- Email: [email protected]
Vibrational spectroscopy and predissociation of UF6 clusters in a supersonic Laval nozzle
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 4 1O-41 1 ( 1997) 299-304
Vibrational spectroscopy and predissociation of UF6 clusters in a supersonic Lava1 nozzle Y. Okada*, S. Tanimura,
H. Okamura, A. Suda, H. Tashiro, K. Takeuchi
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako. Saitama 351-01, Japan
Received 26 August 1996; accepted 6 September 1996
Abstract The FI’-IR spectra of clusters of UF6 seeded in argon were measured in a continuous supersonic Lava1 nozzle flow. We observed absorption peaks of UF6 clusters on both the red- and the blue-shifted sides of the monomer peak (around 627 cm-‘) at a temperature below 130 K. We assigned these peaks using calculated results on the frequency shifts and transition strength of the IR spectra of UF6 clusters. Clusters as large as 10 in size were found to be formed under our experimental conditions. Vibrational predissociation of UF6 clusters was observed after a single shot from a Raman laser tuned to a vibrational mode of the UF6 clusters (614.8 cm-‘) in the Lava1 nozzle. We monitored the concentration of UF6 monomers in the ground state with a diode laser spectrometer, and qualitatively interpreted the time-scale of the predissociation of the UF6 clusters and the recombination of UF6 monomers subsequent to the predissociation in the nozzle. After the UF6 clusters had dissociated in less than 1 ns, the excited monomers formed by the dissociation were deactivated with a time constant of about 0.3 ps via vibrational relaxation induced by collisions with carrier gas (argon) atoms. The recombination of UF6 monomers produced by the vibrational predissociation competed with vibrational relaxation. The time constant of the recombination (about 6 ps) was found to be much larger than that of the relaxation. 0 1997 Elsevier Science B.V. Keywords:
Uranium hexafluoride; nozzle; FT-IR spectra
Molecular
clusters;
Vibrational
1. Introduction Over the last two decades, the isotopically
selective multiphoton dissociation of UF6 using 16 pm infrared laser beams has been studied for its application to uranium enrichment [l-6]. When UF6 molecules were supercooled by adiabatic expansion in a supersonic Lava1 nozzle, the rotation-vibration spectrum of UF6 was sufficiently narrow compared with the * Corresponding author. Tel. 00 81 48 467 9306; fax.: 00 81 48 462 4702; e-mail: [email protected].
spectroscopy;
Vibrational
predissociation;
Supersonic
isotope shift (0.6 cm-‘) of UF6 [7]. Thus one could obtain high separation factors while overcoming the problem of the UF6 molecule having a rather small isotope shift. In the adiabatic expansion, condensation of supercooled UF6 molecules takes place when the concentration of UF6 is high. Condensation of the UF6 monomers raises the temperature and causes the isotopic selectivity to deteriorate. There is then an upper limit of the UF6 concentration for a high value of the separation factor to be obtained. If we could eliminate or loosen this restriction, in other words, if we could
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SOO22-2860(96)09567-l
300
Y. Okada et al./Journal
of Molecular
ensure that the separation factor does not decrease even in the case of the UF6 concentration being higher than the upper limit, the process would have the following two advantages: (i) a large value of the product how rate would be possible, since we could increase the UF6 concentration while the separation factor is kept high; (ii) a much higher value of the separation factor would be possible, since we could achieve a much lower temperature without the occurrence of UF6 condensation while keeping the UFh density high.
(4
Structure 410-411
(1997) 299-304
In the present paper, we report the results of experiments performed to observe the vibrational spectra of UFI, clusters, the vibrational predissociation of these clusters using 16 pm Raman laser radiation, and the formation of UF6 monomers populated in the ground state. We demonstrate that one can utilize this procedure for the vibrational predissociation of UF6 clusters as a technique to increase the concentration of UFf, monomers in an irradiation zone for the molecular laser isotope separation of uranium.
