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tritium isotope separation by CO2 laser-induced multiphoton dissociation of CTF3
Volume
82. number
TRITRJM
CHEMICAL
I
ISOTOPE
MAKIDE.
15 August
LETTERS
1981
SEPARATION
BY CO, LASER-INDUCED Yoshihiro
PHYSICS
MULTJPHOTON
Satoru HAGIWARA,
DISSOCIATION
OF CTF,
Takeshi TOMINAGA
Deparrment of Chemrstry. Faculty of Science, The University of Tokyo, Hongo. Tokyo 113. Japan
and
Kazuo TAKEUCHI and Ryohel NAKANE 77ze Institute of Physrcat and G’zemzcat Research, Wake-shi, Saitama 3.51, Japatl Recerved
8 June 1981
Isotope separatron of trrtmm at ppm concentration level was achieved by COa laser-induced multiphoton dissociation of CTI’s rn CHFa with single-step separation factors exceeding 500. The effects of laser frequency, pulse energy, pulse duration, irradiation geometry, tritium concentration, sample pressure, and buffer gas were investigated_
,
l_ Introduction Extensive studres of infrared (IR) laser-induced multrphoton dissocration (MPD) dcne during last several years have indicated remarkable capabilities in laser Isotope separation [l--3] _Application of this method to deuterium separation !ras been proposed recently [4-6]_ A novel isotope separation technique for tritiurn is needed with the development of nuclear fuel reprocessing plants in Japan. We have reported for the first time successful isotope separation of tritium by CO, laser-induced MPD of a tritium compound [7] _ The compound used was trifluoromethane-T (CTF,), which proved to be the best in the tritiated halomethanes we surveyed extensively. It unexpectedly coincided wrth the compound CDF,, which was proposed by Marling et al. as one of the suitable compounds for deuterium separation [6], although our approach was substantrally different because large isotope shifts were expected in absorption bands of isotopic molecules and no IR spectra of trltiated halomethanes had been observed_ Very recently, Herman and lMarhng [!3] reported the IR spectrum of highly-radioactive CTF3 implying an idea toward laser tritium separation after our successful experimental attempt with the same compound 18
0 009-26
(CTF3) [7] _Their IR spectrum of CTF3 confirmed our results although we started the MPD experiments on the bdsis of the calculated fundamental frequencies of CTF, [9] : the reported IR absorption peak (1077 -‘) [8] is fairly close to our observed peak for ED of CTF, (1075 cm-l) [7] _ In the course of our experiments on the isotopically-selectrve photodrssociation of CTF,, the mixture CTF, f CHF, has proved to be a unique and ideal system to study both the behavior of a trace component by ultrasensitive detection techniques and the behavior of a major component without being disturbed by the excitation or decomposition of the trace component_ In this paper we present the dependencies of the dissociation rates and selectivity of CTF,/CHF, upon the laser frequency, pulse energy, pulse duration, irradiation geometry, tritium concentration, sample pressure, and kind of buffer gas.
2. Experimental Trifluoromethane-T (CTF3) was prepared by the hydrogen isotope exchange of CHF3 with tritiated water in the presence of dimethyl sulfoxide and sodium
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Volume 82, number 1
CHEMICAL. PHYSICS LETTERS
hydroxide_ The procedure is similar to the methods reported previously [7,10] _The concentration of CTF3 obtained was 6 ppm (~8 pCi/rnl gas) or 0.2 ppm (~0.3 pCi/ml); the latter was mainly used throughout the experiment. TEA CO2 lasers (mainly Lumonics model 103-2, and Lumonics 821 for the experiment in section 3.8) were used for irradiation of the sample mixture (CTF3 and CHF,, with/without buffer gas) in a photolysis cell. A laser pulse consisted of a 100 ns width splice (when a mixture of CO, and He was used as a laser gas) followed by a 1 PCStail (when N2 was added to the laser gas: CO2 + N2 + He). Laser pulses of 0.3-l .3 J at 1 (or 2) Hz repetition rate passed through an aperture (14 mm $) were focused into the center of the photolysis cell (20 mm 9, 100 mm long, or 450 mm long for studies in section 3.8) by means of a BaF, lens (75 mm in focal length, or 380 mm for section 3.8). A few CaF, plates were placed before the lens to attenuate pulse energy. The pulse energy for the irradiation was measured at before/behind the cell and before/after the irradiation with a pyroelectric detector (Lumonics 20D) cahbrated to Scientech 364 disk calorimeter_ The laser irradiated sample was condensed in a sample loop cooled in liquid N2 (and the buffer was pumped out), and then injected into a specially-designed radio-gas chromatograph equipped with a TCD and a proportional counter. The amounts of CHF3 and CTF3 were determined from the TCD peak area and the total counts of radioactive peak, with reproducibilities within 0.3% and OS%, respectively.
3. Results and discussion 3.1. Chemical change by laser irradiation Trifluoromethane excited by IR multiphoton absorption decomposed to difluorocarbene (CF& and hydrogen fluoride 17,101, similarly to the thermal decomposition [ 111. The CF2 radicals recombined to form stable tetrafluoroethylene (GF4), and hydrogen fluoride (TF + HF) was eliminated from the gas phase by reaction with the glass wall.
