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Hot hydrogen atoms in a water-vapor microwave plasma source
international journal of hydrogen energy 34 (2009) 9585–9590
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hot hydrogen atoms in a water-vapor microwave plasma source E. Tatarova*, F.M. Dias, C.M. Ferreira Instituto de Plasmas e Fusa˜o Nuclear, Instituto Superior Te´cnico, 1049-001 Lisboa, Portugal
article info
abstract
Article history:
A study of the hydrogen Balmer line shape in a water-vapor, microwave slot-antenna
Received 31 July 2009
excited plasma source operated at 2.45 GHz is reported. The emission profiles of the Ha and
Received in revised form
Hb lines are well fitted by Gaussian profiles. Excited hydrogen atoms are detected in the
25 September 2009
remote plasma zone of the source up to 30 cm distance from the exciting antennas. The
Accepted 27 September 2009
measured Doppler temperature corresponding to the Hb line broadening is about three
Available online 21 October 2009
times higher than the rotational temperature of the hydrogen molecular Fulcher-a band. It has been found clear evidence for the existence of a local source of excited ‘‘hot’’
Keywords:
hydrogen atoms in the ‘‘microwave field free’’ remote plasma zone. The measured Doppler
Hydrogen
broadening of the O(777.4 nm) triplet line indicates that ‘‘hot’’ oxygen atoms, with an
Microwave plasma
energy around 0.3 eV, are also created in this source. Exothermic electron–ion and ion–ion
Spectroscopy
recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are possible local sources of ‘‘hot’’ atoms in the far remote plasma zone. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Water-vapor plasmas attract attention as working media for lasers, light sources and efficient sources of UV radiation, especially in the range below 200 nm [1–5]. For environmental applications, microwave induced water-vapor plasmas are also interesting sources of hydrogen and oxygen atoms due to the high levels of dissociation achieved. Moreover, hydrogen containing plasmas show an unexpected behavior. Anomalous, even extreme, hydrogen line broadening was found in a number of mixed discharge plasmas excited via DC or RF electric fields [6–15]. The Balmer line spectra emitted by these discharges have typical multimode behavior, with widely broadened ‘‘wings’’ (‘‘fast’’ hydrogen) and a sharp top (‘‘slow’’ hydrogen). Some authors explain these results in terms of Doppler shift and broadening due to the acceleration of þ charges (such as Hþ, Hþ 2 and H3 ions) in the high DC electric fields present in the sheath regions of these discharges [14,15].
The acceleration of hydrogen ions in these DC fields is followed by neutralization and generation of fast excited H atoms. There appear to exist some weaknesses in this explanation, as pointed out in [3], having in view the observed symmetry of the line broadening, that is, both red and blue shifts are observed. Phillips and collaborators have detected selective H atom line broadening throughout the volume of low-pressure H2O, helium–hydrogen and argon–hydrogen RF plasmas generated in a capacitively coupled parallel-plate cell [3,7–9]. It was found that a significant fraction of the atomic hydrogen is ‘‘hot’’ with energies higher than 40 eV. The obtained results have been explained in the framework of the ‘‘resonance transfer model’’ proposed by Mills [10–13,16] contrary to the field acceleration models mentioned above. The Balmer lines emitted by microwave discharges usually possess single mode behaviour. As is well known, these discharges are free from strong dc fields. Strikingly excessive, selective Balmer Ha line broadening in He/H2 and Ar/H2
* Corresponding author. E-mail address: [email protected] (E. Tatarova). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.080
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international journal of hydrogen energy 34 (2009) 9585–9590
microwave induced discharges has also been observed by Mills and collaborators and this was considered as an evidence for catalytic production of super hot atomic hydrogen via the ‘‘resonant transfer process’’ [6,16]. In ˚ ) Balmer-a line broadening was particular, excessive (> 2.5 A detected in the emission of low-pressure (0.2 Torr) water vapor plasmas generated with an Evenson microwave cavity (2.45 GHz) [3]. Moreover, an inversion of the line intensities of both the Lyman and Balmer series at distances up to 5 cm from the coupler were also observed. It should be noted that Ha line profiles emitted by microwave discharges under similar conditions but with no excessive broadening have also been reported [7]. Selective hydrogen line broadening has nevertheless been detected when there is no significant broadening of the noble gas lines or the hydrogen molecular lines. In this respect, the Doppler temperatures presented in [18] are questionable due to bad resolution of optical system used and possible self-absorption of Ha emission. Furthermore, hyperthermal hydrogen atoms have surprisingly been detected in atmospheric pressure Ar–H2 microplasma jets, where the H(n ¼ 3) temperatures were found to range from 12,000 to 19,600 K [19]. It is clear that hydrogen line broadening causes controversy and many questions about the mechanisms of ‘‘hot’’ hydrogen atom generation leading to large Balmer-line broadening are presently open. Thus, more experimental observations are currently needed in order to elucidate the mechanisms and processes behind this phenomenon in different types of discharges. The aim of this experimental work is to address some of these problems. This article presents spectroscopic measurements in water-vapor plasma generated by a slot antenna excited, microwave plasma source operating at u/2p ¼ 2.45 GHz at low pressures. Results on the line shape and the emission intensities of excited hydrogen and oxygen atoms, and the emission intensities of the Q-branch of the Fulcher-a band Q ½H2 ðd3 u Þ are presented and discussed [20]. Two distinct temperatures have been determined from the measured hydrogen line shape emissions and rotational distribution of hydrogen molecules.