SUPERSONIC
LAVAL NOZZLE
UF6 +Ar(Ne) I
UF6 +Ar(Ne) *
spectroscopy
/’
NOZZLE DIMENSION x=12-30 mm, h’=1.2 mm, h=l.2+0.22x
FTIR (0.5cm-1 )
mm, d=lO mm
UF6 (P.Omol%)+Ar
(W I
I
MCT DETECTOR
LAVAL NOZZLE
COMPRESSOR
16 pm RAMAN LASER BEAM
(tuned to Cl branch
peak of UF6 ~3,
614.6 cm-l
v=1+0)
Fig. 1. Schematic diagram of the experimental supersonic Lava1 nozzle.
apparatus:
(a) for FT-IR spectroscopy;
DIODE LASER
(b) for the vibrational predissociation
of UF6 clusters in a
Y. Okada et al./Joumal
of Molecular
Structure 410-411
2. Experimental
nozzle. The collimated diode laser beam passed twice through the nozzle in a region which included the Raman laser irradiation region. When a Raman laser pulse (614.8 cm-‘) was applied to the gas in the nozzle for the dissociation of the UF6 clusters, we observed the time-dependent change in the concentration of UF6 monomers populated in the ground state by monitoring the energy of the infrared diode laser beam transmitted through the Lava1 nozzle.
The experimental set-ups for FT-IR spectroscopy [8] and the vibrational predissociation of UF6 clusters 193 are schematically shown in Fig. l(a) and (b), respectively. For both experiments, a mixture of UF6 and the inert gas argon was continuously fed into a plane-symmetric guided Lava1 nozzle equipped with KC1 windows (optical path length, 10 mm). The geometry of this nozzle is shown also in Fig. l(a). In the spectroscopy experiments [8], we changed the pressure at the inlet of the nozzle in order to investigate the effect of cluster formation on the UF6 partial pressure. The resolution of the FT-IR spectrometer was 0.5 cm-‘. In the predissociation experiments [9], we used a pulsed para-Hz Raman laser beam (614.8 cm-‘) which was tuned to a vibrational mode of the UF6 clusters. The wavenumber of the beam emitted from a diode laser (Laser Photonics L5615) was fixed at the v = 1 - 0 transition (627.7 cm-‘) of the v3 vibrational mode of the 238UF6 monomer present in the Lava1
(a)
3. Results and discussion 3.1. Vibrational spectroscopy
1 (b) P(total) = 2.5 Torr P(UF,) = 0.20 Torr ‘\I G;
= 0.10 Torr
ii
, 660
640
620
600
of UF6 clusters
We measured the IR spectra of UF6 gas at a mole fraction of 0.08 in argon at a position 30 mm downstream from the throat of the Lava1 nozzle. Fig. 2 shows typical spectra observed at relatively high values of the total pressure, P(total). In the range 620-635 cm-‘, the P-, Q- and R-branches of UF6 monomers were observed. On the red-shifted side
P(total) = 1.3 Torr P(UF,)
301
(1997) 299-304
560
G+Vw’yIx”v
,
660
k
J
2,
640
620
-.-A.& 600
I
580
1 / j I (c) P(total) = 3.0 Torr
/,I \,q P(UF,J
iy
= 0.24 Torr
, I
!li ; 7
660
640 Wave
Fig. 2. lT-IR
620 number
P(UF,J
= 5.0 Torr = 0.40 Torr
J+i
d L Q--mwv
*,pd \w++ I
(d) P(total)
1,
600 [cm-’
, 580
1
b.F 660
\ “L, ~w++%-./
d 640 Wave
620 number
spectra of UF6 at a mole fraction of 0.08 in argon at a position 30 mm downstream
600 [cm-’
580
1
from the throat of the Lava1 nozzle.
