SINGLE PULSE CARBON-13 ENRICHMENT OF CF3I UNDER IR MPD IN A SHORT GAS DYNAMIC FLOW

An experimental approach is described for obtaining highly enriched residual gas under IR multiphoton dissociation (MPD) of molecules in one cycle of irradiation. The approach is based on utilization of a pulsed molecular flow of small length (Axn <_ cm). The use of a short flow length leads to high values of the dissociation yield of molecules (/3 ___ 1) in the whole volume of the flow. Owing to this, high enrichment of the residual gas is achieved in one cycle of irradiation. The isotopically-selective dissociation of CF3I in the pulsed gasdynamic flow of a small length was studied. About 400-fold enrichment of the 13C isotope in the residual gas was obtained following irradiation of a molecular flow of CF3I of natural isotope composition by a single laser pulse. The selectivity of dissociation was measured to be c > 10.


INTRODUCTION
The important problem in laser isotope separation by the IR MPD of molecules [1,2] is increase of selectivity, especially in the case of molecules with a small isotope shift (like OsO4, UF6...).
The most effective means to increase the selectivity of the dissociation is the cooling of the mixture of isotopomers in gasdynamic jets or flows [3][4][5]. However, in flow conditions problems appear connected with the collection of the products. Because of high chemical activity of the radicals formed in the course of dissociation of molecules and their small concentration in the flow conditions, it is rather difficult to realize the formation and effective collection of the products. Reactions on the walls can dominate and change fundamentally the kinetics and the channels of product formation.
Besides, in the case of dissociation of molecules with a small isotope shift the achievable selectivities are as a rule rather small (_< [3][4][5]. As a result it is impossible to obtain highly enriched product in one cycle of irradiation of initial gas. Therefore, in many cases it is preferable to dissociate molecules containing unwanted isotopes in the mixture and thereby enrich the desirable isotope in the residual gas.
However, in the case of gas dynamic jets and flows, high enrichment factors in the residual gas are impossible to obtain, if circulation of the gas to allow multi-fold irradiation is not realized. So, in the case of continuous molecular flow, which moves along the x-axis with the mean velocity Ux, one can irradiate by the laser only the part / of molecules as determined by the expression: where AXL is the laser beam size in the direction of x-axis, and f is the repetition rate of the laser pulses. Here we have assumed that the laser beam is directed perpendicular to the x-axis, and the flow in the directions of y-and z-axis is confined and totally irradiated by the laser.
With the pulsed-periodic CO2-1asers [6] one can realize the parameters AXL--1 cm and f_ 500 Hz. With a characteristic mean velocity of molecular flow ux5.104cm/s the fraction of irradiated molecules will be /___ 10-2. (Here we have taken into account the fact, that the CO2-1aser pulse duration is 7-p _< las << AXL/UX, and therefore during the irradiating pulse the position of the flow in the space will be practically not changed). If the dissociation yield is equal to/3 0.2, then the fraction of dissociated molecules in the flow will be /; //3 2.10-3. Consequently, the enrichment factor in the residual gas in one cycle of irradiation will be very small.
The situation is better when pulsed molecular flows are used. However, this is still rather small fraction, and consequently, the enrichment factor in the residual gas will be small as well.
In principle another situation is reached, when dissociation is carried out in a short pulsed gas dynamic flow (Axn <_ cm), which one can obtain by means of a pulsed nozzle with a short opening time (rnoz _< 20 gs). In this case one can subject all the flow to irradiation by a highly intense IR laser. The enrichment factor in the residual gas will be mainly determined by the dissociation yield of the resonantly excited molecules. With the dissociation yield /3 one can obtain high enrichment factors in the residual gas as a result of irradiation of a mixture of isotopomers by a single laser pulse even at moderate selectivities, i.e., o >_ 3.
Since the absorption spectra of molecules are narrowed due to cooling in the gasdynamic flow [5], one can obtain comparatively high selectivity (c > 5-10) even at high energy fluences. Owing to this the dissociation of the desired isotopic component is limited.
Such an approach has been developed in this work and its possibilities studied using the CF3I molecule. About 400-fold enrichment of the residual gas by 13C isotope was obtained under irradiation of CF3I of natural isotope composition (_1.1% of 13C, 12C/13C90) in a molecular flow by a single laser pulse. The dissociation yield of 12CF3I molecules and the selectivity of the dissociation in this experiment were measured to be ill2"" and c > 10, accordingly.

THE CHOICE OF THE OBJECT FOR STUDY
The CF3I molecule was chosen for study on the following reasons. It has a rather low dissociation energy (_2.3eV [7]) and effectively dissociates at moderate energy fluences <_ 4 J/cm 2 [8][9][10][11]. Therefore, with this molecule it is easy to realize the conditions at which the dissociation yield/3___ can be achieved. In the dissociation of CF3I stable products (C2F6 and I2) are formed. The IR multiphoton excitation (MPE) and dissociation (MPD) of this molecule are rather 208 G. N. MAKAROV et al.
well studied [8][9][10][11][12]. The IR multiphoton absorption of CF3I was also investigated [13] under molecular beam conditions. CF3I has been considered [14] as a starting substance for the large scale laser separation of carbon isotopes. Furthermore, the IR absorption spectra of 12CF3I and 13CF3I isotopomers are well studied and the isotope shifts in these spectra are measured [15], making it easier to choose selective frequencies.

