ECTROCHIMICA
`ACTA
`“
`[PART B: ATOMIC SPECTROSCOl;f§T“'<~:=;.:
`Vol. 40B, No. 4, 1985
`
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`U1 up 1 r\u\,r111v.uLt-1 AL 1 A
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`PART B: ATOMIC SPECTROSCOPY
`EDITOR-IN-CHIEF
`Dr. P. W. J. M. Boumans, Philips Research Laboratories, 5600 JA Eindhoven, The
`Netherlands
`7
`EDITORS
`Dr. Walter Slavin, Perkin—Elmer Corporation, Main Avenue, Norwalk, CT 06856, USA
`Dr. H. W. Werner, Philips Research Laboratories, 5600 JA Eindhoven, The Nether/ands
`ASSISTANT EDITORS
`Dr. J.. A. C.’ Broekaert, Institute of Spectrochemistry and Applied Spectroscopy (15343),
`P.0. Box 778, 4600 Dortmund, Federal Republic of Germany
`Dr. R. J. Decker, Rhodesia University, Chemistry Department, P.0. Box MP l69, Harare,
`Zimbabwe
`EDITORIAL ADVISORY BOARD
`Chairman: J. D. Winefordner, Gainesville, FL
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`The Netherlands
`H. Falk, Berlin, GDR
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`This material may be protected by Copyright law (Title 17 U.S. Code)
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`666
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`DAVID A. CREMERS et al.
`
`atmospheric pressures or above. The discharge resides near the focus of the laser be;
`independent ofany physical support and does not require a gas flow to stabilize the plasma,
`some other sources. Because the discharge is maintained using optical frequencies (30 TH:
`the plasma is called a “continuous optical discharge” (COD).
`
`small, about 1 mm in diameter, and appeared as a very bright white light. The plasma“,
`initiated by the spark produced by a focused Q-switched Nd :YAG laser pulse superimposed,
`on the focal volume of the cw-beam because the cw powers used to maintain the plasma we
`insufficient to induce optical breakdown. Typically, the focused powers of cw-CO2
`105-107 W/cmz, several orders of magnitude smaller than the 108-109 W/cmz
`
`produced on the surface of a material which is temporarily introduced into the focal volu
`of the cw-beam [15]. Once started, the plasma operates continuously as long as suffici’
`intensity is supplied to the focal volume.
`
`
`
`Fig. 1. Photograph of the COD produced inside a small quartz tube containing Xe at 2000 torr. The ~
`cw-CO2 laser beam entered the cell from the right. The plasma was photographed for 1 s, atf8.6, with
`a neutral density filter (ND) = 3 in front of the camera lens. Then the plasma was turned off and the
`cell photographed under room lights with the ND filter removed.
`
`[19] G. I. KozLov,V.A.Kuzr~;ETsov and V.A. MAsvUKOv,Zh.Tekh.Fiz. 49, 2304 (1979) [Sov. Phys. Teclrl’/1 '
`1283 0979)].
`[20] D. C. SMITH and M. C. FowLi-IR, Appl. Phys. Len. 22, 500 (1973).
`[21] D. R. KEEFER, B. B. HENRIKSEN and W. F. BRAERMAN, J. Appl. Phys. 46, 1080 (1975).
