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Vibrational relaxation of CO2(0001) in pure CO2
Volume
31~ n;mtXr
1
VIBRATIONAL
RELAXATION
OF CO,(OO”l)
IN PURE Co,
R.C. SEPUCI-LA Aerodyrlc
Research,
~IIC.. Bwiirgtorr,
Massachoseffs
01803.
USA
Received 18 October 1974
The rate constant for coLlisionn1 deactivation.of COz(OO”l) in pprc CO2 Ius been measured at room tcmpcnturc using the laser fluorescence technique. The relaxntion rate has beer. found to be (1 .O ? 0.2) X lo-‘” cn13 5-l which
is in f;lvorablc
agreement
with
previously
published
values.
temperature and was capable of withstanding gas pressures up to 4 atm. It was connected to a mechanical vacuum pump and was evacuated to 3 torr prior to each run. Mounted,inside the cell wzs a 5 cm diameter, gold coated spherica’l mirror which formed one end of the laser cavity. The total reflector had a 1 m radius of curvature and focused the laser beam to a minimum cross section at the center of the ~11.
st room
This rlote reports the results of robm-temperature measurements of the vibrational relaxation of C02(OOo 1) by CO, as determined by the laser fluorescencc technique. The vibrational relaxation was obtained from the temporal decay of 9.4 an& 10.4 urn emission from CO, immediately following saturated absorption of 13.6 urn laser radiation. ly 2.5
The experimental apparatus is shown schematicalin fig. 1. The system consisted of a 30 cm dhmeter,
The beam
m long steel absorption
ty of a Lumonics
was collimated
with
a 5 cm dinrz?etcr
KC1
lens which had a 1 m focal length.
cell located inside the caviTEA CO2 laser. The cell was operated
The Gutput end of an 8% trxwAtting Ge flat_
the lnser consisted
ED-ZOO JOULEhlETEFi (TO SCOPE OR STRIP CHART RECORDER)
of
GEN-TEC MODEL PERFORATED
SCREEN : TI7ANShllSSION = 0.0.77
TO DEYTECTION 5X-I EM .I
7 LLJMCWICSMODEL
III
103
SCA’i-iEEUNG
CAL
‘.,
.,‘..’
:
Fig. 1. Schematic
_’
:
of scattering cell and CO2 laser.
I.
.. . .
_l..
.‘.
;
”
75 .,
‘.
:
Volume 31,numbcr.l
:
CHINICAL
PHYSCS ,&ETTERS
:-. . . ... In this configuration, the unpolarized’laser output, monitored by a Cen-Tee Model ED-200.joulemeter,.. _ .was,approximately ‘I 5 joules per pulse. The laser pUulse shape was composed of a 0.2 gs wide initial spike followed by, a Ions tail 2.5 ps wide. About 40% cf the. piilse.energy wascanttited in the initial spike. kt-the focal.volume, the laser beam had x reotangular .c;o& section, 8 mm X, 4 mm, and the peak power density was approximately 2 X, 108, W/cml.- The pulse ‘energy was found to be reproducible to within 5% td ! C@over a span of ,100 consecutive pulses. Because of the-high laser flux at the focal plane,. the CO; gas admitted to the,absorption cell had to be prefiltered to avoid the oicurrence.of laser breakdown: A Millipore Corp. fiter~containing a 0.025 pmpore,filter element was employed to remove,any dust from the gas. Linde instrument grade, 99.99% pure CO, was used for all reported measurements. : The detection system consisted.of a 30 cm diarneter sphericalmirror, which; with an k-tan 2, f/l lens, focussed the CO2 fluorescence onto a Santa Barbara Research Ge:Hg detector. With the use of a fast preampIiEsi (24 ns risetime), the detector output was displayed on an oscilloscope where single-pulse Polaroid oscillograms were recorded. Fluorescence measurements.@re made for CO2 -‘pressures from 0.25.to 2.0 at,m in the absorption cell. At these’pressures and over the optical path from the laser axis to the ob,servotion window (30
cm), the gas was optically.opaque
to, 4.3 pm Cd,
:
,’
...
15 Fcbrutiy
1973
,
‘.