15 August 1981
3.2. Dependence of dissociationand specific radioactivityon number of pulses When the sample mixture (CTF, + CHF,) was irradiated with the 000 l-0200 R( 14) line at 1074.6 cm-1 corresponding to the maximum dissociation rate of CTF3 (see section 3.3), the unchanged fraction of CTF, (1 - X,) decreased exponentially with increasing pulse number showing a pseudo-first order decay over a wide range of conversion (XT): 1 - X, = exp(-+n), where n is the number of pulses and dT denotes the specific dissociation rate of CTF,: dT = --In (1 - XT)/n. Decomposition of CHF~ also followed the pseudo-first order decay when the conversion (dH) was kept below 10%. 1 - XR = exp(-dHn), or dH = -ln(l - X&r, where dH is the specific dissociation rate of CHF, _ The ratio of both dissociation rates gives the selectivity (ST/R) of the dissociatron: ST/R = dT/dH _The specific radioactivity of residual gas, therefore, decreased exponentially with n. Since the amounts of CTF, present in the sample are extremely small, even the complete decomposition of CTF, scarcely affects the overall chemical composition or total pressure of the sample. However, the dissociation of CHF, alters the composition and the total pressure, which in turn affect the specific dissociation rates considerably as will be discussed in section 3.6. The converted fraction of CTF, (XT) was selected between 0.1 and 0.9 in accordance with the ST/B value to obtain better accuracy in quantitative measurements, where X, was usually kept between 0.02 and O-l_ When the ST/R was extremely high (e.g. 500) dT and dH were measured separately in two identical samples but with a different number irradiation pulses n. 3.3. Frequency dependence Specific dissociation rates of CTF3 (dT) and CHF, (dH) were measured against several OO”1-O2oO transition lines of a CO, laser (model 103-2, without N2 in the laser gas, 1 Hz). Fig. la shows the observed vaiues of dT and & at the maximum pulse energy available for each line shown in fig. lb. The maximum d, was obtained at the R(14) line (1074.6 cm-l). Fig. lc shows the dissociation rates normalized to the pulse energy of R(14) (dk and d;f) according to the dependence of dT and dH on the pulse energy (see
section 3.4). The MPD spectrum thus obtained (fig. Ic) 19
Volume 82, number1
1
1
I
I
,
was compared with the linear-absorption LR spectrum of CTF, (fig. id)_ The maximum of the MPD spectrum shifted slightly to the red. The MPD peak shape trailed to the red side and fell sharply on the blue side. Similar phenomenon has been observed and appears to be characteristic of MPD j12,13]. The steep increase of dk on the blue side may be related to the Increase of d;f which corresponds to the red-side tail of the strong v2 absorption band of CHF, (C-F stretching mode) at 1141 cm-l. Since dk decreases with decreasing wavenumber, the selectivity ST/~ was improved on the red side of R(14). However, both the avaIlable maximum pulse energy and d;’ value decrease on the red side, and there are no laser lines between the P and R branches (1060-1068 cm-‘). Therefore, if the better ST/H values are primarily intended, the lines on the red side should be investigated at the cost of the dissociation rate.
L
100 -
YZP-branch 60
-
20
-
100
-
lr\
b 60
-
40
-
20
-
0
‘\ CHFs
(d)
_
(cl
-
CTF3
4 A
00 -
0
3.4. Dependence
l.__
I
I
1090
1000
1070
1050
1060 C cm-’
1040
)
0
80
(a) CTF3
000
100 F
20
0
l
0
1 1090
0
l* I 1080
. .
I
0
CHF3
1070 Wavenumber
.
.
1060 C cm-’
1050
I
-
1040
1
Fig. 1. Frequencydependenceof multiphotondissociationof CTFa presentin CHF3. Sample: 5 Ton CHFa containing0.2 ppm CJJF3.(a) Observeddissociationratesof CJXa (do) and CHFa (dH) for each laser line with availablepulseenergy (Eo) as shown in (b). Asteriskin (%a)indicatesabsenceof laser linesin the region. (c) Dissociationratesof CTFa (dk) and CHFa (dir>normalizedto the same pulse enerm (0.6 J). (d) LinearabsorptionIR spectrumof Cl& (from ref. 181) for comparison.
20
of dT and dH on pulse energy and
pulse duration
I Wavenumber
60
15 August 1981
CHEMICAL PHYSICS LETTERS
Table 1 shows the results of the dissociation of CTF, and CHF, on irradiation with different pulse energy of the R( 14) line (with/without N2 added in the laser gas)_ Specific dissociation rates of resonant CTF, (dT) and non-resonant CHF, (dH) increased with the pulse energy (Eo): dT aE$6io-2 and d, a E$oko-2. While both dissociation rates increased with increasing Eo, the selectivity ST/H decreased_ Dissociation of CTF3 may be considered to follow the 3/2-power law [ 14,151. Since d, corresponds to the fractional volume of the dissociation zone (V) where the laser fluence (Q) exceeds the critical value for dissociation (‘k,) in CTF, , dT should increase with the 3/2-power of E, if the zone is approximated by the double-cone shape (V a ,$i2). Assuming that the dksociation probability (4) outside the zone (G?< a& is negligible, the value of Gc for CTF, is calculated as ~60 J/cm2 at 5 Torr, and 100 J/cm2 at 1 Torr *_ Similarly, +c for CHF, is calculated as 250 J/cm2 at 5 TOIT according to the double-cone geometry_ Since the laser fluence around the focal spot is close to the %C value for CHF, , however, the observed secondpower dependence of d, cannot be explained by * Assumingthat q = (Q/Q& higher than these values.
