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Time-resolved Lyman α photometry in a chemical kinetic investigation of H + C2H4
Volume 3. number
CHEMICAL PHYSICS LETTERS
6
TIME-RESOLVED
LYMAN
KINETIC
INVESTIGATION
J. V. MICHAEL Department
(Y PHOTOMETRY OF
June 1969
IN
A
CHEMICAL
H + C2H4
and DAVID T. OSBORNE *
of ChemiGt-3, Carnegie-Mellor,
Universzty.
Pittsburgh,
Pa. 15213,
USA
Received 1 May 1969 An experunent IS described m whmh reactlons of hydrogen atoms, produced by mercury photosensitization, are followed 111a real time experrment by tune-resolved i.yman (Yphotometry. Application of the technique to the reactron of hydrogen atoms wrth ethylene is descrrbed.
In this paper an experiment is described in which hydrogen atom co1 _mtration is followed oscilloscopically Q Lyman (Y photometry. Lyman (Y photometry has been used by other investigators in discharge flow systems [l-3] and a shock tube [4]. Lyman cy korescence has been used to monitor atom concentrations in one recently reported investigation [5]. This photometric technique has been found to be very sensitive, and the hmit of detection m this laboratory [3] has been found to be 5 X lOlo atoms/cm3 in a 2.4 cm absorption length. Since the method is so sensi’ave the technique of mercury photosensitization as a source of hydrogen atoms suggested itself. Thrs paper is a report of thx research. Hydrogen atoms are produced by the mercury sensitized decomposition of molecular hydrogen. The formation and subsequent depletion of hydrogen atoms may be summarized in terms of the following mechanism:
1.
Hg(‘So)
+ hu -+ Hg(3P1)
2.
Hg(3P1) - H&6)
3.
Hg(3Pl)
4.
H+H+M-1H2+M
5.
H + walls * $H2
6.
H + R - products.
+ hv
+ H2 - Hg(k9)
+ 2H or HgH + H
Some steps which may occur in the process of mercury have been deleted plification. The atoms produced in the process may decay by three reactions: if reactant is present, 6. * NDEA Predoctoral Fellow, 1966-65.
402
primary for simprimary 4, 5, and,
A schematic diagram of the apparatus is shown in fig. 1. A mercury resonance lamp was constructed from 6 mm quartz tubing which was tolled into a flat spiral of 1.5 in. diameter. The electrodes are tungsten, and the lamp is powered by a 1200 V dc supply. This lamp was situated 7 cm from the quartz window of the cell. Smce the atom decay times are rather long, as will be described below, it has not been necessary to build synchronization circuitry. Instead the lamp can simply be operated manua.Ily with the on/off switch cn the power supply. The Lyman LYlamp is constructed from 38 mm glass tubing and is 15 cm long. It has two tungsten electrodes, and it is prepared for operation in exactly the same way as described by Barker and Michael [3]. In order to increase signal size, however, the lamp m this experiment is powered by a 28.8 MHz radio frequency generator of a design described previously [6]. The Lyman a radiation passes through LiF unndows into an oxygen filter section in order to spectrally isolate the Lyman (I line and fmally into the T shaped reaction cell. For best operation it is necessary to purify the oxygen in the filter section by passage through a liquid nitrogen trap. If atoms are present attenuation results, and this can be monitored by the ionization detector [2,3,7] located on the opposite side of the cell. The output from the ionization detector is dropped through lo6 ohms, and the resultant voltage (proportional to ion current and therefore to intensity) is displayed on a Tektronix 549 storage oscilloscope with a lA7 high sensitivity preamplifier. Xxperiments were performed to test the maximum time resolution of the detector with the lo6 ohm load resistance. It was found that
Volume
3, number
6
CHEMICAL
VhCUUM
PHYSICS
June 1969
LETTERS
LINE
Fig. 2. Transmission of Lyman o! in mercury photosensitized H2,C2H4 mrxture. [H2]/fC2H41 = 377. The formatlon occurs during the Light period and the decay occurs during the dark permd. 2G ms/cm time base. D. C. refers to dark current. that first order kinetics are always obeyed in the absence of reactant. Inspection of the mechanism during the dark period (reactions 4, 5, and 6) leads to the following equation: it is found
- q DETECTOR +
Fig. 1. SchematIc diagram of apparatus, R - rf generator; L - Lyman (Y lamp, 0 - oxygen filter; Q - quartz wmdow, N - needle valve; Ml - Hg manometer; P mechamcal pump; T - glass wool trap; DC - dc power supply, s - osclllcscope.
