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Laser isotope separation of heavy elements by infrared multiphoton dissociation of metal alkoxides
Spcctrochimica
0584 - 8539/90 $3.00 + 0.00
Acto, Vol. 46A, No. 4. pp. 643 - 646. 1990.
0 1990 Pergamon Press plc
Printed in Great Britain.
Laser isotope separation of heavy elements by- infrared multiphoton dissociation of metal alkoxides Y. OKADA,S. KATO, S. SARIOKAand K. TAKEUCHI The Institute of Phisical and Chemical Research, Wako, Saitama 351-01, Japan
convenient since we are examining the possibility of the MLIS of heavy elements such as germanium and zirconium by the IRMPD of these compounds induced by CO2 laser. We investigated the MLIS of germanium and zirconium by TEA CO2 laser-induced IRMPD of tetramethoxide (Ge(OCH3)4) and germanium zirconium tetra-tert-butoxide (Zr[OC(CH3)3]4). These alkoxide compounds are more volatile than the Since these two corresponding other alkoxides. compounds are chemically as unstable as other organometallic compounds under normal laboratory conditions, a careful handling procedure had to be developed to exclude contamination by H20 or 02.
Abstract The molecular laser isotope separation (MLIS) of germanium and zirconium was investigated by TEA CO2 laser-induced infrared multiphoton dissociation (IRMPD) of germanium tetramethoxide (Ge(OCH3)4) and zirconium tetra-tert-butoxide (Zr[OC(CH3)3]4). Ge(OCH3)4 at 0.5 Torr was irradiated with a TEA CO2 laser and the maximum of dissociation was observed around 1040 cm-l. The critical fluence for the complete dissociation was found to be as low as 0.86 J/cm2 near the optimum dissociation wavelength. A separation factor for 70Ge with respect to 74Ge was 1.8 by irradiation with the 9P(28) line (1039 cm-l) at room temperature. When 0.5 Torr of Zr[OC(CH3)3]4 was irradiated
EXPERIMENTAL
at 80 OC with a TEA CO2 laser, the
dissociation maximum was found around 984 cm-l. The critical fluence for the complete dissociation was estimated to be about 0.94 J/cm2 near 984 cm-l. Selective dissociation of specific Zr isotopes was not observed at 80 OC within the detection limit of mass spectrometer.
TECHNIQUES
Gaseous Ge(OCH3)4 has a vapor pressure of 3.5 Ton at room temperature and a strong infrared absorption peak at 1054 cm-l due to the Ge04 configuration [2]. Gaseous Zr[OC(CH3)3]4 has a vapor pressure of 3.0 Torr at 80 OC and a strong infrared absorption peak at 1011 cm-l corresponding to the v(C-0)Zr bands. These compounds are highly reactive with H20 and 02. Furthermore, their chemical stabilities depend on the wall materials of irradiation cells. For irradiation experiments. we made use of brass cells with inner walls mechanically polished. In addition, the careful passivation of the innerwalls with these compounds was carried out repeatedly before irradiation. The laser pulses emitted from the TEA CO2 laser (Lumonics 103-2) were used for irradiation. A typical pulse duration was 100 ns. The pulse energy (h) was measured by a pyroelectric detector before and after irradiation. The transmission of the KBr windows was monitored during the course of the experiment. Typical shot-to-shot variation of the pulse energy was found to be within 3%. Tightly focused irradiation geometry with a short focal length BaF2 lens (f.l.=7.5 cm) was adopted in irradiation experiments for determination of the specific dissociation rate b. The laser pulse was focused on the center of the irradiation cell with a path length of 5 or 10 cm. The conversion of the
INTRODUCTION Germanium has five isotopes of atomic mass 70, 72. 73. 74, and 76. One of the germanium isotopes is used as an important substance in the medical market. Zirconium has five isotopes of atomic mass 90, 91, 92, 94, and 96. In view of the fission power market and the future fusion power market, great benefits are anticipated for the uses of enriched or depleted zirconium. In the MLIS of heavy elements, the selection of working substances is one of the most essential factors. We chose the metal alkoxide compounds (M(OR),; M: metal element, R: alkyl group). The advantages of metal alkoxides as the working substances for MLIS [l] are: (1) Alkoxide compounds are available for most of the heavy elements. (2) Metal alkoxides have relatively high volatility compared with other organometallic compounds. (3) They have strong and isotope-selective infrared absorptions corresponding to the v(C-0)M and v(M0) bands within the emitting regions of CO2. NH3. and para-H2 Raman lasers. The criterion (3) is