302
Y. Okada et al.Mournal of Molecular Structure 410-411
(610-615 cm-‘) of the monomer peaks, distinct peaks appeared, and relatively small peaks were present on the blue-shifted side (around 640 cm-‘). With an increase in the total pressure, both the red- and blueshifted peaks became large, and the relative intensity of the monomer peaks decreased. Peaks other than the monomer peaks are thought to be due to homogeneous clusters of UF6, because the temperature was higher than the boiling point of argon at 1 atm, 87 K, and the shapes of the spectra were almost the same whether the carrier gas was argon or neon. When the total pressure was 0.95 Torr or below, we observed no peaks other than those of monomers. We found that the shape of the Q-branch of the UF6 monomers observed from the FT-IR measurements was dependent on the vibrational temperature of the UF6 gas due to the change in the fractional populations in the vibrational states. Under the conditions of formation of the UF6 clusters, we determined the vibrational temperature of the UF6 monomers by comparing the Q-branch observed to that measured when the clustering did not occur at relatively low concentrations and high temperatures, with the vibrational temperatures being confirmed to be equal to the known values of the translational temperatures under thermal equilibrium conditions. In addition, we determined the monomer fractions in the nozzle using values for the integrated absorption area of the monomer in the observed spectra. Fig. 3 shows the dependence of the monomer fraction, X(mono), and the UF6 vibrational temperature, T(vib), on the UF6 partial pressure at a UF,, mole fraction of 0.08 in argon. From Fig. 3(a) it was found that no clustering occurred at UFh pressures below 0.95 Torr, and in the pressure range above 0.95 Torr the monomer fraction decreased due to clustering as the UF6 pressure increases. From Fig. 3(b) we found that the temperature increased with decreasing UF6 monomer fraction. It is reasonable to believe that the increase in temperature was caused by the heat of condensation. We estimated the size of the UF6 clusters shown in Fig. 2 by calculating the frequency shift and transition strength in the IR spectra of the UF6 clusters with an interaction Hamiltonian which contains dipoledipole and dipole-induced dipole effects [ 10,111. The calculations were based on simple models, i.e. a regular triangle for the trimer, a regular tetrahedron for the tetramer, and then closest packed structures for
(1997) 299-304
1.2
0.0
I
I
I
0.1
0.2
0.3
I
240
120 0
P(UF,)
0.4
[ Torr ]
Fig. 3. Dependences on the UF6 partial pressure P(UF,,) of (a) the monomer fraction X(mono) and (b) the vibrational temperature T(vib) of UF6 monomers. (The UF6 mole fraction was 0.08 in argon, and the measurement position was 30 mm downstream from the throat of the Lava1 nozzle.)
the larger clusters. The nearest-neighbour distance was assumed to remain unchanged. Each UF6 molecule was treated as a three-dimensional harmonic oscillator, neglecting Coriolis effects. We used 0.385 D for the Jransition dipole moment of the UF6 molecule, 5.21 A for the nearest-neighbour distance, and 12.3 A” for the polarizability. As a result, we found that clusters mostly with a size of 12 were formed under the conditions shown in Fig. 2(c). 3.2. Vibrational predissociation
of UF6 clusters
We measured the time-dependent change in the concentration of UF6 monomers after a shot from a Raman laser. Fig. 4 shows the responses obtained after averaging 100 pulses of output signals from the HgCdTe infrared detector for monitoring the diode laser beam intensity. The arrow marked on the abscissa indicates the irradiation time of the Raman laser pulses. It should be noted that the increase in detector signal intensity on the ordinate corresponds
Y. Okada et alJJouma1 of Molecular Structure 410-411
303
(1997) 299-304 UF6 CLUSTERS
..1.‘: . ..___ _.. . ..____ _ .... . ....
TIME SCALE
%% OS
PHOTON
t-w 1
VIBRATIONAL
PREDISSOCIATION
EXCITED MONOMERS
COLLISIONS WITH Ar 1 VIBRATIONAL
Fig. 4. Signals measured by the HgCdTe infrared detector: (a) trace obtained with Raman laser radiation present when a mixture of IJF6 and argon was circulated in the Lava1 nozzle; (b) trace obtained with the Raman laser beam blocked when a mixture of UF6 and argon was circulated; (c) trace obtained with Raman laser radiation present when there was no gas in the Lava1 nozzle.