(when 4 < Qd), QC should be
Volume 82, number 1
CHEMICAL
PHYSICS
15 August 1981
LETTERS
Table 1 Dependence of dissociation of CTFs and CHFs on laser pulse energy and pulse duration at the OO” 1-02°0 R(14) line(1074.6
m-l) a) --_Run number 1 2 3 4 5 6 7 8 9 10
Laser gas
1 -XTb)
1 -XHb)
dTb) (1 OS5 pulse-r)
dHb)
E” (J) b,
;puIse)
CO, + He
0.26 0.32 0.38 0.47 0.57
10000 7500 5700 4050 3000
0.812 0.759 0.756 0.794 0.770
0.980 0.981 0.979 0.978 0.973
2.1 3.7 4.9 5.7 8.7
0.20 0.26 0.38 0.54 0.92
COz+Nz+He
0.45 0.55 0.70 1.03 1.26
2900 2000 1500 1000 800
0.905 0.916 0.910 0.871 0.854
0.978 0.975 0.970 0.952 0.946
3.4 4.4 6.3 13.8 19.7
0.77 1.3 2-O 5.0 6.9
(1 O-’ pulse-‘)
a) The experimental conditions are as follows: sample 5 Torr CHFs containing 0.2 ppm CfF3; laser, Lumonics model 103-2; BaF2 lens, 75 mm focal length; photolysis cell, 10 cm long and 45 ml. b)The estimated errors in Eo, 1 - XT, 1 - XH, dT and dH areapproximately k2.5%, &O-7%, cO_4%, '7% and klS%: respectively.
either the double-cone geometry model [ 14,151, or the modified “dog-bone” geometry model proposed by Speiser and Jortner [ 161. An improved model and deconvolution procedure to obtain the intrinsic correlation between G?and q will be proposed elsewhere The addition of N2 into the laser gas increases the pulse energy by the contribution of the 1 PS tail part of the pulse. When dT or dH for such longer pulses (columns 6-10 in table 1) was compared with the data for 100 ~LSpuises, dT was roughly haIved and dH increased by %4O% for the same pulse energy. The decrease in dT may be caused by the lowering of the peak intensity with the same fluence and/or by the increase in the probability of collisional de-excitation of activated CTF, during the long Iaser p&e. The increase in d, may be due to the collisional dissociation of pre-excited CHF3 molecules in the long pulse duration as mentioned later, while energy transfer from the resonant CTF3 cannot affect the dH value. 3.5. Tn%iumconcentration The dependence of d, and dH on the CTF, concentrations was measured over a wide range of about three orders ofmagnitude as well as in neat CHF3 containing no CTF, (fig_ 2). Since dT does not ch_ange with the CTF, concentration, it is confirmed that there is no interaction between CTF, molecules in the dissociation process at ppm level concentrations.
1
I
IrId
0
I
I
,llllll
CTFs
Fig.
I
I
,Ilrrll
01
001
concentratior,
I
I
,lrrrrl
1
1
I
10 C pprn 1
2. Effect of CTF3 concentration on specific dissociation
ratesof CIF3 (0: dT) and CHFJ (0: dH): 0 indicates dH of neat CHFa (containing no CTF3). CHF3 pressure: constant at 5 Torr; laser irradiation- R(l4) line, 100 ns, 0.6 J pulse-
The dH value also remained constant throughout the concentration range and agreed well with the dH value in neat CHF3, indicating that the dissociation of non-resonant CHF, moiecules happens independently of CTF, _Hence the decomposition of CHF3 which decreases the selectivity is not due to energy transfer from excited CTF3 to non-resonant CHF,, but rather to multiphoton absorption and dissociation by CHF, itself. 3.6. hXs7.0Veffi?Cton d-p dH, a?zdS~j~ The d, and d,
values both increased with increas21
I,,,
,
,
,
1,
L,L,
I
(Ar
01
02
15 August 1981
CHEMICAL PHYSICS LETTERS
Volume 82, number 1
05
1
,
,
L
1
,I’,
,
added 1
2
5
10
20
Fig. 3. Dependence of dissociation rates of CTF3 (dT) and CHF, (dH) on the pressure of trifluoromethane (containing 0.2 ppm Cl?F,) with and without argon. -oand -m-: dT and dH in trfluoromethane without argon, respectively_ --n--and - -A- -: dT and dH intrif%rorometbane/argonmix-
3.7. Effect of argon and oxygen as buffi When buffer gas was mixed into the sample, the MPD behavior of CTF, and CHF, changed drastically. With the addition of argon to trifhroromethane with the total pressure kept constant at 10 Torr (fig. 3, a and A), dT remained at the same value with that in 10 Torr trifluoromethane without buffer in spite of the decreased trifluoromethane pressure, while dH showed no change or the same value with d, at each trifluoromethane pressure. In another experiment, argon at different pressure was added to 1 Torr trifluoromethane. When dT and d, were plotted against total pressure (fig. 4, a and Aj, dT changed almost the same as in neat trifluoromethane, while dH scarcely changed in the pressure range below 10 Torr. Both series of experiments indicate that dT depends only on the total pressure irrespective of the collisional partner and dH depends only on the partial pressure of CHF3 itself. These results strongly support the drs-
ture, respectively, with total pressurekept constant at 10 Torr Wfluoromethane/argon = l(Torr)/S(Torr), Z/8,5/5, or 10/O). Laser irradiation: R(14) line at 1074.6 cm-r, 100
CAr
added)
_
b'
-
ns, 0.6 J pulse. *¶I' _,_QH
ing pressure of trifluoromethane (fig. 3, o and a)_ Since d, increased more rapidly than dT , the selectivity ST/R decreased with increasing pressure. The increase in dissociation rate of resonant CTF8 with Increasing pressure may be explained in terms of the collision-induced rotational relaxation of vibrationally ground-state molecules. The collision frequency of CTF, with major component CHF, can be estimated as about 5 times at 5 Torr in the 100 ns pulse duration_ Since CHF, decomposes independently of CTF,, the strong pressure dependence of dH shouldbe attributed to the interaction between CHF8 molecules_ This dependence can be understood by assuming that pre-excited CHF, molecules which have been excited only insufficiently for unimolecular dissociation collide with each other to induce dissociation of one of the two molecules.