643
Y. OKAOAet oz.
644
sample gas after irradiation was measured by an infrared spectrometer to determine the specific dossociation rate b as follows, b=-(l/t)ln(l-X) (1) where X is the overall conversion after t-irradiation pulses. In irradiation experiments for determination of the dissociation probability q and the isotopic selectivity in IRMPD, we used a long focal length BaF2 lens (f.l.=165 cm). Under this mildly focused irradiation geometry, the laser beam within the cell was regarded as virtually unfocused. The dissociation probability q was calculated from the value of b obtained under this geometry. The dissociation selectivity was determined by using a mass spectrometer. RESULTS
AND DISCUSSION
A. Multiphoton Ge(GCH3)4
Dissociation
of
1. The irradlatlon frequency dependence of specific dissociation rate Fig. 1 shows the infrared absorption spectrum and the irradiation frequency dependence of the specific dissociation rate (Multiphoton Dissociation (MPD) spectrum) for Ge(OCH3)4. There was a strong infrared absorption peak at 1054 cm-l for gaseous Ge(GCH3)4. Irradiation of this compound (0.5 Torr) was performed at room temperature using a tightly focused CO2 laser beam. The specific dissociation rates at different pulse energies were normalized to the values at Q=O.35 J using the relation, b a Eol.5 (2). The maximum of dissociation was observed around 1040 cm-l with a red shift of 11 cm-l compared with the absorption peak.
2. Fluence dependence efflclency
of
dissociation
We adopted the fluence dependence dissociation probability, q(a). expressed as @C$
&@@c)n
of the
(3-a)
@Q@c (3-b)
q-_l where Qc is the critical fluence for the complete conversion (q=l). Eqns (3-a) and (3-b) satisfactorily describe the pulse energy (EO) dependence of the specific dissociation rate b under an unfocused irradiation geometry. With unfocused irradiation of 0.5 Torr of Ge(OCH3)4 using the 9P(28) line (1039 cm-l), we observed that the specific dissociation rate b was proportional to the 3.5th power of the pulse energy EO (6 = b3e5). For analyzing experimental data under the condition that the photon absorption of the sample gas is not negligible. we used an analytical model for dissociation efficiency which took account of beam transmittance [3]. Fig. 2 shows a dimensionless reaction volume Y as a function of a dimensionless fluence F for Ge(OCH3)4. Under the condition that F(=@/@,) is below unity, the following relation holds between Y and F. Y=( l/n)( I-Tn)Fn F
1,
, -
,
,
,,1,,
I
I11111.
1039.4 cm4 (T-0.82)
0
no0
1080
1060
1040
Wavenumber
1020
1000
980
lcm?
0.1
1.0 F
Fig. 1. Infrared absorption spectrum (upper side: path legth. 5cm) and specific dissociation rate b (low side) of Gc(GCH3)4. Sample pressure, 0.5 Torr; pulse duration, 1OOns; focal lcgth of lens, 75 mm; aperture diameter, 14 mm; cell The dissociation rate volume 11X cm3. normaliscd at pulse energy __ of 3.5 J.
10
(-‘W&.)
Fig. 2. Dimensionless reaction volume (Y) vs. dimensionless fluence (F) for Ge(OCH3)4. Irradiation wavenumber. 9P(28) sample pressure, 0.5 Torr; pulse ns. Solid line calculated from beam transmittance (T=O.82) analytical model.