2.
Plasma source and experimental setup
A slot antenna excited, surface wave plasma source as described in [21–25] is used. Surface waves propagating radially (r) and azimuthally (4) along the interface between the plasma and a quartz dielectric plate located at the top wall of a large diameter cylindrical metal chamber are the energy source for the plasma (see Fig. 1). The plasma source comprises two different zones due to the exponential decrease of the electric field of the surface mode sustaining the discharge. The first one is the active discharge zone close to the interface (up to an axial distance z ¼ 1 cm), where surface waves propagate and the microwave electric field intensity is finite, and the second one (z > 1 cm) is the remote, ‘‘electric field-free’’ plasma zone. Emission spectroscopy measurements were performed using an ‘‘optical periscope’’ placed inside the plasma as seen in Fig. 1. The optical system provides azimuthal and axial
Fig. 1 – Experimental setup.
resolution of the measurements [24]. The plasma radiation (in the visible range) is detected by the ‘‘optical periscope’’ and guided by an optical fiber into the input slit of a spectrometer. Photons emitted by the plasma are transferred by the optical system into the entrance slit of a SPEX 1250 M spectrometer (2400 g/mm grating) equipped with a Hamamatsu R928 photomultiplier. The emission spectrum in the range 250–850 nm has been investigated. A precise calibration of the whole optical system, using different optical sources has been performed [24]. The detected peaks in the visible range are the atomic hydrogen H(n ¼ 3,4) / H(n ¼ 2) emission lines while those in the infrared region are the 3 p5P0 / 3s5S0 (777.1 nm) and 3p3P /3s3S0 (844.6 nm) emission lines of atomic oxygen. The measured profiles of the Hb line are well fitted by a Gaussian profile (much better than by a Voigt one) as seen in Fig. 2. This line has been used to determine the kinetic temperature of the atoms because Hb is less critical than Ha regarding self-absorption. The error in determining the full width at half maximum (FWHM) is always much less than 1%. The FWHM of each Gaussian profile results from the folding of the Doppler (DlDffi) and the instrumental (DlI) half widths, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DlG ¼ Dl2D þ Dl2I , for the present conditions. The instru˚ . It should mental width was determined to be Dlapp ¼ 0.065 A be noted that the measured line widths of the Hb line are 3–5 times larger than the instrumental width. Further on, a subtractive procedure has been applied in order to determine the ‘‘pure’’ Doppler broadening in the Gaussian part, taking into account the fine structure of the Balmer lines.
