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Overtone and combination bands of SF6 and 238UF6
__ l!&l
10 June 1996
‘2%
‘it-3 ,_
PHYSICS
ELSEYlER
LETTERS
A
Physics Letters A 2 1’5(1996)29 l-29.5
Overtone and combination bands of SF6 and 238UF6* Jian Zhang a, Qinghua Zhong a*1,Nianle Wu ‘, Jianwei Wang a, Jun Zhao ‘, Yizhuang Xu a, Songtao Guo b, Chuntong Ying b a Department of Physics, h Deparrment
Received
17 January
of Engineering
Tsinghua
University,
Physics, Tsinghua
Beijing
Universily,
100084, China Beijing
1996; revised manuscript received 22 March 1996; accepted Communicated by B. Fricke
100084,
China
for publication
25 March 1996
Abstract The absorption spectra for overtone and combination bands of SF6 and 238UFh have been measured using photoacoustic spectroscopy technologies with a CO laser. The band intensity of VI + ~2 + ~3 of 23RUFe and 3~4 of SF6 are reported for the first time as 6.5 + 1.5 and 5.5 f 1.5 m/mole, respectively. PACS: 33.20.E;
33.7O.Fd; 33.7O.Jg; 42.60.B
1. Introduction Recently much attention has been given to the feasibility of the chemical reaction isotope separation by laser activation (CRISLA) method of uranium isotope separation to replace or supplement conventional gaseous diffusion and gas centrifuge technologies [ l51. It has many advantages: it is economical, convenient for poor ore enrichment and the production cost is independent of the manufacturing scale, etc. The infrared spectra of SF6 and 2381_& have been studied extensively [ 6-81. However, the data of overtone and combination bands of SF6 and 238UF6 are not adequate to the demand, not only for CRISLA [ 51 but also for fundamental interests: multiple photon process, photochemistry etc. That is because the absorption cross
* This work was supported by the National Science Foundation and the Tsinghua University of China. ’ Present address: National Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Academia Sinica, Beijing 100080, China. 0375.9601/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved PII SO375-9601(96)00253-8
section at these bands is so low that it is difficult to measure. The photoacoustic effect is the process of chopped radiation absorption by a sample of gas, thus producing heat and pressure waves at the chopping frequency. Photoacoustic spectroscopy is based on measuring the photoacoustic signal as a function of the wavelength of the monochromatic radiation impinging on the sample. The photoacoustic spectroscopy has the ability to measure very small absorption coefficients in a gaseous sample [ 9-101. Therefore, it can be used to study overtone and combination bands of SF6 and 238UFg. In this paper, a novel design of the photoacoustic cell was used for spectrum detection. The band intensity and band width of 238UFg 3~3 were precisely measured. They coincided with those in Ref. [ 61. The band intensities of ~1 + 24 + ~3 of 238UFg and 32~4of SF6 are reported for the first time.
J. Zhung et d/Physics
292
Fig. I. Laser discharge tube. ( I ) Inlets of laser gases. (2) Quartz cooling coils. (3) LN2 bath. (4) Brewster windows. (5) Electrode at the ends. (6) Middle ground electrode. (7) Outlet of working gas. (8) Dewar layer.
