Journal of Applied Mathematics and Physics, 2014, *, ** Published Online **** 2014 in SciRes. http://www.scirp.org/journal/jamp http://dx.doi.org/10.4236/jamp.2014.***** UV multi-element light sources for different applications G.Revalde 1,2 1 Institute of Technical Physics, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P.Valdena 3/7, Riga, LV- 1048, Latvia 2 Ventspils University College, Inzenieru 101, Ventspils, Latvia Email: gitar@latnet.lv Z.Gavare, E.Goško, M.Ziņģe Institute of Atomic Physics and Spectroscopy, University of Latvia, Škunu 4, Riga, LV 1050, Latvia Abstract We present in this paper our experience in creation and optimization special type of light sources, containing multiple filling elements for different application like sensors, medical applications, AAS. Detailed measurements were done for cadmium containing high frequency electrodeless lamps (HFEDLs). The intensities of cadmium, zinc, mercury and argon spectral lines were measured in dependence on power of the excitation generator. The stability of the line intensity over time was measured. Multi-element lamps, containing Cd, Zn and Ar were compared with lamps containing only Cd and Ar. New technology allowed suppressing the buffer gas argon lines. Keywords Cadmium, argon, zinc, mercury plasma, high frequency electrodeless lamp, light sources, UV, benzene detection 1. Introduction In many applications in medicine, biology, pollution detection intensive UV radiation in region of 200 300 nm is needed. Creation of lasers, lasing light in UV spectral region, is still a challenge. Very often mercury vapor lamps are used for these applications because of its very strong resonance radiation in 185 nm and 254 nm region. Our previous research was also devoted to the creation of special type light sources- high frequency electrodeless lamps (HFEDLs), with special attention to mercury for different applications [1,2]. Our current interest is also to optimize light source for benzene and toluene detection in the air and natural gas using atomic absorp-
tion spectrometry (AAS) in UV region where benzene and toluene have strong absorption bands [3]. HFEDLs normally are made from glass or quartz and are filled with metal and buffer gas. By application of electromagnetic excitation, the lamp is heated up to approx. 1000 C or more and metal is evaporated and excited by means of outer electromagnetic field. Metal vapor in the lamp is a working element that emits atomic and sometimes molecular spectrum. Two types of excitation are commonly used: inductive and capacitative excitation [4]. The radiation of the lamps depends on many parameters: excitation type and power, concentration of atoms and molecules in the discharge, temperature, and lamp form and operation conditions. Examples of HFEDLs are shown in Fig.1. Typically HFEDLs are filled with one metal and a buffer gas, like Ar, Ne, Xe or other. In this case the EM field excites the atoms of buffer gas first and then the metal atoms are excited. The processes in such plasma are studied in detail in our previous works [5]. However, in this case we receive emission spectrum, containing spectral lines only from excited atoms of this one filling element and spectral lines of buffer gas. In this paper we report our efforts to create multi-element high frequency light sources with rich spectra in UV region. In this case the problem might be the complex interaction and energy transfer between different elements, so the careful optimization of the filling amount and working conditions is necessary. Figure 1. Examples of HFEDLs. 2. Experimental In this paper we report the results of the investigation of HFED lamps containing (1) Cd + Ar, (2) Cd + Zn + Ar. In first case we have only one working element cadmium, and a buffer gas - Ar. In the second cade we add also Zn, as the second working element. We investigate the lamp performance in dependence of filling elements with the goal to receive more stable and intensive lines in the spectral region from 200 800 nm, with the special interest in the UV region. In this research we primarily study the intensity in emission of atoms Cd and Ar (with and without Zn) and also OH group in dependence on the excitation power. OH group is of interest for temperature estimation in plasma [6]. We only show examples of several spectral lines of each of these elements; however, these spectral lines reflect the on overall physical processes of each element in the lamp. The experimental setup is shown in Fig.2. To observe the lamp spectra, we put the HFEDL in an electromagnetic field of 100 MHz frequency for excitation of the discharge. The voltage of the discharge was changed from 21V to 29V, and the intensity changes of emission spectra were recorded. The spectra of each HFEDL were measured by means of JobinYvon SPEX 1000M high resolution spectrometer (grating 1200 l/mm) with a charge-coupled device (CCD) matrix detector (Thermoelectric Front Illuminated UV Sensitive CCD detector, Symphony, 512 x 2048). * Special description of the title. (dispensable) 2
