Oxygen-iodine lasers that utilize electrical discharges to produce O2(a1Δ) are currently being developed.
The discharge generators differ from those used in chemical oxygen-iodine lasers in that they produce
significant amounts of atomic oxygen and traces of ozone. As a consequence of these differences, the
chemical kinetics of the discharge laser are markedly different from those of a conventional chemical
oxygen-iodine laser (COIL). The reactions of O with iodine include channels that are both beneficial and
detrimental to the laser. The beneficial reactions result in the dissociation of I2 while the detrimental
processes cause direct and indirect removal of I(2P1/2) (denoted I*, the upper level of the laser). We have
examined kinetic processes relevant to the laser through studies of photo-initiated reactions in N2O/CO2/I2
mixtures. The reactions have been monitored using absorption spectroscopy, laser induced fluorescence
and time-resolved emission spectroscopy. It has been established that deactivation of I* by O atoms is a
critical energy loss process. We have determined a rate constant of (1.2±0.1)×10-11 cm3 s-1 for this reaction.
As part of this effort the branching fraction for the formation of O2(a) from the reaction of O(1D) with N2O
was determined to be 0.38. This result has implications for lasers based on photolysis of O3/N2O/I2
mixtures and the formation of O2(a) in the upper atmosphere.
The efficient power operation in a chemical oxygen-iodine laser for subsonic modes has been demonstrated. It is
shown that the substitution of the buffer gas N2 by CO2 does not cause any significant variation in the dependence of the
output power on the degree ofdilution ofthe active medium. The maximum power was 581 W for the flow rate of molecular
chlorine 22 mmole/s that corresponds to a chemical efficiency of &eegr;chem = 29%.
The mechanism by which I2(B) is excited in the chemical oxygen-iodine laser was studied by means of emission
spectroscopy. Using the intensity of the O2(b1&Sgr;,&ngr;'=0) → O2(X3&Sgr;,&ngr;''=0) band as a reference, I2(B) relative number densities were assessed by measuring the I2(B,&ngr;')→ I2(X,&ngr;") emission intensities. Vibrationally excited singlet
oxygen molecules O2(a1&Dgr;, &ngr;'=1) were detected using IR emission spectroscopy. The measured relative density of O2(a1&Dgr;,&ngr;'=1) for the conditions of a typical oxygen-iodine laser medium amounted to ~15% of the total O2 content. Mechanisms for I2(B) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A', A) as an intermediate. It has been suggested that, in the dissociation zone, the I2 A', and A states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1&Dgr;,&ngr;'). In the region downstream of the dissociation zone the intermediate states are populated by iodine atom recombination process. I2(B) is subsequently formed in collisions of I2(A',A) with singlet oxygen. We also conclude that I2(B) does not participate measurably in the I2 dissociation process and that energy transfer from O2(b1&Sgr;) does not excite I2(B) to a significant degree.
Oxygen-iodine lasers that utilize electrical or microwave discharges to produce singlet oxygen are currently being developed. The discharge generators differ from conventional chemical singlet oxygen generators in that they produce significant amounts of atomic oxygen. Post-discharge chemistry includes channels that lead to the formation of ozone. Consequently, removal of I(2P1/2) by O atoms and O3 may impact the efficiency of discharge driven iodine lasers. In the present study we have measured the rate constants for quenching of I(2P1/2) by O(3P) atoms and O3 using pulsed laser photolysis techniques. The rate constant for quenching by O3, 1.8x10-12 cm3 s-1, was found to be a factor of five smaller than the literature value. The rate constant for quenching by O(3P) was 1.2x10-11 cm3 s-1. This was six times larger than a previously reported upper bound, but consistent with estimates obtained by modeling the kinetics of discharge-driven laser systems.
The analysis of a luminescence spectra of oxygen molecules on O2(b1Σg,υ’)→O2(X3Σg-υ") transitions has shown, that vibrationally excited O2(b1Σg+) molecules up to υ=5 are generated in the active medium of chemical oxygen-iodine laser (COIL). Comparison of experimental and calculated results has shown that 4.5 vibrational quanta of oxygen are formed in the active medium of COIL under the deactivation of one singlet oxygen molecule. Dependencies of the threshold of O2(α1Δg) yield and gain on relative population of vibrationally excited oxygen are studied. The threshold of O2(α1Δg) yield increases with rising of the relative population of vibrationally excited oxygen and can be some percents more than it was considered before. The gain coefficient weakly depends on the relative population of vibrationally excited oxygen.
The vibrationally excited oxygen in O2(a1Δg)-I mixture was detected by emission spectroscopy. The analysis of a luminescence spectra of oxygen molecules on O2(b1Σg+,υ') → O2(X3Σg-,υ") transitions has shown, that vibrationally excited O2(b1Σg+) molecules up to υ=5 are generated in the active medium of chemical oxygen-iodine laser (COIL). The highest values of relative O2(b1Σg+,υ=1) population of 22% and O2(b1Σg+,υ=2) of 10% are reached for I2 content in an oxygen flow ≈1%. It is shown theoretically, that the relative populations of O2(X3Σg-), O2(a1Δg) and O2(b1Σg+) molecules at the first and the second vibrational levels are approximately equal because of fast EE energy exchange between oxygen molecules. Up to 20% of oxygen molecules in COIL active medium are vibrationally excited.
The kinetics model of chemical oxygen-iodine laser (COIL) active medium taking into account EE, EV, VV, VT energy transfer processes was proposed. The O2 molecule distribution on the vibrational levels in COIL was calculated. It is suggested that the involving of vibrationally excited O2(a) into pooling reaction can increase the rate of stored in singlet oxygen electronic energy in COIL medium. Approximately 50% of relaxed O2(a) energy transfers into thermal energy.
It is experimentally shown that more than 20% of O2 molecules in chemical oxygen iodine laser (COIL) active medium are vibrationally excited. Calculations show that approximately 4.5 vibrational quanta are formed under deactivating of one singlet oxygen O2(1Δ) molecule. Dependencies of threshold O2(1Δ) yield and gain on relative population of O2(1Δ,υ) are presented. The threshold O2(1Δ) yield increases with rising of the relative population of vibrationally excited oxygen and it can be some percents more than was considered before. The gain coefficient of COIL weakly depends on the degree of vibrational excitation of oxygen.