THE RELATION BETWEEN THE PARAMETERS OF THE SEPARATION PROCESS IN CONDITIONS OF THE PRESENT EXPERIMENT
In conditions when all the molecular flow is irradiated by the laser the concentrations of molecules in a two-component mixture after the action of a laser pulse will be: where N10 and N20 are the concentrations of isotopomers in an initial mixture, /31 and /32 are the dissociation yields of these molecules, accordingly.
Let the laser radiation be in resonance with the molecules marked by index 1. Then/31 >/2. The selectivity of the dissociation will be: The enrichment factor in the residual gas will be determined by the expression: or, taking in to account (2), (3) and (4) it will be: The enrichment factor in the products will be equal to: Nlprod / N10 fllNIO/Nlo__ N2prod N2---=/32 N20 020 /31//32 c Therefore, to obtain the highly enriched product it is necessary to realize a rather high selectivity in the dissociation process, which is not easing achieved in many cases even with cooled gas.
However, one can obtain (even at moderate selectivities c> 3) highly enriched residual gas. As follows from relation (6) Figure 2a. To obtain a short molecular flow a "current loop" type pulsed nozzle [16] was used. The opening time of the nozzle was about 18gs (at half maximum). The diameter of the nozzle aperture was 0.75 mm. The CF3I pressure in the nozzle P0 could be varied from to 5 atm. The number of molecules, N, flowing from the nozzle per pulse depended on P0 and at p0-2 atm N was equal to about 1016 molecules/pulse. The nozzle could operate both in single pulse regime and at repetition rate up to Hz. The opening time of the nozzle, -noz, and the mean flow velocity, Ux, were determined with a pyroelectric detector using a time of flight technique [17,18]. The mean CF3I flow velosity in the excitation zone was Ux (400 4-20)m/s [19].
The molecular flow was formed (Fig. 2b) with the help of two thin (100 gm) metallic strips. In the plane xz they had a variable radius of curvature. The maximum angle of opening of the strips (near the nozzle exit) was about 60 The dimensions of the strips were 2.5 x 2.5cm2. The minimum distance between the strips (near the nozzle excite) was about 1.5 mm and the maximum one was about 8 mm. Since the pulse of molecules flowing from the nozzle had the length Ax--Ux-noz7.2mm, it was totally enclosed in the space between the strips. Exactly at the moment when the molecules flew through this space they were subjected to irradiation.
Excitation was carried out using a line to line tunable TEA CO2laser. The energy in the laser pulse was up to 3 J. The laser radiation was slightly focused (fL m) and directed into the vacuum chamber in the region between the strips which confined the molecular flow. The laser beam was perpendicular to the flow axis. The shortest distance from the nozzle exit to the excitation zone was about 3 mm. In the excitation region the laser beam cross-section was about 6 x 6 mm2. In the y-and z-direction the laser beam irradiated all the space between the strips, while in the x-direction its size AxL _ _ _ 6 mm was less than the flow length Axn _ _ _ 7.2 mm. In order to irradiate all the flow, the laser beam was reflected by small angle back, so that the length of the irradiating volume in the x-direction was about 12 mm. Therefore, all the molecular flow could be irradiated by highly intensive IR laser radiation. The energy fluence in the excitation zone was up to 8 J/cm 2. At such energy fluence the dissociation yield of CF3I is practically equal to unit [11,14].
The synchronization of the CO2-1aser, the pulsed nozzle and the detection system was carried out with the help of the generator of delayed pulses GI-1.