22
"T;_p*H ,
__-i P / A' I'
l '
z
/'
CHF3 .‘a’
I’
t
/
,+” f---f
Fig. 4. Dependence of dissociation rates of ClFa (dT> and Cm3 (d& On the tOtd pm2-e Of trifluOrOmethane/~gOn mixture. - -o- - and - a- -: dT and dH in trifluoromethane without argon, respectively. -A- and -A-: dT and do in 1 Ton trifluoromethane with different pressures of argon, respectively.
Volume 82, number 1
CHEMICAL
PHYSICS LETTERS
sociation scheme proposed in section 3.6: colhsional relaxation of bottle-necking of CTF3 by the other major components, and collision-induced dissociation of preexcited CHF,. From a practical point of view, the addition of argon buffer ideally increased both the dissociation rate of CTF, and the selectivity remarkably. The %c value for CTF, was decreased to -30 J/cm2 at 30 Torr total pressure. When oxygen was added to trifluoromethane as a buffer gas (also as a radical scavenger), the results obtained were very similar to those with argon buffer, although the C2F4 peak disappeared and COF2 was produced instead as the scavenged product, which IS detected as a CO, peak after the gas-chromatographic separation_ 3.8. Inadiation geometry From the results of the systematic studies, the @= values for CTFS and CHF3 were estimated and it was suggested that the introduction of a longer focal-length lens might increase dT, decrease dH, and result in a notable increase of STiH and efficient use of laser power. A 380 mm focal-length lens and 450 mm long cell were employed with the model 821 laser @I2 in the laser gas) since this laser can be operated at high repetition rate up to 20 Hz while a lower selectivity was expected as compared with the model 103-2 laser without N, _ Upon irradiation of 5 Torr neat trifluoromethane with a 0.9 J pulse, the selectivity STjH observed was as high as 120. Furthermore, when a mixture of 5 Torr trifluorometbane and 100 Torr argon was irradiated with 0.53 J pulses, a larger dissocration rate for CTF, (dT = 2.4 X 10m4) was obtained with a smaller rate for CHF, (dH = 4.6 X 10s7), thus with extremely high selectivity of x5 10. Since the peak intensity of fluence at the focal spot decreases when the longer focal-length lens is employed, the probability for occurrence of laser-induced dielectric breakdown decreases, and a much higher operating pressure can be employed: this may be useful for the survey of scaling-up of the laser isotope separation method. A detailed discussion of irradiation geometry will be published elsewhere 1171.
15 August 1981
4. Concluding remarks The selective decomposition of a tritium compound (with single-step separation factors exceeding 500 directly measured by radio-gaschromatography) was achieved by using a CO2 TEA laser and trifluoromethane as a working molecule. Typical tritium concentrations expected in the waste light water from nuclear fuel reprocessing plants are 0.1-l Ci/Q, or 0.03-0.3 ppm T in H; these are in the concentration level we studied in this work. Since the laser power is selectively deposited in the trace component (CTF& our study has shown the laser method can become one of the most feasible methods for the removal of tritium from waste water. Although we have observed that the presence of CDF, in the mixture interferes the selective dissociation of CTF, at the R(14) line (1074.6 cm-l), we have already reported the selective decomposition of CDF, in a CHF,/CDF,/CTF, ternary mixture by using a different CO, laser line [lo], by which such interference can be removed_
References [l ] V.S. Letokhov and C.B. Moore, in: Chemical and bio-
[2]
[3] 141 [S]
[61 171
chemical applications of lasers, Vol. 3, ed. C.B. Moore (Academic Press, New York, 1977) p. 1. C-D. Cantrell, SM. Freund and J.L. Lyman, in: Laser handbook, Vol. 3, ed. M-L. Stitch (North-Holland, Amsterdam, 1979) p. 485. V-S. Letokhov, Phys. Today 33 (Nov. 1980) p_ 34. 1-P. Herman and JB. Marling, Chem. Phys. Letters 64 (1979) 75. SAG. Tuccio and A. Hartford Jr., Chem. Phys. Letters 65 (1979) 234. J.B. Marling, 19. Herman and S.J. Thomas, J. Chem.
Phys. 72 (1980) 5603. Y. Makide, S. Hagiwara, 0. Kurihara, K. Takeuchi,
Y. Ishiluxwa, s. Arab, T. Tominaga, I. Inoue and R. Nakane, J. NucL Sci. Technol. 17 (1980) 645. 181 19. Herman and JB. Marling, J. Phys. Chem. 85 (1981) 493. 191 Y. Lshikawa, S. Arai and R. Nakane, J. NucL Sci. TechnoL 17 (1980) 275; presented at the 1979 Annual Meeting of the Atomic Energy Society of Japan, Tokai, Oct. 1979. 1101 Y. Makide, S. Hagiwara,T. Tominaga, 0. Kurihara and R. Nakane, Intern. J. AppL Radiat. Isotopes, to be published.-
23
Volume 82, number 1
CHEMICAL PHYSICS LETTERS
[I 1 ] E. Tschuikow-Roux and J.E. Marte, J. Chem. Phys. 42 (1965) 2049. [ 121 R-V. Ambartzumian, Yu. A. Corokhov, VS. Letokov, G.N. Makarov and AA_ Puretzkii, JETP Letters 23 (1976) 22. [I 31 R.V. Ambartzumian and VS. Letokhov, in: Chemical and biochemical apphcations of lasers, Vol. 3, ed. C.B. Moore (Academic Press, New York, 1977) p- 167.
24
1.5 August 1981
1141 J L. Lyman, SD. Rockwood and SM. Freund, J. Chem. Phys. 67 (1977) 4545. [lS] W. Fuss and T-P. Cotter, Appl. Phys. 12 (1977) 265. [16] S. Speiser and J. Jortner, Chem. Phys. Letters 44 (1976) 399. [ 171 K. Takeuchi, Y. Makide, S. Kate, 0. Kurihara, T. Tominaga, I. Inoue and R. Nakane, to be published.
