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D6 Reaction chemistry of the UF6 lisosep process
Volume 18, n u m b e r I
OPTI('S C O M M U N I ( A T I O N S
In the experiments reported here, the infrared double resonance technique has been used to confirm this conjecturc by direct observation. A single pulse from a m o d e locked, transverse discharge, atmospheric pressure CO 2 laser oscillating on the P(20) line of the 10u transition was used to excite a m i x t u r e of from 1 0 0 - 5 0 0 mtorr SF 6 and 0 100 torr of He or At. Pulse widths (fwhm) were 2 nS: pulse energies were ~ 1 0 mJ. A low power cw CO 2 probe laser beam was also passed through the sample. Changes in the absorption o1" the probe are a direct measure of the population of excited vibrational states. Absorption changes were measured by focusing the probe laser onto a Ge:Hg infrared detector capacitively coupled into a oscilloscope. Detection system overall resetime was <1 ns and its falltime was ~3 ns. Oscilloscope traces of the probe laser power on any CO 2 line from P(22) to P(36) show an increase in absorption induced by the high power (107 W cm - 2 ) laser pulse which is not exponential, always has the same shape, undergoes its full increase in 4 ns, and is independent of SF 6 pressure over the range 1 0 0 - 5 0 0 mtorr and buffer gas pressure from 0 - 1 0 0 tort. Consequently, these traces are interpreted as an integral of the nanosecond p u m p i n g pulse itself. Evidently, steady state p u m p i n g conditions exist, i.e., the upper level m u s t be emptied and the lower level replenished at a rate m u c h greater than the p u m p i n g rate. Evidence of this condition is obtained when the cw laser is t u n e d to the P(20) line and thus probes the transition being directly p u m p e d . Here a transient decrease in absorption occurs which falls far short of full saturation of the transition and whose falltime constant is ~4 ns, close to the detector limit, Similar results have been obtained in BC13. These rates ( > 1 0 tO s -1 torr - 1 ) are more t h a n 104 times gas kinetic and provide direct confirmation of t h e so called "collisionless" V- V energy transfer processes postulated by other authors to account for rapid, isotope specific dissociation of SF 6, BC13 and other polyatomics.
!t~ly !9"76
collisional V(V +_q ) aspects of laser-excited Ut:~ interacting with HCt. Ref. [2] gives for the chemical reaction of Ul:~ with H('I tile formulas: UF~ + HC1 ~ UF 5 ~ + HI: + CI( -46 kcal/mote) ,
~la)
C1 + C1 --~ C12 (+57.16 kcal/mole) ,
(t b}
where the quantities in brackets are the estimated heats of formation. However we believe that at total pressures below 20 tore the CO 2 laser-induced reaction in an excess of ttCI, is instead: UF 6 + HCI ~ UF5CI + HF(+22.4 _+ 10 kcal/mole)
(2a)
or possibly : UI:6+ HCI(v = 0 ; J = 0, 1) ~ UF6HCI; UF6HC1 + hv L ~ UF6HCI* ~ UFsC1 + H F ,
(2b)
where UF6HCI is a short-lived metastable molecular complex. The 2 kcal/mole delivered by CO 2 laser p h o t o n s are inadequate to provide the energy required to drive the endothermic reaction (la). Reaction (1) m a y occur at higher pressures with the aid of a third b o d y or on walls, or it m a y be achieved on walls at high temperatures with (2) as an intermadiate step: 2UFsC1 ~ 2UF 5 $ + C12 ( - 8 0 . 5 k c a l / m o l e ) .