`
`identical gases and pressures [4, 10, 17]. The temperature of the COD depends upon theigas
`and at 1520 torr has been mmsured spectroscopically to be a maximum of 14000 Ki Xe
`
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`
`Evaluation of the continuous optical discharge
`
`667
`
`3000 K in Ar, and 22000 K in N2 [16]. These are much higher than the temperatures
`31-acterizing other continuous excitation sources: arcs (4000-5000 K), ICP (60()0~8000 K)
`d a microwave discharge (5000—7000 K). The temperature of the COD can approach that
`fained by sparks (20 000 K. or higher) and is due to penetration of the high frequency optical
`ation into the core of the plasma [16]. At laser frequencies, which are typically above the
`sma frequency [22], absorption occurs mainly via free—free transitions (inverse
`femsst1‘ahlung) associated with electron—ion collisions [23]. For comparison, at radio and
`crowave frequencies, which are below the plasma frequency, plasma heating occurs
`771-ough direct plasma—electric field interactions characterized by larger absorption coef-
`clents. Consequently, only the outer layers of these plasmas are heated. The higher
`imperaturc of the COD is also related to its greater input energy density (p) compared to
`re conventional discharges. For example, for a 0.1-cm dia. plasma, a laser power of 45 W,
`‘d assuming 50% absorption by the plasma [7], p = 5 x 103 W cm”, vs p = 188 W cm‘ 3
`an ICP of l.5 kW and a volume of 8 cm3. The electron density of the Ar COD is
`out 10” cm”, at least an order of magnitude greater than other common continuous
`durces [4].
`tudics of the COD to date have concentrated mainly on measurements of its physical
`roperties in various gases under different conditions. Several theoretical models of the COD
`ave been developed to account for these properties [2429] and a few applications have
`em suggested [30—32]. An almost complete listing of the previous studies is presented in the
`eferences cited herein. To our knowledge, the COD has not previously been investigated as
`n excitation source for atomic emission spectroscopy. The high temperatures and electron
`ensity of this plasma should provide improved excitation for species diflicult to detect with
`ooler conventional cw sources. In addition, the COD is a single source which combines the
`h temperature of a spark with the continuous operation of a do are, the goal behind the
`evelopment of many other types of spectroscopic sources [33]. In this paper we present the
`ults of a preliminary examination of this plasma for spectrochemical analysis. Particular
`mphasis is placed on characteristics of COD generation, variation of plasma properties with
`perating parameters, and analytical performance.
`
`2. EXPERIMENTAL
`
`Apparatus
`schematic of the apparatus used to generate the COD and record the emission spectrum is shown in
`1g. 2. The experimental conditions are listed in Table l. The gas cell was evacuated to a pressure
`50 um before adding plasma gas. The cw—CO2 and pulsed Nd : YAG laser radiations were focused
`me the same volume of the cell at right angles as shown. The focal volume of the pulsed beam was
`usted to overlap that of the cw-CO2 beam by moving the glass imaging lens slightly using an X YZ
`
`
`
`] The plasma frequency (vp) is given by V; = (eznc)/(41rZe0mc), Where e, He, and me are the electronic charge,
`density, and mass, respectively, and so is the vacuum permittivity. At frequencies below vp, EM waves interact
`directly with the plasma to induce heating. At frequencies above vp, the waves only interact with individual
`electrons and ions to heat the plasma. For an electron density of8 X 10”‘ cm”, vp : 2.5 X 10” Hz, which is
`about an order of magnitude below the CO2 laser frequency of 30 TI-lz.
`’
`] T. P. HUGHES, Plasmas and Laser Light. John Wiley, New York (1975).
`] A. A. Kuunnov, T. YA. PoPovA and N. G. PREOBRAZENSKY, Pros. ofthe XIII Int. Conf. on Phenomena in
`Ionized Gases, 1977, Part II, p. 899. Berlin, GDR (1977).
`_
`25] G. I. KozLov and l. K. SELEZNEVA, Zh. Tekh. Fiz. 48, 386 (1978) [Son Phys. Tech. Phys. 23, 227 (1978)].
`] M. V. GERASIMENKO, G. I. Kozrov, V. A. KUZNFTSOV, Pis’ma Zh. Tekh. Fiz. 6, 485 (1980) [Son Tech. Phys.
`‘ Lett. 6, 203 (l980)].
`] Yu. P. RAZIER, Pis’ma Zh. Tekh. Fiz. 7, 938 (1981) [Sov. Tech. Phys. Lett. 7, 404 (198l)].
