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Isotope selective ir-laser multffhoton dissociation of CF3Cl and selectivity enhancement with substrate pressure
Volume 78, number 3
CHEMICAL
PHYSICS LETTERS
15 March
1981
ISOTOPE SELECTIVE IR-LASER MULTIPHOTON DISSOCIATION OF CF3Ci AND SELECTIVITY ENHANCEMENT WITH SUBSTRATE PRESSURE SK_ SARKAR, V. PART~SA~T~Y, A. PANDEY, R.V.S. RAMA R.AO and Jai P. MITT-AL” Bfiabi~aAtomic Resetwdz Cenrre, Trombay, Bombay-400 095, Indur Received 23 October 1980; in fmal form 15 December 1980
Photolysis of CF3CI with a 9.6 pm CO2 laser has indicated that carbon-13 selectsvrty increases when the excitation frequency is sliihtly red-shifted from the ~1 band centreat R(18). A selectmrty enhancement with substrate pressure was aiso observed if the P(l8) line, which is far red-shifted, is used at low fluence.
1. Introduction Isotope sekctlve multIphoton dissociation (MPD) was reported for SF6 in 1975 by a number of laboratories using pu!sed CO, laser radiation [ 1,2]. Smce then, a large amount of theoretical and experimental work was done in this field, mostly on poiyatomlc moE ecules including a few triatomic systems [3] _There is much current interest in the MPD of simple freons in the context of C-l 3 enrichment_ The CF3X system (where X = Cl, Br or I) is well known for isotope selecttve IR MPD [4]. Of the three molecules, CF,Cl and CF3Br offer better isotopic selectivities at room temperature @I. Gauthier et al. [6] have reported an unusual increase in the selectivity factor cwwith increase of substrate pressure in the multiphoton dissociation of CF3Br under low-fluence, off-resonant irradiation. The selectivity factor goes through a maximum and then decreases as the CF3 Br pressure is increased beyond a certain range. They attrrbute this phenomenon to “differences in the up-pumping rates for the two isotopic species reiative to the down-pumping rates induced by coliision with cold bath molecules”. According to the model proposed, this phenomenon should in general be true for any polyatomic molecule when the minor isotopic species is subjected to selective multiphoton excitation. Except for slight differences in spectroscopic and thermodynamic properties, CF3Cl closely * Address for correspondence: Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400
0 009-26
14/8 1 /OOOO-00~/$02.50
085, India.
0 Non-Hoard
resembIes CF3 Br. We have therefore chosen CF, Cl as the Investigating system to establish whether the above phenomenon exists for other polyatomics as well. Experiments were performed for two different laser irradiation conditions and carbon-13 enrichment in the C,F6 product was determined as a function of substrate pressure and laser line. III the near-resonant, strong-focusing irradiation (focal fluence = 500 J/cm2), selectivity was found to increase as the excitation frequency was progressively red-shifted from the “CF3 Cl p1 band centre (1077.4 cm-l). However, increase of substrate pressure under similar laser fluences had a deleterious effect on seIectivity. On the other hand, in the off-resonant irradiation of it band, pressure enhancement of selectivity was mdeed observed, provided the excitation fhrence was kept iaw, i.e. =G10 J/cm2. Higher fluence in off-resonant excitation was found to destroy the selectivity enhancement for a given pressure.
