Eur. Phys. J. D (2013) 67: 31 DOI: 10.1140/epjd/e2012-30642-x
THE EUROPEAN PHYSICAL JOURNAL D
Regular Article
Light emission from heteronuclear Ar-Kr doubly ionized excimer molecules Alexey B. Treshchalov1, Alexander A. Lissovski1 , and Andreas. F. Ulrich2,a 1 2
Institute of Physics, University of Tartu Riia 142, 51014 Tartu, Estonia Physik Department E12, Technische Universit¨ at M¨ unchen James-Franck-Str. 1, 85748 Garching, Germany Received 18 October 2012 / Received in final form 18 December 2012 c EDP Sciences, Societ` Published online 6 March 2013 – a Italiana di Fisica, Springer-Verlag 2013 Abstract. Light emission from a ∼100 mbar Ar-Kr mixture excited by a pulsed discharge is described. The discharge was arranged to form a homogeneous cathode layer and spatial filtering was used to measure time-dependent spectra emitted from a region where electrons accelerated in the cathode sheath induce the light emission. Novel excimer bands were observed around a wavelength of 315 nm in addition to the better known so called third excimer continua of Ar and Kr. A tentative assignment for these bands to charge-transfer transitions: ArKr2+ → Kr+ + Ar+ + hν is provided and discussed in the context of earlier works on heteronuclear ionic excimer molecules. Predictions for the wavelength positions of similar emission bands are provided for other combinations of noble gases. The rate constants for the formation of heteronuclear ArKr2+ excimers in three-body reactions and two-body collisional quenching of Kr 2+∗ ions by Ar atoms have been determined from the time dependence of the ArKr2+ emission.
1 Introduction Light emission from rare gases plays an important role in the lighting industry in the form of low and high pressure lamps and as a buffer gas e.g. in mercury containing lighting devices. The fundamental processes for light emission are well understood in these cases. Rare gas excimer light sources emitting in the vacuum ultraviolet spectral range have been developed in the last decades in various forms [1–7]. Here, the light emission processes are also well understood as far as the neutral excimer molecules are concerned. They are based on the so called first and second excimer continua. The origin of the so called third continua emitted from pure rare gases at longer wavelengths than the second continua are still a subject of ongoing research although a great deal of information has been collected on this issue over the last decades using various excitation methods [8–13]. It is commonly accepted that the origin of the third continua has to be attributed to energetically high lying levels, preferentially in doubly ionized diatomic molecules. In this publication we go one step beyond this status and study the analogue of the third continuum emitted from a Ar-Kr gas mixture. Thereby we extend an interpretation given in reference [8] to doubly ionized mixed rare gas molecules. Using a qualitative assignment and quantitative estimation of the expected emission wavelength analogue to processes described in reference [8] we assign the emission bands observed around 315 nm in our experiments to a a
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charge-transfer transition accompanied by the emission of a photon: ArKr2+ → Kr+ + Ar+ + hν. The light emission from mixed rare gas molecules has been described in several publications [14–17]. Many of them, such as reference [14], deal with neutral mixed molecules. Rare gas mixtures are also important in the context of laser schemes in dense gases in the infrared and visible spectral range [18–21]. Typically a small amount (1%) of a heavier rare gas is added to the majority gas in all these cases. A limited number of papers describes excimer bands emitted from mixtures with mixing ratios of rare gas species RgI and RgII typically RgI : RgII = 10% : 90% or 90% : 10%, respectively. In reference [22] a detailed study of the emission from mixed molecules has been provided and emission bands have been attributed to a charge transfer transition accompanied by the emission of a photon in the reaction KrAr+ → Kr+ + Ar + hν. Here we apply this concept to the next ionization level: ArKr2+ → Kr+ + Ar+ + hν. This process is closely related to the interpretation given for the third continuum [8,9,11] in which a doubly ionized atom is bound to an atom of the same species with subsequent emission of a photon in a charge-transfer transition such as: + + Kr2+ + 2Kr → Kr2+ 2 + Kr → Kr + Kr + Kr + hν. Note, that the final state of these optical transitions is not unique due to spin orbit splitting of the ground state of the rare gas ions as discussed in references [8,22]. The appearance of three third continuum bands e.g. at 224, 248 and 296 nm for Kr (see Fig. 4 discussed in Sect. 3) strongly supports this assignment of the third continua. Based on the level scheme presented below (Fig. 6) an
