2268 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-17, NO. 12, DECEMBER 1981 Papers Electron Beam iatomic and Triatomic Excimer Abstruct-The spontaneousand stimulated emission characteristics ENERGY, eV 12'10 9 8 7 4 .~~--~~~~.-~~~~-~~~...~~~~~.~~6 5 ~.2 of three recentlyreported broad-band blue-green rare gas-halide ex- Ar? Ktl Xe? cimers, XeF (C+A) at 486 nm, Xe2CI at 518 nm, and KrzF centered RAPE GCSE5 6 6 IrFn.n5 KrCl KIF XeBrUb" YeCl XeFIB-XIn at 436 nm, are reviewed. The influence of different halogen donors RG HCuDES and buffer gases as well as optimization of both the gas mixture and y 4'0 K,O RG 3LIDES E1 8 opticalresonator configuration for wavelength tuning were studied. E CI. CIF Br I. BrF IF ,bq.., Rd. The kinetic mechanisms which describe the formation and quenching hALOGENS o^ 'G of Xe2Cl* are discussed. Problems with achieving optimum gain as well HgCl Z"l MlL HALICES 0 0 d1' as understanding both molecular and atomic absorptions in electron XaF ic -4, beam excited raregas-halide mixtures aredetailed. XeF {C-AI ezE ill F Kr,F Xe,Cl [WF] L',6 "~ , , . .x "7 ." . ," Rg>X TRIMERS f5l CZ I. INTRODUCTION ~ ~-~~~ ~-~~r~. ~1~~ , ~ ~., ~ ~~ -~ ---- ~~~~~~~ INCE 1975, numerous excimers have been reported that 103 2c3 300 ioc 530 600 650 Sare capable of generating high power laser reduction in the WAVELENGTH. nrn __ visible and ultraviolet spectrum [l] [5]. The relatively sim- Fig. 1. Wavelengths and tuning ranges of the various rare gas, rare gas- - halide, rare gas-oxide, halogen, mercury halide, and broad-band RgX ple techniques required to pump such lasers, aswell as their and Rg2X excimer lasers. For Ar2F and Xe2F, thespectral ranges of demonstrated high efficiency, have made them useful lasers in fluorescence are indicated. many interesting applications. Recently,there has been in- terestin exploring the broadbandwidth emission,which is the wavelength selecting element, a tuning range of 4 nm for observed from several diatomic and triatomic excimers as an Ar2 excimer laser at 127 nm [7 J and -2 nm for ArF, KrF, wavelength tunable lasersources (see Fig. 1). Alternatively, XeC1, and XeF (B +X) excimers [8], [9] has been achieved. conversionof narrow-band UV excimer laser emission into In this paper, the general characteristics of three new electron longer wavelength tunable dye laser radiation has been dem- beam pumped lasers, XeF (C--A) [lo], Xe2Cl [ll], and onstrated [4] .l Wavelength tuning ofan excimer was first Kr2F [12], capable of broad-band tunability will be discussed. achieved over a 6 nm tuning range with the Xe, laser centered Fig. 2 summarizes the complexes associated with high pressure at 172 nm [6]. With either a prism or diffraction grating as mixtures of the rare gases argon, krypton, and xenon with the halogens fluorine and chlorine. The pure rare gas excimers, Manuscript received July 20, 1981. This work was supported by the Arf, Krf, and Xef with UV emissions at 126, 146, and 172 Office of Naval Research, the National Science Foundation, and the Robert A. Welch Foundation. nm, respectively, are shown in the first column.The other F. K. Tittel and W. L. Wason, Jr. are with the Department of Electri- columns list their respective exciplex transitions:the short cal Engineering and Rice Quantum Institute, Rice University, Houston, wavelength B+X transition,the longerwavelength broad- TX. -+ G. Marowsky is with the Max-Planck Institute, Gottingen, Germany. band C A transition, and the emission of the triatomic ex- M. C. Smayling was with the Department of Electrical Engineering ciplex. For Ar2F* and Xe,F*, the wavelengths listed are the and Rice Quantum Institute, Rice University, Houston, TX. He is now center of the fluorescenceemission, whereas the data given with Texas Instruments, Houston,TX. 'Commercial excimer laser pumped dye laser systems are offered by for Kr2F* and Xe2Cl* correspond to the center of the laser several manufacturers. emission, as obtained in the experimentsdescribed below. 0018-9197/81/1200-2268$00.75 b 1981 IEEE TITTEL et al.: BROAD-BAND DIATOMIC AND TRIATOMIC EXCIMER LASERS 2269 I Rg X. Rg+X Fig. 4. Chemical structure and potential energy diagrams for a typical (a) diatomic RgX* and (b) Rg2X* triatomic molecule. Fig. 2. Raregas halide complexes and their associated emission wave- lengths. The C -+ A transition of ArF has not yet been identified. Ar LOOtorr Botm Ar Xe 1.2 torr CCIL F CI Br 1 J n I m 300 350 400 450 500 550 lArl WAVELENGTH, nm - I -- 36 54 Fig. 5. Xenon pressure dependenceof XeCl (B -t X)and Xe2Cl fluores- cence spectra. ATOMIC NUMBER - Fig. 3. Relative dissociation energy of rare gas-halide excimers versus the atomic number of the rare gas; represents the pure rare gas Excimer lasers operate on electronic transitions involving a excimers such as Ne;, AI;, Kr;, and Xe;; represents a B -+X ex- bound ionic excited state and a repulsive, or at most weakly