2. Experimental
setup
The laser discharge tube is depicted in Fig. 1. It consists of two symmetrically arranged discharge paths. The length of the liquid-nitrogen jacket of the laser tube made of quartz glass is 89 cm. The internal diameter of the laser discharge tube is 20 mm. Cylindrical electrodes made of an Al plate are employed as both anode and cathode, arranged at the ends. The anode in the middle is ground. A self-sustained DC glow discharge is employed for the laser excitation. The resonator is arranged in the external resonator configuration. The laser cavity is comprised of a gold coating glass plane reflector and an Al-coating grating with 300 lines/mm (its blazed wavelength is 5.3 pm> in a Littrow configuration. The outcoupling efficiency (by zero-order diffraction) of the grating is roughly a constant over the measuring range (5.2-5.6 pm) of the photoacoustic spectra. The laser cavity length is kept constant by four Invar steel rods with an expansion coefficient of 0.5 x 10e6 K-‘. A step motor controlled by a microcomputer drives the blazed grating and the frequency of the laser radiation. The laser radiation is modulated by an intracavity mechanical chopper operating at 12.5 Hz. Because the average power of this CO laser is several watts [ 1 l-13], the photoacoustic cell is also placed in a laser cavity so that the signal to noise ratio will be much higher. The structure of the photoacoustic cell is shown in Fig. 2. It consists of a vacuum Dewar, a cooling chamber and a sample gas cell. They are all made of stainless steel. This temperature controlling system is for studying the thermal-band-
Letters A 215 (1996) 291-295
effect (which is very strong for UF6). In order to reduce the background signal from Brewster windows we open the other two windows at the side of the sample cell so that the light reflected from the Brewster window (because of any misalignment due to the large dimensions of the cell and cavity) can get out of the cell. A pre-amplifier is near the microphone. The photoacoustic cell is typically filled with a small partial pressure of sample gas (less than 133 Pa), to which a much higher (about 13333 Pa) argon pressure is added. The responsivity of the photoacoustic cell is defined as K uwa - ~SlWPS~~
(1)
where US is the photoacoustic voltage, W is the average power of the CO laser radiation, 9, is the partial pressure of the sample gas and LYis the absorption coefficient of the sample gas. The responsivity Ku,, was calibrated by the absorption coefficients of the CO laser in NO gas. The spectroscopic data of the l0 vibrational band of the fundamental electronic state of nitric oxide [ 141 have been used to calibrate the responsivity of the cell. We found K,,,=lO.9 V/ (W cm-’ ) . The minimum detectable absorption coefficient of this photoacoustic instrument is 1.5 x 10e7 cm-‘. The photoacoustic detector system is shown in Fig. 3. The photoacoustic voltage signal is measured by a lock-in amplifier and the laser power is detected by a laser power meter. A two-Y recorder draws a picture of the photoacoustic voltage and the laser power as a function of the laser frequency. The vacuum and gas handling system are all anticorrosion. Before the photoacoustic spectroscopy experiment the gas cell and the vacuum system are conditioned at an ambient temperature (290 K) with UF6 for several hours so that no depletion of the UFg occurs during the following experiment.
3. Results The band intensity is defined as [ 151 S=
.I
K(v) dv.
b&l
For XF6 3~3 and 3~4 bands, we have [ 161
(2)
J. Zhung et al./Physics
Fig. 2. Photoacoustic gascell.
cell.
( 1) CaF2 windows. (2) Microphone.
Letters A 215 (1996) 291-295
(3) Preamplifier.
where K(V) is the monochromatic absorption coefficient, N is Avogadro’s number, h is the Planck constant, c is the velocity of light in vacuum, ~0 is the band center and l&s12 is the square of the quantummechanical matrix element of the molecule-fixed z component of the molecular electric-dipole moment operator. Fig. 4 shows the band contour of 2112+ ~4,113+ ~4 + v6, ~1 + ~4 + ~5 and 3~4 bands in natural SF6 at 254 K. The band at the right side of Fig. 4 is the sum of the 2~2 + ~4, ~3 + ~4 + V6 and VI + ~4 + ~5 bands of SF6. The three combination bands cannot be distinguished. It seems to be a P, Q, R branch structure because the 2~2-t~~ combination band is dominant, From the band contour, we can calculate the sum of the three band intensities 5, = 14.7 * 1.7 m/mole. This is consistent with an earlier result [7]. The band at the left side of Fig. 4 is the SF6 3~4 band, which is weaker than the 229 + ~4 band. The band intensity for SF6 32~4is about 5.5 f 1.5 m/mole. The dipole moment for the SF6 3~4 band is )&s12 = 6.3 x lop4 Debye. The band contours of 3~s and VI+ u2 + ~3 of 238UF6 at 250 K are shown in Fig. 5. The width at halfmaximum of the 3~s absorption band is 33 cm-‘. The
(5) Cooling
chamber.
(6) Sample
2
1 t
(4) Thermometer.