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Eur. Phys. J. D (2013) 67: 31
Table 1. Positions (nm) of the third continuum bands for (experimental data [9]) and heteronuclear homonuclear Rg2+ 2 (calculated in this work) RgII Rg2+ doubly ionized excimers. I 2+
He Ne Ar Kr Xe
He 70–80 63 49 46 43
2+
Ne 130 99 68 62 56
2+
Ar – 1719 189 150 122
2+
Kr – – 328 224 167
2+
Xe – – 1288 455 270
extension of the concept to the mixed doubly ionized molecules can be worked out. Assuming a binding energy of about 1 eV for these molecules in the excited state and an identical shape for the repulsive potential of the singly ionized species in the lower state, transition energies on the order of 3.78 (8.27) eV with a corresponding emission wavelength of 328 (150) nm could be predicted for Ar-Kr combinations: Kr2+ and Ar (Ar2+ and Kr). Similar predictions are listed in Table 1 for other combinations of rare gases. The selection of the combination of argon and krypton for the demonstrational experimental study described below was based on the predicted wavelength region around 300 nm (Kr2+ and Ar combination) which is easily accessible and also outside other well known strong emission bands which could make it difficult to disentangle the mixed molecule bands from the overall emission spectrum.
L
Sparks
C
HV
c
R2
R1
a
Trigger 1-100 Hz
Chamber Fig. 1. A high-impedance (C = 0.33 nF, L ≈ 500 nH) electrical circuit used for the low-current excitation of the discharge. Charging voltages between 5 and 15 kV were used, depending on the gas pressure. compressor
getter
discharge chamber
gas flow
mirror
anode cathode
UV-VIS spectrograph
ICCD
PC
HV 1-20 kV
trigger
Fig. 2. Schematic drawing of the discharge cell and the spectroscopic setup for time dependent measurements of the light emission from the discharge.
2 Experiment In previous publications [23,24] about discharge excitation of argon at very high gas pressure (up to 10 bar) a compact thyratron-switched charge-transfer device has been described. The circuit with a low-inductive peaking capacitor placed inside the chamber ensures a very steep build-up of the discharge current and an energy deposition in the gas with a very high power density. For this highcurrent pumping regime, however, it was difficult to obtain an excitation in the form of a single pulse because the resistance of the plasma during the breakdown becomes lower than the critical resistance of the excitation circuit. Moreover, the discharge usually lost its uniformity during the excitation pulse and seeds of micro-arcs (filaments) appeared. These filaments started from hot cathode spots, which exploded several nanoseconds after the maximum of the discharge current pulse. The local breakdown of the cathode layer in the spots is caused by a field-enhanced thermionic emission of electrons from microprotrusions, rapidly developing into an explosive emission. In this work a low-current pulsed excitation regime was organized by a specially designed high-impedance thyratron-switched circuit. The electrical scheme of the circuit is shown in Figure 1. A transversely excited volume discharge with a repetition rate of 25 Hz was initiated between two parallel cylindrically profiled pure tungsten electrodes. The diameter of the electrodes was 6 mm, their
length 22 mm, and the gap spacing 3 mm. The width of the discharge was about 2 mm. Automatic VUV preionization for the main discharge was provided by a sliding discharge on the surface of a sapphire plate with a thickness of 0.6 mm which was placed laterally to the cathode electrode. After a thyratron breakdown a steep (∼20 ns) build-up of the high voltage on the cathode electrode was produced. During the breakdown of the main discharge gap the energy from the storage capacitor C was loaded into the plasma from a high-impedance thyratron circuit. In this regime the resistivity of the discharge plasma remains high during the excitation pulse. Therefore, without resistor R2 in Figure 1, an over-damped regime of the discharge with a long excitation pulse is available. For shortening of the excitation pulse, the resistor R2 (10 Ω) was placed parallel to the discharge gap. This scheme produces predominantly a short-pulse (∼5 ns), low-current excitation which leads to a very homogeneous volume discharge with a stable cathode layer in a gas pressure range from 40 to 250 mbar. As was shown in [24], high-energy (up to 150–250 eV) electrons are created in a strong electric field of the cathode fall and injected into the negative glow zone of the discharge. These electrons can effectively produce doubly ionized atoms. A schematic drawing of the experimental apparatus used for the spectroscopic diagnostics of the discharge plasma is shown in Figure 2. An image of the discharge was