international journal of hydrogen energy 34 (2009) 9585–9590
9587
structure splitting must be taken into account. The corresponding temperatures have been determined from Doppler broadening assuming a Maxwellian distribution of the atoms according to the well known formula rffiffiffiffiffi T ; M
DlD ¼ 7:16 107 l
(1)
where M is the molecular weight (M ¼ 1 for hydrogen), l is the central wavelength, and T is the temperature in Kelvin. The rotational temperature of hydrogen molecules has been determined in a series of separate measurements. The energy separations between rotational levels within a given vibrational transition are usually small compared to the thermal translational energy. Nearly all gas kinetic collisions produce a change in the rotational quantum number, whereas collisions producing a change in vibrational quantum number are much less frequent. Consequently, the relative rotational population distribution in a sufficiently long-lived vibrational state has a Boltzmann distribution, with a rotational temperature that reflects the gas kinetic temperature. The rotational line intensities I for a Boltzmann distribution are given by Eupper hc I ¼ exp kT S
The fine structure splitting consists of seven closely related components corresponding to transitions between sublevels s, p, and d. Each line is Doppler broadened so that the actual spectrum is the sum of seven Gaussians [26]. As known, when the kinetic temperature decreases the fine structure splitting starts to influence the Balmer lines shape. Fig. 3 presents calculated Hb line spectra corresponding to the transition H(n ¼ 4) / H(n ¼ 2) for eight different kinetic temperatures. As seen, for temperatures lower than about 1,000 K the fine
where I is the relative intensity divided by the nuclear and magnetic degeneracies, S is the Ho¨nl–London factor and Eupper is the upper level energy in reciprocal centimeters. Thus, using a classical Boltzmann plot, the slope of ln(I/S) vs the upper level energy is (hc/kTr) [20]. Hydrogen has the advantage that its rotational lines are well separated and easily resolved (see Fig. 4). In order to determine the H2 rotational temperature, Tr, the Q-branch of the Fulcher-a band Q P in the rotational spectrum ½d3 u ðv ¼ 0Þ/a3 þ g ðv ¼ 0Þ wavelength range 600–606 nm has been used as shown in Fig. 4. The rotational distribution of the Fulcher-a band line intensities nearly follows Boltzmann’s law, so that the rotational temperature can be calculated taking into account the Ho¨nl–London factors [20].
Fig. 3 – Fine structure influence on Hb line profile.
Fig. 4 – Fulcher-a band of molecular hydrogen.
Fig. 2 – Hb line profile fitted by a Gaussian profile.
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international journal of hydrogen energy 34 (2009) 9585–9590
Experimental results and discussion
The active discharge of our plasma source is sustained by a TM240 surface mode according to the experimental results. The azimuthal variation of the Ha, Hb and 777.1 nm oxygen atomic line intensities in the discharge zone (at a constant axial distance Dz ¼ 1 cm from the interface) are presented in Figs. 5 and 6. The theoretical azimuthal variation of the total electric field is also presented here as a solid line [17,25]. As seen, the observed variations in the line intensities follow the azimuthal pattern peculiar to the electric field of the TM240 surface mode. Typical Ha and Hb profiles recorded in the discharge and in the remote plasma zone at different axial distances, for a fixed azimuthal position 4 ¼ 162 , are shown in Figs. 7 and 8. As seen, the line profiles do not change along the transition from the discharge to the remote plasma. Excited hydrogen atoms are even observed in the far remote plasma zone up to 30 cm from the dielectric plate. The shape of the line does not change in spite of the decrease in the line emission intensity (see below). Figs. 9 and 10 show the kinetic temperature derived from the broadening of the Hb line and the rotational temperature derived from the Q(0–0) branch of the H2 Fulcher-a band as a function of the azimuthal position in the discharge, for a fixed axial distance Dz ¼ 1 cm. The values of Trot vary azimuthally between 570 K and 720 K (Fig. 10) while the Doppler temperature corresponding to the Hb line varies in the range 1800–2100 K for the same conditions. Thus, the Doppler Hb temperature is much higher than the rotational temperature, the latter being indicative of the kinetic temperature of the background gas. These results demonstrate that the H atom kinetic temperature significantly differs from the background gas temperature in the discharge. Thus, some selective heating mechanisms of excited H atoms do occur in the present source.
Fig. 5 – Azimuthal dependence of the intensity of atomic spectral lines in the discharge zone (Dz [ 1 cm) of water vapor plasma source.
Fig. 6 – Azimuthal dependence of the intensity of oxygen atomic spectral lines at 777.4 nm in the discharge zone (Dz [ 1 cm) of water vapor plasma source.
The measured axial variations of the Hb Doppler broadening demonstrate that the corresponding temperatures are nearly axially constant up to 20 cm from the interface. Therefore, there is little effect of the electric field intensity on the H atom temperature along the z-axis, in spite of the strong axial decrease of the electric field intensity (note that the field vanishes at about 2 cm from the interface as seen in Fig. 11). Further on, the Doppler temperatures of hydrogen atoms are significantly higher than the gas temperature.
Fig. 7 – Ha line profile recorded at different axial distances.
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Fig. 10 – Azimuthal variation of rotational temperatures Trot.
Fig. 8 – Hb line profile recorded at different axial distances.
‘‘Hot’’ hydrogen atoms were also detected in the far remote ‘‘electric field free’’ plasma. This is indicative of the presence of local sources of hot atoms. As far as recombination processes are generally dominant in remote plasmas, exothermic electron–ion and ion-ion recombination processes are likely to be the sources of ‘‘hot’’ atoms in the present far remote plasma zone. Furthermore, distributed DC potentials arising from strong axial and radial density gradients may also contribute to the process of ‘‘hot’’ atom generation.
