Butane
100
Fa& W
16 14
types of excitation butane’= 100
Pyrolysis at 280 “C
c60 64
23 10
42 13 100
6.6 2.5 100
in the gas phase. Expressed
rf discharge (Tesla coil)
Micrbwave discharge
1.50 230
1000 670
200 30 100
as r&&e
Laser
spark
420 70 100
yields with
Laser induced. decomposition
34 86
36 72
26 10 100
36 5 100
a) Our.ob&ved
pr oduct distribution is‘identical to that observed by Ausloos and Steacie [7]. with the full arc of a Hanovia SH medium pressure mercury lamp. Irradiated through an LIP window with the continuum resulting from a microwave discharge in argon at
‘b) had&cd
s,
of the starting material, residual concentrations of anticipated products from blank experiments fluctuated UP to 1013 molecules, thus limiting the sensitivity for detecting ph,otochemical decomposition to = 1013 ,events. : In early gas phase experiments, before the sensitivity of decomposition to ihe condition of the window surfaces was established [5] , we observed decomposition of azoethane using both normal burst and Q-switched laser pulses. Yields were high and variable, and the product distribution was distinctly different from t&t of the well characterized photodissociation of azoethane in the near ultraviolet.[4,6] . In particular, propant and propylene were found, which are typical of the decomposition of azoethane by many mechanisms other .than the near W photolysis. In table 1 relative product yields for, different modes of excitation of atioethtie in the gas phase are-compared with the pro-duct
distribution
from
these
laser-indueed
decorriposilions shown in the last column. The presence of propane and propyle& in the laser experiments, in contrast to their absence from the near W photolysis, is a strong indication ‘&at a two-photon photodecomposition’& 3ot the principal mechvlism of product ‘forma&i? ii the? experiments. We concluded that these high yield decdmpositions might have been surface induced. To’.test thii hypothesis, samples of gaseousazo&hane tit pressures of 6 torr were.irradiated with ten
.. :
:
‘.
150-200 torr.
induced by each laser pulse in the gas phase. From this value, we can obtain an upper limit of the two photon absorption cross section 8, as defined by Speiser et al. [II Y = a$&VI’A
t(h~)-~,
where our limit on the yield Y is as above; a (=0.753), a dimensionless constant to allow for the gaussian distribution of the light flux; the quantum yield [4,6] r$= 0.80; N is the number of molecules in the effective volume of azoethane exposed to the intense laser beam; I is the photon flux; Ar the laser pulse envelope half-width; h is Plank’s constant; and u the laser frequency, 4.3 X lOI set-’ . In these pho tolyses with the laser beam focussed, the photon flux increases rapidly with proximity to the focal point, whereas N in a given cross section perpendicular to the laser beam increases with distance from the focal point. An appropriate numerical integration was therefore made in estimating a maximum two-photon cross section. The upper limit of 6. then becomes 10~~~ cm4sec phbton-1 molecule-l, a factor of twenty he!ow the cross section reported by Speiser et al.i. In a series of liquid phase experiments in a cell that was not iigorously cleaned, a seemingly controllah!e $ The.vohime of azoethane
irradiated by the ruby laser in the experiments of ref. [I] is 60 ci$(Sp&ser, private cornmuniutiok). Llsing this volume, the‘two-photon cross set” tion Ff S+er et al. [l] becomes 2.3 X 10m5f cm4sec’ .. photon-’ mole$e-‘; ..
:,
,..
_/’
.’
,:
Volume
13, humber-1
CHEMiCAL
PHYSIdS
LETTERS
1 Februd ..
1972
-
atmosphere’of 0,. Irradiation of a sample d&led into this clean cell gave a reduction’in product of over ” an order of magnitude. Only very small amountsof. ‘. product with the see distribution asndted above could be ob&ined after repeated 1Cng’: it was concluded that the products observed in the$e experiments. resulted from surface contamination, not fromhomogeneous photodissociation, and therefore.in’the liquid phase an upper limit of = 1013 reactions per
laser pulse could be’attributed to two-photon dissociation. Although laser decompositions may be induced in azoethane, we have been unable to detect two-photon
photodecomposition at the 1012-1013 event level.
I
I
I
2
IO
20
LASER
POWER
Fig. 1 . Dependence of propane yield from the laser-induced decomposition of liquid azoethane on laser power (arbitrary units). Slope = 2.
decomposition was observed. Ethane, ethylene, propane, propylene and butane were produced with relative yields of 32:70:24:5: 100. Yields of all products monitored were second order in light intensity as exemplified by propane in fig. 1. Examination of the sample celt revealed that the surface in contact the
with
liqtiid developed a well defined gray, hazy pattern
after several laser pulses. To test the effect of this surface phenomenon on the product yield, an experiment was performed in which the laser was held at constant power and the liquid samples were irradiated with successively larger numbers of pu!ses, varying from one to six. It was found that the product yield was greatest for the first few pulses and subsequent irradiation gave very little additional product. Further evidence that the products observed in this study resulted from surface effects tias obtained by thorough cleaning of the sample cell by repeated flaming, both under vacuum and .wheri filled with ari
Quantitative observations beloti this level are experimentally difficult because even where suff$iently sensitive detection is available, chemitial purity places a restriction on minimum detectable amounts of product. Should an appropriately clean @tern be available, at these low product levels further complications will be introduced by high energy electromagnetic radiation and charged particles emitted from the container surface [S ] . The failure to observe photo-. dissociation products from two-photon absorption.to the 1B, state both in gas and liquid phases may be thought surprising in light of the initial qualitative assessment that azoethane would be a favorable model system for such processes. However, semi-quantitative calculations of product yields, made’under the assumption that two-photon absorption in azoethane occurs via an intermediate state [ 181, indicate that the experimental sensitivity at the laser powers used in this early work is just at the threshold for detection. Finally, caution must be exercised in evaluating evidence for two-photon
photochemical
processes.
Ascribing two-photon mechanisms to systems in which second-order dependence of product yields upon incident extremely
photon
flux has not been denionstrated
is
tenuous,. Even
when second-order dependence of products upon light intensity is observed, the possibility of surface effects must be borne in mind, as the present examp1.e clearly demonstra!& ;,
Some of this work was performed at Princeton .. University with the p’atial support of the U.S. Office -of Naval Retiarch. ‘, .,’
‘,
.’ “i?l M.B:‘Rbbin. :
,‘. -.
R;R.-Hart (1967) :564.
Soc.89
:,
‘.:..:
;:
.,
.;
and N=A_ Kukbler
:
J- A&. Chem.
‘. -
[4] h. Cerfontain ahd-Kcd. Kutkke, Can. J.&em. 36 (1958) 344. [Sl .D.&. Rousseau; GB.._I$r&i’and W.E. Falconer,:J. Appl. ‘. phys. 39 (1968) ,333@. ISI W-C. Woishawand’O& Rice, J. Chem. Phys. 46 (1967) : 2021. .. ‘. I::. (71 P. Ausloos and E-W-R. Sfeacie; Bull. Sm. Chim. (1954)87. [81R. PanteU,. F. Pradere, J. Hanus, M. Schott and H. Purhoff, J. Chem. Phys 46 (1967) 3507.
: