December 1, 1995 / Vol. 20, No. 23 / OPTICS LETTERS 2423

Xenon excimers produced from xenon clusters in aquasi-continuous-wave jet discharge

Kenichi Mitsuhashi and Tatsushi Igarashi

Research and Development, Ushio, Inc., 1194 Sazuchi, Bessho-cho, Himeji, Hyogo 671-02, Japan

Motofumi Komori, Toshiaki Takada, Eiji Futagami, Junji Kawanaka,Shoichi Kubodera, Kou Kurosawa, and Wataru Sasaki

Department of Electrical and Electronic Engineering, Miyazaki University, Miyazaki 889-21, Japan

Received August 17, 1995

A new process of formation of xenon excimers from xenon clusters in a quasi-cw jet discharge is experimentallyverif ied. Xenon excimer emission characteristics show a strong correlation with those of xenon clustersproduced in a supersonic gas jet. A xenon cluster density of more than 1016 cm23 is obtained at the dischargepoint. The corresponding xenon excimer peak power at 174 nm is 300 mW, with a power conversion efficiencyof 0.6%. This vacuum-ultraviolet emission lasts ,5 ms (FWHM). 1995 Optical Society of America

Compact vacuum-ultraviolet (VUV) light sources are inhigh demand for potential applications in many fieldssuch as materials processing, life science, and atomicspectroscopy. Rare-gas excimers are one of the mostpromising species, emitting in the wavelength range of100–200 nm, where window materials are available forthe applications.

Since the first report of argon excimer emission ina supersonic gas jet discharge,1 there have been nu-merous efforts with similar supersonic jet dischargeschemes.2 – 8 Efthimiopoulos and Stoicheff 2 reportedargon excimer emission in a pulsed jet discharge, andDube et al. observed various rare-gas excimer emis-sions in dc jet discharges.3 Phillips et al.4 duplicatedtheir pulsed jet discharge scheme to obtain argonexcimer emission but were unable to identify the emis-sion. Tucker and co-workers, however, observed argonexcimer emission5 and krypton excimer emission6 in anintense pulsed jet discharge by improving the plenumpressure of the gas jet.

Following the first report by Efthimiopoulos andStoicheff,1 Sasaki et al.7 reported argon and kryptonexcimer emission in a pulsed jet discharge, and theysuggested new excimer production kinetics that aredifferent from those described in Ref. 1. In such asupersonic gas jet discharge, excimer formation kinet-ics other than the conventional three-body collisionalprocess may be possible in which clusters (especiallydimers) produced in a supersonic gas jet are directlyexcited to produce excimers. Searles et al.8 pointedout the contribution of this cluster excitation in apulsed jet discharge indirectly by comparing the ex-perimental data with their kinetics simulation. How-ever, there are, to our knowledge, no experimentalreports that verify excimer production by a cluster exci-tation process rather than by the three-body collisionalprocess in a supersonic gas jet discharge.

In this Letter we report the experimental verif ica-tion of xenon excimer emission originating from xenoncluster excitation in a quasi-cw gas jet discharge.Cluster characteristics in a supersonic jet were mea-

0146-9592/95/232423-03$6.00/0

sured with a time-of-f light (TOF) mass spectrometer,which showed the strong correlation with the excimeremission characteristics. A xenon cluster density ofmore than 1016 cm23 was obtained at the dischargepoint. The corresponding xenon excimer peak powerat a center wavelength of 174 nm was 300 mW, with apower conversion efficiency of 0.6%. This VUV emis-sion had long pulse widths ranging from 4 to 10 ms(FWHM), which we controlled by changing the stagna-tion pressure.

Figure 1 is a schematic diagram of the experimentalapparatus for the measurement of the excimer emis-sion. A quartz Laval nozzle coupled to a pulsed valve(General Valve, Inc.) provided a xenon-gas jet insidea vacuum chamber. The nozzle had a throat diameterof 0.3 mm and a length of 6 mm. The opening angle ofthe throat was 12±. Orthogonally to the gas jet f low, apair of electrodes was placed 3.5 mm below the nozzle.An aluminum rod with a diameter of 10 mm acted asan anode, and a cathode was made of a tungsten needlewith a 1.5-mm diameter. The electrode distance wasoptimized to be 3.5 mm.