ciplex; o represents a C-t A exciplex; * stands for triatomic exci- plexes. Thedouble lines indicatethose trimers whichhave shown bound, lower state that rapidly dissociates. The lowest elec- laser action. tronic level is designated the X state, with higher levels labeled A, B, C, etc. [Fig. 4(a)]. Due to the weakly bound nature of 11. CHEMICALPHYSICS the X state, the allowed B +X transition is inherently narrow Homogeneously broadened excimer emission bands have banded. The C+A transition, however, terminateson the been observed on the long wavelength side of most B +X elec- highly repulsive A state, which gives rise to the broad band- tronic laser transitions as incidental companion fluorescence width of this transition. For the triatomic exciplex [Fig. 4(b)] [l], [13]. These emission spectra are attributed to diatomic Rg2X*, the ground state is repulsive, similar to the A state of C -+ A transitions and triatomic homonuclear and heteronu- the dimer, and thus, the emission is characteristically broad clear rare gas-halide excitiers. banded. : In Fig. 3, the dissociation energies of the rare gas-halide ex- The formation of the trimer depends on several factors. It is cimers listed in Fig. 2 are plotted versus atomic number of the strongly enhanced by increasing both the buffer gas pressure rare gas. Each energy is normalized to the dissociation energy and the concentration of the rare gas. High pressure favors ex- of its respeetiw lare gas excimer Art, Krt, and Xet . The un- cimer formation by, increasing therecombination rate of stable Ne; and NeF* exciplexes have been adde{ for complete- atomic excited states into the molecular upper level.Fig. 5 ness. From this figure it is apparent that: 1) the dissociation depicts the effect of xenon pressure by showing the relative energy of the RgX exciplex decreases with increasing atomic Xe2C1* fluorescence emission for two different Xe' pressures number, and 2) the dissociation energy increases regularly for inelectron beam excited Ar/Xe/CC14 mixtures.The XeCl" a particular rare gas with F, Cl, Br, and I. This diagram is use- fluorescence intensity is not only reduced byopening the for- ful in the search for wavelength positions of new unknown mation channel towards Xe, C1*, but also by increased nonradi- candidates for broad-band C+A transitions (marked by the ative quenching of XeCI" due to the higher Xe partial pressure, rectangular symbols) or new trimers (marked by the triangular as discussed in Section VI. symbols) such as Ar2CI* or the bromides of Kr and Xe. Because of the broad bandwidth characteristics of these ex- Schematic potential energy curves for the various RgX" and cimers, the stimulated emission cross section of the bound-free RgzX" excimer lasers under consideration are shown in Fig. 4. transitions is small. Therefore, pumping rates larger than those *' 2270 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-17, NO. 12, DECEMBER 1981 TO OMA I* PHOTODIODE ACCELER- tt FARADAY CAGE , PDP 11/23 I INTRA CELL OPTICAL RESONATOR FARADAY CUP PROBE TRANSIEWT pdqPLOTTER GRAPHICS ;I PRESSURE GAUGE TO BUFFER GASES +VACUUM SYSTEM Fig. 7. Schematic diagram of the transverse electron beam pumped laser cell. Fig. 6. Blockdiagram of the experimental arrangement and the data acquisition system. and Faraday probe, which can both be mounted inside the laser cell, monitor energy deposition and current density along of atomic level lasers are necessary for reasonable gain. This the optical axis. With an 8 cm X 1 cm cathode and 6 atm of requires intense electron beam or other powerful pumping ex- argon, the current density is -100 A/cm2 and the energy den- citation (fast discharge or optical) of the excimer species in sity is -2 J/cm2 at a distance of 2.5 cm from the anode foil. order to achieve large excited state populations. The forma- The laser cell is connected to an all stainless steel gas-vacuum tion of the excimer relies primarily on ion recombination and manifold. The quality of the gases used in the experiments chemical reactive collisional processes involving excited rare and the filling techniques employed are important factors in gas species and an appropriate halogen donor. obtaining consistent experimental results, since quenching by ., even small amounts of impurities can be severe. The rare gases used-neon, argon, krypton, and xenon-were of research 111. EXPERIMENTAL DETAILS grade, 99.995 percent pure. Halogen donors such as CCI4 and The basic experimental arrangement, illustrated in Fig. 6, NF3 were of spectrophotometric grade, with a purity of 99t consists of an electron beam excitation source, a high pressure percent. The halogen donors were allowed to flow into the re- laser cell, and various electrical and optical instrumentation. action celi slowly to permit precise pressure measurement of Direct electron beam excitation is one of the most general less than 0.1 torr.