82. number
TRITRJM
CHEMICAL
I
ISOTOPE
MAKIDE.
15 August
LETTERS
1981
SEPARATION
BY CO, LASER-INDUCED Yoshihiro
PHYSICS
MULTJPHOTON
Satoru HAGIWARA,
DISSOCIATION
OF CTF,
Takeshi TOMINAGA
Deparrment of Chemrstry. Faculty of Science, The University of Tokyo, Hongo. Tokyo 113. Japan
and
Kazuo TAKEUCHI and Ryohel NAKANE 77ze Institute of Physrcat and G’zemzcat Research, Wake-shi, Saitama 3.51, Japatl Recerved
8 June 1981
Isotope separatron of trrtmm at ppm concentration level was achieved by COa laser-induced multiphoton dissociation of CTI’s rn CHFa with single-step separation factors exceeding 500. The effects of laser frequency, pulse energy, pulse duration, irradiation geometry, tritium concentration, sample pressure, and buffer gas were investigated_
,
l_ Introduction Extensive studres of infrared (IR) laser-induced multrphoton dissocration (MPD) dcne during last several years have indicated remarkable capabilities in laser Isotope separation [l--3] _Application of this method to deuterium separation !ras been proposed recently [4-6]_ A novel isotope separation technique for tritiurn is needed with the development of nuclear fuel reprocessing plants in Japan. We have reported for the first time successful isotope separation of tritium by CO, laser-induced MPD of a tritium compound [7] _ The compound used was trifluoromethane-T (CTF,), which proved to be the best in the tritiated halomethanes we surveyed extensively. It unexpectedly coincided wrth the compound CDF,, which was proposed by Marling et al. as one of the suitable compounds for deuterium separation [6], although our approach was substantrally different because large isotope shifts were expected in absorption bands of isotopic molecules and no IR spectra of trltiated halomethanes had been observed_ Very recently, Herman and lMarhng [!3] reported the IR spectrum of highly-radioactive CTF3 implying an idea toward laser tritium separation after our successful experimental attempt with the same compound 18
0 009-26
(CTF3) [7] _Their IR spectrum of CTF3 confirmed our results although we started the MPD experiments on the bdsis of the calculated fundamental frequencies of CTF, [9] : the reported IR absorption peak (1077 -‘) [8] is fairly close to our observed peak for ED of CTF, (1075 cm-l) [7] _ In the course of our experiments on the isotopically-selectrve photodrssociation of CTF,, the mixture CTF, f CHF, has proved to be a unique and ideal system to study both the behavior of a trace component by ultrasensitive detection techniques and the behavior of a major component without being disturbed by the excitation or decomposition of the trace component_ In this paper we present the dependencies of the dissociation rates and selectivity of CTF,/CHF, upon the laser frequency, pulse energy, pulse duration, irradiation geometry, tritium concentration, sample pressure, and kind of buffer gas.
2. Experimental Trifluoromethane-T (CTF3) was prepared by the hydrogen isotope exchange of CHF3 with tritiated water in the presence of dimethyl sulfoxide and sodium
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0 North-Holland
Publishing Company
Volume 82, number 1
CHEMICAL. PHYSICS LETTERS
hydroxide_ The procedure is similar to the methods reported previously [7,10] _The concentration of CTF3 obtained was 6 ppm (~8 pCi/rnl gas) or 0.2 ppm (~0.3 pCi/ml); the latter was mainly used throughout the experiment. TEA CO2 lasers (mainly Lumonics model 103-2, and Lumonics 821 for the experiment in section 3.8) were used for irradiation of the sample mixture (CTF3 and CHF,, with/without buffer gas) in a photolysis cell. A laser pulse consisted of a 100 ns width splice (when a mixture of CO, and He was used as a laser gas) followed by a 1 PCStail (when N2 was added to the laser gas: CO2 + N2 + He). Laser pulses of 0.3-l .3 J at 1 (or 2) Hz repetition rate passed through an aperture (14 mm $) were focused into the center of the photolysis cell (20 mm 9, 100 mm long, or 450 mm long for studies in section 3.8) by means of a BaF, lens (75 mm in focal length, or 380 mm for section 3.8). A few CaF, plates were placed before the lens to attenuate pulse energy. The pulse energy for the irradiation was measured at before/behind the cell and before/after the irradiation with a pyroelectric detector (Lumonics 20D) cahbrated to Scientech 364 disk calorimeter_ The laser irradiated sample was condensed in a sample loop cooled in liquid N2 (and the buffer was pumped out), and then injected into a specially-designed radio-gas chromatograph equipped with a TCD and a proportional counter. The amounts of CHF3 and CTF3 were determined from the TCD peak area and the total counts of radioactive peak, with reproducibilities within 0.3% and OS%, respectively.
3. Results and discussion 3.1. Chemical change by laser irradiation Trifluoromethane excited by IR multiphoton absorption decomposed to difluorocarbene (CF& and hydrogen fluoride 17,101, similarly to the thermal decomposition [ 111. The CF2 radicals recombined to form stable tetrafluoroethylene (GF4), and hydrogen fluoride (TF + HF) was eliminated from the gas phase by reaction with the glass wall.
