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Quenching of electronically-excited UF6 by selected inorganics
CHEMICAL
Volume 54, number 3
QUENCHING
PHYSICS
OF ELECTRONICALLY-EXCITED
15 hiarch 1978
LETTERS
UF6 BY SELECTED INORGANiCS*
F-B. WAMPLER, R.C. OLDENBORG and W-W. RICE Los AIamos Scientific Labotctory. University of California, Los Alamos, New Mexico 87.545. USA Received
18 October
1977
Rate constants at 25°C have been evaluated for the quenching of electronicallyexcited UF6 by selected inorganic molecules. Values for the rate constants at 25°C (units of 10” Q/mole s) for 02, N2, and Ar are l-0,0.025, and 0.0097. respectively. N2 and Ar quenching exhibit a saturation effect at high pressures and their rate constants were evaluated from lowpressure data uhere the quenching follows a hnear Stern-Volmer relationship.
1_ Introduction
The quenching of electronically-excited uranium hexafluoride molecules, * UFg , in the gas phase has been reported by various research groups [l-6] using the technique of laser-induced time-resolved emission of *UF,. In our recent work on the quenching of *UF6 by selected organic molecules [6] and in our earlier study on *UF6 quenching by UFs [S] we noted that the Stem-Volmer plots for *UF, quenching do not always follow a linear Sterner-Volmer relationship. The Stem-Volmer plots for UF, and C2F6 both were noted to reveal a downward curvature at high quencher pressures. de Witte et al. [4] have reported similar findings for UFg seIfquenching. We have investigated the following inorganic molecules as quenchers for *UF,: He, Ar, N2, CO, SFs, F,, O,, and Ha. Except for 02, all of the inorganic quenchers investigated show saturation of lifetimes at high pressures. We wish to report representative findings here for O,, N2, and Ar. A later detailed paper wii! report on a mathematical formulation of the above-mentioned quenching data according to Freed’s saturation theory on bimolecular-induced non-radiative intersystem crossings [7] _
* Work performed under the auspices of the United States Energy Research and Development Administration.
560
2. Experimental The experimental apparatus and technique have been described in an earlier report from this laboratory [S] and only the essential features will be repeated here. A N2 pumped dye laser, linewidth 0.15 A, is operated at 393.5 nm to optically excite the UFg _The time-resolved emission from *UF, is monitored with a RCA C3 1034 photomultiplier tube. Scattered laser light is discriminated against by placing a dielectric interference filter having a central wavelength of 422 nm and a fwhm of 20 nm in front of the photomultiplier tube. Research grade 02, N2, and Ar are used without further purification. A metal bellows pump is used to insure proper mixing of the reagents before commencing an experiment. All the observed lifetimes represent single exponential decays.
3. Results
and discussion
Fig. I depicts the Stern-Volmer plot for 02_ Fig. 2 gives the Stem-Volmer plot for N2 and Ar. All data were taken at &JF~ = 3.00 torr, &&akn = 393.5 nm, and room temperature. Values determined from figs. 1 and 2 for the rate constants at 25°C (units of 1011 Q/ mole s) for the quenching of *UFgby O,, N2, and Ar are 1 .O, 0.025, and 0.0097, respt%tively. Note that the quenching rate constants for N2 and Ar are derived
15 March 1978
CHEMICAL PHYSICS LETTERS
Volume 54. number 3
ing route contains a significant physical quenching contribution. For N2 and Ar quenching of *UF6 there is no obvious accordance
TIN. 1. Stern-Volmer plot for 02. Excitation wavelength is 393.5 nm, mF, = 3.00 torr.
chemical reaction pathway, with theory, the quenching
and thus, in
data reveal a
saturation effect. Lewis et al. [9] have interpreted the ultravrolet spectroscopy of UFs. For our excitatron wavelength of 393.5 nm, *UF, may be considered as an electronically excited state having a doublet-doublet character or being a mixed state of singlet and trrplet character. Thus, *IiF should possess those properties which are characteristic of electronically-excited states having an unpaired spin. Oxygen 1s known to quench such electronic states very efficrently, particularly if a chemical reaction channel exists. Our quenching rate constant value for 0, is 1.0 X 10’ l Q/mole s whrch certainly is consistent with a very efficient quenching pathway. Recall for N, that the low pressure quenching data &ve a value of 0.025 X 101’ Q/mole s. Thus, 0, is 40 times more efficient in removing *UF, than N,. We conjecture that some form of chemical quenching is proceeding with 02, *UF6 collnions. A better understanding of this reaction must await further experimentation.