Figure 2. Experimental set-up. 4. Results and discussion The emission spectrum example of the lamp with one working element filling with element Cd and buffer gas Ar is shown in Fig.3. We can observe several lines in UV region, very intense Cd line of 228,8 nm and several argon line in the region from 700 nm to 850 nm. Figure 3. Emission spectrum example of Cd+Ar high-frequency electrodeless lamp by 21 V excitation voltage. The spectrum of the lamp with multi-element filling with elements Cd, Zn and buffer gas Ar is shown in Fig.3. As we can observe, there are more l strong lines in UV region, in addition of Cd line of 228 nm there is also Zn line of 213 nm. Small amount of mercury radiation also can be observed due to the fact that the filling mixture for cadmium contains also mercury. We tried also to put additional additives to increase the intensity of Zd and Cd lines and to decrease the Ar lines. First results of the new technology are shown in Fig. 4. To understand the excitation processes, we did measurements of the intensity dependence on the excitation voltage. 3
Figure 4. Spectrum of Cd+Zn+Ar high frequency electrodeless lamp at the excitation voltage of 21 V. Intensity dependences on voltage for 4 lines of cadmium and argon plasma emission from Cd+Ar and Cd+Zn+Ar HFEDLs are shown in Fig.5. The graphs show argon decreasing intensity and cadmium increasing intensity as we increase the voltage. The problem in the case of monometal filling by Cd is that Ar lines are more intense than Cd lines by voltages up to 28 V. By adding Zn (Fig.5,b)) we see that the intensity of Cd lines increase more rapidly and already at 26-27 V, the lines are on the same intensity. Still, it can be observed that the optimum voltage is in the range of 28-29 V. a) b). Figure 5. Intensity dependence on voltage for Ar spectral lines 763,5 nm and 811,5 nm and Cd spectral lines of 228,8 nm and 643,85 nm from two different HFEDs with filling a) Cd + Ar and b) Cd + Zn + Ar. 4
Conclusion In our work we report the results of our research and optimization of multi element high frequency electrodeless lamps containing working metals cadmium and zinc and buffer gas Ar with the goal to increase the spectral line amount and intensity of separate lines in UV region 200-250 nm. Our experiments proved that by adding Zn to Cd+Ar HFEDLs, the intensities of Cd lines, in particular line of 228,8 nm increase. Our experiments showed that multielement HFEDLs are promising. To improve the performance of such lamps further research is necessary. Theoretical modelling of plasmas processes should be done in future.. Acknowledgements G.Revalde acknowledges the European Social Fund grant within the project Elaboration of Innovative Functional Materials and Nano materials for Application in Environment Control Technologies DP/1.1.1.2.0/13/APIA/VIAA/30. Z.Gavare, E.Goško acknowledge the Latvian State research programme Multifunctional materials and composites, photonics and nanotechnologies project Nr.1. Photonics and materials for photonics. References [1] Revalde, G., Denisova, N., Gavare, Z., Skudra, A. (2005) Diagnostics of capillary mercury-argon high-frequency electrodeless discharge using line shapes, Journal of Quantitative Spectroscopy & Radiative Transfer, 94, 311-324. [2] Revalde, G., Skudra, A., Zorina, N., Sholupov, S. (2007) Investigation of Hg resonance 184.9 nm line profile in a low-pressure mercury argon discharge, Journal of Quantitative Spectroscopy & Radiative Transfer 107, 164 172 (2007). doi:10.1016/j.jqsrt.2007.01.046. [3] Alnis, J., Revalde, G., Vrublevskis, A., Gavare, Z. (2014) Optical diagnostic method for benzene in air, Proc. of SPIE 9421, 94210E. [4] Kazantsev, S.A., Khutorshchikov,V.I., Guthöhrlein, G. H., Windholz L. (1998) Practical Spectroscopy of High-Frequency Discharges, New York: Plenum Press. [5] Denisova, N., Revalde, G., Skudra, A., Skudra, J. (2011) Spatial Diagnostics of Hg/Ar and Hg/Xe Discharge Lamps by Means of Tomography, Japanese Journal of Applied Physics 50 (8), 08JB03-5. [6] Gavare, Z., Svagere, A., Zinge, M., Revalde, G., Fjodorov, V. (2013) Determination of gas temperature of hogh-frequency low-pressure electrodeless plasma using mmolecular spectra of hydrogen and hydroxyl- radical, Journal of Quantitative Spectroscopy & Radiative Transfer 113, 1676-1682. 5