15 August 1981
3.2. Dependence of dissociationand specific radioactivityon number of pulses When the sample mixture (CTF, + CHF,) was irradiated with the 000 l-0200 R( 14) line at 1074.6 cm-1 corresponding to the maximum dissociation rate of CTF3 (see section 3.3), the unchanged fraction of CTF, (1 - X,) decreased exponentially with increasing pulse number showing a pseudo-first order decay over a wide range of conversion (XT): 1 - X, = exp(-+n), where n is the number of pulses and dT denotes the specific dissociation rate of CTF,: dT = --In (1 - XT)/n. Decomposition of CHF~ also followed the pseudo-first order decay when the conversion (dH) was kept below 10%. 1 - XR = exp(-dHn), or dH = -ln(l - X&r, where dH is the specific dissociation rate of CHF, _ The ratio of both dissociation rates gives the selectivity (ST/R) of the dissociatron: ST/R = dT/dH _The specific radioactivity of residual gas, therefore, decreased exponentially with n. Since the amounts of CTF, present in the sample are extremely small, even the complete decomposition of CTF, scarcely affects the overall chemical composition or total pressure of the sample. However, the dissociation of CHF, alters the composition and the total pressure, which in turn affect the specific dissociation rates considerably as will be discussed in section 3.6. The converted fraction of CTF, (XT) was selected between 0.1 and 0.9 in accordance with the ST/B value to obtain better accuracy in quantitative measurements, where X, was usually kept between 0.02 and O-l_ When the ST/R was extremely high (e.g. 500) dT and dH were measured separately in two identical samples but with a different number irradiation pulses n. 3.3. Frequency dependence Specific dissociation rates of CTF3 (dT) and CHF, (dH) were measured against several OO”1-O2oO transition lines of a CO, laser (model 103-2, without N2 in the laser gas, 1 Hz). Fig. la shows the observed vaiues of dT and & at the maximum pulse energy available for each line shown in fig. lb. The maximum d, was obtained at the R(14) line (1074.6 cm-l). Fig. lc shows the dissociation rates normalized to the pulse energy of R(14) (dk and d;f) according to the dependence of dT and dH on the pulse energy (see
section 3.4). The MPD spectrum thus obtained (fig. Ic) 19
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1
1
I
I
,
was compared with the linear-absorption LR spectrum of CTF, (fig. id)_ The maximum of the MPD spectrum shifted slightly to the red. The MPD peak shape trailed to the red side and fell sharply on the blue side. Similar phenomenon has been observed and appears to be characteristic of MPD j12,13]. The steep increase of dk on the blue side may be related to the Increase of d;f which corresponds to the red-side tail of the strong v2 absorption band of CHF, (C-F stretching mode) at 1141 cm-l. Since dk decreases with decreasing wavenumber, the selectivity ST/~ was improved on the red side of R(14). However, both the avaIlable maximum pulse energy and d;’ value decrease on the red side, and there are no laser lines between the P and R branches (1060-1068 cm-‘). Therefore, if the better ST/H values are primarily intended, the lines on the red side should be investigated at the cost of the dissociation rate.
L
100 -
YZP-branch 60
-
20
-
100
-
lr\
b 60
-
40
-
20
-
0
‘\ CHFs
(d)
_
(cl
-
CTF3
4 A
00 -
0
3.4. Dependence
l.__
I
I
1090
1000
1070
1050
1060 C cm-’
1040
)
0
80
(a) CTF3
000
100 F
20
0
l
0
1 1090
0
l* I 1080
. .
I
0
CHF3
1070 Wavenumber
.
.
1060 C cm-’
1050
I
-
1040
1
Fig. 1. Frequencydependenceof multiphotondissociationof CTFa presentin CHF3. Sample: 5 Ton CHFa containing0.2 ppm CJJF3.(a) Observeddissociationratesof CJXa (do) and CHFa (dH) for each laser line with availablepulseenergy (Eo) as shown in (b). Asteriskin (%a)indicatesabsenceof laser linesin the region. (c) Dissociationratesof CTFa (dk) and CHFa (dir>normalizedto the same pulse enerm (0.6 J). (d) LinearabsorptionIR spectrumof Cl& (from ref. 181) for comparison.
20
of dT and dH on pulse energy and
pulse duration
I Wavenumber
60
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CHEMICAL PHYSICS LETTERS
Table 1 shows the results of the dissociation of CTF, and CHF, on irradiation with different pulse energy of the R( 14) line (with/without N2 added in the laser gas)_ Specific dissociation rates of resonant CTF, (dT) and non-resonant CHF, (dH) increased with the pulse energy (Eo): dT aE$6io-2 and d, a E$oko-2. While both dissociation rates increased with increasing Eo, the selectivity ST/H decreased_ Dissociation of CTF3 may be considered to follow the 3/2-power law [ 14,151. Since d, corresponds to the fractional volume of the dissociation zone (V) where the laser fluence (Q) exceeds the critical value for dissociation (‘k,) in CTF, , dT should increase with the 3/2-power of E, if the zone is approximated by the double-cone shape (V a ,$i2). Assuming that the dksociation probability (4) outside the zone (G?< a& is negligible, the value of Gc for CTF, is calculated as ~60 J/cm2 at 5 Torr, and 100 J/cm2 at 1 Torr *_ Similarly, +c for CHF, is calculated as 250 J/cm2 at 5 TOIT according to the double-cone geometry_ Since the laser fluence around the focal spot is close to the %C value for CHF, , however, the observed secondpower dependence of d, cannot be explained by * Assumingthat q = (Q/Q& higher than these values.