References [l] 0
100
300
200 P”
400
so0
tTORRI
FIN. 2 Stern-Volmer plots for N2. circles, and for Ar, tnangles. Excitation wavelength is 393.5 nm, &JF6 = 3.00 torr. The dashed lines are extrapolated initial low pressure slopes.
from the low-pressure data where a linear StemVolmer relationship exists. The dashed lines in fig. 2 represent the extrapolated low-pressure slopes. It is easily noted that there is pronounced deviation from these initial slopes at high quencher pressures. According to Freed’s theory [7] and its application by Su et al. [8] to saturation effects in SO, (3BI) lifetime data, this deviation from a nonlinear Stem-Volmer equation is to be anticipated only when the mechanistic quench-
[2] (31 [4] [5] [6] [7] [S]
[9]
A.A. \ndreoni and H. Bucher, Chem. Phys. Letters40 (1976) 237. A.A .\ndreoni, R. Cubeddu, S. DeSrlvestri and F. Zavaga, Chem. Phys. Letters 48 (1977) 431. P. Benettr. R. Cubeddu, CA. Sacch~, 0. Svelte and F. Zaraga, Chem. Phys. Letters 40 (1976) 140. 0 de Witte, R Dumonchin and hl. hhchon, Chem. Phys. Lette-s 40 (1977) 505. I -9. Wampler, R.C. Oldenborg dnd W-W. Rtce, Chem Phys. Letters 54 (1978) 554. F B. Wampler, R.C. Oldenborg and W.W. Rice, Chcm. Phys. Letters 54 (1978) 557. K-F. Freed, Chem. Phys. Letters 37 (1976) 47. F. Su, F-B. Wampler, J.W. Bottenherm, D.L. Thorsell, J-G- Calvert and E-K. Damon, Chem. Phys. Letters 51 (197_) 150. W-B. Lewis. L-B. Asprcy, L-H. Jones, R.S. McDowell, SW. Rabideau and A H. Zeltmann, J. Chem. Phys. 65 (1976) 2707.
561
Volume 54, number 3
QUENCHING
PHYSICS
OF ELECTRONICALLY-EXCITED
15 hiarch 1978
LETTERS
UF6 BY SELECTED INORGANiCS*
F-B. WAMPLER, R.C. OLDENBORG and W-W. RICE Los AIamos Scientific Labotctory. University of California, Los Alamos, New Mexico 87.545. USA Received
18 October
1977
Rate constants at 25°C have been evaluated for the quenching of electronicallyexcited UF6 by selected inorganic molecules. Values for the rate constants at 25°C (units of 10” Q/mole s) for 02, N2, and Ar are l-0,0.025, and 0.0097. respectively. N2 and Ar quenching exhibit a saturation effect at high pressures and their rate constants were evaluated from lowpressure data uhere the quenching follows a hnear Stern-Volmer relationship.
1_ Introduction
The quenching of electronically-excited uranium hexafluoride molecules, * UFg , in the gas phase has been reported by various research groups [l-6] using the technique of laser-induced time-resolved emission of *UF,. In our recent work on the quenching of *UF6 by selected organic molecules [6] and in our earlier study on *UF6 quenching by UFs [S] we noted that the Stem-Volmer plots for *UF, quenching do not always follow a linear Sterner-Volmer relationship. The Stem-Volmer plots for UF, and C2F6 both were noted to reveal a downward curvature at high quencher pressures. de Witte et al. [4] have reported similar findings for UFg seIfquenching. We have investigated the following inorganic molecules as quenchers for *UF,: He, Ar, N2, CO, SFs, F,, O,, and Ha. Except for 02, all of the inorganic quenchers investigated show saturation of lifetimes at high pressures. We wish to report representative findings here for O,, N2, and Ar. A later detailed paper wii! report on a mathematical formulation of the above-mentioned quenching data according to Freed’s saturation theory on bimolecular-induced non-radiative intersystem crossings [7] _
* Work performed under the auspices of the United States Energy Research and Development Administration.