In research with pulsed jet discharges describedpreviously,1 – 6 one of the discharge electrodes was in-serted inside a nozzle and a discharge current was

Fig. 1. Schematic diagram of the experimental apparatus.

1995 Optical Society of America

2424 OPTICS LETTERS / Vol. 20, No. 23 / December 1, 1995

sent through the nozzle. When the discharge currentwas parallel to the gas f low, the gas density gradientat the nozzle exit could not be unambiguously deter-mined. It was thus difficult to distinguish the excimeremission that originated from xenon atoms and/or clus-ters. The transverse discharge configuration that wehave used in the research reported in this Letterpermitted unambiguous determination of the densitydistribution at the discharge point, leading to the de-termination of the major species contributing to the ex-cimer emission.

We applied a high voltage (, 3 kV) between theelectrodes by charging a 0.53-mF capacitor (C inFig. 1) with a dc high-voltage supply (2HV). Whenlaboratory-grade xenon gas was injected from the noz-zle for 5 ms (FWHM), the impedance between the elec-trodes decreased, and the discharge was initiated. Wecontrolled the discharge current by changing a stabi-lized resistor (Rs), and the current was monitored by a300-V resistor (Rm), both inserted into a dischargecircuit. The pulsed gas valve was operated at 0.5 Hz.

The discharge current had a pulse duration of 40 ms(FWHM) independently of the stagnation pressure,and the peak current was adjusted between 0 and5 mA. The VUV emission of the atomic xenon reso-nance line at 147 nm had a pulse width of 50 ms(FWHM), which showed a time behavior similar to thatof the current pulse.

Xenon excimer emission was detected by a photo-multiplier tube with a plastic scintillator coupled to a0.2-m VUV spectrometer. Spectral information wasacquired with a boxcar integrator. To evaluate anabsolute VUV output power of the xenon excimeremission, we used a calibrated VUV photomultiplier(Hamamatsu, R972) with a calibrated bandpass filter(Acton Corporation). Output signals of the boxcar in-tegrator and the VUV photomultiplier were monitoredby a 400-MHz digital oscilloscope.

In the spectral range 120–230 nm the only observedspectra were xenon excimer emission centered at174 nm with a 9-nm spectral width (FWHM) and thexenon resonance line at 147 nm. Xenon-ion lineswere not observed in the wavelength region. Further-more, no other lines originating from the electrode ornozzle material were detected. The rather narrowexcimer bandwidth may ref lect the low clustertemperature3 or vibrational relaxation during thedischarge.

In a separate experimental setup, clusters producedin a supersonic gas jet were monitored by a TOF massspectrometer with a drift length of 1 m. The pressureinside the spectrometer was kept below 1027 Torr. Afrequency-quadrupled Nd:YAG laser (0.2 mJ in 5 ns)was used for three-photon ionization of the xenonand was focused at a point 43 cm from the nozzlewith an interaction volume of 0.6 mm3. To make adirect comparison between the discharge data andthe mass spectroscopic data we used the identicalnozzle and valve to produce xenon clusters inside themass spectrometer. Atomic and cluster ions travelingthrough a drift tube were detected by a microchannelplate.

We obtained time-resolved mass spectra of clustersby changing the delay time between gas injec-

tion and YAG laser irradiation. Figure 2 showsthe time-resolved ion signals of xenon atoms anddimers as a function of the delay time of theionizing YAG laser at different stagnation pres-sures. As the stagnation pressure increases, thexenon dimer signals appear earlier, indicatingthe different dimer formation kinetics. The timedurations of the dimer signals become shorterwhen the stagnation pressure increases. Simi-lar time dependence was observed for the other xenonclusters. In the case of the xenon atomic signals, thepeak behavior is less clear.