with this resls+ance an mduced pulse decays 111 120 PS. Typical decay times for hydrogen atoms are found to be about three orders of magnitude greater than thm response limit. In preparation for an experiment a known pressure of reactant gas (H2 or a mixture of a small amount of R in Hz) 1s expanded into the evacuated cell. The mercury vapor pressure may be regulated w&h a coolant at known temperature surrounding the glass wool trap. After sufficient time has elapsed for equilibration of the mercury vapor pressure, the mercury resonance lamp is quic!dy turned on and then off. A typical record of the detector response is shown in fig. 2. When the lamp is turned on, the atom concentration builds up rapidly to a steady state. When the lamp is extinguished, the atoms decay and the signal returns to IO. Intensity can be related to hydrogen atom concentration by Beer’s law provided the transmittance is always greater than 75% [3]. Thus, the temporal behavior of hydrogen atom concentration can be obtained, and
= 2k4[H12[M] + k5[H] + k6[H][R]
.
(1)
Reaction 4 is negligible wzth the small atom concentration used since first order kinetics strictly holds. Also the decay constants are invariant over a 3 X lo4 change m pressure as seen in table 1, and this further supports the contention that reaction 4 1-snegligible. Therefore, eq. (I) may be simpllfled to -
9 =(kg + kG[R])fH]
concentration than atom concentration, If reactant
is very
.
much greater
ln [H-J= - (k5 i- k6[Rj)f f c .
(3)
With no reactant added it is conceivable that mercury might react homogeneously with hydrogen atoms m a reaction of type 6. In order to test this hypothesis the c_oncentration of mercury was varied with coolants surrounding the gIass wool trap. As expected the steady state concentration of hydrogen atoms decreased drasticalLy with a decrease in mercury concentration. Lz fact, no atoms were detected when all of the mercury was removed by liquid nitrogen. Table 1 also shows the decay constants obtained at four mercury pressures. Even with a two hundred decrease in mercury concentration the decay times are invariant within experimental error. Thus, mercury does not react with hydrogen atoms homogeneously. 403
CHEMICAL PHYSICS LETTERS
Volume 3, number6
Table 1 kob&+
PHg(Tom)
4.5 5.1 5.0 5.6 5.5 4.6 6.9 6.0 5.8 5.0 6.2 6.0 * 5.7 * 5.6 f 6.8 f 7.5 7.1 5.2 6.8 4.0
Table 2 2.1 212 2.1
2.0
2.1
2.0 2.0 Z:8 2.0 2.0 2.c 2.0 2.0 2.0 2.0 i:: 2.0
PT(Torr)
PH2tToi-r)
G.O1O 0.14 0.34 2.0 2.0
1.5 1.5 1.5 1.5
X lo3
June 1969
0.52 I.01 1.47 3.01 5.13 7.11 10.06 14.99 25.3 75.0
0.32 0.03
0.04 0 15 0.20 0.50 1.1 5.0 10.1 15.0 25.0 50-O 104 201 403 605
. I
Inspection of table 1 shows that the decay c on&ants in the absence of reactant are invariant with pressure within expemmental error. Tlus is undoubtedly due to the high value of 1.9 x 103/P cm2 Torr s-l for the binary diffusion coefficient 0~ hydrogen atoms in molecular hydrogen [8]. From Einstein’s dBZusion formula *, X2 = 2Dt,the average time for dmplacement to the wall even at pressures 111excess of 100 Torr is always less than the average time for removal of the atoms (reciprocal of the observed time constants in table 1). Thus, the time constants that are presented m table 1 refer simply to removal at the wall, and diffusion to the wall is never rate determimng. This implies that the atoms survive many collisions with the walls an 4 indicates that uniform atom concentrations v+l result even rf the rate of photosensitization is not uniform across the length of the cell. This technique has been applied to the reaction of hydrogen atoms with ethylene in excess molecular hydrogen. Inspection of eq. (3) mdicates that the observed time constant will mcrease with increasmg ethylene concentration. This behavtor is found to hold, and the resultant slope of kobs against [C2H4] yields the blmolecular rate constant for hydrogen atoms with ethylene. Table 2 shows that the bimolecular rate constant increases with increasing total pressure. These results can be compared to recent results obtained 111flow discharge studies [lO,ll] where the &luent gas is helium. Also * For a discwslon of the diffusion phenomen see W. A. Noyes Jr. : nd P. A. Lelghton [S].