Fig. 9 – Azimuthal variation of Doppler temperatures THb .
In addition, the shape of the fine structure components of the O(777.4 nm) triplet line shows that ‘‘hot’’ oxygen atoms with an energy of approximately 0.3 eV are also generated [27]. The axial variations of the emissions of excited hydrogen atoms, at 656.3 nm [Ha line (n ¼ 3/n ¼ 2)], and oxygen atoms at 777.4 nm are shown in Fig. 12. The observed decay of the line intensities should be correlated to some extent with the axial exponential decay of the TM240 surface mode total electric field intensity (shown by the black line). As observed, the discharge zone of the plasma source, with non-zero microwave electric field intensity, extends up to about 2 cm from the interface. The intensity of the lines decays exponentially, but much slower than electric field intensity. The decay is by nearly one order of magnitude in the ‘‘electric field free’’ remote plasma zone up to Dz ¼ 20 cm. These variations reflect the axial variation of the population density of the excited H atoms. Nevertheless, we have found clear evidence for the existence of a local source of excited ‘‘hot’’
Fig. 11 – Axial variation of Doppler THb temperatures.
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international journal of hydrogen energy 34 (2009) 9585–9590
references
Fig. 12 – Axial dependence of the intensity of Ha and O (777 nm) atomic lines in water vapor plasma source (p [ 1 mbar, points-experiment, lines-exp. fitting). Black line is the axial variation of the total electric field intensity.
hydrogen atoms in the remote plasma zone. A detailed investigation including a numerical calculation of chemical and ion compositions is necessary to explain the presence of these ‘‘hot’’ atoms in the far remote plasma zone. This will be the subject of further investigations.
4.
Conclusion
Emission spectroscopy was used for the diagnostic of a largescale, slot-antenna excited microwave plasma source operating in water vapor at low-pressure conditions. The Doppler temperatures corresponding to the broadening of the Hb line at 486.1 nm are about 3 times higher than the rotational temperatures determined from the Q-branch of the Fulcher-a band Q P ½d3 u ðv ¼ 0Þ/a3 þ g ðv ¼ 0Þ. We have found clear evidence for the existence of a local source of excited ‘‘hot’’ hydrogen atoms in the ‘‘electric field free’’ remote plasma zone. The Doppler broadening of the O (777.4 nm) triplet line indicates that ‘‘hot’’ oxygen atoms are also generated. Electron–ion and ion-ion recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are likely to be the sources of ‘‘hot’’ atoms in the far remote plasma zone, but a detailed kinetic analysis is necessary to elucidate this question.
Acknowledgment This study was funded by FCT/FEDER in the framework of the project ‘‘Ecological Engineering Plasma Laboratory’’ POCI/FIS/ 61679/2004.
[1] Mills R, Ray P, Mayo R. Appl Phys Lett 2003;82:1679. [2] Mills R, Ray P, Mayo R, Nansteel M, Dhadapani B, Phillips JJ. Plasma Phys 2005;71:877. [3] Phillips J, Chen Chun Ku, Mills R. Int J Hydrogen Energy 2008; 33:2419. [4] Sawada M, Tomai T, Ito T, Fujivara S, Terashima K. J Appl Phys 2006;100:123304. [5] Sun B, Kunitomo S, Igarashi C. J Phys D: Appl Phys 2006;39: 3814. [6] Mills R, Ray P, Nansteel M, Cen X, Mayo R, He J, et al. IEEE Trans Plasma Sci 2003;31:338. [7] Phillips J, Chen Chun Ku. Int J Hydrogen Energy 2008;33:7185. [8] Phillips J, Chen Chun Ku, Akhar K, Dhadapani B, Mills R. Int J Hydrogen Energy 2007;32:3010. [9] Phillips J, Chen, CK, Shiina T. Evidence of the production of hot atomic hydrogen in hydrogen–helium plasmas. Paper #0509127. ArXiv.org. [10] Mills R, Voigt A, Ray P, Nansteel M, Dhadapani B. Int J Hydrogen Energy 2002;27:671. [11] Mills R, He J, Chang Z, Good W, Lu Y, Dhadapani B. Int J Hydrogen Energy 2007;32:2573. [12] Mills R, Onuma T, Lu Y. Int J Hydrogen Energy 2001;26:749. [13] Mills R, Ray P. Int J Hydrogen Energy 2002;27:301. [14] Adamov M, Obradovich B, Kuraica