to a decrease in the diode laser beam intensity detected, and vice versa. Fig. 4(a) shows a trace obtained with the Raman laser radiation (fluence, about 20 mJ cm-*) present when a gas mixture of UF6 and argon was circulated in the Lava1 nozzle loop. The signal intensity of the HgCdTe infrared detector increased rapidly during 2 ps after the Raman laser irradiation and subsequently slowly decreased. The trace in Fig. 4(b) shows the response obtained with the Raman laser beam blocked before the nozzle when a mixture of UF6 and argon was circulated. Fig. 4(c) shows the trace obtained with the Raman laser radiation present when there is no gas in the Laval nozzle. Neither of these traces (Fig. 4(b) and 4(c)) shows such a signal increase as that seen in Fig. 4(a). It is reasonable to believe that the signal increase shown in Fig. 4(a) resulted not from artifacts but from an increase in concentration of UF6 monomers populated in the ground state as a result of the vibrational predissociation of the UF6 clusters. Fig. 5 shows a schematic representation of the vibrational predissociation of the UF6 clusters and a qualitative interpretation of the time-scale, taking the case of the UF6 dimer as an example. We believe that UF6 clusters dissociate on a time-scale of less than
RELA%ATION
- 0.3 us
MONOMERS
IN GROUND STATE
:..: .____ _ _..-. . -.._._ %sae 1 RECOMBINATION
- 6 us
_.._..-”
UF6 CLUSTERS
Fig. 5. Schematic representation of the vibrational predissociation of UF6 clusters and qualitative interpretation of the time-scale, taking the case of the UF6 dimer as an example.
1 ns. The UF6 dimers dissociate to form UF6 monomers populated both in the ground state and in vibrationally excited states. The excited UF6 monomers are subsequently deactivated via vibrational relaxation induced by collisions with carrier gas (argon) atoms. Through this deactivation, the concentration of UF6 monomers in the ground state increases. We estimated the time constant of this vibrational relaxation of excited UF6 monomers in the argon bath to be about 0.3 ps under our experimental conditions, which corresponds to about 20 collisions, using the experimental data reported by Gilbert et al. [3]. It should be noted that the rising part of the curve shown in Fig. 4(a) does not reflect the correct response because the response time of the HgCdTe infrared detector used was as slow as 1 ps.
304
Y. Okada et al./Journal of Molecular Structure 410-411
A competition takes place between the vibrational relaxation mentioned above and the recombination of UF6 monomers produced by vibrational predissociation. The former process increases the number of UF6 monomers in the ground state while the latter process decreases it. The time constant of the decrease in the observed signal intensity in Fig. 4(a) is estimated to be about 4 ps from the portion of the curve after about 2 PCShas elapsed from the Raman laser irradiation. This decrease results from the following two processes: (i) the recombination of UF6 monomers; and (ii) the disappearance of the produced UF6 monomers from the field of vision of the diode laser beam in the supersonic gas flow. The time constant of process (ii) is calculated to be about 12 ps. Using values for the time constants of signal decay in Fig. 4(a) and process (ii), we obtain a time constant of about 6 ps for process (i). Therefore, we found that after the predissociation, the recombination of UFG monomers populated in the ground state took place much more slowly than the vibrational relaxation of excited UF6 monomers.
4. Conclusions Spectra of homogeneous UF6 clusters were observed in a supersonic Lava1 nozzle by carrying out FT-IR measurements. Distinct peaks due to the clusters were present in the range from 610 to 615 cm-’ on the red side of the monomer peaks. In the predissociation of UF6 clusters vibrationally excited by Raman laser radiation (614.8 cm-‘), the observed change in the concentration of UF6 monomers indicated that the recombination of the UF6 monomers took place much more slowly than the vibrational
(1997) 299-304
predissociation of the UF6 clusters and the relaxation of UF6 monomers formed by vibrational predissociation. We found that most of the UF6 monomers formed by the vibrational predissociation can be utilized for laser isotope separation by applying laser radiation to the isotopically selective dissociation of UF6 monomers prior to the slow recombination of these monomers. These experimental results verified that we can apply this vibrational predissociation technique to a high-UF6-concentration reactor for the molecular laser isotope separation of uranium.
