WDS' Proceedings of Contributed Papers, Part II, 7,. ISBN 978-8-7378-- MATFYZPRESS Population of Excited States of Hydrogen Leading to Balmer Series Emission and Their Threshold Energies Studied by Electron Induced Fluorescence in Methane Molecule M. Danko, J. Országh, A. Ribar, Š. Matejčík Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská dolina, 8448 Bratislava, Slovakia. S. Denifl Institute for Ion Physics and Applied Physics, University of Innsbruck, Technikerstrasse, A-6 Innsbruck, Austria. Abstract. Electron induced fluorescence spectra of methane were measured for electron energies 3 ev in wavelength range 36 66 nm. The intensities of hydrogen Balmer series lines were plotted as a dependence of the principal quantum number n of the upper states for each measurement to estimate how these hydrogen states are populated. Relative cross sections of excitation reactions were measured as well and the thresholds of the excitation processes were determined. Introduction Methane is the simplest stable (non-radical) hydrocarbon molecule. It makes an important percentage of gas content in atmospheres of jovian planets (Jupiter, Saturn, Uranus, and Neptune), and some of their moons (e.g. Titan or Triton) [Lutz et al., 976]. Solar radiation causes ionization of particles in the planetary atmospheres, leading to production of free electrons with a spectrum of energies. These electrons can react with atoms and molecules present there, causing different kinds of physical and chemical reactions. From that perspective, study of electron induced processes in methane brings interesting results for the astrophysical and astrochemical research. Excitation of methane molecule to a higher electronic state causes fragmentation of the molecule. There is a consensus, that there is no stable electronic excited state for methane. The statement was supported both by experiment [Aarts et al., 97], [Beenakker et al., 97], where only spectral lines and bands of excited fragments were obtained, and by theory [Gil et al., 988]. Our study is also in agreement with this suggestion. The emission spectrum of methane in visible region induced by electrons with energy up to ev contains CH and CH + bands, and H Balmer lines [Hesser et al., 97]. There are similar electron impact induced fragmentation channels for many other hydrocarbons both smaller, e.g. ethane, ethylene and acetylene [Beenakker et al., 97], and larger, e.g. isoxazole [Linert et al., ]. Thanks to this fact we can extrapolate our knowledge gained from electron induced fluorescence processes measurements in methane molecule, to other, more complex organic compounds. The study of electron induced processes in methane gives us also an opportunity to better understand the processes in high temperature edge plasmas in tokamaks and diagnose them, since hydrogen (working gas) and carbon atoms (directly or sputtered from divertor plates) can form light hydrocarbon radicals in the wall region [EFDA, ]. In the present study we focused on hydrogen fragment processes leading to Balmer series emission. Relative intensities of Balmer lines in spectra induced by electrons with different energies contain information about how the hydrogen excited states are populated at concrete conditions [Motohashi et al., 996]. Detection of fluorescence radiation is a powerful method for studying excited states of atoms and molecules. Since there is no use of mass spectrometry, it is one of few methods that allow studying not only ions, but also neutral particles in a relatively simple way. Excitation, leading to emission of a fluorescence photon, can be achieved optically by a photon, or by a particle, usually an electron. The electron impact excitation of studied molecule allows us to study also states that are forbidden for optical excitation. Processes that are investigated by electron induced fluorescence method are electron