M, Konjevic N. IEEE Trans Plasma Sci 2003;31:444. [15] Cvetanovic N, Kuraica M, Konjevic N. J Appl Phys 2005;97: 033302. [16] Mills R, Ray P. J Phys D: Appl Phys 2003;36:1535. [17] Tatarova E, Dias FM, Ferreira C, Puac N. J Appl Phys 2007;101: 063306. [18] Jovicevic S, Sakan N, Ivkovic M, Konjevic´ N. J Appl Phys 2009; 105:013306. [19] Oliveira C, Souza Correa JA, Gomes MP, Sismanoglu BN, Amorim J. Appl Phys Lett 2008;93:041503. [20] Tomasini L, Rousseau A, Gousset G, Leprince P. J Phys D: Appl Phys 1996;29:1006. [21] Sugai H, Ghanashev I, Nagatsu M. Plasma Sources Sci Technol 1998;7:192. [22] Ghanashev I, Nagatsu M, Sugai H. Jpn J Appl Phys 1997;36: 4704. [23] Ghanashev I, Nagatsu M, Sugai H. Jpn J Appl Phys 1997;36: 337. [24] Dias FM, Martins A, Tatarova E. ‘‘Wave fields, probe, and spectroscopic measurements of a large –volume top-excited surface wave discharge’’ in: Contributed papers of frontiers in low temperature plasma diagnostics V. (Villaggio Cardigliano, Italy); April 3–7, 2003. p. 144–7. [25] Tatarova E, Dias FM, Henriques J, Ferreira CM. IEEE Trans Plasma Sci 2005;33:866. [26] Lavrov BP, Kaning M, Ovtchinnikov VL, Ropcke J. ‘‘Fine structure of Balmer-a lines and determination of translational temperature of hydrogen and deuterium containing plasmas’’ in: Proceedings of frontiers in low temperature plasma diagnostics II (Bad Honnef, Germany); February 17–21, 1997. p. 169–72. [27] Capitelli M, Ferreira CM, Gordiets BF, Osipov AI. Plasma kinetics in atmospheric gases. Berlin: Springer; 2000.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hot hydrogen atoms in a water-vapor microwave plasma source E. Tatarova*, F.M. Dias, C.M. Ferreira Instituto de Plasmas e Fusa˜o Nuclear, Instituto Superior Te´cnico, 1049-001 Lisboa, Portugal
article info
abstract
Article history:
A study of the hydrogen Balmer line shape in a water-vapor, microwave slot-antenna
Received 31 July 2009
excited plasma source operated at 2.45 GHz is reported. The emission profiles of the Ha and
Received in revised form
Hb lines are well fitted by Gaussian profiles. Excited hydrogen atoms are detected in the
25 September 2009
remote plasma zone of the source up to 30 cm distance from the exciting antennas. The
Accepted 27 September 2009
measured Doppler temperature corresponding to the Hb line broadening is about three
Available online 21 October 2009
times higher than the rotational temperature of the hydrogen molecular Fulcher-a band. It has been found clear evidence for the existence of a local source of excited ‘‘hot’’
Keywords:
hydrogen atoms in the ‘‘microwave field free’’ remote plasma zone. The measured Doppler
Hydrogen
broadening of the O(777.4 nm) triplet line indicates that ‘‘hot’’ oxygen atoms, with an
Microwave plasma
energy around 0.3 eV, are also created in this source. Exothermic electron–ion and ion–ion
Spectroscopy
recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are possible local sources of ‘‘hot’’ atoms in the far remote plasma zone. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Water-vapor plasmas attract attention as working media for lasers, light sources and efficient sources of UV radiation, especially in the range below 200 nm [1–5]. For environmental applications, microwave induced water-vapor plasmas are also interesting sources of hydrogen and oxygen atoms due to the high levels of dissociation achieved. Moreover, hydrogen containing plasmas show an unexpected behavior. Anomalous, even extreme, hydrogen line broadening was found in a number of mixed discharge plasmas excited via DC or RF electric fields [6–15]. The Balmer line spectra emitted by these discharges have typical multimode behavior, with widely broadened ‘‘wings’’ (‘‘fast’’ hydrogen) and a sharp top (‘‘slow’’ hydrogen). Some authors explain these results in terms of Doppler shift and broadening due to the acceleration of þ charges (such as Hþ, Hþ 2 and H3 ions) in the high DC electric fields present in the sheath regions of these discharges [14,15].