`] S. MULLER and J. Uurhwnuscn, Physica 112C, 259 (1982).
`19] G. G. GLADUSII and A. N. YAVUKHIN, Kuantovayu Eleklron. (Moscow) 10, 1399 (1983) [S01]. J. Quantum
`_
`Electron. 13, 908 (l983)].
`[30] R. W. THOMPSON, E. J. MANISTA and D. L. ALGER, Appl. Phys. Lett. 32, 610 (1978).
`i’ ] Laser Focus, Dec. 1977. p. 20.
`32] N. H. KEMP and R. G. Roor, J. Spacecraft 16, 65 (1979).
`] P. W. J. M. BOUMANS, Analytical Eniissimr Spectroscopy, Ed. E. L. GROVE, Part II, Chap. 6. Marcel Dekker,
`New York (1972).
`
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`668
`
`DAVID A. CREMERS el al.
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`
`
`S/GA/AL
`
`Fig. 2. Top view diagram of the apparatus used to generate the COD and monitor the emission
`spectrum. The large gas cell is shown here. The cw-CO2 laser beam was focused using a mirror (dashed
`line) or lens (shown here). The COD was sometimes started by replacing the side window by a pair of
`electrodes shown in the insert. PB: pulsed laser beam to start the COD; TS: translation stage to
`position the lens focusing the pulsed beam; W: quartz window; SW: salt window; C: chopper; QL;
`quartz lens; P: photomultiplier tube; M: monochromator; LIA: lock-in amplifier; R: chart recorder; E;
`electrode; B: vacuum bellows; G: pressure gage; V: vacuum pump; Xe: Xe gas supply. The tesla coil was
`connected to wire l-lV.
`
`
`
`translation stage. The mode of the cw-CO; laser was adjusted to be TEM00. The laser power was not
`stabilized via feedback control but it varied by less than l ‘X, over a 4-h period. The laser was operated
`broadband with a maximum available power of about 70 W. The pulsed laser was operated at about-
`I Hz and was turned off following ignition of the COD. The cell pressure reached a constant value
`within ~ 1 min after ignition.
`-
`For measurements of line intensities and line widths, the plasma light was mechanically chopped '
`imaged on the entrance slit of a scanning monochromator with unit magnification, and detected with a :
`photomultiplier tube (PMT). The PMT signal was processed by a lock-in amplifier (LIA). The“
`monochromator wavelength was scanned over spectral features and the LIA output recorded on a chart —
`recorder. The spectral response of the detection system from 350 to 960nm was determined using I
`standard lamp calibration. Phase sensitive detection with the LIA minimized noise introduced by
`slight regular fluctuation of the plasma intensity discussed below.
`A
`Some measurements of the stability of the COD were made by tuning the monochromator C
`wavelength to a spectral feature (emission line or background), storing the PMT current on a capacitor —'
`over an “exposure” period of 5 s, and then digitizing the accumulated voltage. This method was used ',
`
`because intensity data could be obtained over a much shorter period compared to scanning over th V
`,
`spectral line. Measurements were repeated 16 times to obtain statistical information.
`To spectrally resolve the intensity profile of the COD, the light at the rnonochromator exit slit was
`imaged with unit magnification on a vertically oriented photodiode array. The voltages from each ofthe —[
`512 photodiodes of the array were digitized and read into the computer memory for averaging.
`2.2. Gas cells
`
`
`
`lens or spherical mirror, as shown. The side windows were u.v. grade fused silica. These windows and the *
`rear flange of the cell were often replaced by various devices to test different analytc introduction
`techniques or nonlaser methods of starting the COD, some of which are discussed below. The fron,
`brass flange ofthc cell became very hot during prolonged operation and was water cooled to prevent the _.
`salt window from cracking. It was not necessary to cool the rest of the cell.