Fig.‘2. Measured C02-v~
collision&
relaxation
time in pure
coz. After each laser pulse, the fluorescence signal? V, decreased expsnentially wiyl time as V = V0 exp(-r/T), where Vu is a constant, and r is the relation time. For each indi\idual,run, the fluorescence signal was least-square fi:; to this functional form, and a’ each pressure, the relaxation time was determined. The resuits are.shown in fig. 2 where the relax&n time is shown plotted as a function of gas pressure. Except
,.
for the point at 2 atm, each data point represe_nts an average over 10 to 20 individual runs. The value at 2 atm is an average over 5 runs. As shown in the figure, T decreased from approximately 16 to 2 US over the pressure range of 0.25 to 2.0 atm. The rate constant, k; determined from these data using k-l=]C02]
emission. However, under the same conditions, the optiCa1 transmission for the 9.4 and lC!.4 pm CO, -bands varied from 0.94 to 0.29, so that the observed ,’ fiudrescence was attributed only to these two bands: .’
‘.
r,
Table 1 gomption
of mtisured
rate cdmtants
k x 10’4 (cm” s-l)
. .:
”
.0.58 0.97,. OS?:
:
‘,
:
,.
‘.
1.02 1.19
.’ ‘, ,‘.
.,
1.h + 0.09 .; ~. :I.O.:k d.2
..
..
:
4.3 gm fluorescence ,,-
.’
.,
:;
spectrophork
‘..
‘-
;
Las& fluoresknce (4.3 pm) Laser fluckescence (4.3 /.~tyri; : tier-gain _‘.,’ laser fluorescence (4.3 inl) laser fluorescenk(9.4 and ,10.4 urn)
~ . ..’ ;- ,:.’
,“,
[3]
.’
[41 I4.-61 [71 [8] present study ,,
:
:
.,
[ll [2]-
-.
spectrophone.
183. 330 385‘ ,362k 30: 324 f 65
Ref.,
.’
335*5
,’
..
..
M&hod
: (7p) (km -1,-l; 188. :313
_. ,:, 1.03 i a.62
’
at.295 K
.‘:.
.’
: ,. .:
,’
where [CO-,] is.the CO, number density, was found to be k = (1 .O+ 0.2) X lo-l4 cm3 s-l at 298 K; i.e., (7~)” = 324 t 65 tori-l s-l, wliere p is the CO, pressure. The error represents the standard deviation for an average.ofapproximately 50 individual runs. A comljarison of the measured value of the rate constant with previously published values is shown in table 1. The value obtained by a fluorescence tech-, ,nique that employed the periodic absorption and emission of 4.3 pm radintion is almost a factor of two smaller than the present result. Similarly, the results obtained by the spectrophone technique tend to be smaller than the value reported here, although one value is.within the error bounds of the present result. The agreement is very good, however, between. the value reported here and those determined by the laser fluqrescence technique that employ&d 4.3 /.ml emission. From the tab’lc, it appears that the laser flugrcscencc measurements to date indicate a value of 1 .O X lo-l4 cm? s-l for this relaxation rate constant at room temperature. Unfortunately, this,teclmique does not readily provide a means for determining the mechanism for the collisional deactivation. Consequently, this rate constant must be attributed to the overall
reaction
c0*(00”1) where
+ co2
+ co;
COY represents
mollcule state.
in the 0310,
+ CO2,
a vibrationally exited CO2 1110,0200 or 01’0 vibrational
This work was supported by the Office of Nova1 Research and the Advanced Research Projects Agency under ARPA Order 1806 and Contract NO00 I473-c-0025.
References [I] J.T. &I&
Prob. Phys. Sot. (Londcn)
91 (1967)
439.
[2] P.V. Slobodskaya, Opt. Spectry. 22 (1967) 14. [3] T.L. Cottrcll, I.hl. McF’nrlanc, A.W. Reed and A.H. Young, Trans. Faraday !3oc. 62 (1966) 2655. [4] W.A.-ROSSC~ Jr., A.D. Wood and E.T. Gerry, 5. Chcm. Phys. 50 (1969) 4996. [5] L.O. Hacker, h1.A. Kovacs, C.K. Rhodes, G.W. Flynn and A. Javan, Phys. Rev. Letters 17 (1966) 233. [6] CB. MOOX, R.E. Wood, B. HU and J.T. Yxdlcy, I. Chcm. Phys. 46 (1967) 4222. [7] P.K. Cheo, J. Appl. Phys. 38 (1967) 3563. [8] C.W. von Rqscnberg Jr. and A. Lowenstein, Phys. 59 (1973) 2751.