(when 4 < Qd), QC should be
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PHYSICS
15 August 1981
LETTERS
Table 1 Dependence of dissociation of CTFs and CHFs on laser pulse energy and pulse duration at the OO” 1-02°0 R(14) line(1074.6
m-l) a) --_Run number 1 2 3 4 5 6 7 8 9 10
Laser gas
1 -XTb)
1 -XHb)
dTb) (1 OS5 pulse-r)
dHb)
E” (J) b,
;puIse)
CO, + He
0.26 0.32 0.38 0.47 0.57
10000 7500 5700 4050 3000
0.812 0.759 0.756 0.794 0.770
0.980 0.981 0.979 0.978 0.973
2.1 3.7 4.9 5.7 8.7
0.20 0.26 0.38 0.54 0.92
COz+Nz+He
0.45 0.55 0.70 1.03 1.26
2900 2000 1500 1000 800
0.905 0.916 0.910 0.871 0.854
0.978 0.975 0.970 0.952 0.946
3.4 4.4 6.3 13.8 19.7
0.77 1.3 2-O 5.0 6.9
(1 O-’ pulse-‘)
a) The experimental conditions are as follows: sample 5 Torr CHFs containing 0.2 ppm CfF3; laser, Lumonics model 103-2; BaF2 lens, 75 mm focal length; photolysis cell, 10 cm long and 45 ml. b)The estimated errors in Eo, 1 - XT, 1 - XH, dT and dH areapproximately k2.5%, &O-7%, cO_4%, '7% and klS%: respectively.
either the double-cone geometry model [ 14,151, or the modified “dog-bone” geometry model proposed by Speiser and Jortner [ 161. An improved model and deconvolution procedure to obtain the intrinsic correlation between G?and q will be proposed elsewhere The addition of N2 into the laser gas increases the pulse energy by the contribution of the 1 PS tail part of the pulse. When dT or dH for such longer pulses (columns 6-10 in table 1) was compared with the data for 100 ~LSpuises, dT was roughly haIved and dH increased by %4O% for the same pulse energy. The decrease in dT may be caused by the lowering of the peak intensity with the same fluence and/or by the increase in the probability of collisional de-excitation of activated CTF, during the long Iaser p&e. The increase in d, may be due to the collisional dissociation of pre-excited CHF3 molecules in the long pulse duration as mentioned later, while energy transfer from the resonant CTF3 cannot affect the dH value. 3.5. Tn%iumconcentration The dependence of d, and dH on the CTF, concentrations was measured over a wide range of about three orders ofmagnitude as well as in neat CHF3 containing no CTF, (fig_ 2). Since dT does not ch_ange with the CTF, concentration, it is confirmed that there is no interaction between CTF, molecules in the dissociation process at ppm level concentrations.
1
I
IrId
0
I
I
,llllll
CTFs
Fig.
I
I
,Ilrrll
01
001
concentratior,
I
I
,lrrrrl
1
1
I
10 C pprn 1
2. Effect of CTF3 concentration on specific dissociation
ratesof CIF3 (0: dT) and CHFJ (0: dH): 0 indicates dH of neat CHFa (containing no CTF3). CHF3 pressure: constant at 5 Torr; laser irradiation- R(l4) line, 100 ns, 0.6 J pulse-
The dH value also remained constant throughout the concentration range and agreed well with the dH value in neat CHF3, indicating that the dissociation of non-resonant CHF, moiecules happens independently of CTF, _Hence the decomposition of CHF3 which decreases the selectivity is not due to energy transfer from excited CTF3 to non-resonant CHF,, but rather to multiphoton absorption and dissociation by CHF, itself. 3.6. hXs7.0Veffi?Cton d-p dH, a?zdS~j~ The d, and d,
values both increased with increas21
I,,,
,
,
,
1,
L,L,
I
(Ar
01
02
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CHEMICAL PHYSICS LETTERS
Volume 82, number 1
05
1
,
,
L
1
,I’,
,
added 1
2
5
10
20
Fig. 3. Dependence of dissociation rates of CTF3 (dT) and CHF, (dH) on the pressure of trifluoromethane (containing 0.2 ppm Cl?F,) with and without argon. -oand -m-: dT and dH in trfluoromethane without argon, respectively_ --n--and - -A- -: dT and dH intrif%rorometbane/argonmix-
3.7. Effect of argon and oxygen as buffi When buffer gas was mixed into the sample, the MPD behavior of CTF, and CHF, changed drastically. With the addition of argon to trifhroromethane with the total pressure kept constant at 10 Torr (fig. 3, a and A), dT remained at the same value with that in 10 Torr trifluoromethane without buffer in spite of the decreased trifluoromethane pressure, while dH showed no change or the same value with d, at each trifluoromethane pressure. In another experiment, argon at different pressure was added to 1 Torr trifluoromethane. When dT and d, were plotted against total pressure (fig. 4, a and Aj, dT changed almost the same as in neat trifluoromethane, while dH scarcely changed in the pressure range below 10 Torr. Both series of experiments indicate that dT depends only on the total pressure irrespective of the collisional partner and dH depends only on the partial pressure of CHF3 itself. These results strongly support the drs-
ture, respectively, with total pressurekept constant at 10 Torr Wfluoromethane/argon = l(Torr)/S(Torr), Z/8,5/5, or 10/O). Laser irradiation: R(14) line at 1074.6 cm-r, 100
CAr
added)
_
b'
-
ns, 0.6 J pulse. *¶I' _,_QH
ing pressure of trifluoromethane (fig. 3, o and a)_ Since d, increased more rapidly than dT , the selectivity ST/R decreased with increasing pressure. The increase in dissociation rate of resonant CTF8 with Increasing pressure may be explained in terms of the collision-induced rotational relaxation of vibrationally ground-state molecules. The collision frequency of CTF, with major component CHF, can be estimated as about 5 times at 5 Torr in the 100 ns pulse duration_ Since CHF, decomposes independently of CTF,, the strong pressure dependence of dH shouldbe attributed to the interaction between CHF8 molecules_ This dependence can be understood by assuming that pre-excited CHF, molecules which have been excited only insufficiently for unimolecular dissociation collide with each other to induce dissociation of one of the two molecules.