1039.4 cm-l; duration, 100 the observed by using an
645
Laserisotopeseparationof heavy elements found to fit the Eqn. (4) well. The critical fluence 4, was then calculated to be as low as 0.86 J/cm2. This value indicated that the IRMPD of Ge(OCH3)4 was quite efficient. 3. Laser
isotope
separation
of Ge
After unfocused irradiation, the isotopic composition of the residual gas sample was measured with a mass spectrometer. Germanium has five isotopes of atomic mass 70, 72, 73. 74, and 76. The separation factor for a specified germanium isotope (mass number: i) with respect to another isotope (mass number: j). Si/j. is defined as the ratio of the correspondmg specific dissociation rates as follows, Svj=bi/bj (6). The specific dossociation rate for a specified isotopic compound, biv is defined by bi=-( l/t)ln( l-Xi) (7) where Xi is the conversion of the specified isotopic compound after t-irradiation pulses. The Xi is easily obtained as. Xi=l-(di’/di)(l-X) (8) where di and di’ indicate the isotopic composition before and after laser irradiation. Fig. 3 shows the separation factors for specified germanium isotopes with respect to 74Ge. A separation factor for 70Ge was 1.8 by irradiation with the 9P(28) line (1039cm- ‘) at room temperature. This is the fist report to our knowledge on the experimental isotope selectivities in laserinduced MPD of germanium tetramethoxide. This value is close to that reported by Tiee and Wittig [4] for the isotope separation of selenium which has mass number similar to germanium. B. Multiphoton Dlssoclatlon Zr[OC(CH3)314 1. Irradiation frequency specific dissociation Liquid
Zr[OC(CH3)3]4
dependence rate
5 u)
-
5
_
0
I
I
O-0
o-0,;
I 70
I - 0
1039.4 ml-’
_ 0
1053.8 on-’ I
\
I 72
I 73
I 74
I 70
Ge mass number
Fig. 3. Separation factors for specified germanium isotopes with respect to 74Ge. Irradiation wavenmbers. 9P(28) 1039.4 cm-l and 9P(12) 1053.9 cm-l; sample pressure, 0.5 Torr; pulse duration, 100 ns; focal length of lens, 165 cm. The values for 76 Ge are not shown because of relatively large experimental error.
of
has a strong infrared
absorption peak corrrespondmg to the v(C-0)Zr
band
(absorption peak=101 1 cm-‘) and the MPD spectrum obtained by irradiation of 0.5 Torr of Zr[GC(CH3)3]4 at 80 OC under tightly focused irradiation geometry. The gas temperature had to be kept relatively high to obtain a sufficient vapor pressure. The dissociation maximum was observed around 984 cm-l within the frequency range we studied. There was a red shift of 27 cm-l compared with the absorption peak. Fluence dependence efficiency
, -
of
at 1002 cm-l [5]. Fig. 4 shows the infrared absorption spectrum of gaseous Zr[OC(CH3)3]4
2.
2.0
of
dlssoclaton
We investigated the fluence dependence of the dissociation probability 4. Unfocused irradiation was conducted for 0.5 Torr of Zr[OC(CH3)3]4 at 80
Fig. 4. Infrared absorption spectrum (upper side; path legth, 5 cm) and specific dissociation rate b (low side) of Zr[OC(CH3)3]4. Sample pressure, 0.5 Ton; pulse duration, 100 ns; focal length of lens, 75 mm; a true diameter, 14 mm; cell volume, 86 cm r . The dissociation rate normalized at pulse energy of 0.15 J.
OC. Fig. 5 indicates the fluam dependence of q. The data were found to satisfy Eqn. (3-a) with 4,(@/@,)4.3. The critical fluence 45, was found to be as low as 0.94 J/cm2. This value indicates, as in the case of Ge(OCH3)4 dissociation, that the IRMPD of Zr[GC(CH3)3]4 occurred very efficiently.
Y. @‘ADA et al.
646
extremely high. The separation factor for 70Ge with respect to 74Ge was 1.8 by irradiation with the 9P(28) line (1039 cm-‘) at room temperature. This value showed the possibility of laser isotope separation of Ge by TEA CO2 laser-induced IRMPD. By irradiation of Zr[GC(CH3)3]4 with a TEA CO2 laser, the dissociation maximum was observed around 984 cm-l. This compound dissociated efficiently at low fluences. Isotope selective dissociation of Zr[OC(CH3)3]4 was not observed at the temperature we studied. Reduction of gas temperature by supersonic expansion combined with multi-frequency IR excitation may enhance the separation factor significantly. REFERENCES
,s.