Eur. Phys. J. D (2013) 67: 31
projected onto the vertical entrance slit of a spectrometer by a concave aluminum-coated mirror. The lower part of the image corresponded to the cathode region and the upper one to the anode region. The spatial resolution at the discharge can be estimated to about 0.03 cm. UV-VIS spectra were detected by a f = 0.3 m, D = f /4 imaging spectrograph (Shamrock 303i) with a 300 l/mm grating (spectral resolution 0.27 nm/pixel) and a ns-gated intensified CCD camera (ICCD, Intraspec V, Andor Technology). A spectral region selected by this spectrograph in one exposure is restricted to about Δλ = 190 nm by the size of the ICCD photocathode (18 mm) and the dispersion of the spectrograph. The available spectral range determined by the sensitivity of the ICCD photocathode was 200–850 nm. Discharge triggering and ICCD gate pulses were synchronized by a computer controlled delay generator DG-535 (Stanford Research Systems Inc.) with a total jitter of about ±1 ns. More precise time dependencies of the light emission at specific wavelength were recorded by a UV photomultiplier tube (R3377, Hamamatsu) with a rise time of 0.7 ns, operated in the pulsed current mode on a load of 50 Ω. All spectra presented in this paper were not corrected for the spectral response of the system. In summary, the spectroscopic setup allowed to record space- and time dependent spectra emitted from different zones across the discharge gap (positive column and near-electrode zones) at various delay times with respect to the onset of the discharge. It is well known that the VUV emission from rare gas Rg∗2 excimer molecules is very sensitive to gaseous impurities. Therefore special attention was paid to the purity of the gas system. In our experiments the following purities of the rare gases were used: Ar – 6.0; Kr – 4.8. The discharge chamber with a volume of ∼5 l was made of aluminum and the gas handling system was evacuated by a turbomolecular, oil-free pumping system to 10−5 mbar prior to gas filling. Nevertheless, the continuous outgassing of materials inside the discharge chamber led to an accumulation of gaseous impurities in the gas after a pure rare gas filling. To avoid these contaminants we used continuous gas recirculation through a heated getter used as gas purifier (Omni III 200, NuPure) during the experiments. The gas circulation was organized by a diaphragm compressor (N143ST.9E, KNF). The purifier removes H2 O, O2 , CO, and CO2 contaminants at room temperature. With the getter heated to about 300 ◦ C, it removes also N2 and CH4 impurities.
3 Experimental results and data analysis A typical set of raw data is shown in Figure 3 to illustrate what type of information can be retrieved from the measurements. This figure shows a two- dimensional image obtained from the ICCD camera for a 20 ns long exposure recorded 5 ns after the onset of a discharge in 100 mbar krypton. The horizontal axis provides the wavelength information and the vertical axis corresponds to the position along the spectrometer slit which correlates to the position between the discharge electrodes- cathode is in the
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Fig. 3. Raw data showing the emission from a pulsed discharge in Kr at 100 mbar, detected by a ICCD camera for the spectral regions in the (a) UV and (b) VIS. The exposure time of the ICCD was 20 ns and the delay from the beginning of the discharge 5 ns. A pseudo-colour scale is used for the intensity. The broad third continuum bands of Kr at 224, 248 and 296 nm as well as several Kr+∗ lines in the range of 340–400 nm with the most intense line at 377.8 nm are emitted exclusively from the negative glow zone near the cathode electrode. Emission from the Kr∗ lines in the range of 427–450 nm and the most intense line at 557.0 nm is distributed uniformly along the gap between the electrodes (mainly the positive column of the discharge).
