Increase in the Efficiency of the Isotope-Selective Infrared Laser Multiphoton Dissociation of ¹¹BCl₃ Molecules in a Mixture with SF₆ Serving As a Sensitizer and an Acceptor of Radicals

A strong increase in the efficiency of the isotope-selective infrared laser multiphoton dissociation of ¹¹BCl₃ molecules in the natural mixture with ¹⁰BCl₃ by radiation of a pulsed CO₂ laser in the case of admixture of SF₆ molecules, which serve as a sensitizer and simultaneously acceptors of radicals, Cl atoms formed in the dissociation of BCl₃ molecules, has been detected. The yield and selectivity of dissociation of ¹¹BCl₃ molecules increase by several times and the threshold energy density for the dissociation of molecules decreases significantly in the case of their irradiation in the mixture with SF₆ compared to irradiation without SF₆. This property allows the single-frequency isotope-selective dissociation of ¹¹BCl₃ molecules by unfocused laser radiation at a moderate energy density (≈3–5 J/cm²), which is important and relevant for the practical implementation of the laser separation of boron isotopes.

1 INTRODUCTION

Interest in the laser separation of boron isotopes has increased recently [1–4] because of their application in important industrial branches, in medicine [5–7], and in space experiments [8]. Boron in nature exists in the form of two isotopes ¹⁰B (about 19.8%) and ¹¹B (about 80.2%) [5, 6]. The thermal neutron capture cross section of ¹⁰B is very large, about 3837 b [9] (1 b = 10^–24 cm²). This cross section for most nuclides is about several or fractions of barn. For this reason, materials enriched in the ¹⁰B isotope (boric acid, boron carbide, and other compounds) are widely used in the nuclear power industry. The ¹¹B isotope is used in electronics industry as a dopant in the manufacture of semiconductor products [6, 7]. Consequently, the development of methods of efficient technologies for the separation of boron isotopes is important and relevant.

The most efficient method of molecular laser separation of isotopes is currently the selective infrared multiphoton dissociation of molecules by the radiation of a pulsed periodic CO₂ laser [10, 11]. Therefore, it is reasonable to use this method in the technological process of separation of boron isotopes. It was successfully applied in the practical technology for laser separation of carbon isotopes [12, 13].

For the molecular laser separation of isotopes with existing efficient pulsed CO₂ lasers, infrared absorption bands of the chosen molecules should be in the generation range of the CO₂ laser and have a fairly large ($\geqslant {\kern 1pt} 5{-} 10$ cm^–1) isotopic shift. In addition, a quite low energy density of laser radiation for the efficient isotope-selective dissociation of these molecules is desired.

One of the most suitable compounds for the laser separation of boron isotopes is the gaseous BCl₃ compound. Infrared absorption bands of ${{\nu }_{3}}$ vibrations of ¹¹BCl₃ (≈954.2 cm^–1) and ¹⁰BCl₃ (≈993.7 cm^–1) molecules [14] are in resonance with the P and R branches of the 10.6-μm band of the CO₂ laser, respectively. The isotopic shift between the indicated infrared absorption bands of ¹¹BCl₃ and ¹⁰BCl₃ molecules is $\Delta {{\nu }_{{{\text{is}}}}} \approx 39.5$ cm^–1 [14]. The isotope-selective laser infrared multiphoton dissociation of molecules was demonstrated for the first time just with BCl₃ molecules [15]. The isotope-selective dissociation of BCl₃ molecules in the radiation of a high-power pulsed CO₂ laser with the use of various acceptors of radicals (H₂, NO, H₂S, D₂S, HBr, etc.) to suppress the association of BCl₂ radicals and Cl atoms formed at the dissociation of BCl₃ molecules [16] was studied in detail in numerous subsequent works [16–26].