`The small gas cell is shown in detail in Fig. 3. The plasma was generated inside a quartz tube 3.3-Cm’
`long by 1.5-cm dia. to provide a small volume (~ 7 cm3) and to eliminate the need for separate windows
`to ignite the plasma and View the spectral emissions. These \vcre accomplished by imaging direcl1Y ‘
`through the tube walls. The 0.18-cm wall thickness of the quartz tube limited the gas pressure to ‘
`S 2800 torr. Analyte gases were introduced into the COD by injection either into a flowing gas stream .
`
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`
`Evaluation of the continuous optical discharge
`
`669
`
`Table 1. Experimental apparatus and conditions
`
`A. COD generation apparatus
`(1) cw-CO2-laser
`wavelength
`power
`beam diameter
`beam divergence
`
`,
`
`(2) Nd':YAG pulsed laser (ignition)
`wavelength
`g
`pulse duration
`pulse energy
`repetition rate
`(3) Sample cells
`material
`size
`large cell (Fig. 2)
`small cell (Fig. 3)
`gas pressure measurements
`
`(4) Optics
`10.6 pm focusing lens
`
`1.06 pm focusing lens
`cw-C0; input window
`(both cells)
`side windows (large cell)
`B. Detection system
`(1) Spectrometer
`gratings
`
`slit width
`slit height
`second order filter
`
`(2) Phutomultiplier tubes
`
`(3) Chart recorder
`(4) Chopper
`(5) Photodiode array
`evaluation board
`
`C. Solid sample introduction
`(1) Nd:YAG pulsed laser
`wavelength
`pulse duration
`pulse energy
`repetition rate
`(2) Lens
`
`Advanced Kinetics MSL-100
`106 pm
`45 W typical
`6 mm
`6 mrad
`
`Quanta—Ray DCR—lA
`1.06 am
`15 ns
`250 ml/pulse
`1 Hz to initiate COD
`
`brass
`
`20L><10Hxl0Wcm
`9 L x 9 H x 9 W cm
`MKS 254 flow controller/Baratron,
`0~l0000 torr head
`
`2.54 cm dia., 2.54 cm fl, positive meniscus
`ZnSc, anti—reflection coated for 10.6 pm
`2.54 cm dia., 7.5 cm ii
`5 cm dia., 1.25 cm thick NaCl
`
`1 cm thick, 5 cm dia. quartz
`
`Spex industries 0.5 m Czerny Turner
`1200 lines mm”, 500 nm blaze, A 2 350 nm
`3600 lines mm 1, 300 nm blaze, .3. < 350 nm
`2 10 pm
`100 ,um—2 cm
`Corning CS 3—70, 1. 2 600 nm
`RCA C3l034, 14004600 V, l 2 350 nm
`Hamamatsu R955, 800 1200 V, /l < 350 nm
`Houston Instruments, Omniscribe
`Laser Precision CTX-534
`Reticon RL512S
`RC1024-S~2
`
`ILS NT-901-I
`1.06 pm
`15 ns
`75 mJ/‘pulse (attenuated)
`10 Hz
`10 cm H
`
`Ssing through the cell from front to rear or into a static gas fill using the rear latex seal shown in Fig. 3.
`_c seal was positioned out of view of the COD because it was melted by the large fraction of 10.6-pm
`radiation transmitted through the plasma volume. The front salt window flange and the rear analytc
`be was air cooled using convection and only a slight
`ection port were water cooled. The quartz tu
`azing ot‘ the tube directly above the COD was observed due to the high temperatures which reached
`°C at the outer wall. The inside tube diameter was restricted to a minimum of 1.5 cm to prevent the
`Sed Nd: YAG pulses from striking and damaging the tube walls.
`
`3. RESULTS
`
`n this section, results are presented describing some characteristics of plasma generation
`e using combined lower laser powers and lower gas pressures than previously reported.
`
` mat:ww>.~mw.
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`DAVID A. CREMERS er al.
