HF-Chemical Laser Based on Hypofluorite (CF3OF or CF2(OF)2)-Containing Mixtures

The possibility of using fluoro-organic hypofluorites (CF3OF or CF2(OF)2 as a source of atomic fluorine for hydrogen oxidation in the working mixture of HF-chemical lasers, with pulsed CO2-laser 
pumping, was proved theoretically and experimentally. The results suggest an increase of the generation 
energy due to the chain reaction energy developed in the chemical laser working mixture.

The idea of HF-chemical laser development was successfully solved in 1969.1 However despite such a "mature" age researchers are still attracted by the problems of the development of new mixtures for HF-lasers, especially those which work in chain-reaction mode. Such mode suggests the increase of the generation power and of the efficiency. One of the most significant features providing the operability of a chemical laser is the method of initiation. 2 By the initiation we mean the way of obtaining fluorine atoms in the working mixture of the laser. One of the possible methods of fluorine atom formation is dissociation of the fluorine-containing molecule. In particular, the bond energy of molecular fluorine is 159.2 kJ/mol. Dissociation may be made by UV irradiation on wavelength 2 < 350 nm. As a result of strong absorption of UV pumping energy in the laser mixture the concentration of fluorine atoms necessary for laser starting will decrease along the excitation area.
IR multiquantum excitation of fluorine-containing molecules may be used as another possible source of photoinitiation. 3 In this case the gas volume irradiated with the pumping radiation forms an antireflecting area, which provides uniform initiation of the mixture through the length of the cell. 4 In this work we use the product of multiquantum dissociation of fluoro-organic hypofluorite molecules (trifluoromethylenehypofluorite, CF3OF, and trifluoromethylene-bis-hypofluorite, CF2(OF)2), irradiated with a pulsed CO2-1aser, as a source of atomic fluorine. The rather weak bond energy of O-F, equal to 182.2 kJ/mol for CF3OF and 166 kJ/mol for CF2(OF)2, makes the above molecules rather attractive for the purpose. 5 CF3OF dissociation under excitation with the pulsed CO2-1aser was shown in, 6  (2) Atomic fluorine formed in the above reactions initiates the usual scheme of HFchemical laser reactions: F" + H2 HF* + H" Vibrationally excited mo]e,cules of hydrogen fluoride formed in the above and in the further reactions are the source of fluorescence with a maximum at 2.7 l.tm. These results allow us to expect hydrogen fluoride molecule generation.
To simplify the calculations let us consider a three-level model of HF-laser, i.e. let us consider the first three vibrational levels of this molecule: a zero, one and two excited levels. The induced transitions in this system posess the highest intensity with rotational quantum number J 6. z Such ]imitation in the consideration of the frequencies of the induced transitions should not distort the real processes significantly, as laser radiation is the most intense at these levels and starts first in time, according to the experimental resutls. ' '9 As the durations of HF-generation pulse and COz-]aser pumping pulse are of similar order of magnitude, the model of instant initiation will not meet the requirements of the problem. Laser dissociation of CF3OF was approximated by an exponential expression which provided the dissociation of about I0% of the initial substance (experimental results given in6'7) in i gsec (COz-laser pulse duration). In I gsec the increase of concentrations according to these laws was stopped according to the program.
In our case the generation will take place at frequences v_, 3693.
A system consisting of 27 kinetic equations given in 8 was used in our calculations.
The respective system of differential equations contained unknown values of the The solution stability was obtained by the automatic step selection, which was 10 -8-10-10 sec. The calculations were made both for isotermal case and for the gas self-heating due to exothermicity of the reactions. The condition of the gas self-heating in a cylindric container of an infinite radius length may be expressed as follows 10: where 2is thermal conductivity coefficient, To-iS inflammation temperature, W i-is the rate of a respective reaction with thermal effect Qi and energy, E i. As it is known from the kinetic theory of gases when the distance of the molecule free path is much lower than the thickness of heat conducting layer, then 2 depends on the pressure. For the pressure value of 1 Torr the distance of the molecule free path, 1 0.008 cm, i.e. the 1 cm thick layer will be already large and table values of 2 can be used. Figure 1 shows the results of computations. It follows from eq. (6) that when the temperature of the gas increases significantly the condition of thermal explosion will always be fulfilled. The generation pulse becomes shorter and the power increase is negligible. The used three-level calculation value for HF-laser does not allow to Time, t,/as describe inversion decrease due to the gas heating. Calculation without considering chain processes, which corresponds to the substitution of CF 3 OF for SF6, shows the decrease of the generation intensity and shortening of its duration.