induced excitation (), electron induced ionization excitation (), and electron induced dissociative excitation with leftover excited fragments (3). e( ε ) + Y e ( ε ) A ( X ) A ( C ) + e ( ε ) A ( B ) + hν + e ( ε ) ε) + A( X ) A ( C) + e( ε ) + e( ε ) A ( B) + hν + e( ε ) e( ε ) e( ε) + M ( X ) M ( C) + e( ε ) Y ( C) + [ M Y ] + e( ε ) Y ( B) + [ M Y ] + hν + e( ε ) M ( X ) M ( C) + e( ε ) + e( ε ) Y ( C) + [ M Y ] + e( ε ) + e( ε ) ( B) + [ M Y ] + hν + e( ε ) + e( ε ) + () e ( + () Radiative electron attachment can occur after electron impact, too. The cross section of this process is usually too low to be observed in practice. Experiment Our experimental apparatus for electron induced fluorescence processes research is shown in Figure. There is a trochoidal electron monochromator (TEM) of our own design [Matuska et al., 9] set inside the vacuum chamber pumped to ultra high vacuum of 8 mbar. Electrons are emitted from a hairpin tungsten filament, and subsequently TEM produces an electron beam with narrow electron energy distribution ( mev), with typical current of 3 na, measured on faraday cup at the back of the TEM by picoammeter Keithley 648. Magnetic field of about 3 T necessary for the TEM operation is produced by a pair of coils outside of the vacuum chamber. The electron beam collides with an effusive molecular beam produced by a capillary of.3 mm inner diameter. The working pressure in the chamber is 4 mbar. Fluorescence photons are produced. They are gathered by system of plano-convex lenses and focused onto a slit of ¼m Czerny-Turner optical monochromator. After wavelength selection the signal is detected by a low-noise (. cps), thermoelectrically cooled R4P Hamamatsu photomultiplier operating in wavelength range of 8 7 nm. (3) (4) Figure. Electron induced fluorescence apparatus. Trochoidal electron monochromator (TEM) forms an energetically uniform electron beam which collides with a beam of molecules. Fluorescence radiation is directed by lenses to an optical monochromator and detected by photomultiplier.
Results and Discussion Measurements of electron induced fluorescence processes in methane were done. Series of emission fluorescence spectra in the wavelength region of 36 66 nm were obtained for the incident electron energy of 3 ev. Figure shows one such spectrum. It contains radiation of methane fragments and no bands what confirms conclusion of previous researchers that there is no stable electronically excited state of the molecule. High sensitivity of our apparatus allows us to see all hydrogen Balmer lines from alpha to the mixture of Rydberg states close to the ionization potential. Relative emission cross sections, or emission functions were also measured for Balmer lines of hydrogen, which results from the fragmentation of methane molecule. The measurements are presented in Figure 3. We were able to measure emission functions for all the lines from H α (n = 3 ) to H η (n = 9 ). Electron energy calibration was done by the peak energy position of (,) vibrational 3 3 line of the second positive system of nitrogen N ( C Π u B Π g ) at 337 nm [Zubek, 994], and by the onset energy of He line at 447.4 nm. Threshold energies were estimated from measured functions by fitting the functions with a convolution of linear and Gaussian function. The obtained values are displayed and compared with previous results in the Table. Probable fragmentation channels were proposed and threshold energies for these channels were calculated, using enthalpy parameters of fragments from NIST Chemistry Webbook database [NIST]. These calculations were used also to check, if the energy calibration used for the estimation is correct it must not be lower than the calculated values. Our thresholds are lower, or correspond [Motohashi et al., 996] to the thresholds published previously. They lie below the excitation channel e+ CH+H +H after we consider the deviations for the measured values, which are ±. ev. These deviations were estimated by measuring of electron energy distribution function by retarding potential method. There could be larger deviations for H γ, H ε, and H ζ emission functions due to their intensity sharing with CH bands. We subtracted the CH emission functions, but since it is not a perfectly exact method, some shift to the higher energies may have been introduced. The results indicate that we were able to observe fragmentation and excitation channel e+ CH 3 +H, which was previously considered as not active during excitation leading to Balmer lines emission. It can be argued, that [Motohashi et al., 996] observed this channel too, but since they used a simple electron gun without an electron monochromator (resulting in broad electron energy distribution), the difference from the higher threshold could have been attributed to the standard deviation of the electron energy. 