References [II R.J. Jensen and A. Sullivan, AIChE Symp. Ser., 78 (1982) 61. 121P. Rabinowitz, A. Stein and A. Kaldor, Opt. Lett., 7 (1982) 212. [31 M. Gilbert, J. M. Weulersse, P. Isnard and G. Salvetat, in Laser Applications in Chemistry, SPIE Proc., 669 (1986) 10. [41 K. Takeuchi, H. Tashiro, S. Kato, K. Midorikawa, T. Oyama, S. Satooka and S. Namba, J. Nucl. Sci. Technol., 26 (1989) 301. [51 Cl. Schweizer, in P. Louvet, P Noe and Soubbaramayer (Eds,), Proc. 2nd Workshop on Separation Phenomena in Liquids and Gases, Versailles, Vol. 2, Centre d’Etudes Nucleaires de Saclay and Cite Scientifique Parts et Technopoles, Ile de France Sud, 1989, p. 653. [61 Y. Kuga, S. Endoh, S. Satooka, Y. Okada, H. Tashiro and K. Takeuchi, J. Nucl. Sci. Technol., 28 (1991) 870. [71 M. Takami, T. Oyama, T. Watanabe, S. Namba and R. Nakane, Jpn. J. Appl. Phys., 23 (1984) L88. [a S. Tanimura, Y. Okada and K. Takeuchi, J. Phys. Chem., 100 ( 1996) 2842. [91 Y. Okada, H. Okamura, J. Onoe, A. Suda, H. Tashiro and K. Takeuchi, Appl. Phys., B63 (1996) 57. [lOI M. Snels and J. Reuss, Chem. Phys. Lett., 140 (1987) 543. [Ill S. Tanimura, Y. Okada and K. Takeuchi, Chem. Phys. Lett., 241 (1995) 62.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 4 1O-41 1 ( 1997) 299-304
Vibrational spectroscopy and predissociation of UF6 clusters in a supersonic Lava1 nozzle Y. Okada*, S. Tanimura,
H. Okamura, A. Suda, H. Tashiro, K. Takeuchi
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako. Saitama 351-01, Japan
Received 26 August 1996; accepted 6 September 1996
Abstract The FI’-IR spectra of clusters of UF6 seeded in argon were measured in a continuous supersonic Lava1 nozzle flow. We observed absorption peaks of UF6 clusters on both the red- and the blue-shifted sides of the monomer peak (around 627 cm-‘) at a temperature below 130 K. We assigned these peaks using calculated results on the frequency shifts and transition strength of the IR spectra of UF6 clusters. Clusters as large as 10 in size were found to be formed under our experimental conditions. Vibrational predissociation of UF6 clusters was observed after a single shot from a Raman laser tuned to a vibrational mode of the UF6 clusters (614.8 cm-‘) in the Lava1 nozzle. We monitored the concentration of UF6 monomers in the ground state with a diode laser spectrometer, and qualitatively interpreted the time-scale of the predissociation of the UF6 clusters and the recombination of UF6 monomers subsequent to the predissociation in the nozzle. After the UF6 clusters had dissociated in less than 1 ns, the excited monomers formed by the dissociation were deactivated with a time constant of about 0.3 ps via vibrational relaxation induced by collisions with carrier gas (argon) atoms. The recombination of UF6 monomers produced by the vibrational predissociation competed with vibrational relaxation. The time constant of the recombination (about 6 ps) was found to be much larger than that of the relaxation. 0 1997 Elsevier Science B.V. Keywords:
Uranium hexafluoride; nozzle; FT-IR spectra
Molecular
clusters;
Vibrational
1. Introduction Over the last two decades, the isotopically
selective multiphoton dissociation of UF6 using 16 pm infrared laser beams has been studied for its application to uranium enrichment [l-6]. When UF6 molecules were supercooled by adiabatic expansion in a supersonic Lava1 nozzle, the rotation-vibration spectrum of UF6 was sufficiently narrow compared with the * Corresponding author. Tel. 00 81 48 467 9306; fax.: 00 81 48 462 4702; e-mail: [email protected].
spectroscopy;
Vibrational
predissociation;
Supersonic
isotope shift (0.6 cm-‘) of UF6 [7]. Thus one could obtain high separation factors while overcoming the problem of the UF6 molecule having a rather small isotope shift. In the adiabatic expansion, condensation of supercooled UF6 molecules takes place when the concentration of UF6 is high. Condensation of the UF6 monomers raises the temperature and causes the isotopic selectivity to deteriorate. There is then an upper limit of the UF6 concentration for a high value of the separation factor to be obtained. If we could eliminate or loosen this restriction, in other words, if we could
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SOO22-2860(96)09567-l
300
Y. Okada et al./Journal
of Molecular
ensure that the separation factor does not decrease even in the case of the UF6 concentration being higher than the upper limit, the process would have the following two advantages: (i) a large value of the product how rate would be possible, since we could increase the UF6 concentration while the separation factor is kept high; (ii) a much higher value of the separation factor would be possible, since we could achieve a much lower temperature without the occurrence of UF6 condensation while keeping the UFh density high.