lower part of the image. The image shows clearly that different spectral features are emitted from different positions in the discharge gap. The molecular bands below 275 nm as well as Kr+∗ lines are emitted only from the cathode sheath whereas Kr∗ lines fill the whole discharge gap. Varying the delay between the discharge and the gate for the ICCD camera provides information about the time dependence of the light emission and thereby the gas kinetic processes in the discharge. The main result concerning the emission of Ar-Kr mixtures is shown in Figure 4. Spectra of mixtures with 100 mbar total pressure containing 50 mbar Kr and 50 mbar Ar and 25 mbar Kr and 75 mbar Ar, respectively are plotted together with a spectrum obtained from 100 mbar pure Kr for comparison. For pure Ar the emission signal in this spectral range is negligible – only a small red tail from the 190 nm third continuum band is detected, therefore it is not shown in Figure 4. Data were recorded with 30 ns exposure time and with 30 ns delay time with respect to the onset of the discharge optimized for maximum intensity of the third continua. Note that the short wavelength cut-off of all spectra near 200 nm is due to the strongly decreasing sensitivity of the Shamrock 303i spectrometer at that wavelength. The excitation conditions as well as the data recording procedure were the same for all three spectra and they are also plotted with the same relative scale. Note that the Kr continua below 300 nm scale linearly with the Kr content in the gas mixture. The interesting features in the spectra of the mixtures are the two
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Intensity, a.u.
Kr2+ 2
1.0 Kr 100 mbar Kr 50 mbar, Ar 50 mbar Kr 25 mbar, Ar 75 mbar
Kr 100 mbar Kr 25 mbar, Ar 75 mbar
Intensity, a.u.
1.0
Eur. Phys. J. D (2013) 67: 31
ArKr2+
0.5
0.5
ArKr2+
Kr2+ 2
Kr+* 0.0
0.0 200
250
300 Wavelength, nm
350
400
Fig. 4. Emission spectra of Kr and Ar-Kr mixtures in a wavelength range from 200 to 400 nm. All spectra were recorded at 100 mbar total pressure and with the mixture compositions indicated in the figure. They were extracted from raw data as shown in Figure 3 for a region near the cathode and for a time interval of 30 ns with 30 ns delay with respect to the onset of the discharge. The intensity ratios are as recorded in the experiment. Along with the three third continuum bands of Kr at 224, 248 and 296 nm [8], two new bands at 315 and 328 nm appear with Ar additives to Kr.
peaks at 315 and 328 nm which are clearly visible both for the 50 to 50% and the 25 to 75% Kr-Ar mixture but are completely absent for pure krypton and pure argon. To show the new emission bands more clearly in comparison with the third continuum emission of krypton, again for 100 mbar gas pressure, a krypton spectrum with good signal to noise ratio is plotted together with the 25 mbar Kr, 75 mbar Ar spectrum (see Fig. 5). Here the intensity of the mixture was scaled to the intensity of the pure krypton spectrum in the region of the third continuum of krypton. The new emission bands are rather broad extending over roughly 50 nm between about 290 and 340 nm. In the structure we locate two peaks at 315 and 328 nm which we attribute to the radiative decay of doubly ionized heteronuclear ArKr2+ molecules as will be described below.
4 Interpretation of the data As was mentioned in the introduction we interpret the emission of the mixed gases in analogy to the interpretation of the third continua of the pure rare gases [8] and the charge transfer process in singly ionized mixed molecules [22]. This can be explained by the potential diagram shown in Figure 6. A schematic energy level diagram of Ar and Kr is shown in this figure with emphasis on the singly and doubly ionized species. The linear energy scale was used according to the data from [25]. Doubly ionized argon Ar2+ and krypton Kr2+ represent an energy level
200
250 300 Wavelength, nm
350
Fig. 5. Spectra emitted from the cathode region for Kr and Kr-Ar. The shape of the newly found ArKr2+ continua bands is better visible in this figure (in comparison with Fig. 4) in which the Kr2+ bands are scaled approximately to the same 2 intensity.