Fig. 5. Dissociation probability (4) vs. fluence (‘P) for Zr[GC(CH3)3]4. Irradiation wavenumber, lOR(34) 984.4 cm-l; sample pressure, 0.5 Ton; pulse duration, 1OOns; focal length of lens, 165 cm. 3. Laser
isotope
separation
of Zr
The isotopic composition of the residual gas sample was measured similarily with a mass spectrometer. Selective dissociation of any Zr isotopic compound was not observed at 80 OC within the detection limit if the mass spectrometer. It was partly because the temperature was so high that the hot band absorption occurred. Contrary to the case of MLIS of Ge. zirconium compounds do not have enough volatility at ambient temperature partly because of the large mass number of the central Zr atom. Therefore, reduction of gas temperature by supersonic expansion may be needed. Lahoda of Westinghouse Research & Development [6] reported the isotopic selectivity of 1.9 for 90Zr/91Zr by irradiation at 993 -l of similar compound, ir;bC(CH3)2C2i5]4:ezled in a molecular beam. A significant enhancement of the separation factor is expected also for zirconium tetra-tert-butoxide at such low temperautre where the bands disappear, and vibrational structure is sharper. CONCLUSION In order to study the molecular laser isotope separation of germanium and zirconium, we investigated the characteristics of infrared multiphoton dissociation of germanium tetramethoxide (Ge(GCH3)4) and zirconium tetra-tertbutoxide (Zr[OC(CH3)3]4). By irradiation of Ge(OCH3)4 with a TEA CO2 laser. the dissociation maximum efficiency
was observed around 1040 cm-l. of dissociation of this compound
The was
[l] D. C. Bradley, R. C. Mehrotra and D. P. Gaur. Metal Alkoxides. Academic Press, New York (1978). [2] .O. H. Johnson and H. E. Fritz, J. Am. Chem. Sot. 75. 718 (1953). [3] S. Kato, J. Ichikawa. T. Ishii and K. Takeuchi, to be submitted. [4] J. J. Tiee and C. Wittig, J. Chem. Phys. 69, 4756 (1978). [5] C. T. Lynch, K. S. Maxdiyasni. J. S. Smith snd W. J. Crawford, Anal. Chem. 36. 2332 (1964). [6] E. Lahode, Proc. Second Isotope Separation Workshop, 74 (1985).
0584 - 8539/90 $3.00 + 0.00
Acto, Vol. 46A, No. 4. pp. 643 - 646. 1990.
0 1990 Pergamon Press plc
Printed in Great Britain.
Laser isotope separation of heavy elements by- infrared multiphoton dissociation of metal alkoxides Y. OKADA,S. KATO, S. SARIOKAand K. TAKEUCHI The Institute of Phisical and Chemical Research, Wako, Saitama 351-01, Japan
convenient since we are examining the possibility of the MLIS of heavy elements such as germanium and zirconium by the IRMPD of these compounds induced by CO2 laser. We investigated the MLIS of germanium and zirconium by TEA CO2 laser-induced IRMPD of tetramethoxide (Ge(OCH3)4) and germanium zirconium tetra-tert-butoxide (Zr[OC(CH3)3]4). These alkoxide compounds are more volatile than the Since these two corresponding other alkoxides. compounds are chemically as unstable as other organometallic compounds under normal laboratory conditions, a careful handling procedure had to be developed to exclude contamination by H20 or 02.
Abstract The molecular laser isotope separation (MLIS) of germanium and zirconium was investigated by TEA CO2 laser-induced infrared multiphoton dissociation (IRMPD) of germanium tetramethoxide (Ge(OCH3)4) and zirconium tetra-tert-butoxide (Zr[OC(CH3)3]4). Ge(OCH3)4 at 0.5 Torr was irradiated with a TEA CO2 laser and the maximum of dissociation was observed around 1040 cm-l. The critical fluence for the complete dissociation was found to be as low as 0.86 J/cm2 near the optimum dissociation wavelength. A separation factor for 70Ge with respect to 74Ge was 1.8 by irradiation with the 9P(28) line (1039 cm-l) at room temperature. When 0.5 Torr of Zr[OC(CH3)3]4 was irradiated
EXPERIMENTAL
at 80 OC with a TEA CO2 laser, the
dissociation maximum was found around 984 cm-l. The critical fluence for the complete dissociation was estimated to be about 0.94 J/cm2 near 984 cm-l. Selective dissociation of specific Zr isotopes was not observed at 80 OC within the detection limit of mass spectrometer.