The acceleration of hydrogen ions in these DC fields is followed by neutralization and generation of fast excited H atoms. There appear to exist some weaknesses in this explanation, as pointed out in [3], having in view the observed symmetry of the line broadening, that is, both red and blue shifts are observed. Phillips and collaborators have detected selective H atom line broadening throughout the volume of low-pressure H2O, helium–hydrogen and argon–hydrogen RF plasmas generated in a capacitively coupled parallel-plate cell [3,7–9]. It was found that a significant fraction of the atomic hydrogen is ‘‘hot’’ with energies higher than 40 eV. The obtained results have been explained in the framework of the ‘‘resonance transfer model’’ proposed by Mills [10–13,16] contrary to the field acceleration models mentioned above. The Balmer lines emitted by microwave discharges usually possess single mode behaviour. As is well known, these discharges are free from strong dc fields. Strikingly excessive, selective Balmer Ha line broadening in He/H2 and Ar/H2
* Corresponding author. E-mail address: [email protected] (E. Tatarova). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.080
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international journal of hydrogen energy 34 (2009) 9585–9590
microwave induced discharges has also been observed by Mills and collaborators and this was considered as an evidence for catalytic production of super hot atomic hydrogen via the ‘‘resonant transfer process’’ [6,16]. In ˚ ) Balmer-a line broadening was particular, excessive (> 2.5 A detected in the emission of low-pressure (0.2 Torr) water vapor plasmas generated with an Evenson microwave cavity (2.45 GHz) [3]. Moreover, an inversion of the line intensities of both the Lyman and Balmer series at distances up to 5 cm from the coupler were also observed. It should be noted that Ha line profiles emitted by microwave discharges under similar conditions but with no excessive broadening have also been reported [7]. Selective hydrogen line broadening has nevertheless been detected when there is no significant broadening of the noble gas lines or the hydrogen molecular lines. In this respect, the Doppler temperatures presented in [18] are questionable due to bad resolution of optical system used and possible self-absorption of Ha emission. Furthermore, hyperthermal hydrogen atoms have surprisingly been detected in atmospheric pressure Ar–H2 microplasma jets, where the H(n ¼ 3) temperatures were found to range from 12,000 to 19,600 K [19]. It is clear that hydrogen line broadening causes controversy and many questions about the mechanisms of ‘‘hot’’ hydrogen atom generation leading to large Balmer-line broadening are presently open. Thus, more experimental observations are currently needed in order to elucidate the mechanisms and processes behind this phenomenon in different types of discharges. The aim of this experimental work is to address some of these problems. This article presents spectroscopic measurements in water-vapor plasma generated by a slot antenna excited, microwave plasma source operating at u/2p ¼ 2.45 GHz at low pressures. Results on the line shape and the emission intensities of excited hydrogen and oxygen atoms, and the emission intensities of the Q-branch of the Fulcher-a band Q ½H2 ðd3 u Þ are presented and discussed [20]. Two distinct temperatures have been determined from the measured hydrogen line shape emissions and rotational distribution of hydrogen molecules.
2.
Plasma source and experimental setup
A slot antenna excited, surface wave plasma source as described in [21–25] is used. Surface waves propagating radially (r) and azimuthally (4) along the interface between the plasma and a quartz dielectric plate located at the top wall of a large diameter cylindrical metal chamber are the energy source for the plasma (see Fig. 1). The plasma source comprises two different zones due to the exponential decrease of the electric field of the surface mode sustaining the discharge. The first one is the active discharge zone close to the interface (up to an axial distance z ¼ 1 cm), where surface waves propagate and the microwave electric field intensity is finite, and the second one (z > 1 cm) is the remote, ‘‘electric field-free’’ plasma zone. Emission spectroscopy measurements were performed using an ‘‘optical periscope’’ placed inside the plasma as seen in Fig. 1. The optical system provides azimuthal and axial
Fig. 1 – Experimental setup.
resolution of the measurements [24]. The plasma radiation (in the visible range) is detected by the ‘‘optical periscope’’ and guided by an optical fiber into the input slit of a spectrometer. Photons emitted by the plasma are transferred by the optical system into the entrance slit of a SPEX 1250 M spectrometer (2400 g/mm grating) equipped with a Hamamatsu R928 photomultiplier. The emission spectrum in the range 250–850 nm has been investigated. A precise calibration of the whole optical system, using different optical sources has been performed [24]. The detected peaks in the visible range are the atomic hydrogen H(n ¼ 3,4) / H(n ¼ 2) emission lines while those in the infrared region are the 3 p5P0 / 3s5S0 (777.1 nm) and 3p3P /3s3S0 (844.6 nm) emission lines of atomic oxygen. The measured profiles of the Hb line are well fitted by a Gaussian profile (much better than by a Voigt one) as seen in Fig. 2. This line has been used to determine the kinetic temperature of the atoms because Hb is less critical than Ha regarding self-absorption. The error in determining the full width at half maximum (FWHM) is always much less than 1%. The FWHM of each Gaussian profile results from the folding of the Doppler (DlDffi) and the instrumental (DlI) half widths, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DlG ¼ Dl2D þ Dl2I , for the present conditions. The instru˚ . It should mental width was determined to be Dlapp ¼ 0.065 A be noted that the measured line widths of the Hb line are 3–5 times larger than the instrumental width. Further on, a subtractive procedure has been applied in order to determine the ‘‘pure’’ Doppler broadening in the Gaussian part, taking into account the fine structure of the Balmer lines.