560
2. Experimental The experimental apparatus and technique have been described in an earlier report from this laboratory [S] and only the essential features will be repeated here. A N2 pumped dye laser, linewidth 0.15 A, is operated at 393.5 nm to optically excite the UFg _The time-resolved emission from *UF, is monitored with a RCA C3 1034 photomultiplier tube. Scattered laser light is discriminated against by placing a dielectric interference filter having a central wavelength of 422 nm and a fwhm of 20 nm in front of the photomultiplier tube. Research grade 02, N2, and Ar are used without further purification. A metal bellows pump is used to insure proper mixing of the reagents before commencing an experiment. All the observed lifetimes represent single exponential decays.
3. Results
and discussion
Fig. I depicts the Stern-Volmer plot for 02_ Fig. 2 gives the Stem-Volmer plot for N2 and Ar. All data were taken at &JF~ = 3.00 torr, &&akn = 393.5 nm, and room temperature. Values determined from figs. 1 and 2 for the rate constants at 25°C (units of 1011 Q/ mole s) for the quenching of *UFgby O,, N2, and Ar are 1 .O, 0.025, and 0.0097, respt%tively. Note that the quenching rate constants for N2 and Ar are derived
15 March 1978
CHEMICAL PHYSICS LETTERS
Volume 54. number 3
ing route contains a significant physical quenching contribution. For N2 and Ar quenching of *UF6 there is no obvious accordance
TIN. 1. Stern-Volmer plot for 02. Excitation wavelength is 393.5 nm, mF, = 3.00 torr.
chemical reaction pathway, with theory, the quenching
and thus, in
data reveal a
saturation effect. Lewis et al. [9] have interpreted the ultravrolet spectroscopy of UFs. For our excitatron wavelength of 393.5 nm, *UF, may be considered as an electronically excited state having a doublet-doublet character or being a mixed state of singlet and trrplet character. Thus, *IiF should possess those properties which are characteristic of electronically-excited states having an unpaired spin. Oxygen 1s known to quench such electronic states very efficrently, particularly if a chemical reaction channel exists. Our quenching rate constant value for 0, is 1.0 X 10’ l Q/mole s whrch certainly is consistent with a very efficient quenching pathway. Recall for N, that the low pressure quenching data &ve a value of 0.025 X 101’ Q/mole s. Thus, 0, is 40 times more efficient in removing *UF, than N,. We conjecture that some form of chemical quenching is proceeding with 02, *UF6 collnions. A better understanding of this reaction must await further experimentation.
References [l] 0
100
300
200 P”
400
so0
tTORRI
FIN. 2 Stern-Volmer plots for N2. circles, and for Ar, tnangles. Excitation wavelength is 393.5 nm, &JF6 = 3.00 torr. The dashed lines are extrapolated initial low pressure slopes.
from the low-pressure data where a linear StemVolmer relationship exists. The dashed lines in fig. 2 represent the extrapolated low-pressure slopes. It is easily noted that there is pronounced deviation from these initial slopes at high quencher pressures. According to Freed’s theory [7] and its application by Su et al. [8] to saturation effects in SO, (3BI) lifetime data, this deviation from a nonlinear Stem-Volmer equation is to be anticipated only when the mechanistic quench-
[2] (31 [4] [5] [6] [7] [S]
[9]
A.A. \ndreoni and H. Bucher, Chem. Phys. Letters40 (1976) 237. A.A .\ndreoni, R. Cubeddu, S. DeSrlvestri and F. Zavaga, Chem. Phys. Letters 48 (1977) 431. P. Benettr. R. Cubeddu, CA. Sacch~, 0. Svelte and F. Zaraga, Chem. Phys. Letters 40 (1976) 140. 0 de Witte, R Dumonchin and hl. hhchon, Chem. Phys. Lette-s 40 (1977) 505. I -9. Wampler, R.C. Oldenborg dnd W-W. Rtce, Chem Phys. Letters 54 (1978) 554. F B. Wampler, R.C. Oldenborg and W.W. Rice, Chcm. Phys. Letters 54 (1978) 557. K-F. Freed, Chem. Phys. Letters 37 (1976) 47. F. Su, F-B. Wampler, J.W. Bottenherm, D.L. Thorsell, J-G- Calvert and E-K. Damon, Chem. Phys. Letters 51 (197_) 150. W-B. Lewis. L-B. Asprcy, L-H. Jones, R.S. McDowell, SW. Rabideau and A H. Zeltmann, J. Chem. Phys. 65 (1976) 2707.
561