`
`YGAS FLOW PORT
`
`
`
`Fig. 3. Diagram of the small gas cell, photographed in Fig. 1, used in analytical experiments with a
`static Xe fill. Gas samples were introduced through the injection port using a syringe. Solid samples
`were laser ablated from a metal strip positioned below the COD (see Section 3.3). WCC: water cooling
`coil.
`
`Xenon was used because of the difificulty of producing the plasma in Ar at the ‘low pressures
`(<3 atm) and low laser powers (< 70 W) available to this study. A minimum of 80W is
`needed to generate the COD in Ar at 3040 torr [I 0]. The lower operating pressures of the Xe
`COD also meant greater ease of sample introduction and less stringent materials
`requirements. The maintenance threshold of Xe is lowest ol'a1l stable inert gases because ofits
`relatively low ionization potential and thermal conductivity. The cw-CO2 laser beam was
`imaged into the gas cell horizontally because of the significantly greater cw maintenance
`thresholds required to generate the COD with a vertical beam
`Although Xe is expensive
`and would not normally be employed in a routine analysis, the results obtained here should
`indicate general trends applicable to a COD generated in other inert gases.
`"
`Initial attempts at generating the COD used a 2.8-cm focal length spherical copper mirror
`similar to those used in other studies [3, 4, 10]. The resulting plasma was unstable, as
`indicated by large irregular intensity fluctuations, and remained operating for at most 30 3;
`This behavior was attributed to the small diameter (6 mm) of the CO2 laser beam. The COD
`intercepted a significant portion of the central intense region of the incoming laser beam
`before it reached the mirror. This not only reduced the beam intensity reflected off the mirror
`back into the focal volume, but could also induce instability because energy was incident on
`the plasma from behind the focus. The plasma is spatially stable near the focus of the las’
`beam due to the decreasing intensity gradient as the plasma moves toward the focusing,
`
`In addition, with mirror focusing strong convection currents and density gradients from the;
`COD distort the incoming laser beam [17] and this effect will be more pronounced with
`smaller beam diameters. Instability in the COD noted here using mirror focusing was
`probably not as apparent in previous work because of the larger laser beam diameters, which
`were typically 2 cm or more [3, 7, 19]. In this study, a stable plasma was generated at lower X
`pressures and laser powers than previously reported in the literature using a 2.54-cm focal‘
`length best-form ZnSe lens. For example, at 1875 torr the COD was maintained with on
`25 W, whereas a minimum of about 80 W was reported previously [4]. The lower threshold is I
`
`probably due to beam focusing with a lens instead of a mirror. Zinc-selenide was used for lens.
`material because of its low absorption at 10.6 pm and absence of “thermal runaway
`exhibited by germanium at high cw laser powers.
`For a C02 laser beam power of 45 W, the plasma could be reliably maintained in Xe at:
`pressures between 1335 and 3200 torr using the large cell. With the small cell the minimum
`pressure was about ll50 torr. The lower maintenance threshold results from insulficien ‘
`absorption of the laser beam by the plasma to compensate for energy losses due to thermal
`
`
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`
`Evaluation of the continuous optical discharge
`
`671
`
`ductivity, convection, and radiation [16]. The upper threshold results from increased
`ative losses at the higher gas densities.
`he COD was ignited in most cases using the laser spark generated by focusing a 2 MW
`' AG pulse within the focal volume ofthe 10.6-um beam. Careful alignment of the focal
`mes was necessary to produce ignition and movements of 0.03 cm were critical. Small
`ges in the alignment led to the observation of unique phenomena: one pulse would start
`COD but the next pulsc would turn it off and the third pulse would turn it on, and so
`rth. When the focal volumes were close, a bright light flash was observed from the chamber
`0 e COD ignited but quickly went out. Trial and error proved to be the best method of
`‘gning the focal volumes. Although most convenient, plasma ignition need not be
`mplished with a laser pulse. As noted above, the COD has been started in other studies
`mg the plasma formed on a tungsten wire introduced into the focal volume of the cw beam.