(4
Structure 410-411
(1997) 299-304
In the present paper, we report the results of experiments performed to observe the vibrational spectra of UFI, clusters, the vibrational predissociation of these clusters using 16 pm Raman laser radiation, and the formation of UF6 monomers populated in the ground state. We demonstrate that one can utilize this procedure for the vibrational predissociation of UF6 clusters as a technique to increase the concentration of UFf, monomers in an irradiation zone for the molecular laser isotope separation of uranium.
SUPERSONIC
LAVAL NOZZLE
UF6 +Ar(Ne) I
UF6 +Ar(Ne) *
spectroscopy
/’
NOZZLE DIMENSION x=12-30 mm, h’=1.2 mm, h=l.2+0.22x
FTIR (0.5cm-1 )
mm, d=lO mm
UF6 (P.Omol%)+Ar
(W I
I
MCT DETECTOR
LAVAL NOZZLE
COMPRESSOR
16 pm RAMAN LASER BEAM
(tuned to Cl branch
peak of UF6 ~3,
614.6 cm-l
v=1+0)
Fig. 1. Schematic diagram of the experimental supersonic Lava1 nozzle.
apparatus:
(a) for FT-IR spectroscopy;
DIODE LASER
(b) for the vibrational predissociation
of UF6 clusters in a
Y. Okada et al./Joumal
of Molecular
Structure 410-411
2. Experimental
nozzle. The collimated diode laser beam passed twice through the nozzle in a region which included the Raman laser irradiation region. When a Raman laser pulse (614.8 cm-‘) was applied to the gas in the nozzle for the dissociation of the UF6 clusters, we observed the time-dependent change in the concentration of UF6 monomers populated in the ground state by monitoring the energy of the infrared diode laser beam transmitted through the Lava1 nozzle.
The experimental set-ups for FT-IR spectroscopy [8] and the vibrational predissociation of UF6 clusters 193 are schematically shown in Fig. l(a) and (b), respectively. For both experiments, a mixture of UF6 and the inert gas argon was continuously fed into a plane-symmetric guided Lava1 nozzle equipped with KC1 windows (optical path length, 10 mm). The geometry of this nozzle is shown also in Fig. l(a). In the spectroscopy experiments [8], we changed the pressure at the inlet of the nozzle in order to investigate the effect of cluster formation on the UF6 partial pressure. The resolution of the FT-IR spectrometer was 0.5 cm-‘. In the predissociation experiments [9], we used a pulsed para-Hz Raman laser beam (614.8 cm-‘) which was tuned to a vibrational mode of the UF6 clusters. The wavenumber of the beam emitted from a diode laser (Laser Photonics L5615) was fixed at the v = 1 - 0 transition (627.7 cm-‘) of the v3 vibrational mode of the 238UF6 monomer present in the Lava1
(a)
3. Results and discussion 3.1. Vibrational spectroscopy
1 (b) P(total) = 2.5 Torr P(UF,) = 0.20 Torr ‘\I G;
= 0.10 Torr
ii
, 660
640
620
600
of UF6 clusters
We measured the IR spectra of UF6 gas at a mole fraction of 0.08 in argon at a position 30 mm downstream from the throat of the Lava1 nozzle. Fig. 2 shows typical spectra observed at relatively high values of the total pressure, P(total). In the range 620-635 cm-‘, the P-, Q- and R-branches of UF6 monomers were observed. On the red-shifted side
P(total) = 1.3 Torr P(UF,)
301
(1997) 299-304
560
G+Vw’yIx”v
,
660
k
J
2,
640
620
-.-A.& 600
I
580
1 / j I (c) P(total) = 3.0 Torr
/,I \,q P(UF,J
iy
= 0.24 Torr
, I
!li ; 7
660
640 Wave
Fig. 2. lT-IR
620 number
P(UF,J
= 5.0 Torr = 0.40 Torr
J+i
d L Q--mwv
*,pd \w++ I
(d) P(total)
1,
600 [cm-’
, 580
1
b.F 660
\ “L, ~w++%-./
d 640 Wave
620 number
spectra of UF6 at a mole fraction of 0.08 in argon at a position 30 mm downstream
600 [cm-’
580
1
from the throat of the Lava1 nozzle.