2. Experimental An indigenously built, line-tunable, helical TEA CO2 laser was used. The: pulse energy was adjusted to be 750 mJ on all lines used. A typical pulse consisted of a 250 ns spike carrying two-thirds of the total energy with a tail extending up to a few microseconds. Laser lines for irradiation were selected and verified using a spectrum analyser (Optical Eng. Co.) and the laser was operated at a repetition rate of 20 pulses per minute. The beam (diameter 1.4 cm) was focused with an (f = Publishing Company
479
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
15 c-m) BaF2 lens. Depending on the fluence reqcirements of the experiments, the Pyrex reactlon cell (9 cm long, 5 cm diameter) was positioned with the focal spot either at the centre of the cell (strong focusmg) or 3 cm away from the exit window (weak f’ocusing). CF,Cl (Matheson) was purified each time before use by b&o-to-bulb distillation between two traps kept at -160°C (isopentane slush) and -196°C (liquid nitrogen) GC-MS analysis of blank CF3 Cl by a VG Mlcromass 7070F mass spectrometer did not reveal any impurity TypicaIIy 1000 or 2000 pulses were given depending on whether the experiment mvolved nearresonant, strong-focusing or off-resonant, weak-focusing geometry. All the expenments were carried out at room temperature C, Fs yield and CF, Cl depletion were measured by anIR spectrometer (Perkm-Elmer model 577) at 1250 cm-l and 1217 cm-l respectively. The
isotopic distribution of the CzF6 species was determined mass spectrometrically using C,FS peaks at nr/e 119, 120 and 121. The selectivity factor cx was evaluated by the expression (13C/12C) cu = (13C/12C)
ratio in product
C2F6
ratio in reactant
CF,CI-
A home-made, low-power (= 1 W) line-tunable cw CO, laser was used to determine the absorption of various CO, laser lines by CF,CI m its v1 band (symmetric C-F stretchmg) at 9.6 ,um. The experimental arrangement and spectrum obtained are shown m fig. l_
3. Results and discussion 3.1. rVea]-resonatzt
-
T1
cw
Ge BEAV SPLITTER
co2
LASER
p
10
0,
R 9
DETECTORS
64”
Fig. 1. IR-absorption
BAND of
r OF
&SORPTION CELL
P CO2 LASER
CFaCI for various lines of
the
9.6 pm
band of the CO? laser. Pressure of CFJCI: 20 Torr and 9 cm optical path used. On the top, the experimental arraxement
is shown.
480
and strong-focusing
exntatiou
As the wavelength of the laser hne chosen for Irradiation was moved to the low-frequency side of the absorptron band (13CF3C1: 1077.4 cm-l), the selectivity with respect to C-l 3 enriched product C, F6 continued to increase, as shown in fig.2. It generally agrees with the trend reported m the hterarure [4]. Beyond this wavelength range, the yields become small and measured (Yvalues, though high, are imprecise_ In the above experiments the substrate pressure was kept at 1 Torr. We have also observed that the selectivity progresslvely decreases with an Increase in substrate pressure. The loss of selectivity at higher pressure can be Interpreted as a result of energy transfer scrambling reactions_ The primary decomposition channel in the CF,X system has been reported [7,8] to involve C-X bond sassion. From the molecular beam studies of Sudbo et al. [7], it was concluded that the IR laser excited CF3X species have mean dissociative lifetimes of l--10 ns. On the basis of Cl-atom elimination, modelled by a “loose complex” in the critical configuration and using spectroscopic and thermodynamic data of refs. [9,10], a full RRKM trearment of CF3Ci was made (fig. 3). The RRKM plot indicates that excitation levels l-2 photon excess above the dissociation threshold (3.5 eV) would have sufficiently fast dissociation rates (~2 X lo8 s-l) to be able to compete with the excitation rate and further excitation becomes lifetime (~5 ns) limited. Other product channels, requiring still higher levels of excitation, are not observed.
Volume
78, number 3
CHEMICAL
PHYSICS
15 March 1981
LETTERS
EXCESS
ABSORBED
PHOTONS
123L5 EG(cm-‘lx. 1530
2000 300 EXCESS
~000 som ENERGY
1Ci’
6000 7000
Et1 cm-‘)
Frg. 3. Calculated RRKM rate of CFsCI decomposition as a function of the number of photons in excess of the dissociation threshold. Vibrational state denstty N(Ez) versus vrbratronal energy content E,* is shown in the mset
R-BRANCH CO2 LASER LINE (9 6 qrn band) Kg. 2.lXperimental arrangement (top): (a) TEA CO2 laser, (2) Bafz lens, (3) reactron cell, (4) gas chromatogroph. (5) mass spectrometer, (6) CRO, (7) recorder, (8) detector. Bottomfrequency dependence of (Y and C2F.s yteld Substrate pressure1 Torr; incident fluencer 0 5 J/cm2.