It was established that oxygen is a quite good acceptor of radicals to achieve a high selectivity and a high dissociation yield for BCl₃ molecules [17–19, 22]. The final products of the infrared multiphoton dissociation of BCl₃ molecules in the presence of oxygen are B₂O₃ and Cl₂ [17]. The solid B₂O₃ compound is deposited in the form of a film on the walls and windows of the cell, where the gas is irradiated. It was shown in [17, 23] that acceptors of radicals are hardly responsible for any significant increase in the selectivity and yield of dissociation of BCl₃, as well as for the reduction of the threshold for the dissociation of molecules.

Previous studies show that the efficient infrared multiphoton dissociation of BCl₃ molecules requires pump laser radiation with fairly high energy densities ($\Phi \geqslant 20{\kern 1pt}$ J/cm²) [23, 24] mainly because of a high binding energy of the molecules (≈110 kcal/mol [27]). Consequently, only focused laser beams can induce the infrared dissociation of molecules. At the same time, the use of focused laser beams usually reduces the isotopic selectivity of the dissociation of molecules [10, 11].

The selectivity of dissociation of ¹⁰BCl₃ and ¹¹BCl₃ molecules excited by single-frequency laser radiation is α(¹⁰B/¹¹B) ≤ 8 and α(¹¹B/¹⁰B) ≤ 2.5, respectively [22–24]. In this case, the dissociation yields for molecules (recalculated to the laser-irradiated volume of the gas in the cell) were low (${{\beta }_{{10}}} \approx {{\beta }_{{11}}} \leqslant (2{-} 8) \times {{10}^{{ - 4}}}$ [24]). The selectivity and yields of the dissociation of BCl₃ molecules pumped by two-frequency laser radiation are much higher (α(¹⁰B/¹¹B) ≥ 8; ${{\beta }_{{10}}} \geqslant 10{\kern 1pt} \%$; ${{\beta }_{{11}}} \geqslant 20{\kern 1pt} \%$) [28, 29]. In this case, the threshold for the dissociation of molecules decreased significantly (to ${{\Phi }_{1}}$, ${{\Phi }_{2}} \geqslant 2{-} 3$ J/cm²). However, the practical technological implementation of the laser separation of boron isotopes with the two-frequency dissociation of molecules is very difficult.

In this work, we find that the use of SF₆ molecules as sensitizers and acceptors of radicals in the isotope-selective laser infrared molecule dissociation of BCl₃ molecules significantly increases the efficiency of the dissociation of ¹¹BCl₃ molecules. This is very important and relevant for the technology of the laser separation of boron isotopes.

2 EXPERIMENT

The experimental setup (Fig. 1) included a frequency-tunable pulsed CO₂ laser, forming optical elements, a 112-mm-long stainless steel gas cell 24.2 cm³ in volume with BaF₂ windows for the irradiation of studied molecules, and calorimetric (TPI-2-5) and pyroelectric (SensorPhysics Model 510) detectors to measure the energies incident and transmitted through the cell. The pulse of the CO₂ laser consisted of the leading peak with a FWHM duration of about 80 ns and the tail part with a FWHM duration of about 750 ns, which contained approximately one-third of the energy of the pulse. The laser beam was collimated by a long-focus lens (f = 1 m) to the cell. The energy in the laser pulse was 0.7 to 3.0 J, depending on the lasing frequency and on the partial composition of the laser mixture. The laser frequency was varied in the range of 9.2–10.8 μm. To determine the frequencies of radiation lines of the CO₂ laser, we used an optoacoustic detector with the NH₃ reference gas. The transverse energy density distribution and the area of the cross section of the laser beam were determined by scanning of a microprobe across the laser beam in the mutually perpendicular directions at the input and output of the cell.

Fig. 1.

(Color online) Schematic of the experimental setup: (1) mirror, (2) long-focus lens, (3) attenuators of laser radiation, (4) splitter plate, (5) detector of radiation, (6) cell with the irradiated gas, and (7) absorber of radiation.