9 6 3 spectrum Ee - = 4eV CH (A X Π) (,)+(,) 43,nm H β (4 ) 486,4nm H α (3 ) 66,9nm Intensity [a.u.] 7 7 CH(B Σ X Π) (,) 387,nm H η (9 ) 383,6nm H(inf ) (Rydberg states) 364,6nm H ζ (8 )+ CH(B Σ X Π) 389,nm H ε (7 ) 397,nm CH + (A X) 4,8nm H δ (6 ) 4,nm H γ ( ) 434,nm CH (A X Π) (,) 43,nm 36 37 38 39 4 4 4 43 44 4 46 47 48 6 66 Wavelength [nm] Figure. Emission fluorescence spectrum of methane induced by impact of electron with energy of 4 ev. Spectrum contains only radiation of fragments. Corrected for optical sensitivity of apparatus. 3
4 8 Hα (66.9 nm) 3 3 Hβ (486.4 nm) 6 4 6 7 8 9 3 4 3 3 4 4 6 6 7 7 8.4...8.6. 6 7 8 9 3 4 3 3 4 4 6 6 7 7 8 3 4. 3. 3..... 4 4 Hγ (434. nm) Hδ (4. nm) 3 3. 7 8 9 3 4-3 3 4 4 6 6 7 7 8 6 4 8 6 4 Hε (397. nm) 3 3 4 4 6 6 7 7 8 4. 3. 3..........9.8.7.6..4.3 6 7 8 9 3 4 4. 4. 3. 3........8.6.4...8.6.4. 8 9 3 3 3 4 4 6 6 7 7 8 Hζ (389. nm, smoothed). 8 9 3 4 -. 3 3 4 4 6 6 7 7 8.9.8.7.6..4.3.. Hη (383.6 nm) 4 3.6...4.4.3 8 9 3 4 6 3 3 4 4 6 6 7 7 8 Figure 3. Emission functions of Balmer lines of hydrogen fragments from methane molecule. The threshold energies were determined by fitting with convolution of linear and Gaussian function. Lines for H γ, H ε, and H ζ were mixed with CH emission bands radiation, so we subtracted their emission functions to get clean hydrogen functions..3 4
Table. Calculated and measured threshold energies for Balmer lines of hydrogen. Results compared with estimations of previous works and fragmentation channels proposed. Balmer s line Wavelength [nm] H α (3-) 66.9 H β (4-) 486.4 H γ (-) 434. H δ (6-) 4. H ε (7-) 397. H ζ (8-) 389. H η (9-) 383.6 Possible fragmentation e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H e+ CH 3 +H e+ CH+H +H e+ CH +H+H e+ CH+H+H Peak intensities of hydrogen Balmer lines H α to H η were plotted against the principal quantum number n of the upper electronic states for each measured fluorescence emission spectrum. Allometric k fit I = an, where I is intensity of a Balmer line, n is the principal quantum number, and a is a constant, was applied on the data. At the spectrum induced by ev electrons H β was omitted from fitting, since its deviation was relatively large in comparison with all the other spectra, and caused value of the k coefficient to be % higher than the value obtained after its exclusion. A correction for intensity was necessary in all the spectra for H ζ (8-) line, since it was strongly shared with CH ( B Σ X Π) system. This was achieved by subtraction of emission function for CH ( B Σ X Π) (,) line at 387. nm from emission function of H ζ at 389. nm. The value of the coefficient k indicates how the hydrogen excited electronic states leading to Balmer emission are populated. If all n, l, m states in hydrogen atom are populated equally (according to their statistical weights (l+)), the intensities of the lines are proportional to k = 3 I nl nl 3 ( l + ) h A α = ν n () nl n' l' Calculated threshold energies [ev] 6.6.3.4.8 7.3.96.7 6.48 7.6.7.38 6.79 7.79.44.4 6.96 7.89.4.64 7. 7.9.6.7 7. 8..6.76 7.7 where nl is the upper state and n l lower state, A is the Einstein coefficient of spontaneous emission [Motohashi et al., 996]. n' l' n' l' Measured threshold energies [ev].8.7.3 Previously determined first threshold values [ev].; 6.; 36 [Motohashi, 996].9 [Aarts, 97].; 6.3 [Motohashi, 996] ; 7.4; 36. [Donohue, 977].8 [Aarts, 97].7; 8 [Motohashi, 996].6; 8 [Donohue, 977].3 [Aarts, 97].94 ; 7 [Motohashi, 996].77.89.9