the entrance window and 20 J/cm2 at the exit window. Typical results are given in table 1_ These data cIearly indicate that IXincreases with pressure, reaches a maxrmum of -65 and then drops to 429 at 20 Torr. Gauthier et al. [6] have reported increase in selectivrty for CF,Br over an extended pressure regron up to 55
3.2.Off-resonant and weak-focusing excitation Among the off-resonant P-branch lines, we find P(18) (off-resonant by 29 cm-l for uCF3C1 and 58 cm-l for 12CF3C1 band centres) is moderately absorbed by CF3C1 (cf. fig. 1). This absorption is comparable to that for the R(12) line for which an or value of 9.25 was obtamed with reasonable C,F, yield in strongfocusing conditions (cf. fig.2). Hence, the P(18) line was chosen for high-pressure, off-resonant irradiation experiments at low fluence. The beam optics employed in these experiments is such that the sample is exposed to an average fhrence of -23 J/cm2 - Since the beam converges through the cell, a fluence gradient does exist throughout the irradiation volume, the fluence variation being 1 J/cm2 at
Table 1 Pressure dependence focusing excitatron
of LYvalues under off-resonant,
Cforr)
Number pulses
45 10 20
Pressure
4.7 a)
of
weak-
a
CzF, yreld (mTorr) b)
2000 2000 2000
45 65 4.29
=l = 1-2 = 3
2000
4.03
15
a) In this experiment off-resonant, strong-focusmg irradiation was done. b, Yield of CzF6 was based on mass spectrometric signal though CIFs pressures were drastically modified by a large excess of CFsCI.
481
Volume
78, number
3
CHEMICAL
PHYSICS
Torr. The comparatively smaller pressure range observed m our work can be explained by differences in excitation geometry in the two cases, Le. an almost pcralIe beam of average fluence x 5 J/cm2 in Gauthkr’s experiments compared to our converging beam geometry wmch includes regions of higher fluence. There is clear mdicatron that selectivity versus pressure is a strong functron of the excitation fiuence. It is demonstrated by the drastic fall in rsotopic selectrvtty from 45 to 4 at 45 Torr under strong-focusing geometryThis phenomenon. which is not uncommon m the literature [l l] _IS attributed to “overdrrvmg to dissoctatron of both species at higher fluence”_
4. Conclusion In summary. we have carried out C-l 3 enrichment by MPD of CF2Cl We have also shown that isotope selectwrty contmues to increase wtth CF,CI pressure up to 10 Torr m off-resonant, weak-focusmg excitation.
Acknowledgement The authors wish to thank Dr. P-RX. Rao for his keen Interest m tlus work. It rs a pleasure to thank Dr.
482
LETTERS
15 March 1981
G. Ramanan and Mr. K_N_ Bhide for their help in the mass spectrometric analysis.
References [ 11 J L. Lyman, R.J. Jensen, J. Rink, C P. Robinson and S D. Rockwood, Appl Phys Letter.. 27 (1975) 87. 17-1 R V Ambartzumian, Y A Gorokov, V S. Letokhov and G.N. hlakarov, JETP Letters 21 (1975) 171. [3] N. Bloembergen and E. Yabbnovitch, Phys. Today (May 1978) p_ 25 [4] M. Droum, M. Gauthier, R Pilon, P.A. Hackett and C Wrlli& Chem. Phys. Letters 60 (1978) 16 [Sj &f_ Gauthrer, P A. Hackett, M. Drouin, R. PiJon and C. W&s, Can. 5. Chem. 56 (1978) 2227. [6] M Gauthier. W.S. Nrpp, P-A_ Hackett and C Whis. Chem. Phys Letters 69 (1980) 372_ [71 Aa. S. Sudbo, P.A. Schulz, E.R. Grant, Y.R. Shen and Y.T. Lee, J. Chem. Phys- 70 (1979) 912. [81 E. Wurzberg, L J. Kovalenko and P.L. Houston, Chem. Phys. 35 (1978) 317. [91 E.K. Piyer and W S. Benedict, J. Res Natl. Bur. Std. US 47 (1951) 202. Tables, NSRDS-NBS 37 (US 1101 JANAF Thermochemical Govt Printing Offices, Washington, 1971). 1111 J.B. hlarlmg and 1-P. Herman, Appl. Phys. Letters 34 (1979) 439, D M Cox, R B. Hall, 3 A. Horsley, G.&f Kramer, P. Rabinowitz and A. Kaldor, Science 205 (1979) 390.