(3)
ttowever with the HC1/UF 6 gas mixture at T = 295 K and p ~< 20 torr, the data obtained indicate that the chemical reaction proceeds by exothermic reactions (2a) or (2b). Having chosen the most plausible reaction formula dictated by thermochemical considerations, our data indicate further that for chemical reaction (2a) or (2b) ro proceed, the excitation of a specific vibration m u s t be involved, n a m e l y the v 3 vibration of UF 6 or vr vibration of the metastable molecule UF6HCI. That is, reactions (2a) or (2b) should be written: UF6(u3) + HC1 --* UF5C1 + HF*(v = 1, or v = 2) ,
(4a
or:
UF6HC1 (Ur)--, UFsCI + HF*(v = 1, or v = 2) . D6
REACTION CHEMISTRY OF THE UF 6 LISOSEP PROCESS Jeff W. EERKENS
AiResearch Manufacturing Company. A Division of the Garrett Corporation, 2525 West 190th Street, Torrance, California 90509, USA The first successful laser isotope separation of u r a n i u m hexafluoride by the LISOSEP process~t was carried o u t in 1972/1973 by irradiation of a 1/4 UF6/HC1 gas m i x t u r e with a special filtertuned isotope-selective c o n t i n u o u s CO 2 laser that operated in the (v 3 + v4+ v6) absorption band of UF 6. Some spectral considerations for the process were reviewed in a previous paper and publication [ 1 ]. Here we review the chemical reaction and *Patents have been applied for. 32
(4b)
These reactions are almost thermoncutral if, as indicated, the HF molecule f o r m e d is in the first (11.8 kcal]mole) or second (23 kcal/mole) vibrationally excited state. Collisional energy transfer theory and chemical reaction rate theory show that t h e r m o n e u t r a l or " r e s o n a n t " reactions are very efficient [3]. At present, we think reaction (4a) is operative, b u t reaction (4b) cannot be ruled out. For the former case we have the following scenario: at r o o m temperature, UF 6 is collisionally excited to m a n y (multiple) levels of its three low-energy vibrations v4, v 5, v6, and also to one or more levels of its three high-energy vibrations ul, v2, and v 3. In the presence o f HCI, rapid reaction takes place only with v3-excited u F 6 molecules, but w i t h o u t laser irradiation the effective reaction rate is controlled by the eollisional translation ~ vibration (TV) rate of v 3 excitation which is slow [3]. With a continuously operated CO 2 laser irradiating the UF6/HC1 m i x t u r e however, the v 3 population can be increased one-hundred-fold or more, since VT deactivation and s p o n t a n e o u s decay o f v 3 (r 3 ~
ISOTOPE SEPARATION 4 sec [4] ) are both slow. Thus the chemical reaction (CR) rate is also increased one-hundred-fold and controlled by the laser pump rate of v3, that is by the power level of the laser. The experimentally observed reaction rates can be predicted by the above scenario and the improved V(V + T) theory given in [3]. Whereas unexcited UF6, and UF 6 excited to any level of u I , u2, v 5, or v6 vibrations, interacts via a L e n n a r d Jones potential (LJP), UF 6 excited to the u 3 or v4 vibration interacts via a Dipole-Dipole Stockmeyer (DDS) or Quadrupole Dipole Stockmayer (QDS) intermolecular potential, whose enhancement factors are larger than for the LJP case [3]. For most UF 6 - U F 6 VT or TV collisions involving v3, the weaker LJP or QDS interactions thus apply, while for HC1-UF 6 interactions involving u 3 the stronger DDS potential is in operation. The u 3 dipoles in these interactions are not permanent but oscillating in time. However because the collision time is on the order of, or shorter than the vibration oscillation time, an effective nonzero dipole field is acting during the collision. The interaction between UF~(v3) and HC1 thus involves a strong DDS field which has a high "attractive enhancement factor" [3]. Coupled with the resonant (thermo-neutral) condition of reaction (4a) a high CR rate results. If reaction (2b) or (4b) is correct, the isotope shifts and vibrations of UF6HC1 must be considered instead of UF 6. Work is in progress to check this alternative to (2a) or (4a). In conclusion, we report here two new results: (a) An efficient chemical reaction important for the laser enrichment of UF6, and (b) the possibility that specific-vibration-catalyzed chemical reactions can be induced by laser photons. Result (b) is quite significant and should find application in many other laser-induced chemical reaction schemes. References [ 1 ] J.W. Eerkens, to be published in May, 1976 issue of Applied Physics, Springer-Verlag. [21 R.L. Farrar and D.F. Smith, AEC Oak Ridge Report K-L-3054 (March 1972). [3] J.W. Eerkens, Review of Molecular Energy Transfer Theory and its Application in Photochemical Laser Isotope Separation' 1o be released for publication in 1976. [4] J.W. Eerkens, Rocket Radiation Handbook Vol. II (December 1973), U.S. Air Force FTD No. CW-01-01-74; ADA007-946.
D7
"~('
~HT -H~T. ~PUMP L A S E R ~ : : ~ ~ :,~ .
i
PROBE
[I
LASER
;
;i
I
SCOPE
®
.
.Br
L~ i
~
~
-HT
ICONTROLLERt
I SUPPLY
PUMP Roe~
"%%/]/
// (~7~1
MONOCHROMATORS ~ , ~
Fig. 1. Diagram of the double resonance setup for the measurement of V - V transfer in highly excited CO.