kh(cm3/raolec.sec) x 1013 2.00 l 2.76 f 2.99 * 4.15 f 5.24 f 6.26 f 5.93 f 6.50 f 6.33 f 5.57 f
0.20 0.26 0.41 0.37 0.98 0.68 0.90 1.01 0.68 0.63
results for this reactive system have been obtarned in the previously cited [5] fluorescence experiment. Withm the error of the present results, the agreement with Westenberg and deHaas and Harker and Michael is excellent, and the results from these three investigations are superImposable. This demonstrates the apphcabihty of the present technique to measure rate constants of hydrogen atoms with various molecules reliably. We gratefully acknowledge the support of the Atomic Energy Commission under Contract No. AT(30-l)-3794.
REFERENCES [l] I.Tanaka and J.R.McNesby, [2] [3] [4] [5] [6] [A [8] [S]
[lo] [ll]
J. Chem. Phys. 36 (1962) 3170. J. V. Michael and R. E. Weston Jr., J. Chem. Phys. 45 (1966) 3632. J.R. Barker and J.V.Michael. J. Opt. Sot. Am. 58 (1968) 1615. A. L.IAyerson and W, S.Watt, J. Chem. Phys. 49 (1968) 425. W. Braun and M. Lenzi, Disc. Faraday Sot. 44 (1967) 252. B. Budmk, R. Novick and A. Lumo. Appl. Opt. 4 (1965) 229. J. H. Carver and P.Mitchell, J. Sci. Instr. 41 (1984) 555. B. Kbouw, J. E. Morgan and H. 1. Schrff, J. Chem. Phys. 50 (1969) 66. W.A.Noyes Jr. and P.A.Leighton. The photochemistry of gases ‘(Remh.>ld Publmhutg Corp., New York, 1941) 182. A.A.Westenberg and N.deHaas, J. Chem. Phys. so (1989) 707. J-R-Barker and J. V. Michael, J. Chem. Phys., to be published.
CHEMICAL PHYSICS LETTERS
6
TIME-RESOLVED
LYMAN
KINETIC
INVESTIGATION
J. V. MICHAEL Department
(Y PHOTOMETRY OF
June 1969
IN
A
CHEMICAL
H + C2H4
and DAVID T. OSBORNE *
of ChemiGt-3, Carnegie-Mellor,
Universzty.
Pittsburgh,
Pa. 15213,
USA
Received 1 May 1969 An experunent IS described m whmh reactlons of hydrogen atoms, produced by mercury photosensitization, are followed 111a real time experrment by tune-resolved i.yman (Yphotometry. Application of the technique to the reactron of hydrogen atoms wrth ethylene is descrrbed.
In this paper an experiment is described in which hydrogen atom co1 _mtration is followed oscilloscopically Q Lyman (Y photometry. Lyman (Y photometry has been used by other investigators in discharge flow systems [l-3] and a shock tube [4]. Lyman cy korescence has been used to monitor atom concentrations in one recently reported investigation [5]. This photometric technique has been found to be very sensitive, and the hmit of detection m this laboratory [3] has been found to be 5 X lOlo atoms/cm3 in a 2.4 cm absorption length. Since the method is so sensi’ave the technique of mercury photosensitization as a source of hydrogen atoms suggested itself. Thrs paper is a report of thx research. Hydrogen atoms are produced by the mercury sensitized decomposition of molecular hydrogen. The formation and subsequent depletion of hydrogen atoms may be summarized in terms of the following mechanism:
1.
Hg(‘So)
+ hu -+ Hg(3P1)
2.
Hg(3P1) - H&6)
3.
Hg(3Pl)
4.
H+H+M-1H2+M
5.
H + walls * $H2
6.
H + R - products.
+ hv
+ H2 - Hg(k9)
+ 2H or HgH + H
Some steps which may occur in the process of mercury have been deleted plification. The atoms produced in the process may decay by three reactions: if reactant is present, 6. * NDEA Predoctoral Fellow, 1966-65.
