ABSTRACT
Laser radiation from molecular fluorine transitions at 156.71, 157.48, and 157.59 nm was observed for the first time in 1978 by J.R.Woodworth and J.K.Rice [1,2]. They used ebeam excitation to achieve population inversion in a mixture of He (1500 mbar) and F2 (5.3 mbar). The observed stimulated emission was assigned to the 3Пg→3Пu transitions of the F2 molecule. The intensity of the laser pulse was 7 MW/cm
2 and the efficiency was 3.5%. The experimental setup used by Woodworth et al. was complicated and the system had to be pumped and filled back after each shot. Plummer et al. [3] in 1979 used a different more convenient technique by pumping mixtures of F2 and He in a fast ultraviolet (UV)-preionized discharge. This technique proved successful for pumping of the excimer lasers and it was more flexible. The cathode electrode whose position was fixed consisted of a stainless steel mesh. The discharge volume could be varied by moving one electrode in order to find the optimum electrode spacing for higher laser output. In 1985 Cefalas et al. [4] developed a simple UV preionized F2 laser of the fastdischarge type to measure the small signal gain by the passive cell absorption method. The gain with this equipment at optimum working conditions of 2 atm total gas pressure and 2 cm electrode spacing was found to be 3.2% cm−1. The output energy was measured as 12 mJ per pulse and the pulse duration was 10 ns despite the use of gases with high concentration of impurities (0.1% impurities). With this experimental apparatus it was proved that the molecular fluorine laser had the potential to be a powerful VUV laser similar to the excimer lasers. Following its development, higher output energy of 15 mJ per pulse was achieved by Ishchenko et al. [5]. The above values of output energies were half those predicted by the theoretical calculations of Ohwa and Obara [6], if one considers only the dissociative collision of F2 molecules by ion-ion recombination of energy transfer reaction and neglecting the direct excitation of F2 molecules by electron
impact or energy transfer from He*-, He**-, and -excited atoms or clusters. The predictions of Ohwa and Obara were confirmed by Yamada et al. [7], who
developed an F2 laser that delivered 112 mJ per pulse at 8 atm total gas pressure. The electric circuit in this apparatus was similar to that developed by Cefalas et al. (1985) of the fast charge transfer type with UV preionization. The innovating point in the geometry
of this cavity was the small distance between the electrodes of 10 mm, providing stable discharge even at higher gas pressure and hence improving the laser’s performance. In an effort to investigate the physical parameters and limits of the F2 molecular laser, Cefalas et al. used two discharge laser heads driven by a spark-gap switch to measure the smallsignal gain and the saturation intensity in the oscillator-amplifier configuration [8,9]. The small-signal gain coefficient was measured to be 5.2±0.4% cm−1 at 3 atm total pressure and 1.5 cm electrode spacing. It was 4.1±0.4% cm−1 at 2 atm total pressure and 2 cm electrode spacing. The values of saturation intensities were found to be 5MW/cm2 and 4.6 MW/cm2, respectively. The dependences of the energy output and efficiency of the F2 laser on the pump power (up to 40 MW/ cm3) have been studied by Kuznetsor and Sulakshin [10]. The maximum lasing efficiency was 0.05% and the laser radiation energy was 120 mJ at pressure of the He-F2 mixture of 3 atm. A theoretical kinetics model successfully described the characteristics of the F2 laser output. The authors concluded that at high gas pressure operation, where dissociation of the F2 (A′) state is accelerated, higher discharge pumping power is achieved [11]. The small-signal gain and the saturation intensity of the discharge pumped F2 laser, operated at higher pressures (<10 atm) and higher excitation rates (7-39 MW/cm3), were measured [12] with a similar oscillator-amplifier configuration to that used by Cefalas et al. [8,9]. The small-signal net gain with this apparatus now reaches 37±4% cm−1 at an excitation rate of approximately 26 MW/cm3 for a 6-atm gas pressure. The saturation intensity, which depends on a nonsaturation absorption coefficient, was estimated. The output energy and the temporal
behavior of a molecular laser pumped by a coaxial electron beam have been measured in gas mixtures of He/ F2 and He/Ne/F2 [13]. The highest output energy of 172 mJ has been obtained in a mixture of He/Ne/F2 (19.9%/80%/0.1%) at a pressure of 12 bar, correspond-ing to a specific output energy of 10.8 Joule/lt and an intrinsic efficiency of 2.6%. An electron beam pumped molecular laser with pulse width up to 160 ns, and output energy of 1.7 J (optical flux of 4.6 MW/cm2) has been realized [14] by the same group. The widths of the laser pulses seem to be limited by the duration of the excitation pulse (160 ns). For specific output powers up to 100 kW/cm3, no signs of selfterminating laser pulses, due to bottlenecking in the lower laser level, have been observed. The application of a prepulse-main pulse excitation scheme utilizing a saturable magnetic switch in combination with x-ray preionization has resulted in the generation of long optical pulses from a molecular fluorine laser [15]. Optimum laser pulse durations of 70 ns (full width at half maximum) have been obtained in a gas mixture of helium and 3 mbar fluorine at a total pressure of 2 bar. Laser pulse duration was limited by instabilities in the electric discharge. The laser pulse duration was found to decrease with increasing fluorine pressure and to saturate with increasing current density.