CHEMICAL
PHYSICS LETTERS
15 March
1981
ISOTOPE SELECTIVE IR-LASER MULTIPHOTON DISSOCIATION OF CF3Ci AND SELECTIVITY ENHANCEMENT WITH SUBSTRATE PRESSURE SK_ SARKAR, V. PART~SA~T~Y, A. PANDEY, R.V.S. RAMA R.AO and Jai P. MITT-AL” Bfiabi~aAtomic Resetwdz Cenrre, Trombay, Bombay-400 095, Indur Received 23 October 1980; in fmal form 15 December 1980
Photolysis of CF3CI with a 9.6 pm CO2 laser has indicated that carbon-13 selectsvrty increases when the excitation frequency is sliihtly red-shifted from the ~1 band centreat R(18). A selectmrty enhancement with substrate pressure was aiso observed if the P(l8) line, which is far red-shifted, is used at low fluence.
1. Introduction Isotope sekctlve multIphoton dissociation (MPD) was reported for SF6 in 1975 by a number of laboratories using pu!sed CO, laser radiation [ 1,2]. Smce then, a large amount of theoretical and experimental work was done in this field, mostly on poiyatomlc moE ecules including a few triatomic systems [3] _There is much current interest in the MPD of simple freons in the context of C-l 3 enrichment_ The CF3X system (where X = Cl, Br or I) is well known for isotope selecttve IR MPD [4]. Of the three molecules, CF,Cl and CF3Br offer better isotopic selectivities at room temperature @I. Gauthier et al. [6] have reported an unusual increase in the selectivity factor cwwith increase of substrate pressure in the multiphoton dissociation of CF3Br under low-fluence, off-resonant irradiation. The selectivity factor goes through a maximum and then decreases as the CF3 Br pressure is increased beyond a certain range. They attrrbute this phenomenon to “differences in the up-pumping rates for the two isotopic species reiative to the down-pumping rates induced by coliision with cold bath molecules”. According to the model proposed, this phenomenon should in general be true for any polyatomic molecule when the minor isotopic species is subjected to selective multiphoton excitation. Except for slight differences in spectroscopic and thermodynamic properties, CF3Cl closely * Address for correspondence: Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400
0 009-26
14/8 1 /OOOO-00~/$02.50
085, India.
0 Non-Hoard
resembIes CF3 Br. We have therefore chosen CF, Cl as the Investigating system to establish whether the above phenomenon exists for other polyatomics as well. Experiments were performed for two different laser irradiation conditions and carbon-13 enrichment in the C,F6 product was determined as a function of substrate pressure and laser line. III the near-resonant, strong-focusing irradiation (focal fluence = 500 J/cm2), selectivity was found to increase as the excitation frequency was progressively red-shifted from the “CF3 Cl p1 band centre (1077.4 cm-l). However, increase of substrate pressure under similar laser fluences had a deleterious effect on seIectivity. On the other hand, in the off-resonant irradiation of it band, pressure enhancement of selectivity was mdeed observed, provided the excitation fhrence was kept iaw, i.e. =G10 J/cm2. Higher fluence in off-resonant excitation was found to destroy the selectivity enhancement for a given pressure.