302
Y. Okada et al.Mournal of Molecular Structure 410-411
(610-615 cm-‘) of the monomer peaks, distinct peaks appeared, and relatively small peaks were present on the blue-shifted side (around 640 cm-‘). With an increase in the total pressure, both the red- and blueshifted peaks became large, and the relative intensity of the monomer peaks decreased. Peaks other than the monomer peaks are thought to be due to homogeneous clusters of UF6, because the temperature was higher than the boiling point of argon at 1 atm, 87 K, and the shapes of the spectra were almost the same whether the carrier gas was argon or neon. When the total pressure was 0.95 Torr or below, we observed no peaks other than those of monomers. We found that the shape of the Q-branch of the UF6 monomers observed from the FT-IR measurements was dependent on the vibrational temperature of the UF6 gas due to the change in the fractional populations in the vibrational states. Under the conditions of formation of the UF6 clusters, we determined the vibrational temperature of the UF6 monomers by comparing the Q-branch observed to that measured when the clustering did not occur at relatively low concentrations and high temperatures, with the vibrational temperatures being confirmed to be equal to the known values of the translational temperatures under thermal equilibrium conditions. In addition, we determined the monomer fractions in the nozzle using values for the integrated absorption area of the monomer in the observed spectra. Fig. 3 shows the dependence of the monomer fraction, X(mono), and the UF6 vibrational temperature, T(vib), on the UF6 partial pressure at a UF,, mole fraction of 0.08 in argon. From Fig. 3(a) it was found that no clustering occurred at UFh pressures below 0.95 Torr, and in the pressure range above 0.95 Torr the monomer fraction decreased due to clustering as the UF6 pressure increases. From Fig. 3(b) we found that the temperature increased with decreasing UF6 monomer fraction. It is reasonable to believe that the increase in temperature was caused by the heat of condensation. We estimated the size of the UF6 clusters shown in Fig. 2 by calculating the frequency shift and transition strength in the IR spectra of the UF6 clusters with an interaction Hamiltonian which contains dipoledipole and dipole-induced dipole effects [ 10,111. The calculations were based on simple models, i.e. a regular triangle for the trimer, a regular tetrahedron for the tetramer, and then closest packed structures for
(1997) 299-304
1.2
0.0
I
I
I
0.1
0.2
0.3
I
240
120 0
P(UF,)
0.4
[ Torr ]
Fig. 3. Dependences on the UF6 partial pressure P(UF,,) of (a) the monomer fraction X(mono) and (b) the vibrational temperature T(vib) of UF6 monomers. (The UF6 mole fraction was 0.08 in argon, and the measurement position was 30 mm downstream from the throat of the Lava1 nozzle.)
the larger clusters. The nearest-neighbour distance was assumed to remain unchanged. Each UF6 molecule was treated as a three-dimensional harmonic oscillator, neglecting Coriolis effects. We used 0.385 D for the Jransition dipole moment of the UF6 molecule, 5.21 A for the nearest-neighbour distance, and 12.3 A” for the polarizability. As a result, we found that clusters mostly with a size of 12 were formed under the conditions shown in Fig. 2(c). 3.2. Vibrational predissociation
of UF6 clusters
We measured the time-dependent change in the concentration of UF6 monomers after a shot from a Raman laser. Fig. 4 shows the responses obtained after averaging 100 pulses of output signals from the HgCdTe infrared detector for monitoring the diode laser beam intensity. The arrow marked on the abscissa indicates the irradiation time of the Raman laser pulses. It should be noted that the increase in detector signal intensity on the ordinate corresponds
Y. Okada et alJJouma1 of Molecular Structure 410-411
303
(1997) 299-304 UF6 CLUSTERS
..1.‘: . ..___ _.. . ..____ _ .... . ....