THE PROCEDURE OF COLLECTION OF THE RESIDUAL GAS AND PRODUCTS AND THEIR ANALYSIS
In addition to the main pump outlet the vacuum chamber had another (bypass) pump outlet, in which a liquid nitrogen trap, a gas cell with ENRICHMENT OF CF3I BY IR MPD 213 small trap and a manometer were assembled. The vacuum chamber could be pumped also through this channel. We used in this channel a forvacuum pump. The procedure of irradiation and collection of the gas was as follows. At first the vacuum chamber and the nitrogen trap were pumped down to about 10 .5 Torr by turbomolecular pump. After that the pump channel was closed, the cryogenic trap was cooled and the irradiation of CF3I molecules in the flow was started. The residual CF3I gas and the products (mainly C2F6) were collected in the trap.
The irradiation cycle consisted of from 50 to 500 pulses. During the irradiation time the pressure in the vacuum chamber did not increase above 10 .2 Torr. For example, the increase of pressure in the vacuum chamber for 500 pulses was equal to Apc h ANkr/gch7.10 -3 Torr. (Here AN 5.1018 is the number of molecules delivered from the nozzle into the chamber for 500 pulses and gch 2"10 4 cm is the volume of the vacuum chamber). The effective pressure of the CF3I molecules in the irradiation zone was peff NkT/Vn-0.2 Torr (Vn _ _ _ cm 3 is the volume of the flow).
After the irradiation cycle was finished, the vacuum chamber was filled with oxygen to pressure of about Torr. After a while the gas from the chamber was slowly pumped through the bypass channel. In this way only the oxygen was pumped while the CF3! and C2F6 collected in the trap. Following this, gas from the trap was transferred into the gas cell for the analysis.
The analysis of the gas was carried out using a"Specord-75 IR" IR spectrophotometer. The absorption of the gas in the region from 600 to 1400 cm -, where the most intensive absorption bands of CF3I and C2F6 lie, was recorded. The enrichment of the residual gas in the 3C or 2C isotope was determined on the IR absorption of CF3I in the region of the /'4 vibrational band (1187cm -1 for the 2CF3I [15]), where the absorption spectra of 2CF3I and 3CF3I isotopomers are rather well resolved (AUis___33cm - [15]). More accurately the enrichment factor in the residual CF3I as well as in the product C2F6 was determined from mass-spectra taken with MX-7303 massspectrometer. The isotope composition of CF3I was determined on the CF3 I+ ion peaks (m/e-196 and 197) and the isotope composition of the C2F6 was done on C2 Fion peaks (m/e 119, 120 and 121).
The results obtained are shown in Figures 3(a-d). Figure 3a presents the absorption spectrum of nonirradiated CF3I molecules. The ratio of isotopomers in the mixture was measured to be natural. Figure 3b shows the spectrum of the CF3I molecules irradiated in the flow by the 9R (10) laser line at a fluence _ _ _ 8 J/cm 2. In this experiment the laser beam crossed the molecular flow only in one direction. The reflecting mirror was not mounted. Therefore not all the molecules in flow were irradiated by the laser (Axz: < Axn, Ax 6 mm, Ax 7.2 mm). Almost 7-fold enrichment of the residual CF3I in the 13C isotope was obtained (K] 6.8). Figure 3c shows the absorption spectrum of the CF3I irradiated in the flow by the 9R(12) laser line under conditions where all molecules were excited by the laser (reflecting mirror was mounted). The excitation energy fluence was 3.5 J/cm 2. At such energy fluence the dissociation yield of CF3I is less than unity. More than 5-fold enrichment of the residual gas in the 13C isotope was observed 5.2) Finally, Figure 3d presents the absorption spectrum of 13 CF3I irradiated by the 9R(10) laser line in conditions, when all the molecules in the flow were excited by intensive laser radiation (Axz 12 mm, ff _ 7.5 J/cm2). One can see, practically all 12CF3I molecules, which were contained in the natural mixture (_99%) have dissociated. The residual CF3I mainly consists of 13CF3I molecules (> 82%). In this experiment about 400-fold enrichment of the residual CF3I by 3C isotope was obtained. The selectivity ofthe dissociation was measured to be c12 > 10. We could not measure values of OZ12 > 10 because the dynamic range ofthe measurable intensities ofmass peaks in our mass-spectrometer was less than 103 The dissociation yield of 2CF3I in this experiment was measured to be/312 99.8%.
As described above, under the conditions of the present experiments the measured enrichment factor is not the result of a number of gas in the 12C isotope took place. For example, the irradiation of CF3I by the 9P(24) line in conditions similar to those for Figure 3b (Axr 6 mm, _ 8 J/cm2) resulted in more than 4-fold enrichment of the residual gas in the 12C i.sotope (K] 4.1). The selectivity of dissociation was measured to be c _ 11. Not very high selectivity in the case of excitation of 13CFI molecules is mainly connected with the fact that a rather intense combination band u + u of aCFI molecules lies near the ul vibrational band of ICF3I [15]. Note, that enrichment factors in the residual gas in the case of excitation of ICFI were less compared with those for the case of excitation of ICFI. In our opinion, this is connected with that the scrambling reaction of a:CF: radicals with the a:zCF:I molecules decreased the total dissociation yield of I:CF:I. The quantity of product CF6 formed was established to be strongly dependent on the concentration of CFI in the flow. This is probably explained by the mechanism of C:zF6 formation via pair collisions of CF3 radicals.

CONCLUSION
A pulsed gas dynamic molecular flow of small length (Axn < cm, -n <_ 20gs) was used to obtain a high degree of enrichment in the residual gas in single pulse IR MPD. The isotope-selective IR MPD of CF3I was studied in this fashion. Conditions were realized at which the whole flow could be irradiated by an intense IR laser radiation and the dissociation yield of resonantly excited molecules reached the values /3 1. About 400-fold enrichment of the residual gas in the 13C isotope was obtained following irradiation of CF3I of natural isotope ENRICHMENT OF CF3I BY IR MPD 217 composition in a single laser pulse. The dissociation yield and the selectivity were measured to be ill2 and O12 10, accordingly. Since, in the approach described, high values of the enrichment factors in the residual gas can be obtained at moderate selectivities a > 2-3, it is probably applicable also to the heavy molecules with a small isotope shift. This approach can be applied to the deep cleaning of gases from admixtures. To increase of the productivity of the process and to make more effective use of the laser radiation pulsed slit nozzles will be preferable.