2. Experimental An indigenously built, line-tunable, helical TEA CO2 laser was used. The: pulse energy was adjusted to be 750 mJ on all lines used. A typical pulse consisted of a 250 ns spike carrying two-thirds of the total energy with a tail extending up to a few microseconds. Laser lines for irradiation were selected and verified using a spectrum analyser (Optical Eng. Co.) and the laser was operated at a repetition rate of 20 pulses per minute. The beam (diameter 1.4 cm) was focused with an (f = Publishing Company
479
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
15 c-m) BaF2 lens. Depending on the fluence reqcirements of the experiments, the Pyrex reactlon cell (9 cm long, 5 cm diameter) was positioned with the focal spot either at the centre of the cell (strong focusmg) or 3 cm away from the exit window (weak f’ocusing). CF,Cl (Matheson) was purified each time before use by b&o-to-bulb distillation between two traps kept at -160°C (isopentane slush) and -196°C (liquid nitrogen) GC-MS analysis of blank CF3 Cl by a VG Mlcromass 7070F mass spectrometer did not reveal any impurity TypicaIIy 1000 or 2000 pulses were given depending on whether the experiment mvolved nearresonant, strong-focusing or off-resonant, weak-focusing geometry. All the expenments were carried out at room temperature C, Fs yield and CF, Cl depletion were measured by anIR spectrometer (Perkm-Elmer model 577) at 1250 cm-l and 1217 cm-l respectively. The
isotopic distribution of the CzF6 species was determined mass spectrometrically using C,FS peaks at nr/e 119, 120 and 121. The selectivity factor cx was evaluated by the expression (13C/12C) cu = (13C/12C)
ratio in product
C2F6
ratio in reactant
CF,CI-
A home-made, low-power (= 1 W) line-tunable cw CO, laser was used to determine the absorption of various CO, laser lines by CF,CI m its v1 band (symmetric C-F stretchmg) at 9.6 ,um. The experimental arrangement and spectrum obtained are shown m fig. l_
3. Results and discussion 3.1. rVea]-resonatzt
-
T1
cw
Ge BEAV SPLITTER
co2
LASER
p
10
0,
R 9
DETECTORS
64”
Fig. 1. IR-absorption
BAND of
r OF
&SORPTION CELL
P CO2 LASER
CFaCI for various lines of
the
9.6 pm
band of the CO? laser. Pressure of CFJCI: 20 Torr and 9 cm optical path used. On the top, the experimental arraxement
is shown.
480
and strong-focusing
exntatiou
As the wavelength of the laser hne chosen for Irradiation was moved to the low-frequency side of the absorptron band (13CF3C1: 1077.4 cm-l), the selectivity with respect to C-l 3 enriched product C, F6 continued to increase, as shown in fig.2. It generally agrees with the trend reported m the hterarure [4]. Beyond this wavelength range, the yields become small and measured (Yvalues, though high, are imprecise_ In the above experiments the substrate pressure was kept at 1 Torr. We have also observed that the selectivity progresslvely decreases with an Increase in substrate pressure. The loss of selectivity at higher pressure can be Interpreted as a result of energy transfer scrambling reactions_ The primary decomposition channel in the CF,X system has been reported [7,8] to involve C-X bond sassion. From the molecular beam studies of Sudbo et al. [7], it was concluded that the IR laser excited CF3X species have mean dissociative lifetimes of l--10 ns. On the basis of Cl-atom elimination, modelled by a “loose complex” in the critical configuration and using spectroscopic and thermodynamic data of refs. [9,10], a full RRKM trearment of CF3Ci was made (fig. 3). The RRKM plot indicates that excitation levels l-2 photon excess above the dissociation threshold (3.5 eV) would have sufficiently fast dissociation rates (~2 X lo8 s-l) to be able to compete with the excitation rate and further excitation becomes lifetime (~5 ns) limited. Other product channels, requiring still higher levels of excitation, are not observed.
Volume
78, number 3
CHEMICAL
PHYSICS
15 March 1981
LETTERS
EXCESS
ABSORBED
PHOTONS
123L5 EG(cm-‘lx. 1530
2000 300 EXCESS
~000 som ENERGY
1Ci’
6000 7000
Et1 cm-‘)
Frg. 3. Calculated RRKM rate of CFsCI decomposition as a function of the number of photons in excess of the dissociation threshold. Vibrational state denstty N(Ez) versus vrbratronal energy content E,* is shown in the mset
R-BRANCH CO2 LASER LINE (9 6 qrn band) Kg. 2.lXperimental arrangement (top): (a) TEA CO2 laser, (2) Bafz lens, (3) reactron cell, (4) gas chromatogroph. (5) mass spectrometer, (6) CRO, (7) recorder, (8) detector. Bottomfrequency dependence of (Y and C2F.s yteld Substrate pressure1 Torr; incident fluencer 0 5 J/cm2.