402
primary for simprimary 4, 5, and,
A schematic diagram of the apparatus is shown in fig. 1. A mercury resonance lamp was constructed from 6 mm quartz tubing which was tolled into a flat spiral of 1.5 in. diameter. The electrodes are tungsten, and the lamp is powered by a 1200 V dc supply. This lamp was situated 7 cm from the quartz window of the cell. Smce the atom decay times are rather long, as will be described below, it has not been necessary to build synchronization circuitry. Instead the lamp can simply be operated manua.Ily with the on/off switch cn the power supply. The Lyman LYlamp is constructed from 38 mm glass tubing and is 15 cm long. It has two tungsten electrodes, and it is prepared for operation in exactly the same way as described by Barker and Michael [3]. In order to increase signal size, however, the lamp m this experiment is powered by a 28.8 MHz radio frequency generator of a design described previously [6]. The Lyman a radiation passes through LiF unndows into an oxygen filter section in order to spectrally isolate the Lyman (I line and fmally into the T shaped reaction cell. For best operation it is necessary to purify the oxygen in the filter section by passage through a liquid nitrogen trap. If atoms are present attenuation results, and this can be monitored by the ionization detector [2,3,7] located on the opposite side of the cell. The output from the ionization detector is dropped through lo6 ohms, and the resultant voltage (proportional to ion current and therefore to intensity) is displayed on a Tektronix 549 storage oscilloscope with a lA7 high sensitivity preamplifier. Xxperiments were performed to test the maximum time resolution of the detector with the lo6 ohm load resistance. It was found that
Volume
3, number
6
CHEMICAL
VhCUUM
PHYSICS
June 1969
LETTERS
LINE
Fig. 2. Transmission of Lyman o! in mercury photosensitized H2,C2H4 mrxture. [H2]/fC2H41 = 377. The formatlon occurs during the Light period and the decay occurs during the dark permd. 2G ms/cm time base. D. C. refers to dark current. that first order kinetics are always obeyed in the absence of reactant. Inspection of the mechanism during the dark period (reactions 4, 5, and 6) leads to the following equation: it is found
- q DETECTOR +
Fig. 1. SchematIc diagram of apparatus, R - rf generator; L - Lyman (Y lamp, 0 - oxygen filter; Q - quartz wmdow, N - needle valve; Ml - Hg manometer; P mechamcal pump; T - glass wool trap; DC - dc power supply, s - osclllcscope.
with this resls+ance an mduced pulse decays 111 120 PS. Typical decay times for hydrogen atoms are found to be about three orders of magnitude greater than thm response limit. In preparation for an experiment a known pressure of reactant gas (H2 or a mixture of a small amount of R in Hz) 1s expanded into the evacuated cell. The mercury vapor pressure may be regulated w&h a coolant at known temperature surrounding the glass wool trap. After sufficient time has elapsed for equilibration of the mercury vapor pressure, the mercury resonance lamp is quic!dy turned on and then off. A typical record of the detector response is shown in fig. 2. When the lamp is turned on, the atom concentration builds up rapidly to a steady state. When the lamp is extinguished, the atoms decay and the signal returns to IO. Intensity can be related to hydrogen atom concentration by Beer’s law provided the transmittance is always greater than 75% [3]. Thus, the temporal behavior of hydrogen atom concentration can be obtained, and
= 2k4[H12[M] + k5[H] + k6[H][R]
.
(1)
Reaction 4 is negligible wzth the small atom concentration used since first order kinetics strictly holds. Also the decay constants are invariant over a 3 X lo4 change m pressure as seen in table 1, and this further supports the contention that reaction 4 1-snegligible. Therefore, eq. (I) may be simpllfled to -
9 =(kg + kG[R])fH]
concentration than atom concentration, If reactant
is very
.
much greater
ln [H-J= - (k5 i- k6[Rj)f f c .