`is method did not work with the low laser powers used here. However, it was possible to
`rt the plasma using a tesla coil connected to the assembly shown in Fig. 2 consisting of a
`of electrodes positioned inside a bellows and flange assembly. The flange was mounted in
`ce ofa quartz side Window. One electrode was connected to a grounded metal tube passing
`
`ivated, a bright series of sparks was formed between the two electrodes. As the spark was
`nually swept through the focal volume of the C0; laser beam, the COD was easily ignited.
`e electrodes were then retracted out of the way.
`Unsuccessful attempts were made to generate the COD with up to 60W of 1.06-pm
`diation from a multimode cw-Nd:YAG laser. Because laser heating of a plasma via inverse
`Bremsstrahlung varies as A2 [23], the failure to form the COD was probably due to the 100
`es lower absorption of the plasma at 1.06 pm compared to 10.6 hm.
`
`. Plasma characteristics
`
`.2.1. Eflects of pressure and laser power. Figure 4 shows the Xe COD spectrum, not
`rected for detection system spectral response, over the 200—940 nm region at
`two
`ressures. Emissions from neutral and once-ionized Xe were observed having upper levels in
`he 9.6—l3.9 eV range. Because of the high background intensity, due to the high COD
`
`ed in Table 2. In general, Xel intensities of 100 or more on a scale in which the strongest Xel
`11615 104 [34], could be observed. Because of the high intensity of the COD, slit widths less
`han 10 um were used to prevent saturation of the detection system.
`0 he high electron density of the COD broadens emission line widths via the Stark elfect.
`
`Table 2. Relative intensities of some Xe lines observed in the COD*
`
`Wavelength (nm)
`
`Rel. Int. Wavelength (nm)
`
`Rel. Int.
`
`450.10
`452.47
`462.43
`467.12
`473.42
`480.70
`482.97
`484.33
`491.65
`492.32
`
`0.030
`0.018
`0.013
`0.033
`0.037
`0.013
`0.008
`0.016
`0.010
`0.003
`
`529.22 (Xe II)
`764.20
`823.16
`826.65
`828.01
`834.68
`840.92
`881.94
`895.23
`904.55
`
`0.006
`0.044
`0.839
`0.029
`0.259
`0.146
`0.070
`1.000
`0.278
`0.344
`
`*)(e pressure = 2000 torr; cw-C02 laser power = 45 W. Lines are
`from Xe 1 unless otherwise indicated.
`
`4] A. R. STRIGANOV and N. S. SVENTITSKII, Tables of Spectral Lines of Neutral and Ionized Atoms. 1191/Plenum,
`New York (1968).
`
`A
`
`
`
`Energetiq Ex. 2081, page 9 - |PR2015-01277
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`Energetiq Ex. 2081, page 9 - IPR2015-01277
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`672
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`DAVID A. CREMERS et a1.
`
`Measurements of the half-width of the 450.10 and 834.68-nm Xe I and the 529.22-nm Xe,
`lines were obtained over the pressure range of 1402 to 2204 torr. The average widths we
`0.13, 0.17 and 0.13 nm, respectively. Only the 450.10«nm line showed a pressure dependeng
`decreasing from 0.14 to 0.12 as the pressure decreased. The instrumental resolution
`0.086 nm, determined using a low pressure Hg lamp. was not removed from the
`experimental line widths.
`_
`The COD resides near the focus of the CO2 laser beam that supplies it with energy, at
`position dependent upon several parameters including laser energy and gas pressu;
`Changes in these parameters moved the plasma along the axis of the focused laser beam. F
`example, decreasing the Xe pressure from 3175 to 1336 torr moved the plasma 0.5m
`toward the focus. Similarly, at a pressure of 2000 torr, changing the laser power from 25
`60 W moved the plasma away from the focus by 0.3 mm. These results are understood b
`considering the motion of the plasma in a parallel laser beam: the plasma moves toward‘!