V',V'+ 1 kv,v 1 ~.
CO(V
CO(V)+CO(V'),
I)+CO(V'+
1)+-AE
has been intensively investigated when both v and v' have large values. In this case the sample is a CO He mixture excited by an electrical discharge [ 1 ]. The p u m p laser is Q-switched by a rotating mirror (200 ns half-intensity width, 50 W peak power). The intensity of the single-line, single-mode laser oscillating on line center can be further stabilized [2] (see experimental setup in fig. 1). Data acquisition is done by means of a digital signal averager. Thus we record double resonance signals (transient absorption or gain on the probe beam) even when probe transition is located in the vibrational ladder far from the pump transition, which gives much information. Measurements have been performed on a very wide range of v-values between v = 1 and v = 29 (see table 1 ). Results are compared to Sharma and Brau's theory [3] by solving the appropriate system of coupled differential equations for each experiment, with rate constants values obtained from a first Born approximation calculation. The V - v master rate equations can be numerically inverted V ' Vv_ ' + l 1 lJ • , we assume the proportlon. to gl"V e 1t ~e rate constants kv, , , ~ V ' o'+l ality oi xv,~_ 1 to the vibrational matrix elements IRv,v_ 1 i2 and IRv,+l,v,I 2. Table 1 Location of the pump and probe transitions in V - V transfer experiments.
D7
TIME-RESOLVED I N F R A R E D LASER DOUBLE RESONANCE IN CO: V - V RATES IN EXCITEDSTATE-EXCITED-STATE COLLISIONS EXTENDED UP TO V = 29, AND INVESTIGATION OF R - R , T ROTATIONAL RELAXATION Ph. BRECHIGNAC
Laboratoire de Photophysique-Mol6culaire - CN.R.S., Universit6 Paris-Sud, 91405 Orsay, France A time-resolved double resonance technique has been used to measure the kinetics of the various collisional energy transfer processes in CO gas, by means of two CO lasers. The V - V exchange reaction:
PROBE
O
l
'
l
l
l
l
TRANSITION
l
l
l
l
l
l
l
2- 1 3- 2 ; - 3 s-4
7-
6
9-
g
11 - I 0
20-19 2]-22 26-25 27 - 2 6 26-27j
33
OPTI('S C O M M U N I ( A T I O N S
In the experiments reported here, the infrared double resonance technique has been used to confirm this conjecturc by direct observation. A single pulse from a m o d e locked, transverse discharge, atmospheric pressure CO 2 laser oscillating on the P(20) line of the 10u transition was used to excite a m i x t u r e of from 1 0 0 - 5 0 0 mtorr SF 6 and 0 100 torr of He or At. Pulse widths (fwhm) were 2 nS: pulse energies were ~ 1 0 mJ. A low power cw CO 2 probe laser beam was also passed through the sample. Changes in the absorption o1" the probe are a direct measure of the population of excited vibrational states. Absorption changes were measured by focusing the probe laser onto a Ge:Hg infrared detector capacitively coupled into a oscilloscope. Detection system overall resetime was <1 ns and its falltime was ~3 ns. Oscilloscope traces of the probe laser power on any CO 2 line from P(22) to P(36) show an increase in absorption induced by the high power (107 W cm - 2 ) laser pulse which is not exponential, always has the same shape, undergoes its full increase in 4 ns, and is independent of SF 6 pressure over the range 1 0 0 - 5 0 0 mtorr and buffer gas pressure from 0 - 1 0 0 tort. Consequently, these traces are interpreted as an integral of the nanosecond p u m p i n g pulse itself. Evidently, steady state p u m p i n g conditions exist, i.e., the upper level m u s t be emptied and the lower level replenished at a rate m u c h greater than the p u m p i n g rate. Evidence of this condition is obtained when the cw laser is t u n e d to the P(20) line and thus probes the transition being directly p u m p e d . Here a transient decrease in absorption occurs which falls far short of full saturation of the transition and whose falltime constant is ~4 ns, close to the detector limit, Similar results have been obtained in BC13. These rates ( > 1 0 tO s -1 torr - 1 ) are more t h a n 104 times gas kinetic and provide direct confirmation of t h e so called "collisionless" V- V energy transfer processes postulated by other authors to account for rapid, isotope specific dissociation of SF 6, BC13 and other polyatomics.