(3)
With no reactant added it is conceivable that mercury might react homogeneously with hydrogen atoms m a reaction of type 6. In order to test this hypothesis the c_oncentration of mercury was varied with coolants surrounding the gIass wool trap. As expected the steady state concentration of hydrogen atoms decreased drasticalLy with a decrease in mercury concentration. Lz fact, no atoms were detected when all of the mercury was removed by liquid nitrogen. Table 1 also shows the decay constants obtained at four mercury pressures. Even with a two hundred decrease in mercury concentration the decay times are invariant within experimental error. Thus, mercury does not react with hydrogen atoms homogeneously. 403
CHEMICAL PHYSICS LETTERS
Volume 3, number6
Table 1 kob&+
PHg(Tom)
4.5 5.1 5.0 5.6 5.5 4.6 6.9 6.0 5.8 5.0 6.2 6.0 * 5.7 * 5.6 f 6.8 f 7.5 7.1 5.2 6.8 4.0
Table 2 2.1 212 2.1
2.0
2.1
2.0 2.0 Z:8 2.0 2.0 2.c 2.0 2.0 2.0 2.0 i:: 2.0
PT(Torr)
PH2tToi-r)
G.O1O 0.14 0.34 2.0 2.0
1.5 1.5 1.5 1.5
X lo3
June 1969
0.52 I.01 1.47 3.01 5.13 7.11 10.06 14.99 25.3 75.0
0.32 0.03
0.04 0 15 0.20 0.50 1.1 5.0 10.1 15.0 25.0 50-O 104 201 403 605
. I
Inspection of table 1 shows that the decay c on&ants in the absence of reactant are invariant with pressure within expemmental error. Tlus is undoubtedly due to the high value of 1.9 x 103/P cm2 Torr s-l for the binary diffusion coefficient 0~ hydrogen atoms in molecular hydrogen [8]. From Einstein’s dBZusion formula *, X2 = 2Dt,the average time for dmplacement to the wall even at pressures 111excess of 100 Torr is always less than the average time for removal of the atoms (reciprocal of the observed time constants in table 1). Thus, the time constants that are presented m table 1 refer simply to removal at the wall, and diffusion to the wall is never rate determimng. This implies that the atoms survive many collisions with the walls an 4 indicates that uniform atom concentrations v+l result even rf the rate of photosensitization is not uniform across the length of the cell. This technique has been applied to the reaction of hydrogen atoms with ethylene in excess molecular hydrogen. Inspection of eq. (3) mdicates that the observed time constant will mcrease with increasmg ethylene concentration. This behavtor is found to hold, and the resultant slope of kobs against [C2H4] yields the blmolecular rate constant for hydrogen atoms with ethylene. Table 2 shows that the bimolecular rate constant increases with increasing total pressure. These results can be compared to recent results obtained 111flow discharge studies [lO,ll] where the &luent gas is helium. Also * For a discwslon of the diffusion phenomen see W. A. Noyes Jr. : nd P. A. Lelghton [S].
kh(cm3/raolec.sec) x 1013 2.00 l 2.76 f 2.99 * 4.15 f 5.24 f 6.26 f 5.93 f 6.50 f 6.33 f 5.57 f
0.20 0.26 0.41 0.37 0.98 0.68 0.90 1.01 0.68 0.63
results for this reactive system have been obtarned in the previously cited [5] fluorescence experiment. Withm the error of the present results, the agreement with Westenberg and deHaas and Harker and Michael is excellent, and the results from these three investigations are superImposable. This demonstrates the apphcabihty of the present technique to measure rate constants of hydrogen atoms with various molecules reliably. We gratefully acknowledge the support of the Atomic Energy Commission under Contract No. AT(30-l)-3794.
REFERENCES [l] I.Tanaka and J.R.McNesby, [2] [3] [4] [5] [6] [A [8] [S]
[lo] [ll]
J. Chem. Phys. 36 (1962) 3170. J. V. Michael and R. E. Weston Jr., J. Chem. Phys. 45 (1966) 3632. J.R. Barker and J.V.Michael. J. Opt. Sot. Am. 58 (1968) 1615. A. L.IAyerson and W, S.Watt, J. Chem. Phys. 49 (1968) 425. W. Braun and M. Lenzi, Disc. Faraday Sot. 44 (1967) 252. B. Budmk, R. Novick and A. Lumo. Appl. Opt. 4 (1965) 229. J. H. Carver and P.Mitchell, J. Sci. Instr. 41 (1984) 555. B. Kbouw, J. E. Morgan and H. 1. Schrff, J. Chem. Phys. 50 (1969) 66. W.A.Noyes Jr. and P.A.Leighton. The photochemistry of gases ‘(Remh.>ld Publmhutg Corp., New York, 1941) 182. A.A.Westenberg and N.deHaas, J. Chem. Phys. so (1989) 707. J-R-Barker and J. V. Michael, J. Chem. Phys., to be published.