international journal of hydrogen energy 34 (2009) 9585–9590
9587
structure splitting must be taken into account. The corresponding temperatures have been determined from Doppler broadening assuming a Maxwellian distribution of the atoms according to the well known formula rffiffiffiffiffi T ; M
DlD ¼ 7:16 107 l
(1)
where M is the molecular weight (M ¼ 1 for hydrogen), l is the central wavelength, and T is the temperature in Kelvin. The rotational temperature of hydrogen molecules has been determined in a series of separate measurements. The energy separations between rotational levels within a given vibrational transition are usually small compared to the thermal translational energy. Nearly all gas kinetic collisions produce a change in the rotational quantum number, whereas collisions producing a change in vibrational quantum number are much less frequent. Consequently, the relative rotational population distribution in a sufficiently long-lived vibrational state has a Boltzmann distribution, with a rotational temperature that reflects the gas kinetic temperature. The rotational line intensities I for a Boltzmann distribution are given by Eupper hc I ¼ exp kT S
The fine structure splitting consists of seven closely related components corresponding to transitions between sublevels s, p, and d. Each line is Doppler broadened so that the actual spectrum is the sum of seven Gaussians [26]. As known, when the kinetic temperature decreases the fine structure splitting starts to influence the Balmer lines shape. Fig. 3 presents calculated Hb line spectra corresponding to the transition H(n ¼ 4) / H(n ¼ 2) for eight different kinetic temperatures. As seen, for temperatures lower than about 1,000 K the fine
where I is the relative intensity divided by the nuclear and magnetic degeneracies, S is the Ho¨nl–London factor and Eupper is the upper level energy in reciprocal centimeters. Thus, using a classical Boltzmann plot, the slope of ln(I/S) vs the upper level energy is (hc/kTr) [20]. Hydrogen has the advantage that its rotational lines are well separated and easily resolved (see Fig. 4). In order to determine the H2 rotational temperature, Tr, the Q-branch of the Fulcher-a band Q P in the rotational spectrum ½d3 u ðv ¼ 0Þ/a3 þ g ðv ¼ 0Þ wavelength range 600–606 nm has been used as shown in Fig. 4. The rotational distribution of the Fulcher-a band line intensities nearly follows Boltzmann’s law, so that the rotational temperature can be calculated taking into account the Ho¨nl–London factors [20].
Fig. 3 – Fine structure influence on Hb line profile.
Fig. 4 – Fulcher-a band of molecular hydrogen.
Fig. 2 – Hb line profile fitted by a Gaussian profile.
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international journal of hydrogen energy 34 (2009) 9585–9590
Experimental results and discussion
The active discharge of our plasma source is sustained by a TM240 surface mode according to the experimental results. The azimuthal variation of the Ha, Hb and 777.1 nm oxygen atomic line intensities in the discharge zone (at a constant axial distance Dz ¼ 1 cm from the interface) are presented in Figs. 5 and 6. The theoretical azimuthal variation of the total electric field is also presented here as a solid line [17,25]. As seen, the observed variations in the line intensities follow the azimuthal pattern peculiar to the electric field of the TM240 surface mode. Typical Ha and Hb profiles recorded in the discharge and in the remote plasma zone at different axial distances, for a fixed azimuthal position 4 ¼ 162 , are shown in Figs. 7 and 8. As seen, the line profiles do not change along the transition from the discharge to the remote plasma. Excited hydrogen atoms are even observed in the far remote plasma zone up to 30 cm from the dielectric plate. The shape of the line does not change in spite of the decrease in the line emission intensity (see below). Figs. 9 and 10 show the kinetic temperature derived from the broadening of the Hb line and the rotational temperature derived from the Q(0–0) branch of the H2 Fulcher-a band as a function of the azimuthal position in the discharge, for a fixed axial distance Dz ¼ 1 cm. The values of Trot vary azimuthally between 570 K and 720 K (Fig. 10) while the Doppler temperature corresponding to the Hb line varies in the range 1800–2100 K for the same conditions. Thus, the Doppler Hb temperature is much higher than the rotational temperature, the latter being indicative of the kinetic temperature of the background gas. These results demonstrate that the H atom kinetic temperature significantly differs from the background gas temperature in the discharge. Thus, some selective heating mechanisms of excited H atoms do occur in the present source.