To determine absolute cluster densities in a super-sonic gas jet at a discharge point, we calibrated TOFsignals detected by a microchannel plate by measur-ing the signals when a TOF chamber was filled withxenon gas with a known pressure. The pressure wastypically less than 1026 Torr to minimize the signal at-tenuation. The calibrated TOF signals were then com-pared with TOF signals in a supersonic xenon-gas jet.

Figure 3 shows time-resolved xenon excimer emis-sion at 174 nm at three stagnation pressures. Themeasured pulse widths of the excimer emission rangedfrom 4 to 10 ms, depending on the stagnation pres-

Fig. 2. Time-resolved spectra of xenon atoms and dimersin a supersonic xenon gas jet.

Fig. 3. Time histories of the xenon excimer emission (solidcurves) and of the dimer densities (circles) at differentstagnation pressures.

December 1, 1995 / Vol. 20, No. 23 / OPTICS LETTERS 2425

Fig. 4. Xenon excimer emission power as a function of theXe 2 density.

sure of the gas jet. In general, the pulse width of theemission becomes narrower and the emission intensityincreases as the stagnation pressure increases. Thispulse width of the xenon excimer emission is muchshorter than that of the current pulse (50 ms). Inother words, the excimer emission is terminated dur-ing the discharge current. This indicates that the timebehavior of the xenon excimer emission is determinednot by the discharge current but by the condition of thexenon cluster preparation in a supersonic gas jet.

The circles in Fig. 3 represent the time-resolvedxenon dimer (Xe2) mass spectra reproduced fromFig. 2. Almost complete coincidence between the timehistories of the dimer density and of the xenon excimeremission is found. The coincidence of the time be-haviors strongly indicates that the xenon excimerswere produced from fast direct excitation of xenondimers. The well-known excimer formation processby means of three-body collisions seems to be negligiblein the present jet discharge condition.

Because the xenon excimers are produced by thethree-body collision process and/or dimer excitation,the excimer output intensity at a steady state is

I sXe2pd ~ fane 1 bfXepggfXe2g 1 gfXepgfXeg2 , (1)

where a and b are the rate coefficients of the electronimpact excitation and of the energy from metastableatoms, respectively. g is the well-known three-bodyproduction rate coefficient. The rate of electron deex-citation of the xenon excimer is assumed to be muchsmaller than the spontaneous-emission rate of the ex-cimer state, which was supported in our quasi-cw glowdischarge condition. Inasmuch as the pulse widths ofthe current and the resonance line are much longerthan that of the excimer emission, ne and [Xep] areassumed to be constant during the excimer emission.

Figure 4 represents a xenon excimer peak outputpower at 174 nm as a function of the xenon dimerdensity at a discharge point. The excimer outputpower increases linearly with the xenon dimer density.Because of this linear dependence, it is conceivablethat the first term in relation (1) is dominant over thesecond term, supporting excimer production from thedirect excitation of the dimers. Note that the xenonexcimer output power also shows the linear increasewith the xenon atomic density.

The maximum output power of the xenon excimeremission at 174 nm was 300 mW, with a power conver-sion efficiency of 0.6%. An estimated xenon excimerdensity was thus of an order of magnitude of 1011–1012 cm23, resulting in a dimer-to-excimer conversioneff iciency of 1024 –1025. (Uncertainty was caused bythe estimate of the emission volume.)

In summary, we have experimentally verif ied a newformation process of xenon excimers from xenon clus-ters in a quasi-cw jet discharge. Xenon excimer emis-sion characteristics showed strong correlation withthose of xenon clusters produced in a supersonic gas jet.A xenon cluster density of more than 1016 cm23 was ob-tained at the discharge point. The xenon excimer peakpower at 174 nm was 300 mW, with a power conversioneff iciency of 0.6%. We could control the pulse widthsof the VUV emission in the range 4–10 ms (FWHM) bychanging the xenon stagnation pressure of the super-sonic gas jet.

This research was partially supported by a grant-in-aid for scientific research from the Ministry ofEducation, Science and Culture, Japan.

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