22
"T;_p*H ,
__-i P / A' I'
l '
z
/'
CHF3 .‘a’
I’
t
/
,+” f---f
Fig. 4. Dependence of dissociation rates of ClFa (dT> and Cm3 (d& On the tOtd pm2-e Of trifluOrOmethane/~gOn mixture. - -o- - and - a- -: dT and dH in trifluoromethane without argon, respectively. -A- and -A-: dT and do in 1 Ton trifluoromethane with different pressures of argon, respectively.
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sociation scheme proposed in section 3.6: colhsional relaxation of bottle-necking of CTF3 by the other major components, and collision-induced dissociation of preexcited CHF,. From a practical point of view, the addition of argon buffer ideally increased both the dissociation rate of CTF, and the selectivity remarkably. The %c value for CTF, was decreased to -30 J/cm2 at 30 Torr total pressure. When oxygen was added to trifluoromethane as a buffer gas (also as a radical scavenger), the results obtained were very similar to those with argon buffer, although the C2F4 peak disappeared and COF2 was produced instead as the scavenged product, which IS detected as a CO, peak after the gas-chromatographic separation_ 3.8. Inadiation geometry From the results of the systematic studies, the @= values for CTFS and CHF3 were estimated and it was suggested that the introduction of a longer focal-length lens might increase dT, decrease dH, and result in a notable increase of STiH and efficient use of laser power. A 380 mm focal-length lens and 450 mm long cell were employed with the model 821 laser @I2 in the laser gas) since this laser can be operated at high repetition rate up to 20 Hz while a lower selectivity was expected as compared with the model 103-2 laser without N, _ Upon irradiation of 5 Torr neat trifluoromethane with a 0.9 J pulse, the selectivity STjH observed was as high as 120. Furthermore, when a mixture of 5 Torr trifluorometbane and 100 Torr argon was irradiated with 0.53 J pulses, a larger dissocration rate for CTF, (dT = 2.4 X 10m4) was obtained with a smaller rate for CHF, (dH = 4.6 X 10s7), thus with extremely high selectivity of x5 10. Since the peak intensity of fluence at the focal spot decreases when the longer focal-length lens is employed, the probability for occurrence of laser-induced dielectric breakdown decreases, and a much higher operating pressure can be employed: this may be useful for the survey of scaling-up of the laser isotope separation method. A detailed discussion of irradiation geometry will be published elsewhere 1171.
15 August 1981
4. Concluding remarks The selective decomposition of a tritium compound (with single-step separation factors exceeding 500 directly measured by radio-gaschromatography) was achieved by using a CO2 TEA laser and trifluoromethane as a working molecule. Typical tritium concentrations expected in the waste light water from nuclear fuel reprocessing plants are 0.1-l Ci/Q, or 0.03-0.3 ppm T in H; these are in the concentration level we studied in this work. Since the laser power is selectively deposited in the trace component (CTF& our study has shown the laser method can become one of the most feasible methods for the removal of tritium from waste water. Although we have observed that the presence of CDF, in the mixture interferes the selective dissociation of CTF, at the R(14) line (1074.6 cm-l), we have already reported the selective decomposition of CDF, in a CHF,/CDF,/CTF, ternary mixture by using a different CO, laser line [lo], by which such interference can be removed_
References [l ] V.S. Letokhov and C.B. Moore, in: Chemical and bio-
[2]
[3] 141 [S]
[61 171
chemical applications of lasers, Vol. 3, ed. C.B. Moore (Academic Press, New York, 1977) p. 1. C-D. Cantrell, SM. Freund and J.L. Lyman, in: Laser handbook, Vol. 3, ed. M-L. Stitch (North-Holland, Amsterdam, 1979) p. 485. V-S. Letokhov, Phys. Today 33 (Nov. 1980) p_ 34. 1-P. Herman and JB. Marling, Chem. Phys. Letters 64 (1979) 75. SAG. Tuccio and A. Hartford Jr., Chem. Phys. Letters 65 (1979) 234. J.B. Marling, 19. Herman and S.J. Thomas, J. Chem.
Phys. 72 (1980) 5603. Y. Makide, S. Hagiwara, 0. Kurihara, K. Takeuchi,
Y. Ishiluxwa, s. Arab, T. Tominaga, I. Inoue and R. Nakane, J. NucL Sci. Technol. 17 (1980) 645. 181 19. Herman and JB. Marling, J. Phys. Chem. 85 (1981) 493. 191 Y. Lshikawa, S. Arai and R. Nakane, J. NucL Sci. TechnoL 17 (1980) 275; presented at the 1979 Annual Meeting of the Atomic Energy Society of Japan, Tokai, Oct. 1979. 1101 Y. Makide, S. Hagiwara,T. Tominaga, 0. Kurihara and R. Nakane, Intern. J. AppL Radiat. Isotopes, to be published.-
23
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[I 1 ] E. Tschuikow-Roux and J.E. Marte, J. Chem. Phys. 42 (1965) 2049. [ 121 R-V. Ambartzumian, Yu. A. Corokhov, VS. Letokov, G.N. Makarov and AA_ Puretzkii, JETP Letters 23 (1976) 22. [I 31 R.V. Ambartzumian and VS. Letokhov, in: Chemical and biochemical apphcations of lasers, Vol. 3, ed. C.B. Moore (Academic Press, New York, 1977) p- 167.
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1.5 August 1981
1141 J L. Lyman, SD. Rockwood and SM. Freund, J. Chem. Phys. 67 (1977) 4545. [lS] W. Fuss and T-P. Cotter, Appl. Phys. 12 (1977) 265. [16] S. Speiser and J. Jortner, Chem. Phys. Letters 44 (1976) 399. [ 171 K. Takeuchi, Y. Makide, S. Kate, 0. Kurihara, T. Tominaga, I. Inoue and R. Nakane, to be published.