J. Chem.
31~ n;mtXr
1
VIBRATIONAL
RELAXATION
OF CO,(OO”l)
IN PURE Co,
R.C. SEPUCI-LA Aerodyrlc
Research,
~IIC.. Bwiirgtorr,
Massachoseffs
01803.
USA
Received 18 October 1974
The rate constant for coLlisionn1 deactivation.of COz(OO”l) in pprc CO2 Ius been measured at room tcmpcnturc using the laser fluorescence technique. The relaxntion rate has beer. found to be (1 .O ? 0.2) X lo-‘” cn13 5-l which
is in f;lvorablc
agreement
with
previously
published
values.
temperature and was capable of withstanding gas pressures up to 4 atm. It was connected to a mechanical vacuum pump and was evacuated to 3 torr prior to each run. Mounted,inside the cell wzs a 5 cm diameter, gold coated spherica’l mirror which formed one end of the laser cavity. The total reflector had a 1 m radius of curvature and focused the laser beam to a minimum cross section at the center of the ~11.
st room
This rlote reports the results of robm-temperature measurements of the vibrational relaxation of C02(OOo 1) by CO, as determined by the laser fluorescencc technique. The vibrational relaxation was obtained from the temporal decay of 9.4 an& 10.4 urn emission from CO, immediately following saturated absorption of 13.6 urn laser radiation. ly 2.5
The experimental apparatus is shown schematicalin fig. 1. The system consisted of a 30 cm dhmeter,
The beam
m long steel absorption
ty of a Lumonics
was collimated
with
a 5 cm dinrz?etcr
KC1
lens which had a 1 m focal length.
cell located inside the caviTEA CO2 laser. The cell was operated
The Gutput end of an 8% trxwAtting Ge flat_
the lnser consisted
ED-ZOO JOULEhlETEFi (TO SCOPE OR STRIP CHART RECORDER)
of
GEN-TEC MODEL PERFORATED
SCREEN : TI7ANShllSSION = 0.0.77
TO DEYTECTION 5X-I EM .I
7 LLJMCWICSMODEL
III
103
SCA’i-iEEUNG
CAL
‘.,
.,‘..’
:
Fig. 1. Schematic
_’
:
of scattering cell and CO2 laser.
I.
.. . .
_l..
.‘.
;
”
75 .,
‘.
:
Volume 31,numbcr.l
:
CHINICAL
PHYSCS ,&ETTERS
:-. . . ... In this configuration, the unpolarized’laser output, monitored by a Cen-Tee Model ED-200.joulemeter,.. _ .was,approximately ‘I 5 joules per pulse. The laser pUulse shape was composed of a 0.2 gs wide initial spike followed by, a Ions tail 2.5 ps wide. About 40% cf the. piilse.energy wascanttited in the initial spike. kt-the focal.volume, the laser beam had x reotangular .c;o& section, 8 mm X, 4 mm, and the peak power density was approximately 2 X, 108, W/cml.- The pulse ‘energy was found to be reproducible to within 5% td ! C@over a span of ,100 consecutive pulses. Because of the-high laser flux at the focal plane,. the CO; gas admitted to the,absorption cell had to be prefiltered to avoid the oicurrence.of laser breakdown: A Millipore Corp. fiter~containing a 0.025 pmpore,filter element was employed to remove,any dust from the gas. Linde instrument grade, 99.99% pure CO, was used for all reported measurements. : The detection system consisted.of a 30 cm diarneter sphericalmirror, which; with an k-tan 2, f/l lens, focussed the CO2 fluorescence onto a Santa Barbara Research Ge:Hg detector. With the use of a fast preampIiEsi (24 ns risetime), the detector output was displayed on an oscilloscope where single-pulse Polaroid oscillograms were recorded. Fluorescence measurements.@re made for CO2 -‘pressures from 0.25.to 2.0 at,m in the absorption cell. At these’pressures and over the optical path from the laser axis to the ob,servotion window (30
cm), the gas was optically.opaque
to, 4.3 pm Cd,
:
,’
...
15 Fcbrutiy
1973
,
‘.