`
`losses from thermal conduction, convection, and radiation. The absorption coefiicicnt varie
`with pressure (p) as 11 z p2 [16] so that as the pressure increases, AI also increases for a give
`laser power, and the plasma moves further away from the focus, into a region of reduce
`intensity, until absorption is again balanced by the loss mechanisms. In the same way, as th
`laser power is increased at constant pressure, AI increases and the plasma moves toward th
`focusing lens, as observed.
`Gas pressure and laser power also elfect the spectral excitation characteristics of the CO
`For example, pressure changes alter the intensities of the background signal and the Xe I an
`Xe II lines as shown in Figs 4 and 5. During these measurements, the position of the imagin
`A
`A
`
`
`
`
`
`2006- torr
`
`
`
`
`I404 torr
`
`200
`
`275
`
`350
`
`550
`
`7'50
`
`950
`
`Wavelength, nm
`Fig. 4. Spectrum from 200 to 950 nm of the Xe COD at two pressures, not corrected for detection
`system response. Different PMTs and gratings were used to monitor the regions above and below
`350 nm. The horizontal line under each spectrum indicates the baseline signal with the COD off.
`Emission lines marked by a triangle are off-scale. A filter was placed at the entrance slit beginning at
`the wavelength indicated by the arrows to eliminate the second order spectrum.
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`Energetiq Ex. 2081, page 10 - |PR2015-01277
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`Energetiq Ex. 2081, page 10 - IPR2015-01277
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`Evaluation of the continuous optical discharge
`
`673
`
`(°~V)
`
`Ratios
`
`28
`
`32
`
`XeI.Xe11.B65Intensities,V
`
`16
`
`20 24
`
`|O“°'- Pressure.
`
`tort
`
`Fig. 5. Variation of the intensities of (1) 450.10 nm Xe I and (1') adjacent background at 448.00 mm
`(BG 1), and (2) 529.22 nm Xe II and (2') adjacent background at 527.00 nm (BG II), and the ratios (3)
`[Xe I/BG 1] and (4) [Xe 11/BG II], as a function of xenon gas pressure. The laser power was 46 W.
`
`ns was adjusted at each pressure to maintain the most intense portion of the plasma over the
`trance slit. As the pressure was decreased from 3175 to about 1900 torr the intensities from
`1 three sources increased. As the pressure was reduced further, the Xe 1 intensity increased
`ntil about 1700 torr and the Xe I1 intensity peaked at 1600 torr. The net result was that the
`tie of the emission line to background intensity steadily increased as the pressure was
`duced from 3175 to about 1500 torr. These observations agree with measurements reported
`elow, which indicate the temperature of the COD increases somewhat with decreasing
`ressure. Different behavior was observed by changing the laser power. As the power
`ccreased from 55 to 25 W, the intensities of Xel (450.10 nm), Xe 11 (529.22 nm), and the
`djacent backgrounds all increased slightly, but the ratios of these lines to background
`‘mained essentially constant.