293
I
1 Fig. 3. Schematic of photoacoustic detector system. ( 1) Mirrors. (2) Laser discharge tube. (3) Chopper. (4) Photoacoustic cell. (5) Vacuum and gas handling system. (6) Grating. (7) Power meter. (8) Lock-in amplifier. (9) Recorder.
band intensity So,3 is 34 f 4 m/mole. We derive the dipole moment for the 238UF6 3~s overtone band as I,CL$,~~~ = 1.6 x lop3 Debye. The ~1 + ~2 + ~3 band of 2’8UF6 is also measured. The band intensity of the 238UF6 VI + ZJ~+ vs band is 6.5 & 1.5 m/mole. The accurate measurement of this weak band which has not been reported in the literature and a signal to noise ratio of more than 24.7
J. Zhnng et d/Physics
294
Letters A 215 (1996) 291-295
45 a 35
30 T
:
25
P 7
20 5
. '3 I
15
10
5
0
l,.ser
Frequency
lcm”1
Fig. 4. Overtone and combination bands of SF6 at T = 254 K laser frequency (cm-‘)
7 I .e*o Lsacr
1.860 Frequency
I
,880
ic&'l
Fig. 5. VI -t vz f ~3 and 3~3 bands of 23xUF6 at T = 250 K,
1,900
1.920
.I. Zhung et d/Physics Table I Band intensities
Letters A 215 (1996) 291-295
295
of SF6 and 238UFg Band center (cm-‘)
Previous work
band intensity (m/mole)
This work
dipole moment (Debye)
1845
5.5
1902 1911 1913
14 [71
1873
38
161
1824
dB all indicate the reliability of our measurement. A comparison between the result obtained in this work and those determined earlier [ 6,7] is given in Table 1.
Acknowledgement This work was supported by the National Science Foundation (No. 96870034) and the Tsinghua University.
References I I 1J.W. Eerkens,
band intensity (m/mole)
R.P. Griot, J.H. Hardin and R.J. Smith, lntracavity CO laser induced photochemistry of the UF6 + HCl reaction, Conf. on Lasers and electro-optics, San Francisco, California, June 1986. 12 1 J.W. Eerkens, Appl. Phys. 10 (1976) 15. I3 I J.W. Eerkens, Laser separation of isotopes, Australia Patent Abridgement, AUB-66607/81 ( 1981). I4 I J. Zhang, N.L. Wu, J. Zhao, Y.Z. Xu, ES. Zhang, G.Q. Li, X.Q. Song, S.T. Guo and C.T. Ying, Laser Tech. (1996). in press I in Chinese].
dipole moment ( Debye
)
6.3 x lO-4
14.7
1.6 x lo-”
34
1.6 x lO-3
6.4
I51 H.B. Ding, Z.Y. Shen and C.H. Zhang, Proc. SPIE 1859 (1993) 234. 161 G.A. Laguna, K.C. Kim, C.W. Patterson, M.J. Reisfeld and D.M. Seitz, Chem. Phys. Lett. 75 (1980) 357. 132 [71 C. Chapados and G. Bimbaum, J. Mol. Spectrosc. (1988) 323. t81 B.J. Krohn, B.S. McDowell, C.W. Patterson, N.G. Nereson, M.J. Reisfeld and K.C. Kim, J. Mol. Spectrosc. 132 ( 1988) 285. 191 W. Urban, Infrared Phys. Technol. 36 (1995) 465. [lOI A. Thnoey and M.W. Sigrist, Infrared Phys. Technol. 36 (1995) 585. [Ill Zhong Qinghua, Zhang Jian, Wang Jianwei, Wu Nianle and Xu Yizhuang, Laser Infrared 21 (1991) 42 (in Chinese\. [I21 J. Zhang, N.L. Wu, J.W. Wang, J. Zhao and Y.Z. Xu, Opt. Laser Technol. 26 ( 1994) 83. 1131 J. Zhang, N.L. Wu, J.W. Wang, J. Zhao and Y.Z. Xu, Opt. Laser Technol. 26 (1994) 355. (141 J. Ballard, W.B. Johnston, B.J. Kerridge and J.J. Remedios, J. Mol. Spectrosc. 127 ( 1988) 70. 1151 M.A.H. Smith, B. Fridovich and K. Narahari Rao, in: Molecular spectroscopy: modem research, Vol. 3. Intensities and collision broadening parameters from infrared spectra, ed. K. Narahari Rao ( 1985) p. 111. 1161 K. Fox and W.B. Person, J. Chem. Phys. 64 ( 1976) 5218.