3,7 ev k = -3.7694 7 ev k = -4.69 Intensity [arb] 4 ev k = -3.94 ev k = -4.349 Intensity [arb] 8 ev k = -4.346 ev k = -4.74 Principal quantum number [n] Principal quantum number [n] Figure 4. Dependences of Balmer lines intensity on the principal quantum number of the upper state for fluorescence spectra induced by electrons of energy 3 ev. Allometric fits are displayed, and exponent values are shown in the legend at each graph. Values of k that were obtained from our measurements are lower than 3 and have tendency to drop to their minimum for impacting electron energy between and 7 ev. This means that excited states in hydrogen leading to Balmer emission produced during fragmentation of methane molecule by electron impact are not populated equally. Two processes can be responsible. First, the excited states (mostly lower H α, H β, ) are additionally produced from higher lying super-excited states [Linert et al., ]. Second, the higher excited states (H ζ, H η, ) decay by auto-ionization, thus decreasing intensity of corresponding Balmer lines gained by deexcitation. Significance of these processes grow from 3 ev, culminates between and 7 ev and decreases again up to ev of exciting electron energy. Conclusion A crossed beams experiment employing a trochoidal electron monochromator was used to measure electron induced fluorescence spectra of methane for electron energies 3 ev in wavelength range 36 66 nm, where Balmer lines of hydrogen are present. Relative cross sections of excitation reactions were measured as well and the thresholds of the excitation processes were determined both experimentally, and by calculation. The measured values of thresholds were found to be ev lower than previous measurements, which proved occurrence of e+ CH 3 +H process leading to Balmer emission. The intensities of Balmer lines were plotted as a dependence of the principal quantum number n of the upper states for each measured spectrum to estimate how these hydrogen states are populated. The population of the states was not according to their statistical weight (l+), but auto-ionization and population from super-excited states played some role, most significant in the excitation energy range of 7 ev. 6
Acknowledgments. This work has been supported by projects APVV-733-, VEGA /4/, COST CM8, and by Comenius University grants UK/39/ and UK/4/. References Aarts J.F.M., Beenakker C.I.M., and De Heer F.J.: Radiation from and C H 4 produced by electron impact, Physica 3, 3 44, 97. Beenakker C.I.M., and De Heer F.J.: Dissociative excitation of some aliphatic hydrocarbons by electron impact, Chemical Physics 7, 3 36, 97. Donohue D. E., Schiavone J. A., and Freund R. S.: Molecular dissociation by electron impact: Optical emission from fragments of methane, ethylene, and methanol, J. Chem. Phys. 67, 769 78, 977. EFDA: Plasma Wall Interaction, http://www.efda.org/collaborators/task-forces/plasma-wall-interaction/. Gil T.J., Lengsfield B.H., McCurdy C.W., and Rescigno T.N.: Ab initio complex Kohn calculations of dissociative excitation of methane: Close-coupling convergence studies, Physical Review A, 49, 6, 994. Hesser J.E., and Lutz B.L.: Probabilities for radiation and predissociation II. The excited states of CH, CD, and CH +, and some astrophysical implications, The Astrophysical Journal, 9, 73 78, 97. Linert I., Lachowicz I., Wasowicz T.J., and Zubek M.: Fragmentation of isoxazole molecules by electron impact in the energy range 8 ev, Chemical Physics Letters, 498, 7 3,. Lutz B.L., Owen T., and Cess R.D.: Laboratory band strengths of methane and their application to the atmospheres of Jupiter, Saturn, Uranus, Neptune, and Titan, The Astrophysical Journal, 3, 4, 976. Matuska J., Kubala D., and Matejcik S.: Numerical simulation of a trochoidal electron monochromator, Meas. Sci. Technol., 9, 9. Motohashi K., Soshi H., Ukai M., and Tsurubuchi S.: Dissociative excitation of by electron impact: Emission cross sections for the fragment species, Chemical Physics 3, 369 384, 996. NIST Chemistry webbook, NIST Standard Reference Database Number 69, http://webbook.nist.gov/chemistry/. Zubek M.: Excitation of the 3 state of N by electron impact in the near-threshold region, J. Phys. B: At. C Π Mol. Opt. Phys. 7, 73 8, 994. u 7