of 43.4 and 38.6 eV, respectively. Pairs of singly ionized Ar and Kr atoms represent energies of 28.0 and 31.5 eV, respectively. The energy of a combination of Ar+ and Kr+ lies in between with a value of 29.8 eV. This is shown on the right side of the energy level diagram in Figure 6 (large internuclear distances). The development of the energy levels described above for a reduced internuclear distance is plotted in Figure 6 as the main background for interpreting the emission spectra of mixed rare gases. It is known that the homonuclear doubly ionized diatomic rare gas molecules are bound as 2+ indicated in Figure 6: Ar2+ 2 and Kr2 . The first assumption which we make to interpret the spectra emitted from mixtures is that the corresponding heteronuclear mixed molecules KrAr2+ and ArKr2+ have a binding energy which is very similar to the corresponding homonuclear molecules. The dashed lines in Figure 6 are only slightly shifted to make them visible in the plot. It is also assumed as a second assumption that the potential minimum of all four bound, doubly ionized species is located at the same internuclear distance. Again, the shift in the figure is only introduced to get a clearer picture for showing the optical transitions. As a third approximation we assume that the energy of the repulsive ground states of all combinations of Ar+ and Kr+ ions has the same variation with internuclear distance. This can be justified by the long range electrostatic forces between singly ionized species. With the simplified energy level diagram described above it is possible to predict the wavelength of an emis+ + sion of the type RgI Rg2+ II → RgI + RgII + hν based on the → known wavelength of transitions of the type RgI Rg2+ I + + Rg + hν. In the specific case shown in Figures 4 Rg+ I I and 5 the assumption of an identical upper bound potential curve for Kr2+ and ArKr2+ as well as a unique 2 energy shift independent of internuclear distance of the
Energy, eV
Eur. Phys. J. D (2013) 67: 31
Page 5 of 6 Ar2+
KrAr2+
1
3
P3/2
174 nm ×1.5 224 nm 315 nm ×3.5 377 nm ×0.8
2+ 2
40 ArKr2+
Kr2+
3
P3/2
2+ 2
Kr
Intensity, a.u.
Ar
τ = 60 ns 0.1
τ = 30 ns Ar 100 mbar Kr 50 mbar
190 nm
35
0.01 328 nm 224 nm
30
0 150 nm
Ar+(3P3/2) + Ar+(3P3/2) Kr+(3P3/2) + Ar+(3P3/2/3P1/2) 0.18 eV Kr+(3P3/2) + Kr+(3P3/2)
Internuclear distance Fig. 6. Energy level diagram for doubly (Ar2+ , Kr2+ ) and singly-ionized (Ar+ , Kr+ ) states along with the correspondent schematic potential curves for bound and repulsive molecu2+ lar states. Radiative transitions for homonuclear Kr2+ 2 , Ar2 (experimental data) and heteronuclear ArKr2+ , KrAr2+ (calculated in this work) doubly ionized excimers are shown.
repulsive lower potentials of 29.8 eV – 28.0 eV = 1.8 eV (a combination of Ar+ -Kr+ and Kr+ -Kr+ respectively) can be used for the assignment of the emission which appears in the Ar-Kr mixture. The only additional assumption, which is well established for homonuclear molecules, is that the so called third continuum is due to the + RgI Rg2+ → Rg+ I I + RgI + hν emission process (here KrKr2+ → Kr+ + Kr+ + hν) leading to the triple peaked structure (bands at 224, 248 nm and a weak 296 nm) due to 0.66 eV spin-orbit splitting between 2 P1/2 and 2 P3/2 states of Kr+ ions in Figures 4 and 5. In Figure 6 only one (224 nm) of these three bands is shown. The energy shift of ∼1.8 eV then explains the wavelength shift to wavelengths slightly above 300 nm for a Ar-Kr mixture. The doublet structure (315 and 328 nm bands) in emission of Ar Kr2+ heteronuclear excimers can be explained by the spin-orbit splitting (0.18 eV) of Ar+ ions (2 P1/2 and 2 P3/2 states) as indicated in Figure 6. The simple scheme can be used to predict the wave+ + length region where RgI Rg2+ II → RgI + RgII + hν type emissions can be expected for various combinations of rare gas atoms. This is shown in Table 1. There the band wavelength is tabulated in nanometers. The diagonal elements show the known third continua of the rare gases [9] and the non-diagonal elements the wavelength positions for 2+ the heteronuclear Rg2+ I -RgII and RgI -RgII molecules, respectively as predicted in this work. Spin-orbit splitting
50
100 Time, ns
150
200
Fig. 7. The time evolution of the light emission from a 100 mbar Ar, 50 mbar Kr mixture is shown for a selection of wavelength positions in the spectrum. The time dependences of light emission from the Kr third continuum (224 nm, Kr2+ 2 ) and the newly found band (315 nm, ArKr2+ ) are identical and reflect the disappearance of the same precursor – Kr2+ in two formation reactions (1a) and (1b) (see text). The faster decay of the 174 nm band is caused by the disappearance of Kr2+∗ excimers by two quenching reactions (2a) and (2b). + for Rg+ I and RgII ions was not taken into account in this consideration. Combinations where the energy shift introduced above becomes negative so that no optical transition can be expected are indicated by a dash in the table.