TECHNIQUES
Gaseous Ge(OCH3)4 has a vapor pressure of 3.5 Ton at room temperature and a strong infrared absorption peak at 1054 cm-l due to the Ge04 configuration [2]. Gaseous Zr[OC(CH3)3]4 has a vapor pressure of 3.0 Torr at 80 OC and a strong infrared absorption peak at 1011 cm-l corresponding to the v(C-0)Zr bands. These compounds are highly reactive with H20 and 02. Furthermore, their chemical stabilities depend on the wall materials of irradiation cells. For irradiation experiments. we made use of brass cells with inner walls mechanically polished. In addition, the careful passivation of the innerwalls with these compounds was carried out repeatedly before irradiation. The laser pulses emitted from the TEA CO2 laser (Lumonics 103-2) were used for irradiation. A typical pulse duration was 100 ns. The pulse energy (h) was measured by a pyroelectric detector before and after irradiation. The transmission of the KBr windows was monitored during the course of the experiment. Typical shot-to-shot variation of the pulse energy was found to be within 3%. Tightly focused irradiation geometry with a short focal length BaF2 lens (f.l.=7.5 cm) was adopted in irradiation experiments for determination of the specific dissociation rate b. The laser pulse was focused on the center of the irradiation cell with a path length of 5 or 10 cm. The conversion of the
INTRODUCTION Germanium has five isotopes of atomic mass 70, 72. 73. 74, and 76. One of the germanium isotopes is used as an important substance in the medical market. Zirconium has five isotopes of atomic mass 90, 91, 92, 94, and 96. In view of the fission power market and the future fusion power market, great benefits are anticipated for the uses of enriched or depleted zirconium. In the MLIS of heavy elements, the selection of working substances is one of the most essential factors. We chose the metal alkoxide compounds (M(OR),; M: metal element, R: alkyl group). The advantages of metal alkoxides as the working substances for MLIS [l] are: (1) Alkoxide compounds are available for most of the heavy elements. (2) Metal alkoxides have relatively high volatility compared with other organometallic compounds. (3) They have strong and isotope-selective infrared absorptions corresponding to the v(C-0)M and v(M0) bands within the emitting regions of CO2. NH3. and para-H2 Raman lasers. The criterion (3) is
643
Y. OKAOAet oz.
644
sample gas after irradiation was measured by an infrared spectrometer to determine the specific dossociation rate b as follows, b=-(l/t)ln(l-X) (1) where X is the overall conversion after t-irradiation pulses. In irradiation experiments for determination of the dissociation probability q and the isotopic selectivity in IRMPD, we used a long focal length BaF2 lens (f.l.=165 cm). Under this mildly focused irradiation geometry, the laser beam within the cell was regarded as virtually unfocused. The dissociation probability q was calculated from the value of b obtained under this geometry. The dissociation selectivity was determined by using a mass spectrometer. RESULTS
AND DISCUSSION
A. Multiphoton Ge(GCH3)4
Dissociation
of
1. The irradlatlon frequency dependence of specific dissociation rate Fig. 1 shows the infrared absorption spectrum and the irradiation frequency dependence of the specific dissociation rate (Multiphoton Dissociation (MPD) spectrum) for Ge(OCH3)4. There was a strong infrared absorption peak at 1054 cm-l for gaseous Ge(GCH3)4. Irradiation of this compound (0.5 Torr) was performed at room temperature using a tightly focused CO2 laser beam. The specific dissociation rates at different pulse energies were normalized to the values at Q=O.35 J using the relation, b a Eol.5 (2). The maximum of dissociation was observed around 1040 cm-l with a red shift of 11 cm-l compared with the absorption peak.
