APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 1
3 JULY 2000
Cd–Ne direct current glow discharge: An efficient source of ultraviolet radiation G. M. Petrova) Berkeley Research Associates, Inc., P.O. Box 852, Springfield, Virginia 22150
Ts. Petrova Faculty of Physics, Sofia University, 5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria
A. Ogoyski Department of Physics, Technical University, 2 Student Street, BG-9001 Varna, Bulgaria
A. B. Blagoev Faculty of Physics, Sofia University, 5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria
共Received 16 March 2000; accepted for publication 12 May 2000兲 This investigation aims to present data of a potential light source based on Cd. Recent study of cadmium–neon direct current glow discharge at low and intermediate pressure showed that a significant fraction (⬃80%) of the input power delivered to the plasma can be converted into resonance radiation, which leads us to conclusion that Cd can be an efficient light source, even more efficient than Hg. The conventional fluorescent lamps contain Hg, but environmental concerns urge for mercury-free light sources. The ultraviolet radiation from Cd(5p 1 P 1 ) and Cd(5p 3 P 1 ) resonance states has been investigated and the optimum discharge conditions of a potential light source are determined. At optimum discharge conditions 80%–87% of the input power is converted into resonance radiation. © 2000 American Institute of Physics. 关S0003-6951共00兲02827-8兴
the energy differences between the states in the triplet are smaller compared to those of mercury. A typical example for the populations of the resonance states versus cadmium vapor pressure is plotted in Fig. 2共a兲 at 1 Torr neon pressure, 300 mA discharge current, and tube radius 1.5 cm. The power emitted as resonance radiation and the power per unit volume delivered to the discharge are shown in Fig. 2共b兲. At low cadmium vapor pressure the resonance radiation from Cd(5p 1 P 1 ) state dominates, while above 70 mTorr cadmium vapor pressure the main part of the resonance radiation comes from Cd(5p 3 P 1 ) state. The trapping of radiation has been accounted for both resonance lines. Figure 2共c兲 presents the ratio of power emitted as resonance radiation to the power delivered to the discharge. At cadmium vapor pressure of 50 mTorr this ratio is about 80%, which is higher compared to mercury.
In a series of articles data for the elementary processes of Cd1 and a steady-state collisional-radiative model2 have been presented. The data derived by spectroscopic and probe measurements refer to the lowest lying Cd triplet. Rate coefficients for energy transfer in collisions with neon, cadmium atoms and electrons, as well as the diffusion coefficients of Cd metastable states in neon have been determined. The experimental results enabled us to develop a collisionalradiative model for a Cd–Ne direct current 共dc兲 glow discharge. The model, based on a complete set of equations and data, calculates self-consistently all discharge characteristics: the electron energy distribution function, electron density, mean energy, electron mobility, and diffusion coefficient, dc electric field, populations of excited atoms, and ion densities. Furthermore, the particular creation and loss mechanisms of both resonance and metastable states of Cd have been investigated and analyzed. They turned out to be more favorable for producing ultraviolet 共UV兲 radiation compared to Hg. Experimental investigations in the late 60’s also support our conclusion.3 The objectives of this letter are to investigate the UV radiation efficiency from Cd(5p 1 P 1 ) and Cd(5p 3 P 1 ) resonance states and to determine the optimum discharge conditions at which a potential light source will operate. The cadmium and mercury energy level diagrams are shown in Fig. 1. As elements of the same group, they have similar structure, but some differences can briefly be outlined. The cadmium triplet states are closer to the ground state, the wavelengths of the resonance lines are longer and a兲
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FIG. 1. Energy level diagrams and resonance lines of Cd and Hg. 40
© 2000 American Institute of Physics
Appl. Phys. Lett., Vol. 77, No. 1, 3 July 2000
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FIG. 4. Efficiency of converting the electric power into resonance radiation vs Cd vapor pressure at 1 Torr Ne pressure and different discharge currents.
FIG. 2. Population of Cd(5 p 3 P 1 ) and Cd(5p 1 P 1 ) states 共a兲, total and radiation power losses from Cd(5p 3 P 1 ) and Cd(5p 1 P 1 ) states 共b兲 and the efficiency of converting the electric power into resonance radiation 共c兲 vs Cd vapor pressure at 1 Torr Ne gas pressure, discharge current 300 mA, and tube radius 1.5 cm.
Figure 3 shows this ratio at different neon pressures and discharge current 300 mA. With the neon pressure increasing the efficiency of converting the electric power into resonance radiation increases, passes through a maximum, and then decreases. The optimum neon and cadmium vapor pressures are 1–2 Torr and 30–60 mTorr, respectively. The discharge current 共Fig. 4兲 also influences the efficiency. High efficiency of 87% is achieved at discharge current of 100 mA, but it decreases with the current increasing. The optimum cadmium vapor pressure also depends on the discharge current: it is 80 mTorr at i⫽100 mA and 25 mTorr at i ⫽500 mA. In our former article2 we explained the high efficiency of converting the electric power into resonance radiation with the particular and somewhat unique population and destruc-
FIG. 3. Efficiency of converting the electric power into resonance radiation vs Cd vapor pressure at discharge current 300 mA and different Ne pressures.
tion mechanisms of Cd resonance and metastable states. The smaller energy differences between the states in the cadmium triplet compared to those of mercury leads to more efficient transfer of Cd(5p 3 P 2 ) metastable to the Cd(5p 3 P 1 ) resonance state in collisions with Cd atoms 关 Cd(5p 3 P 0,2)⫹Cd →Cd(5p 3 P 1 )⫹Cd兴 , which explains the increase of efficiency as the Cd vapor pressure increases 关Fig. 2共c兲兴. However, if the Cd vapor pressure exceeds certain critical value, the reverse reaction destroys Cd(5p 3 P 1 ) resonance state and an optimum Cd vapor pressure exists. There is also an optimum buffer gas pressure: low neon pressure results in diffusion losses of cadmium metastable atoms and electrons, while high neon pressure increases the energy power loss in elastic collisions with neon atoms and reduces the efficiency. The electrons also depopulate the Cd(5p 3 P 1 ) resonance state, the efficiency decrease with the discharge current increase and discharge current optimum is not observed. The efficiency of UV radiation from Cd and Hg may be compared. The reported values for Hg are close to 70%. At 30 mW/cm3 Cd has an efficiency of 86% 共p Ne⫽1 Torr, p Cd ⫽50– 60 mTorr, i⫽100 mA, R⫽1.5 cm兲, while at equal power density 共31 W/m, R⫽1.8 cm兲4 the efficiency of Hg is 70%. Cadmium may lead to technological problems, which can be briefly summarized: 共i兲 High temperature to sustain optimal cadmium vapor pressure, about 550–600 K. Nowadays, however, lamps operating in this temperature range do exist.5 High gas temperature may result in increased power losses which will reduce the effectiveness of the UV radiation. 共ii兲 Existence of efficient and stable phosphors for 326.1 and 222.8 nm. 共iii兲 Possible blackening the phosphor with cadmium. 共iv兲 Cadmium may be a potential environmental problem. The optimum conditions of a potential light source based on Cd have been determined. A dc positive column with tube radius 1.5 cm, neon pressure of 1–2 Torr, Cd vapor pressure of 30–80 mTorr, and discharge current 100–300 mA would convert 80%–87% of the power delivered to the discharge into resonance radiation. Cd–Ne dc glow discharge is more
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efficient for producing UV radiation compared to Hg–Ne dc glow discharge. Competitive efficiency may be expected from Zn-noble gas discharge. This work is supported by the Bulgarian National Foundation for Scientific Research under Contract No. F 579/ 1995.
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