TIME SCALE
%% OS
PHOTON
t-w 1
VIBRATIONAL
PREDISSOCIATION
EXCITED MONOMERS
COLLISIONS WITH Ar 1 VIBRATIONAL
Fig. 4. Signals measured by the HgCdTe infrared detector: (a) trace obtained with Raman laser radiation present when a mixture of IJF6 and argon was circulated in the Lava1 nozzle; (b) trace obtained with the Raman laser beam blocked when a mixture of UF6 and argon was circulated; (c) trace obtained with Raman laser radiation present when there was no gas in the Lava1 nozzle.
to a decrease in the diode laser beam intensity detected, and vice versa. Fig. 4(a) shows a trace obtained with the Raman laser radiation (fluence, about 20 mJ cm-*) present when a gas mixture of UF6 and argon was circulated in the Lava1 nozzle loop. The signal intensity of the HgCdTe infrared detector increased rapidly during 2 ps after the Raman laser irradiation and subsequently slowly decreased. The trace in Fig. 4(b) shows the response obtained with the Raman laser beam blocked before the nozzle when a mixture of UF6 and argon was circulated. Fig. 4(c) shows the trace obtained with the Raman laser radiation present when there is no gas in the Laval nozzle. Neither of these traces (Fig. 4(b) and 4(c)) shows such a signal increase as that seen in Fig. 4(a). It is reasonable to believe that the signal increase shown in Fig. 4(a) resulted not from artifacts but from an increase in concentration of UF6 monomers populated in the ground state as a result of the vibrational predissociation of the UF6 clusters. Fig. 5 shows a schematic representation of the vibrational predissociation of the UF6 clusters and a qualitative interpretation of the time-scale, taking the case of the UF6 dimer as an example. We believe that UF6 clusters dissociate on a time-scale of less than
RELA%ATION
- 0.3 us
MONOMERS
IN GROUND STATE
:..: .____ _ _..-. . -.._._ %sae 1 RECOMBINATION
- 6 us
_.._..-”
UF6 CLUSTERS
Fig. 5. Schematic representation of the vibrational predissociation of UF6 clusters and qualitative interpretation of the time-scale, taking the case of the UF6 dimer as an example.
1 ns. The UF6 dimers dissociate to form UF6 monomers populated both in the ground state and in vibrationally excited states. The excited UF6 monomers are subsequently deactivated via vibrational relaxation induced by collisions with carrier gas (argon) atoms. Through this deactivation, the concentration of UF6 monomers in the ground state increases. We estimated the time constant of this vibrational relaxation of excited UF6 monomers in the argon bath to be about 0.3 ps under our experimental conditions, which corresponds to about 20 collisions, using the experimental data reported by Gilbert et al. [3]. It should be noted that the rising part of the curve shown in Fig. 4(a) does not reflect the correct response because the response time of the HgCdTe infrared detector used was as slow as 1 ps.
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A competition takes place between the vibrational relaxation mentioned above and the recombination of UF6 monomers produced by vibrational predissociation. The former process increases the number of UF6 monomers in the ground state while the latter process decreases it. The time constant of the decrease in the observed signal intensity in Fig. 4(a) is estimated to be about 4 ps from the portion of the curve after about 2 PCShas elapsed from the Raman laser irradiation. This decrease results from the following two processes: (i) the recombination of UF6 monomers; and (ii) the disappearance of the produced UF6 monomers from the field of vision of the diode laser beam in the supersonic gas flow. The time constant of process (ii) is calculated to be about 12 ps. Using values for the time constants of signal decay in Fig. 4(a) and process (ii), we obtain a time constant of about 6 ps for process (i). Therefore, we found that after the predissociation, the recombination of UFG monomers populated in the ground state took place much more slowly than the vibrational relaxation of excited UF6 monomers.
4. Conclusions Spectra of homogeneous UF6 clusters were observed in a supersonic Lava1 nozzle by carrying out FT-IR measurements. Distinct peaks due to the clusters were present in the range from 610 to 615 cm-’ on the red side of the monomer peaks. In the predissociation of UF6 clusters vibrationally excited by Raman laser radiation (614.8 cm-‘), the observed change in the concentration of UF6 monomers indicated that the recombination of the UF6 monomers took place much more slowly than the vibrational
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predissociation of the UF6 clusters and the relaxation of UF6 monomers formed by vibrational predissociation. We found that most of the UF6 monomers formed by the vibrational predissociation can be utilized for laser isotope separation by applying laser radiation to the isotopically selective dissociation of UF6 monomers prior to the slow recombination of these monomers. These experimental results verified that we can apply this vibrational predissociation technique to a high-UF6-concentration reactor for the molecular laser isotope separation of uranium.
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