10 June 1996
‘2%
‘it-3 ,_
PHYSICS
ELSEYlER
LETTERS
A
Physics Letters A 2 1’5(1996)29 l-29.5
Overtone and combination bands of SF6 and 238UF6* Jian Zhang a, Qinghua Zhong a*1,Nianle Wu ‘, Jianwei Wang a, Jun Zhao ‘, Yizhuang Xu a, Songtao Guo b, Chuntong Ying b a Department of Physics, h Deparrment
Received
17 January
of Engineering
Tsinghua
University,
Physics, Tsinghua
Beijing
Universily,
100084, China Beijing
1996; revised manuscript received 22 March 1996; accepted Communicated by B. Fricke
100084,
China
for publication
25 March 1996
Abstract The absorption spectra for overtone and combination bands of SF6 and 238UFh have been measured using photoacoustic spectroscopy technologies with a CO laser. The band intensity of VI + ~2 + ~3 of 23RUFe and 3~4 of SF6 are reported for the first time as 6.5 + 1.5 and 5.5 f 1.5 m/mole, respectively. PACS: 33.20.E;
33.7O.Fd; 33.7O.Jg; 42.60.B
1. Introduction Recently much attention has been given to the feasibility of the chemical reaction isotope separation by laser activation (CRISLA) method of uranium isotope separation to replace or supplement conventional gaseous diffusion and gas centrifuge technologies [ l51. It has many advantages: it is economical, convenient for poor ore enrichment and the production cost is independent of the manufacturing scale, etc. The infrared spectra of SF6 and 2381_& have been studied extensively [ 6-81. However, the data of overtone and combination bands of SF6 and 238UF6 are not adequate to the demand, not only for CRISLA [ 51 but also for fundamental interests: multiple photon process, photochemistry etc. That is because the absorption cross
* This work was supported by the National Science Foundation and the Tsinghua University of China. ’ Present address: National Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Academia Sinica, Beijing 100080, China. 0375.9601/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved PII SO375-9601(96)00253-8
section at these bands is so low that it is difficult to measure. The photoacoustic effect is the process of chopped radiation absorption by a sample of gas, thus producing heat and pressure waves at the chopping frequency. Photoacoustic spectroscopy is based on measuring the photoacoustic signal as a function of the wavelength of the monochromatic radiation impinging on the sample. The photoacoustic spectroscopy has the ability to measure very small absorption coefficients in a gaseous sample [ 9-101. Therefore, it can be used to study overtone and combination bands of SF6 and 238UFg. In this paper, a novel design of the photoacoustic cell was used for spectrum detection. The band intensity and band width of 238UFg 3~3 were precisely measured. They coincided with those in Ref. [ 61. The band intensities of ~1 + 24 + ~3 of 238UFg and 32~4of SF6 are reported for the first time.
J. Zhung et d/Physics
292
Fig. I. Laser discharge tube. ( I ) Inlets of laser gases. (2) Quartz cooling coils. (3) LN2 bath. (4) Brewster windows. (5) Electrode at the ends. (6) Middle ground electrode. (7) Outlet of working gas. (8) Dewar layer.