`3.2.2. Temperature. Detailed measurements of the COD temperature have been reported
`reviously [8, 9, 11, 17]. The intention here was to determine how the temperature and
`ectron density varied as certain experimental parameters were changed to ascertain the
`ability of this source. The COD temperature was measured using the Saha equation and a
`oltzmann plot. Because of the asymmetry of the horizontal plasma, the resulting data were
`ot Abel inverted. However, only light from the center of the plasma image (10-um wide by
`00-pm high) was monitored by masking the monochromator slits. The resulting tempera-
`ures are therefore population averaged as discussed by BOUMANS [35]. The electron density
`as determined by seeding the Xe with about
`l.3—2.0 7,; hydrogen gas by volume and
`easuring the width of the H, line at 656.28 nm. The Hp line (486.13 nm), which yields more
`ccurate densities [36], was not used because of several Xe I interferences and the about
`ixfold weaker intensity of this line compared to H,,,. To minimize perturbation of the COD,
`' nly the smallest amounts of hydrogen needed to make a measurement were added. Above
`"/0,
`the plasma noticeably decreased in intensity and was extinguished above 5 ‘,’/,,
`oncentration. Using the formula of GRIEM [36], the electron density was deduced by an
`crative process with the Saha temperature. The 450.10-nm Xe I and 529.22-nm Xe 11 lines
`ere used for the Saha analysis. At the pressures of 2766, 2130 and 1601 torr, the temperature
`as measured to be 9630, 10200 and 10 375 K, respectively, using 45 W of laser power. The
`lectron density also varied slightly: at the respective pressures it was 7.39, 8.95 and 7.31
`X 10' 5 cm” 3, which corresponds to about 0.12 "/0 ionization ofthe Xe. The width of the H,
`
`
`
`
`
` 35] P. W. J. M. BOUMANS, Spectrochemical Excilulion. Hilger and Watts, London (1966).
`
`-VT—[36] H. R. GRIEM, Plasma Spectroscopy. McGraw-Hill, New York (1964). The formula used to calculate the
`‘
`electron density (he) is given on p. 305: ne = C(nc, T )A/13”. Here, A}. is the full Stark line width and C(ne,7‘) a
`coefficient tabulated on pp. 5384539.
`
` 5A(R)4O:1v‘I
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`674
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`DAVID A. CREMERS et al.
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`line varied in the range 0.75-0.85 nm as the pressure varied. At 2130 torr, the COD,
`temperature changed by less than 2 “/0 and the electron density by less than 10 9,’, as the laser‘
`power increased from 32 to 48 W.
`To obtain a good measure of the Boltzmann temperature of the plasma, the spread in the
`upper energiesof the emitting levels should be at least 2 eV. Unfortunately, the majority 0f_
`Xel lines for which the oscillator strengths have been measured correspond to upper levejs,
`grouped between 11.4 and 12.0 eV [37]. However, when about 66 "/,§ Kr was added to the Xe
`COD at least 13 lines of Kr I were observed corresponding to upper levels spanning the range
`from 11.44 to 13.49 eV for which the oscillator strengths are known to 1 50 % (Table 3). The
`
`relative intensities of these lines were measured to produce the Boltzmann plot shown in F1’ *
`6. The integrated intensities were determined by tracing the lines on paper, cutting them on’
`Table 3. Data for Kr 1 lines used to construct Boltzmann
`plot*
`
`Wavelength (nm)
`427,40
`43514
`440.00
`445.39
`446.37
`587.09
`76015
`768.52
`769.45
`785.48
`810.44
`811.29
`850.89
`
`g1
`5
`1
`5
`5
`3
`5
`5
`1
`3
`3
`5
`7
`3
`
`10"‘ x A(s‘1)*
`0.026
`0.032
`0.020
`0.0078
`0.023
`0.018
`0.31
`0.49
`0.056
`0.23
`0.13
`0.36
`0.24
`
`Eu(eV)T
`12.82
`13.49
`13.46
`12.82
`12.81
`12.14
`11.55
`12.26
`11.53
`12.14
`1 1.44
`11.44
`12.10
`
`‘Data from: Wavelengths and Transition Probabilities for
`Atoms and Atomic Ions Part II, Transition Probabilities,
`NSRDS-NBS 68, US. Government Printing Office,
`Washington, DC (1980).
`1Here, g and E“ are the statistical weight and energy,
`respectively. of the upper emitting level corresponding to the
`transition having transition probability A, The estimated
`uncertainty of A is 1- 50 "/9.
`
` H
`
`I3
`
`I2
`
`Eu, eV
`
`Fig. 6. Boltzmann plot of Kr I lines in the Xe/Kr COD used to measure the plasma temperature.
`Only the center region of the plasma (10 um W x 100 um H) was monitored. The gas pressure was
`2000 torr and the laser