2. Fluence dependence efflclency
of
dissociation
We adopted the fluence dependence dissociation probability, q(a). expressed as @C$
&@@c)n
of the
(3-a)
@Q@c (3-b)
q-_l where Qc is the critical fluence for the complete conversion (q=l). Eqns (3-a) and (3-b) satisfactorily describe the pulse energy (EO) dependence of the specific dissociation rate b under an unfocused irradiation geometry. With unfocused irradiation of 0.5 Torr of Ge(OCH3)4 using the 9P(28) line (1039 cm-l), we observed that the specific dissociation rate b was proportional to the 3.5th power of the pulse energy EO (6 = b3e5). For analyzing experimental data under the condition that the photon absorption of the sample gas is not negligible. we used an analytical model for dissociation efficiency which took account of beam transmittance [3]. Fig. 2 shows a dimensionless reaction volume Y as a function of a dimensionless fluence F for Ge(OCH3)4. Under the condition that F(=@/@,) is below unity, the following relation holds between Y and F. Y=( l/n)( I-Tn)Fn F
1,
, -
,
,
,,1,,
I
I11111.
1039.4 cm4 (T-0.82)
0
no0
1080
1060
1040
Wavenumber
1020
1000
980
lcm?
0.1
1.0 F
Fig. 1. Infrared absorption spectrum (upper side: path legth. 5cm) and specific dissociation rate b (low side) of Gc(GCH3)4. Sample pressure, 0.5 Torr; pulse duration, 1OOns; focal lcgth of lens, 75 mm; aperture diameter, 14 mm; cell The dissociation rate volume 11X cm3. normaliscd at pulse energy __ of 3.5 J.
10
(-‘W&.)
Fig. 2. Dimensionless reaction volume (Y) vs. dimensionless fluence (F) for Ge(OCH3)4. Irradiation wavenumber. 9P(28) sample pressure, 0.5 Torr; pulse ns. Solid line calculated from beam transmittance (T=O.82) analytical model.
1039.4 cm-l; duration, 100 the observed by using an
645
Laserisotopeseparationof heavy elements found to fit the Eqn. (4) well. The critical fluence 4, was then calculated to be as low as 0.86 J/cm2. This value indicated that the IRMPD of Ge(OCH3)4 was quite efficient. 3. Laser
isotope
separation
of Ge
After unfocused irradiation, the isotopic composition of the residual gas sample was measured with a mass spectrometer. Germanium has five isotopes of atomic mass 70, 72, 73. 74, and 76. The separation factor for a specified germanium isotope (mass number: i) with respect to another isotope (mass number: j). Si/j. is defined as the ratio of the correspondmg specific dissociation rates as follows, Svj=bi/bj (6). The specific dossociation rate for a specified isotopic compound, biv is defined by bi=-( l/t)ln( l-Xi) (7) where Xi is the conversion of the specified isotopic compound after t-irradiation pulses. The Xi is easily obtained as. Xi=l-(di’/di)(l-X) (8) where di and di’ indicate the isotopic composition before and after laser irradiation. Fig. 3 shows the separation factors for specified germanium isotopes with respect to 74Ge. A separation factor for 70Ge was 1.8 by irradiation with the 9P(28) line (1039cm- ‘) at room temperature. This is the fist report to our knowledge on the experimental isotope selectivities in laserinduced MPD of germanium tetramethoxide. This value is close to that reported by Tiee and Wittig [4] for the isotope separation of selenium which has mass number similar to germanium. B. Multiphoton Dlssoclatlon Zr[OC(CH3)314 1. Irradiation frequency specific dissociation Liquid
Zr[OC(CH3)3]4
dependence rate
5 u)
-
5
_
0
I
I
O-0
o-0,;
I 70
I - 0
1039.4 ml-’
_ 0
1053.8 on-’ I
\
I 72
I 73
I 74
I 70
Ge mass number
Fig. 3. Separation factors for specified germanium isotopes with respect to 74Ge. Irradiation wavenmbers. 9P(28) 1039.4 cm-l and 9P(12) 1053.9 cm-l; sample pressure, 0.5 Torr; pulse duration, 100 ns; focal length of lens, 165 cm. The values for 76 Ge are not shown because of relatively large experimental error.
of
has a strong infrared
absorption peak corrrespondmg to the v(C-0)Zr
band
(absorption peak=101 1 cm-‘) and the MPD spectrum obtained by irradiation of 0.5 Torr of Zr[GC(CH3)3]4 at 80 OC under tightly focused irradiation geometry. The gas temperature had to be kept relatively high to obtain a sufficient vapor pressure. The dissociation maximum was observed around 984 cm-l within the frequency range we studied. There was a red shift of 27 cm-l compared with the absorption peak. Fluence dependence efficiency
, -
of
at 1002 cm-l [5]. Fig. 4 shows the infrared absorption spectrum of gaseous Zr[OC(CH3)3]4
2.