2. Experimental
setup
The laser discharge tube is depicted in Fig. 1. It consists of two symmetrically arranged discharge paths. The length of the liquid-nitrogen jacket of the laser tube made of quartz glass is 89 cm. The internal diameter of the laser discharge tube is 20 mm. Cylindrical electrodes made of an Al plate are employed as both anode and cathode, arranged at the ends. The anode in the middle is ground. A self-sustained DC glow discharge is employed for the laser excitation. The resonator is arranged in the external resonator configuration. The laser cavity is comprised of a gold coating glass plane reflector and an Al-coating grating with 300 lines/mm (its blazed wavelength is 5.3 pm> in a Littrow configuration. The outcoupling efficiency (by zero-order diffraction) of the grating is roughly a constant over the measuring range (5.2-5.6 pm) of the photoacoustic spectra. The laser cavity length is kept constant by four Invar steel rods with an expansion coefficient of 0.5 x 10e6 K-‘. A step motor controlled by a microcomputer drives the blazed grating and the frequency of the laser radiation. The laser radiation is modulated by an intracavity mechanical chopper operating at 12.5 Hz. Because the average power of this CO laser is several watts [ 1 l-13], the photoacoustic cell is also placed in a laser cavity so that the signal to noise ratio will be much higher. The structure of the photoacoustic cell is shown in Fig. 2. It consists of a vacuum Dewar, a cooling chamber and a sample gas cell. They are all made of stainless steel. This temperature controlling system is for studying the thermal-band-
Letters A 215 (1996) 291-295
effect (which is very strong for UF6). In order to reduce the background signal from Brewster windows we open the other two windows at the side of the sample cell so that the light reflected from the Brewster window (because of any misalignment due to the large dimensions of the cell and cavity) can get out of the cell. A pre-amplifier is near the microphone. The photoacoustic cell is typically filled with a small partial pressure of sample gas (less than 133 Pa), to which a much higher (about 13333 Pa) argon pressure is added. The responsivity of the photoacoustic cell is defined as K uwa - ~SlWPS~~
(1)
where US is the photoacoustic voltage, W is the average power of the CO laser radiation, 9, is the partial pressure of the sample gas and LYis the absorption coefficient of the sample gas. The responsivity Ku,, was calibrated by the absorption coefficients of the CO laser in NO gas. The spectroscopic data of the l0 vibrational band of the fundamental electronic state of nitric oxide [ 141 have been used to calibrate the responsivity of the cell. We found K,,,=lO.9 V/ (W cm-’ ) . The minimum detectable absorption coefficient of this photoacoustic instrument is 1.5 x 10e7 cm-‘. The photoacoustic detector system is shown in Fig. 3. The photoacoustic voltage signal is measured by a lock-in amplifier and the laser power is detected by a laser power meter. A two-Y recorder draws a picture of the photoacoustic voltage and the laser power as a function of the laser frequency. The vacuum and gas handling system are all anticorrosion. Before the photoacoustic spectroscopy experiment the gas cell and the vacuum system are conditioned at an ambient temperature (290 K) with UF6 for several hours so that no depletion of the UFg occurs during the following experiment.
3. Results The band intensity is defined as [ 151 S=
.I
K(v) dv.
b&l
For XF6 3~3 and 3~4 bands, we have [ 161
(2)
J. Zhung et al./Physics
Fig. 2. Photoacoustic gascell.
cell.
( 1) CaF2 windows. (2) Microphone.
Letters A 215 (1996) 291-295
(3) Preamplifier.
where K(V) is the monochromatic absorption coefficient, N is Avogadro’s number, h is the Planck constant, c is the velocity of light in vacuum, ~0 is the band center and l&s12 is the square of the quantummechanical matrix element of the molecule-fixed z component of the molecular electric-dipole moment operator. Fig. 4 shows the band contour of 2112+ ~4,113+ ~4 + v6, ~1 + ~4 + ~5 and 3~4 bands in natural SF6 at 254 K. The band at the right side of Fig. 4 is the sum of the 2~2 + ~4, ~3 + ~4 + V6 and VI + ~4 + ~5 bands of SF6. The three combination bands cannot be distinguished. It seems to be a P, Q, R branch structure because the 2~2-t~~ combination band is dominant, From the band contour, we can calculate the sum of the three band intensities 5, = 14.7 * 1.7 m/mole. This is consistent with an earlier result [7]. The band at the left side of Fig. 4 is the SF6 3~4 band, which is weaker than the 229 + ~4 band. The band intensity for SF6 32~4is about 5.5 f 1.5 m/mole. The dipole moment for the SF6 3~4 band is )&s12 = 6.3 x lop4 Debye. The band contours of 3~s and VI+ u2 + ~3 of 238UF6 at 250 K are shown in Fig. 5. The width at halfmaximum of the 3~s absorption band is 33 cm-‘. The
(5) Cooling
chamber.
(6) Sample
2
1 t
(4) Thermometer.