3 METHOD

The infrared multiphoton dissociation of BCl₃ molecules selective in boron isotopes was carried out with the tuning of the frequency of the CO₂ laser to the 932.96-cm^–1 10P32 line, which coincides with the long-wavelength wing of the absorption band of the ν₃ vibration of ¹¹BCl₃ molecules (≈954.2 cm^–1 [14]). The absorption band of the ν₃ vibration of molecules SF₆ (≈948 cm^–1 [30]) is redshifted by about 6.2 cm^–1 from the center of the absorption band of ¹¹BCl₃ molecules (Figs. 2a–2c). The absorption bands of both ¹¹BCl₃ and SF₆ molecules under infrared laser multiphoton excitation are redshifted (to the laser frequency) because of the anharmonicity of vibrations and fall in the exact resonance with the laser field [31–34]. The effective infrared multiphoton excitation of both SF₆ and ¹¹BCl₃ molecules occurs and the absorbed energy is transferred from the former to the latter molecules. Since the absorption bands of SF₆ and ¹¹BCl₃ molecules almost coincide, the vibrational–vibrational energy exchange between them is resonant and occurs very effectively [35, 36]. This exchange increases the yield of the dissociation of ¹¹BCl₃ molecules. Since the dissociation energy of SF₆ molecules (≈92 kcal/mol [37]) is lower than that of BCl₃ molecules, SF₆ molecules are also dissociated at high pump energy densities.

Fig. 2.

(Color online) (a) Infrared absorption bands of the ν₃ vibrations of ¹¹BCl₃ and ¹⁰BCl₃ molecules in the 11.2‑cm-long cell at a pressure of 0.35 Torr. (b) Infrared absorption band of the ν₃ vibrations of SF₆ molecules in the cell at a pressure of 0.15 Torr. (c) Infrared absorption bands of the ν₃ vibrations of ¹¹BCl₃, ¹⁰BCl₃, and SF₆ molecules in the cell at pressures of BCl₃ and SF₆ molecules of 0.35 and 0.15 Torr, respectively.

Laser infrared multiphoton dissociation of ¹¹BCl₃ molecules occurs through the photochemical reaction

$$^{{11}}{\text{BC}}{{{\text{l}}}_{3}} + nh\nu \to {{\;}^{{11}}}{\kern 1pt} {\text{BC}}{{{\text{l}}}_{2}} + {\text{Cl}},$$

(1)

where $nh\nu$ means n absorbed infrared laser photons.

The products of dissociation induced by the irradiation of the gas were enriched in the ¹¹В isotope, whereas the residual BCl₃ gas was enriched in the ¹⁰В isotope. Dissociation yields, which are fractions of molecules dissociated in the irradiated volume per laser pulse, were determined from changes in infrared absorption spectra for each of the BCl₃ isotopic components.

Absorption spectra were recorded with an FT-801 infrared Fourier transform spectrometer. The dissociation yields β₁₁ and β₁₀ were calculated from the partial pressures of the respective components ⁱBCl₃ (i = 10, 11) before $({{p}_{{i,0}}})$ and after $({{p}_{i}})$ irradiation by laser pulses using the relation

$${{\beta }_{i}} = {{\Gamma }^{{ - 1}}}[1 - {{({{p}_{i}}{\text{/}}{{p}_{{i,0}}})}^{{1/N}}}],$$

(2)

where $\Gamma \approx 0.083$ is the ratio of the irradiated volume (${{V}_{{{\text{irr}}}}} \approx 2.0{\kern 1pt}$ cm³) to the volume of the cell (${{V}_{{{\text{cell}}}}} =$ 24.2 cm³) and N is the number of pump pulses. The intensities of the absorption band of ¹¹BCl₃ molecules before and after irradiation were measured at a frequency of about 954.2 cm^–1 (approximately at the maximum of the absorption band of ¹¹BCl₃ molecules), at which SF₆ molecules hardly contribute to absorption at pressures used in experiments (see Fig. 2b). The selectivity α(¹¹B/¹⁰B) was determined as the ratio of the dissociation yields of ¹¹BCl₃ and ¹⁰BCl₃ molecules

$$\alpha {{(}^{{11}}}{\text{B}}{{{\text{/}}}^{{10}}}{\text{B}}) = {{\beta }_{{11}}}{\text{/}}{{\beta }_{{10}}}.$$