2.0
of
dlssoclaton
We investigated the fluence dependence of the dissociation probability 4. Unfocused irradiation was conducted for 0.5 Torr of Zr[OC(CH3)3]4 at 80
Fig. 4. Infrared absorption spectrum (upper side; path legth, 5 cm) and specific dissociation rate b (low side) of Zr[OC(CH3)3]4. Sample pressure, 0.5 Ton; pulse duration, 100 ns; focal length of lens, 75 mm; a true diameter, 14 mm; cell volume, 86 cm r . The dissociation rate normalized at pulse energy of 0.15 J.
OC. Fig. 5 indicates the fluam dependence of q. The data were found to satisfy Eqn. (3-a) with 4,(@/@,)4.3. The critical fluence 45, was found to be as low as 0.94 J/cm2. This value indicates, as in the case of Ge(OCH3)4 dissociation, that the IRMPD of Zr[GC(CH3)3]4 occurred very efficiently.
Y. @‘ADA et al.
646
extremely high. The separation factor for 70Ge with respect to 74Ge was 1.8 by irradiation with the 9P(28) line (1039 cm-‘) at room temperature. This value showed the possibility of laser isotope separation of Ge by TEA CO2 laser-induced IRMPD. By irradiation of Zr[GC(CH3)3]4 with a TEA CO2 laser, the dissociation maximum was observed around 984 cm-l. This compound dissociated efficiently at low fluences. Isotope selective dissociation of Zr[OC(CH3)3]4 was not observed at the temperature we studied. Reduction of gas temperature by supersonic expansion combined with multi-frequency IR excitation may enhance the separation factor significantly. REFERENCES
,s.
Fig. 5. Dissociation probability (4) vs. fluence (‘P) for Zr[GC(CH3)3]4. Irradiation wavenumber, lOR(34) 984.4 cm-l; sample pressure, 0.5 Ton; pulse duration, 1OOns; focal length of lens, 165 cm. 3. Laser
isotope
separation
of Zr
The isotopic composition of the residual gas sample was measured similarily with a mass spectrometer. Selective dissociation of any Zr isotopic compound was not observed at 80 OC within the detection limit if the mass spectrometer. It was partly because the temperature was so high that the hot band absorption occurred. Contrary to the case of MLIS of Ge. zirconium compounds do not have enough volatility at ambient temperature partly because of the large mass number of the central Zr atom. Therefore, reduction of gas temperature by supersonic expansion may be needed. Lahoda of Westinghouse Research & Development [6] reported the isotopic selectivity of 1.9 for 90Zr/91Zr by irradiation at 993 -l of similar compound, ir;bC(CH3)2C2i5]4:ezled in a molecular beam. A significant enhancement of the separation factor is expected also for zirconium tetra-tert-butoxide at such low temperautre where the bands disappear, and vibrational structure is sharper. CONCLUSION In order to study the molecular laser isotope separation of germanium and zirconium, we investigated the characteristics of infrared multiphoton dissociation of germanium tetramethoxide (Ge(GCH3)4) and zirconium tetra-tertbutoxide (Zr[OC(CH3)3]4). By irradiation of Ge(OCH3)4 with a TEA CO2 laser. the dissociation maximum efficiency
was observed around 1040 cm-l. of dissociation of this compound
The was
[l] D. C. Bradley, R. C. Mehrotra and D. P. Gaur. Metal Alkoxides. Academic Press, New York (1978). [2] .O. H. Johnson and H. E. Fritz, J. Am. Chem. Sot. 75. 718 (1953). [3] S. Kato, J. Ichikawa. T. Ishii and K. Takeuchi, to be submitted. [4] J. J. Tiee and C. Wittig, J. Chem. Phys. 69, 4756 (1978). [5] C. T. Lynch, K. S. Maxdiyasni. J. S. Smith snd W. J. Crawford, Anal. Chem. 36. 2332 (1964). [6] E. Lahode, Proc. Second Isotope Separation Workshop, 74 (1985).