!t~ly !9"76
collisional V(V +_q ) aspects of laser-excited Ut:~ interacting with HCt. Ref. [2] gives for the chemical reaction of Ul:~ with H('I tile formulas: UF~ + HC1 ~ UF 5 ~ + HI: + CI( -46 kcal/mote) ,
~la)
C1 + C1 --~ C12 (+57.16 kcal/mole) ,
(t b}
where the quantities in brackets are the estimated heats of formation. However we believe that at total pressures below 20 tore the CO 2 laser-induced reaction in an excess of ttCI, is instead: UF 6 + HCI ~ UF5CI + HF(+22.4 _+ 10 kcal/mole)
(2a)
or possibly : UI:6+ HCI(v = 0 ; J = 0, 1) ~ UF6HCI; UF6HC1 + hv L ~ UF6HCI* ~ UFsC1 + H F ,
(2b)
where UF6HCI is a short-lived metastable molecular complex. The 2 kcal/mole delivered by CO 2 laser p h o t o n s are inadequate to provide the energy required to drive the endothermic reaction (la). Reaction (1) m a y occur at higher pressures with the aid of a third b o d y or on walls, or it m a y be achieved on walls at high temperatures with (2) as an intermadiate step: 2UFsC1 ~ 2UF 5 $ + C12 ( - 8 0 . 5 k c a l / m o l e ) .
(3)
ttowever with the HC1/UF 6 gas mixture at T = 295 K and p ~< 20 torr, the data obtained indicate that the chemical reaction proceeds by exothermic reactions (2a) or (2b). Having chosen the most plausible reaction formula dictated by thermochemical considerations, our data indicate further that for chemical reaction (2a) or (2b) ro proceed, the excitation of a specific vibration m u s t be involved, n a m e l y the v 3 vibration of UF 6 or vr vibration of the metastable molecule UF6HCI. That is, reactions (2a) or (2b) should be written: UF6(u3) + HC1 --* UF5C1 + HF*(v = 1, or v = 2) ,
(4a
or:
UF6HC1 (Ur)--, UFsCI + HF*(v = 1, or v = 2) . D6
REACTION CHEMISTRY OF THE UF 6 LISOSEP PROCESS Jeff W. EERKENS
AiResearch Manufacturing Company. A Division of the Garrett Corporation, 2525 West 190th Street, Torrance, California 90509, USA The first successful laser isotope separation of u r a n i u m hexafluoride by the LISOSEP process~t was carried o u t in 1972/1973 by irradiation of a 1/4 UF6/HC1 gas m i x t u r e with a special filtertuned isotope-selective c o n t i n u o u s CO 2 laser that operated in the (v 3 + v4+ v6) absorption band of UF 6. Some spectral considerations for the process were reviewed in a previous paper and publication [ 1 ]. Here we review the chemical reaction and *Patents have been applied for. 32
(4b)
These reactions are almost thermoncutral if, as indicated, the HF molecule f o r m e d is in the first (11.8 kcal]mole) or second (23 kcal/mole) vibrationally excited state. Collisional energy transfer theory and chemical reaction rate theory show that t h e r m o n e u t r a l or " r e s o n a n t " reactions are very efficient [3]. At present, we think reaction (4a) is operative, b u t reaction (4b) cannot be ruled out. For the former case we have the following scenario: at r o o m temperature, UF 6 is collisionally excited to m a n y (multiple) levels of its three low-energy vibrations v4, v 5, v6, and also to one or more levels of its three high-energy vibrations ul, v2, and v 3. In the presence o f HCI, rapid reaction takes place only with v3-excited u F 6 molecules, but w i t h o u t laser irradiation the effective reaction rate is controlled by the eollisional translation ~ vibration (TV) rate of v 3 excitation which is slow [3]. With a continuously operated CO 2 laser irradiating the UF6/HC1 m i x t u r e however, the v 3 population can be increased one-hundred-fold or more, since VT deactivation and s p o n t a n e o u s decay o f v 3 (r 3 ~