(3)

4 RESULTS AND DISCUSSION

The main parameters of selective laser infrared multiphoton dissociation of BCl₃ molecules, namely, the dissociation yields β₁₁ and β₁₀ of ¹¹BCl₃ and ¹⁰BCl₃ molecules, respectively, as well as the selectivity α(¹¹B/¹⁰B) of the dissociation of ¹¹BCl₃ molecules from ¹⁰BCl₃ molecules, were measured in the experiments. The dependences of these parameters on the pressures of the irradiated BCl₃ and SF₆ gases, as well as on the pump laser energy density, were obtained.

Figure 3 presents infrared absorption bands of the ν₃ vibrations of ¹¹BCl₃, ¹⁰BCl₃, and SF₆ molecules (1) before and (2) after laser irradiation and the infrared absorption bands of the 851.4 cm^–1 ν₁ vibration and the 909.1 cm^–1 ν₈ vibration of the formed SF₅Cl product [38]. The initial pressures of BCl₃ and SF₆ molecules in the cell were 0.35 and 0.15 Torr, respectively. The pump laser energy density was 1.91 J/cm². The number of irradiation pulses was N = 1800. In addition to SF₅Cl molecules, we detected BCl₂F, BClF₂, and BF₃ molecules in the products of dissociation; their infrared absorption spectra are not presented in Fig. 3.

Fig. 3.

(Color online) Infrared absorption bands of the ν₃ vibrations of ¹¹BCl₃, ¹⁰BCl₃, and SF₆ molecules (1) before and (2) after laser irradiation and the infrared absorption bands of the formed SF₅Cl product with centers at frequencies of 851.4 cm^–1 (ν₁ vibration) and 909.1 cm^–1 (ν₈ vibration) [38]. The initial pressures of BCl₃ and SF₆ molecules in the cell are 0.35 and 0.15 Torr, respectively. The energy density of exciting laser radiation is 1.91 J/cm² and the number of pulses is N = 1800.

Figure 4 shows the dependences of (a) dissociation yields β₁₁ and β₁₀ of ¹¹BCl₃ β₁₁ and ¹⁰BCl₃ molecules, respectively, and (b) the selectivity α(¹¹B/¹⁰B) of the dissociation of ¹¹BCl₃ molecules from ¹⁰BCl₃ molecules on the pressure of SF₆ in the range of 0.05–2.0 Torr at a fixed pressure of 0.35 Torr of BCl₃ molecules and the energy density of pump radiation of Φ ≈ 2.1 J/cm². It is seen that dissociation yields of ¹¹BCl₃ and ¹⁰BCl₃ molecules in the considered pressure range of SF₆ molecules increase from β₁₁ $\approx$ 4.5 × 10^–4 and ${{\beta }_{{10}}} \approx 3.4 \times {{10}^{{ - 4}}}$ at the irradiation of BCl₃ molecules in the absence of SF₆ to ${{\beta }_{{11}}} \approx 8.5 \times {{10}^{{ - 2}}}$ and ${{\beta }_{{10}}} \approx 7.2 \times {{10}^{{ - 2}}}$, respectively, in the case of the irradiation of BCl₃ molecules in the presence of SF₆ at a pressure of 2.0 Torr.

Fig. 4.

(Color online) (a) Dissociation yields (1) β₁₁ and (2) β₁₀ and (b) the dissociation selectivity α(¹¹B/¹⁰B) versus the pressure of CF₆ molecules at the pressure of BCl₃ molecules of 0.35 Torr and a pump laser energy density of 2.1 J/cm².

Thus, the dissociation yield of BCl₃ molecules irradiated in the presence of SF₆ increases by more than two orders of magnitude. It is noteworthy that the addition of SF₆ molecules to a pressure of only 0.1 Torr increases the dissociation yield of ¹¹BCl₃ molecules by more than an order of magnitude. This also means that the addition of SF₆ molecules strongly reduces the threshold energy density for the dissociation of ¹¹BCl₃ molecules. The dissociation yields of ¹¹BCl₃ and ¹⁰BCl₃ molecules increase monotonically with the pressure of SF₆ in the range of ≈0.5–2.0 Torr.