Fig.‘2. Measured C02-v~
collision&
relaxation
time in pure
coz. After each laser pulse, the fluorescence signal? V, decreased expsnentially wiyl time as V = V0 exp(-r/T), where Vu is a constant, and r is the relation time. For each indi\idual,run, the fluorescence signal was least-square fi:; to this functional form, and a’ each pressure, the relaxation time was determined. The resuits are.shown in fig. 2 where the relax&n time is shown plotted as a function of gas pressure. Except
,.
for the point at 2 atm, each data point represe_nts an average over 10 to 20 individual runs. The value at 2 atm is an average over 5 runs. As shown in the figure, T decreased from approximately 16 to 2 US over the pressure range of 0.25 to 2.0 atm. The rate constant, k; determined from these data using k-l=]C02]
emission. However, under the same conditions, the optiCa1 transmission for the 9.4 and lC!.4 pm CO, -bands varied from 0.94 to 0.29, so that the observed ,’ fiudrescence was attributed only to these two bands: .’
‘.
r,
Table 1 gomption
of mtisured
rate cdmtants
k x 10’4 (cm” s-l)
. .:
”
.0.58 0.97,. OS?:
:
‘,
:
,.
‘.
1.02 1.19
.’ ‘, ,‘.
.,
1.h + 0.09 .; ~. :I.O.:k d.2
..
..
:
4.3 gm fluorescence ,,-
.’
.,
:;
spectrophork
‘..
‘-
;
Las& fluoresknce (4.3 pm) Laser fluckescence (4.3 /.~tyri; : tier-gain _‘.,’ laser fluorescence (4.3 inl) laser fluorescenk(9.4 and ,10.4 urn)
~ . ..’ ;- ,:.’
,“,
[3]
.’
[41 I4.-61 [71 [8] present study ,,
:
:
.,
[ll [2]-
-.
spectrophone.
183. 330 385‘ ,362k 30: 324 f 65
Ref.,
.’
335*5
,’
..
..
M&hod
: (7p) (km -1,-l; 188. :313
_. ,:, 1.03 i a.62
’
at.295 K
.‘:.
.’
: ,. .:
,’
where [CO-,] is.the CO, number density, was found to be k = (1 .O+ 0.2) X lo-l4 cm3 s-l at 298 K; i.e., (7~)” = 324 t 65 tori-l s-l, wliere p is the CO, pressure. The error represents the standard deviation for an average.ofapproximately 50 individual runs. A comljarison of the measured value of the rate constant with previously published values is shown in table 1. The value obtained by a fluorescence tech-, ,nique that employed the periodic absorption and emission of 4.3 pm radintion is almost a factor of two smaller than the present result. Similarly, the results obtained by the spectrophone technique tend to be smaller than the value reported here, although one value is.within the error bounds of the present result. The agreement is very good, however, between. the value reported here and those determined by the laser fluqrescence technique that employ&d 4.3 /.ml emission. From the tab’lc, it appears that the laser flugrcscencc measurements to date indicate a value of 1 .O X lo-l4 cm? s-l for this relaxation rate constant at room temperature. Unfortunately, this,teclmique does not readily provide a means for determining the mechanism for the collisional deactivation. Consequently, this rate constant must be attributed to the overall
reaction
c0*(00”1) where
+ co2
+ co;
COY represents
mollcule state.
in the 0310,
+ CO2,
a vibrationally exited CO2 1110,0200 or 01’0 vibrational
This work was supported by the Office of Nova1 Research and the Advanced Research Projects Agency under ARPA Order 1806 and Contract NO00 I473-c-0025.
References [I] J.T. &I&
Prob. Phys. Sot. (Londcn)
91 (1967)
439.
[2] P.V. Slobodskaya, Opt. Spectry. 22 (1967) 14. [3] T.L. Cottrcll, I.hl. McF’nrlanc, A.W. Reed and A.H. Young, Trans. Faraday !3oc. 62 (1966) 2655. [4] W.A.-ROSSC~ Jr., A.D. Wood and E.T. Gerry, 5. Chcm. Phys. 50 (1969) 4996. [5] L.O. Hacker, h1.A. Kovacs, C.K. Rhodes, G.W. Flynn and A. Javan, Phys. Rev. Letters 17 (1966) 233. [6] CB. MOOX, R.E. Wood, B. HU and J.T. Yxdlcy, I. Chcm. Phys. 46 (1967) 4222. [7] P.K. Cheo, J. Appl. Phys. 38 (1967) 3563. [8] C.W. von Rqscnberg Jr. and A. Lowenstein, Phys. 59 (1973) 2751.
J. Chem.