Nonuniformity of the pumping of the chemical laser active area may have a significant effect of the HF-laser output power. The experiments showed that when pressure varied from 2 to 50 Torr then the absorbed energy value in a 39 cm long cell (which corresponds to the length of the laser cell) varied from 0.6 to 2.0 J (Vlas. 931 cm-1, S 0.16 cm2, (I)--7 J/cm2). Dissociation yield (fluorine atom yield) was also dependent on the gas pressure. We did not manage to obtain experimental data on the CF3OF dissociation yield for high pressures. Therefore the results for CF2(OF)2 molecule dissociation yield were used in the computations. However dissociation yield variation from 10 down to 7% within the respective hypofluorite pressure range from 1 to 21 Torr does not change the chemical laser energy significantly.
The stability of CF3 OF and CF 2 (OF)2 mixtures with oxygen was checked experimentally. It allowed to use standard compositions of HF-chemical laser mixtures with hypofluorite added. CF3 OF (CFz(OF)2): Fz:Oz'SF6"H2 mixture was used. Almost longitudinal pumping was performed with an eximer XeCl-laser. The experimental results showed that the addition of hypofluorites bring about additional losses in the active medium of the chemical laser, which brings about the decrease of the gain factor, and as a result, the decrease of the laser output energy. Such an effect of polyatomic molecules can be explained by the increase of the relaxation rate of hydrogen fluoride vibrationally excited molecules. Besides, fluorophosgen-the final product of CF3OF dissociation-also can absorb the excimer laser radiation and thus decrease the rate of the mixture excitation.
Figure2 shows the schematic diagram of the second experimental unit. Two spherical metal mirrors are used as the chemical laser resonator. The radiation directed out of the resonator with the help of a NaC1 plate was fed to the detector input. Pumping was fulfilled by a pulsed CO2-1aser along the laser cell axis and corresponded to the geometry of a straight beam with-lcm 2 cross-section at the input and output. To eliminate spurious reflections the cell windows were tilted at low angles. The gas mixture is fed into the cell through 2 inlets which are 27 cm apart for better gas mixing. The mixture was pre-prepared in a balast container and then was fed into the pre-evacuated and fluorine-passivated cell. Figure 2 The experimental unit schematic diagram: 1, 2-copper mirrors forming the HF-laser resonator; 3, 4 rotatable copper mirrors; 5 cell with the working gas mixture; 6 rotatable NaC1 plate; 7 filters; 8 detector.
HF-CHEMICAL LASER 269 To check the operability of the laser system HF generation was obtained on SF 6-k-H2 mixture. Then substitution of SF 6 for hypofluorite was made gradually.
The highest intensity of HF-generation was obtained when the working mixture contained one of fluorine-containing molecules. In case of partial disalignment of the resonator generation pulse could be observed on the fluorescent signal background. Blocking one of the mirrors leaves only the fluorescence signal. Figure 3 shows characteristic generation pulses. Change of the laser mixture parameters brings about the pulse shape change as well as delay of the beginning of HF-generation from the moment of pumping pulse.
Addition of 0-25 Torr of Helium into the mixture of CF2(OF)2 (14 Torr)+ H z (6 Torr) resulted first in the increase of the generation intensity more than three times and then in its stabilization and further decrease down to the generation break down. These results demonstrate the effect of thermal processes on the generation. A two-peak structure of the HFgeneration pulse obtained on hypofluorite-containing mixtures shows sufficient contribution of the chain processes into the generation energy. The presence of the second peak seems to be due to the chain processes and simultaneous gas temperature rise. Generation intensity can increase during feeding fluorine into the mixture due to the initiating of a "hot" reaction for forming vibrationally excited SF molecules with the help of fluorine molecules 11. In our experiments fluorine acted as a buffer gas. Application of hypofluorite as an initial source of fluorine atoms shows that "hot" reaction can be used in this case. However Figure 4 shows that additional work is necessary, aimed at the working gas mixture composition improvement to provide positive fluorine effect.
In conclusion it should be mentioned that the possibility to use the molecules of fluoroorganic hypofluorites (such as CF3OF and CF2(OF)2 as hydrogen oxidizers in the working mixtures of HF-chemical lasers has been proved theoretically and experimentally. The pumping energy in this case is sufficiently lower than in case of SF 6 molecules. The experimental results allow us to hope that the energy of the branched chain reaction will be used for the increase of the chemical laser output energy.