The selectivity α(¹¹B/¹⁰B) ≈ 1.3 of the dissociation of ¹¹BCl₃ molecules from ¹⁰BCl₃ molecules irradiated in the absence of SF₆ molecules increases sharply to α(¹¹B/¹⁰B) ≈ 8.0 in the presence of SF₆ at a pressure of ≈0.15–0.2 Torr. With a further increase in the pressure of SF₆ to ≈0.4 Torr, the selectivity decreases rapidly to α(¹¹B/¹⁰B) ≈ 3.0 and then decreases slowly to α(¹¹B/¹⁰B) ≈ 1.0 at a pressure of SF₆ ≥ 1.5 Torr. Thus, the selectivity of the dissociation α(¹¹B/¹⁰B) in the case of irradiation of BCl₃ molecules at a pressure of 0.35 Torr in the presence of SF₆ molecules is maximal at the pressure of SF₆ in the range of 0.15–0.25 Torr. The selectivity α(¹¹B/¹⁰B) in the presence of SF₆ is more than a factor of 3–5 higher than that for BCl₃ molecules irradiated in the absence of SF₆.

Figure 5 presents the dependences of the dissociation yields of ¹¹BCl₃ and ¹⁰BCl₃ molecules, as well as the dissociation selectivity α(¹¹B/¹⁰B), in the mixture of BCl₃ and SF₆ molecules in the cell at their partial pressures of 0.35 and 0.1 Torr, respectively, on the energy density of exciting laser radiation in the range from ≈1.8 to 3.4 J/cm². It is seen that the dissociation yields of BCl₃ molecules increase rapidly with the pump energy density because infrared dissociation at the considered laser energy densities is above-threshold and is comparatively far from saturation in view of a high binding energy of BCl₃ molecules.

Fig. 5.

(Color online) (a) Dissociation yields (1) β₁₁ and (2) β₁₀ and (b) the dissociation selectivity α(¹¹B/¹⁰B) versus the energy density of exciting laser radiation at the pressures of BCl₃ and SF₆ molecules in the cell of 0.35 and 0.1 Torr, respectively.

On the contrary, the selectivity of the dissociation of ¹¹BCl₃ molecules α(¹¹B/¹⁰B) decreases rapidly with an increase in the pump energy density. At the same time, the dissociation selectivity in the presence of SF₆ molecules at all studied pump energy densities is a factor of 2–5 higher than that in the absence of SF₆. The observed dependences of the dissociation yields and selectivity on the pump energy density are characteristic of the isotope-selective laser infrared multiphoton dissociation for numerous types of molecules [10, 11].

5 CONCLUSIONS

A strong increase in the efficiency of isotope-selective infrared laser multiphoton dissociation of ¹¹BCl₃ molecules in the natural mixture with ¹⁰BCl₃ has been detected when the irradiated mixture is supplemented with SF₆ molecules, which resonantly absorb laser radiation and serve as sensitizers and acceptors of radicals. It has been shown that the yield and selectivity of dissociation of ¹¹BCl₃ molecules in the mixture in the presence of SF₆ molecules are one or two orders of magnitude and a factor of 2–5 higher and the threshold energy density for dissociation is about an order of magnitude lower than the respective parameters in the case of the absence of SF₆ molecules. This property allows the single-frequency isotope-selective infrared laser multiphoton dissociation of ¹¹BCl₃ molecules by unfocused laser beams at a moderate energy density ($\leqslant$4–5 J/cm²). The results obtained in this work are important and relevant for the development of a laser technology for separation of boron isotopes because they make it possible to obtain the BCl₃ molecular gas highly enriched in the ¹⁰B isotope because of the dissociation of ¹¹BCl₃ molecules in the natural mixture with ¹⁰BCl₃ molecules.