ISOTOPE SEPARATION 4 sec [4] ) are both slow. Thus the chemical reaction (CR) rate is also increased one-hundred-fold and controlled by the laser pump rate of v3, that is by the power level of the laser. The experimentally observed reaction rates can be predicted by the above scenario and the improved V(V + T) theory given in [3]. Whereas unexcited UF6, and UF 6 excited to any level of u I , u2, v 5, or v6 vibrations, interacts via a L e n n a r d Jones potential (LJP), UF 6 excited to the u 3 or v4 vibration interacts via a Dipole-Dipole Stockmeyer (DDS) or Quadrupole Dipole Stockmayer (QDS) intermolecular potential, whose enhancement factors are larger than for the LJP case [3]. For most UF 6 - U F 6 VT or TV collisions involving v3, the weaker LJP or QDS interactions thus apply, while for HC1-UF 6 interactions involving u 3 the stronger DDS potential is in operation. The u 3 dipoles in these interactions are not permanent but oscillating in time. However because the collision time is on the order of, or shorter than the vibration oscillation time, an effective nonzero dipole field is acting during the collision. The interaction between UF~(v3) and HC1 thus involves a strong DDS field which has a high "attractive enhancement factor" [3]. Coupled with the resonant (thermo-neutral) condition of reaction (4a) a high CR rate results. If reaction (2b) or (4b) is correct, the isotope shifts and vibrations of UF6HC1 must be considered instead of UF 6. Work is in progress to check this alternative to (2a) or (4a). In conclusion, we report here two new results: (a) An efficient chemical reaction important for the laser enrichment of UF6, and (b) the possibility that specific-vibration-catalyzed chemical reactions can be induced by laser photons. Result (b) is quite significant and should find application in many other laser-induced chemical reaction schemes. References [ 1 ] J.W. Eerkens, to be published in May, 1976 issue of Applied Physics, Springer-Verlag. [21 R.L. Farrar and D.F. Smith, AEC Oak Ridge Report K-L-3054 (March 1972). [3] J.W. Eerkens, Review of Molecular Energy Transfer Theory and its Application in Photochemical Laser Isotope Separation' 1o be released for publication in 1976. [4] J.W. Eerkens, Rocket Radiation Handbook Vol. II (December 1973), U.S. Air Force FTD No. CW-01-01-74; ADA007-946.
D7
"~('
~HT -H~T. ~PUMP L A S E R ~ : : ~ ~ :,~ .
i
PROBE
[I
LASER
;
;i
I
SCOPE
®
.
.Br
L~ i
~
~
-HT
ICONTROLLERt
I SUPPLY
PUMP Roe~
"%%/]/
// (~7~1
MONOCHROMATORS ~ , ~
Fig. 1. Diagram of the double resonance setup for the measurement of V - V transfer in highly excited CO.
V',V'+ 1 kv,v 1 ~.
CO(V
CO(V)+CO(V'),
I)+CO(V'+
1)+-AE
has been intensively investigated when both v and v' have large values. In this case the sample is a CO He mixture excited by an electrical discharge [ 1 ]. The p u m p laser is Q-switched by a rotating mirror (200 ns half-intensity width, 50 W peak power). The intensity of the single-line, single-mode laser oscillating on line center can be further stabilized [2] (see experimental setup in fig. 1). Data acquisition is done by means of a digital signal averager. Thus we record double resonance signals (transient absorption or gain on the probe beam) even when probe transition is located in the vibrational ladder far from the pump transition, which gives much information. Measurements have been performed on a very wide range of v-values between v = 1 and v = 29 (see table 1 ). Results are compared to Sharma and Brau's theory [3] by solving the appropriate system of coupled differential equations for each experiment, with rate constants values obtained from a first Born approximation calculation. The V - v master rate equations can be numerically inverted V ' Vv_ ' + l 1 lJ • , we assume the proportlon. to gl"V e 1t ~e rate constants kv, , , ~ V ' o'+l ality oi xv,~_ 1 to the vibrational matrix elements IRv,v_ 1 i2 and IRv,+l,v,I 2. Table 1 Location of the pump and probe transitions in V - V transfer experiments.
D7
TIME-RESOLVED I N F R A R E D LASER DOUBLE RESONANCE IN CO: V - V RATES IN EXCITEDSTATE-EXCITED-STATE COLLISIONS EXTENDED UP TO V = 29, AND INVESTIGATION OF R - R , T ROTATIONAL RELAXATION Ph. BRECHIGNAC
Laboratoire de Photophysique-Mol6culaire - CN.R.S., Universit6 Paris-Sud, 91405 Orsay, France A time-resolved double resonance technique has been used to measure the kinetics of the various collisional energy transfer processes in CO gas, by means of two CO lasers. The V - V exchange reaction:
PROBE
O
l
'
l
l
l
l
TRANSITION
l
l
l
l
l
l
l
2- 1 3- 2 ; - 3 s-4
7-
6
9-
g
11 - I 0
20-19 2]-22 26-25 27 - 2 6 26-27j
33