5 Time dependences of the third continuum emission The time dependence of light emission on the newly discovered ArKr2+ emission band (315 nm) has been measured for a 50 mbar Kr, 100 mbar Ar mixture in comparison with three krypton features emitted from the same +∗ gas: Kr2+∗ at 174 nm, Kr2+ line 2 2 at 224 nm, and a Kr at 377 nm (see Fig. 7). A simple kinetic model was developed to simulate the decay of the third continuum emission bands. The emission of the Kr+∗ line follows essentially the excitation pulse when the singly-ionized (Kr+∗ ) and doubly-ionized (Kr2+ , Kr2+∗ ) species generate in the negative glow zone of the discharge. This active stage of the discharge continues for about 10 ns. After that Kr2+ and Kr2+∗ ions disappear via several chemical reactions 2+ such as three-body formation of Kr2+ ionic 2 and ArKr excimer molecules: k
Kr Kr2+ + Kr + Kr/Ar −− → Kr2+ 2 + Kr/Ar
k
Ar Kr2+ + Ar + Kr/Ar −− → (ArKr2+ ) + Kr/Ar
(1a) (1b)
and two-body quenching reactions [8]: k
−→ Kr2+ + Kr Kr2+∗ + Kr −−2Kr k
Kr2+∗ + Ar −−2Ar −→ Kr2+ + Ar.
(2a) (2b)
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As shown in Figure 7, the emission bands at 224 nm and 315 nm have identical temporal behaviours with a decay time τ = 60 ns, which confirms the suggested assignment of the emission band at 315 nm to ArKr2+ molecules. The key argument is that both emission bands from different ionic excimers reflect the disappearance of the same precursor – Kr2+ in two independent formation reactions (1a) and (1b). If we take for reaction (1a) the rate constant of the three-body formation reaction k3Kr = 1.48 × 10−30 cm6 s−1 [26] and solve the equation for reaction (1b): 1/τ = (k3Kr [Kr] + k3Ar [Ar]) × ([Kr] + [Ar]), then we obtain the rate constant k3Ar = 1.17 × 10−30 cm6 s−1 , which is a bit smaller than for Kr. We assume that in reaction (1b) the rate constants k3Ar and k3Kr are independent from the third particle (Ar or Kr). Similar considerations could be applied for the emission of the 174 nm band. The time behaviour of this band is caused by the disappearance of Kr2+∗ ions by two independent quenching reactions (2a) and (2b). The rate constant for reaction (2a) was determined in reference [8]: k2Kr = 1.4 × 10−11 cm3 s−1 . From the experimental curve with a decay time τ = 30 ns and by solving an equation 1/τ = [Kr] k2Kr + [Ar] k2Ar we obtain k2Ar = 0.8 × 10−11 cm3 s−1 . The fitting of the experimental data shows that the radiative lifetimes for homonuclear Kr2+ and heteronu2 clear ArKr2+ doubly-charged ions are much shorter than the observed emission decay times and not necessary to be taken into account in our calculations. This suggestion is confirmed in several experiments where the radiative lifetime for similar ions was determined as about 5 ns for 2+ Ar2+ 2 [26,27] and 6 ns for Kr2 [28]. The authors gratefully acknowledge partial support from the Estonian Scientific Foundation (Grant No. 7971). One of the authors (A.U.) gratefully acknowledges the support by the ERASMUS program for a lecture visit to Tartu, Estonia.
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