Fig. 5 – Azimuthal dependence of the intensity of atomic spectral lines in the discharge zone (Dz [ 1 cm) of water vapor plasma source.
Fig. 6 – Azimuthal dependence of the intensity of oxygen atomic spectral lines at 777.4 nm in the discharge zone (Dz [ 1 cm) of water vapor plasma source.
The measured axial variations of the Hb Doppler broadening demonstrate that the corresponding temperatures are nearly axially constant up to 20 cm from the interface. Therefore, there is little effect of the electric field intensity on the H atom temperature along the z-axis, in spite of the strong axial decrease of the electric field intensity (note that the field vanishes at about 2 cm from the interface as seen in Fig. 11). Further on, the Doppler temperatures of hydrogen atoms are significantly higher than the gas temperature.
Fig. 7 – Ha line profile recorded at different axial distances.
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Fig. 10 – Azimuthal variation of rotational temperatures Trot.
Fig. 8 – Hb line profile recorded at different axial distances.
‘‘Hot’’ hydrogen atoms were also detected in the far remote ‘‘electric field free’’ plasma. This is indicative of the presence of local sources of hot atoms. As far as recombination processes are generally dominant in remote plasmas, exothermic electron–ion and ion-ion recombination processes are likely to be the sources of ‘‘hot’’ atoms in the present far remote plasma zone. Furthermore, distributed DC potentials arising from strong axial and radial density gradients may also contribute to the process of ‘‘hot’’ atom generation.
Fig. 9 – Azimuthal variation of Doppler temperatures THb .
In addition, the shape of the fine structure components of the O(777.4 nm) triplet line shows that ‘‘hot’’ oxygen atoms with an energy of approximately 0.3 eV are also generated [27]. The axial variations of the emissions of excited hydrogen atoms, at 656.3 nm [Ha line (n ¼ 3/n ¼ 2)], and oxygen atoms at 777.4 nm are shown in Fig. 12. The observed decay of the line intensities should be correlated to some extent with the axial exponential decay of the TM240 surface mode total electric field intensity (shown by the black line). As observed, the discharge zone of the plasma source, with non-zero microwave electric field intensity, extends up to about 2 cm from the interface. The intensity of the lines decays exponentially, but much slower than electric field intensity. The decay is by nearly one order of magnitude in the ‘‘electric field free’’ remote plasma zone up to Dz ¼ 20 cm. These variations reflect the axial variation of the population density of the excited H atoms. Nevertheless, we have found clear evidence for the existence of a local source of excited ‘‘hot’’
Fig. 11 – Axial variation of Doppler THb temperatures.
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references
Fig. 12 – Axial dependence of the intensity of Ha and O (777 nm) atomic lines in water vapor plasma source (p [ 1 mbar, points-experiment, lines-exp. fitting). Black line is the axial variation of the total electric field intensity.
hydrogen atoms in the remote plasma zone. A detailed investigation including a numerical calculation of chemical and ion compositions is necessary to explain the presence of these ‘‘hot’’ atoms in the far remote plasma zone. This will be the subject of further investigations.
4.
Conclusion
Emission spectroscopy was used for the diagnostic of a largescale, slot-antenna excited microwave plasma source operating in water vapor at low-pressure conditions. The Doppler temperatures corresponding to the broadening of the Hb line at 486.1 nm are about 3 times higher than the rotational temperatures determined from the Q-branch of the Fulcher-a band Q P ½d3 u ðv ¼ 0Þ/a3 þ g ðv ¼ 0Þ. We have found clear evidence for the existence of a local source of excited ‘‘hot’’ hydrogen atoms in the ‘‘electric field free’’ remote plasma zone. The Doppler broadening of the O (777.4 nm) triplet line indicates that ‘‘hot’’ oxygen atoms are also generated. Electron–ion and ion-ion recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are likely to be the sources of ‘‘hot’’ atoms in the far remote plasma zone, but a detailed kinetic analysis is necessary to elucidate this question.
Acknowledgment This study was funded by FCT/FEDER in the framework of the project ‘‘Ecological Engineering Plasma Laboratory’’ POCI/FIS/ 61679/2004.
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