the entrance window and 20 J/cm2 at the exit window. Typical results are given in table 1_ These data cIearly indicate that IXincreases with pressure, reaches a maxrmum of -65 and then drops to 429 at 20 Torr. Gauthier et al. [6] have reported increase in selectivrty for CF,Br over an extended pressure regron up to 55
3.2.Off-resonant and weak-focusing excitation Among the off-resonant P-branch lines, we find P(18) (off-resonant by 29 cm-l for uCF3C1 and 58 cm-l for 12CF3C1 band centres) is moderately absorbed by CF3C1 (cf. fig. 1). This absorption is comparable to that for the R(12) line for which an or value of 9.25 was obtamed with reasonable C,F, yield in strongfocusing conditions (cf. fig.2). Hence, the P(18) line was chosen for high-pressure, off-resonant irradiation experiments at low fluence. The beam optics employed in these experiments is such that the sample is exposed to an average fhrence of -23 J/cm2 - Since the beam converges through the cell, a fluence gradient does exist throughout the irradiation volume, the fluence variation being 1 J/cm2 at
Table 1 Pressure dependence focusing excitatron
of LYvalues under off-resonant,
Cforr)
Number pulses
45 10 20
Pressure
4.7 a)
of
weak-
a
CzF, yreld (mTorr) b)
2000 2000 2000
45 65 4.29
=l = 1-2 = 3
2000
4.03
15
a) In this experiment off-resonant, strong-focusmg irradiation was done. b, Yield of CzF6 was based on mass spectrometric signal though CIFs pressures were drastically modified by a large excess of CFsCI.
481
Volume
78, number
3
CHEMICAL
PHYSICS
Torr. The comparatively smaller pressure range observed m our work can be explained by differences in excitation geometry in the two cases, Le. an almost pcralIe beam of average fluence x 5 J/cm2 in Gauthkr’s experiments compared to our converging beam geometry wmch includes regions of higher fluence. There is clear mdicatron that selectivity versus pressure is a strong functron of the excitation fiuence. It is demonstrated by the drastic fall in rsotopic selectrvtty from 45 to 4 at 45 Torr under strong-focusing geometryThis phenomenon. which is not uncommon m the literature [l l] _IS attributed to “overdrrvmg to dissoctatron of both species at higher fluence”_
4. Conclusion In summary. we have carried out C-l 3 enrichment by MPD of CF2Cl We have also shown that isotope selectwrty contmues to increase wtth CF,CI pressure up to 10 Torr m off-resonant, weak-focusmg excitation.
Acknowledgement The authors wish to thank Dr. P-RX. Rao for his keen Interest m tlus work. It rs a pleasure to thank Dr.
482
LETTERS
15 March 1981
G. Ramanan and Mr. K_N_ Bhide for their help in the mass spectrometric analysis.
References [ 11 J L. Lyman, R.J. Jensen, J. Rink, C P. Robinson and S D. Rockwood, Appl Phys Letter.. 27 (1975) 87. 17-1 R V Ambartzumian, Y A Gorokov, V S. Letokhov and G.N. hlakarov, JETP Letters 21 (1975) 171. [3] N. Bloembergen and E. Yabbnovitch, Phys. Today (May 1978) p_ 25 [4] M. Droum, M. Gauthier, R Pilon, P.A. Hackett and C Wrlli& Chem. Phys. Letters 60 (1978) 16 [Sj &f_ Gauthrer, P A. Hackett, M. Drouin, R. PiJon and C. W&s, Can. 5. Chem. 56 (1978) 2227. [6] M Gauthier. W.S. Nrpp, P-A_ Hackett and C Whis. Chem. Phys Letters 69 (1980) 372_ [71 Aa. S. Sudbo, P.A. Schulz, E.R. Grant, Y.R. Shen and Y.T. Lee, J. Chem. Phys- 70 (1979) 912. [81 E. Wurzberg, L J. Kovalenko and P.L. Houston, Chem. Phys. 35 (1978) 317. [91 E.K. Piyer and W S. Benedict, J. Res Natl. Bur. Std. US 47 (1951) 202. Tables, NSRDS-NBS 37 (US 1101 JANAF Thermochemical Govt Printing Offices, Washington, 1971). 1111 J.B. hlarlmg and 1-P. Herman, Appl. Phys. Letters 34 (1979) 439, D M Cox, R B. Hall, 3 A. Horsley, G.&f Kramer, P. Rabinowitz and A. Kaldor, Science 205 (1979) 390.