Resonantly enhanced, three-photon ionization of Xe: Optically pumped rare-gas laser

Abstract

Three-photon ionization of xenon, resonantly enhanced by the Xe 6p [1/2]c state at g0 119 cm ', is observed when a xenon atom simultaneously absorbs one ArF (51733-cm ') and two XeF (28482-cm ) photons. For ArF and XeF laser intensities in the 1 -100-MWcm range, the peak electron density is more than an order-of-magnitude larger than that obtainable by the nonresonant, two-photon ionization of Xe by ArF. The spontaneous radiative lifetime of the 6p[1/2]c level and the rate constant for quenching of the state by background Xe atoms were measured to be 29 +4 ns and 4+1 &&10 'c cm3 s ', respectively. Excitation of the 6p [1/2]c state is both selective and intense and has resulted in iasing on the 6p[1/2]c 6s[3/2]t transition at 828.0 nm, which represents the first optically pumped rare-gas laser.While electron beam and discharge excitation are capable of producing large concentrations of atomic ions in the gas phase, neither possesses the state selectivity that is characteristic of laser pumping but instead produces an array of neutral and ionic species. The large ionization potentials of the rare gases, however, have hindered the widespread application of linear optical pumping techniques and so, in recent years, considerable effort has been expended in developing nonlinear, multiphoton excitation and ionization schemes (involving lasers of high peak power) for the rare gases. Nonresonant multiphoton ionization of the rare gases using ruby' or an harmonic of Nd: YAG (Ref. 2) and resonantly enhanced ionization, exploiting the tunability of a dye laser, have been demonstrated.Due to the small cross sections inherent with nonresonant processes, the former approach requires enormous () 109 -10ta W cm ') intensities to obtain (even for the heavier rare gases) modest ion densities while the latter generally sacrifices laser pulse energy.These limitations can be circumvented by utilizing the high peak power and photon energy combinations that are obtainable with the use of two or more raregashalide excimer lasers. With this laser family, two photon-allowed excited states from 7 to -13 eV can be studied and no more than four photons are required to ionize any of the rare gases.Although neutral excited states of Xe and Kr have been produced with excimer laser radiation of a single wavelength, photoionization of only Xe has been reported. The coefficient for two-photon ionization of Xe at 193 nm (ArF) has been calculated' and recently measured ' and three-photon resonant, fourphoton ionization of Xe at 351 nm (XeF) has been observed. 9The observation of three-photon ionization of Xe (where ht t W h t z = h v3), resonantly enhanced by the 6p[1/2]c state at 80119 cm ' (2ps in Paschen nota-tion), is reported in this Communication. The simultaneous absorption of one ArF (51 733 cm ') and two XeF (28482 cm ') photons by the atom is found, for laser intensities in the 1 -100 MWcm ' range, to enhance the Xe+ ion production rate by at least an order of magnitude over the two ArF photon, nonresonant process. While representing a mode...

A pulsed source for Xe(6s[3/2]1) and Xe(6s′[1/2]1) resonance state atoms using two-photon driven amplified spontaneous emission from the Xe(6p) and Xe(6p′) states

“…In addition to the amplified stimulated emission, the single-color, two-photon excitation of the Xe͑6p, 6pЈ and 7p͒ states in low pressure of Xe can produce other nonlinear Raman scattering processes, and these also may have practical importance. Three-photon resonance ionization 36 and collisional ionization processes also can occur and care must be exercised in choosing experimental conditions to control the degree of ionization.…”

Stimulated directional emission induced by two-photon excitation of the Xe 6p′ and Xe 7p states

“…The first observation of the SE effect in Xe was reported in Ref. 2. Irradiating a cell containing Xe gas at a pressure of 20 Torr with ArF λ = 193 nm and XeF λ = 351 nm lasers, the authors observed stimulated 6p [1/2] 0 → 6s[3/2] 1 emission at λ = 828.0 nm (the combined energy of the λ = 193 and 351 nm photons matches the 6p[1/2] 0 energy).…”

“…In Ref. 2 Xe gas was excited laterally. The length of the volume containing excited Xe atoms was determined by the length of the rectangular laser beams (2.9 cm).…”

“…The studies discussed above [2][3][4] were concerned with the low lying 6p [1/2] of stimulated emission induced by two-photon excitation of higher lying 6p , 7p (see Fig. 1), 8p, and 4f states was reported by Miller.…”