293
I
1 Fig. 3. Schematic of photoacoustic detector system. ( 1) Mirrors. (2) Laser discharge tube. (3) Chopper. (4) Photoacoustic cell. (5) Vacuum and gas handling system. (6) Grating. (7) Power meter. (8) Lock-in amplifier. (9) Recorder.
band intensity So,3 is 34 f 4 m/mole. We derive the dipole moment for the 238UF6 3~s overtone band as I,CL$,~~~ = 1.6 x lop3 Debye. The ~1 + ~2 + ~3 band of 2’8UF6 is also measured. The band intensity of the 238UF6 VI + ZJ~+ vs band is 6.5 & 1.5 m/mole. The accurate measurement of this weak band which has not been reported in the literature and a signal to noise ratio of more than 24.7
J. Zhnng et d/Physics
294
Letters A 215 (1996) 291-295
45 a 35
30 T
:
25
P 7
20 5
. '3 I
15
10
5
0
l,.ser
Frequency
lcm”1
Fig. 4. Overtone and combination bands of SF6 at T = 254 K laser frequency (cm-‘)
7 I .e*o Lsacr
1.860 Frequency
I
,880
ic&'l
Fig. 5. VI -t vz f ~3 and 3~3 bands of 23xUF6 at T = 250 K,
1,900
1.920
.I. Zhung et d/Physics Table I Band intensities
Letters A 215 (1996) 291-295
295
of SF6 and 238UFg Band center (cm-‘)
Previous work
band intensity (m/mole)
This work
dipole moment (Debye)
1845
5.5
1902 1911 1913
14 [71
1873
38
161
1824
dB all indicate the reliability of our measurement. A comparison between the result obtained in this work and those determined earlier [ 6,7] is given in Table 1.
Acknowledgement This work was supported by the National Science Foundation (No. 96870034) and the Tsinghua University.
References I I 1J.W. Eerkens,
band intensity (m/mole)
R.P. Griot, J.H. Hardin and R.J. Smith, lntracavity CO laser induced photochemistry of the UF6 + HCl reaction, Conf. on Lasers and electro-optics, San Francisco, California, June 1986. 12 1 J.W. Eerkens, Appl. Phys. 10 (1976) 15. I3 I J.W. Eerkens, Laser separation of isotopes, Australia Patent Abridgement, AUB-66607/81 ( 1981). I4 I J. Zhang, N.L. Wu, J. Zhao, Y.Z. Xu, ES. Zhang, G.Q. Li, X.Q. Song, S.T. Guo and C.T. Ying, Laser Tech. (1996). in press I in Chinese].
dipole moment ( Debye
)
6.3 x lO-4
14.7
1.6 x lo-”
34
1.6 x lO-3
6.4
I51 H.B. Ding, Z.Y. Shen and C.H. Zhang, Proc. SPIE 1859 (1993) 234. 161 G.A. Laguna, K.C. Kim, C.W. Patterson, M.J. Reisfeld and D.M. Seitz, Chem. Phys. Lett. 75 (1980) 357. 132 [71 C. Chapados and G. Bimbaum, J. Mol. Spectrosc. (1988) 323. t81 B.J. Krohn, B.S. McDowell, C.W. Patterson, N.G. Nereson, M.J. Reisfeld and K.C. Kim, J. Mol. Spectrosc. 132 ( 1988) 285. 191 W. Urban, Infrared Phys. Technol. 36 (1995) 465. [lOI A. Thnoey and M.W. Sigrist, Infrared Phys. Technol. 36 (1995) 585. [Ill Zhong Qinghua, Zhang Jian, Wang Jianwei, Wu Nianle and Xu Yizhuang, Laser Infrared 21 (1991) 42 (in Chinese\. [I21 J. Zhang, N.L. Wu, J.W. Wang, J. Zhao and Y.Z. Xu, Opt. Laser Technol. 26 ( 1994) 83. 1131 J. Zhang, N.L. Wu, J.W. Wang, J. Zhao and Y.Z. Xu, Opt. Laser Technol. 26 (1994) 355. (141 J. Ballard, W.B. Johnston, B.J. Kerridge and J.J. Remedios, J. Mol. Spectrosc. 127 ( 1988) 70. 1151 M.A.H. Smith, B. Fridovich and K. Narahari Rao, in: Molecular spectroscopy: modem research, Vol. 3. Intensities and collision broadening parameters from infrared spectra, ed. K. Narahari Rao ( 1985) p. 111. 1161 K. Fox and W.B. Person, J. Chem. Phys. 64 ( 1976) 5218.