- Email: [email protected]
Time-resolved Fourier transform infrared emission spectroscopy of He2 produced by a pulsed discharge
Chemical Physics Letters 383 (2004) 256–260 www.elsevier.com/locate/cplett
Time-resolved Fourier transform infrared emission spectroscopy of He2 produced by a pulsed discharge Yukio Hosaki 1, Svatopluk Civis 2, Kentarou Kawaguchi
*
Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan Received 12 September 2003 Published online:
Abstract Time-resolved Fourier transform spectroscopy was applied to the study of He2 infrared emission produced by a pulsed discharge in pure He. In the 1800–8000 cm1 region, new electronic bands have been observed in addition to the previously reported transitions. From the observed time profiles of emission spectra, Rydberg states with higher energy than the b3 P state produced 3 þ efficiently in afterglow plasma have been identified. Rotational assignments and a least-squares analysis for the h3 Rþ u –g Rg and 3 þ 1 g3 Rþ –d R bands in the 3200 cm region were carried out to determine the molecular constants. A new band from an unidentified g u 1 state to the d3 Rþ region with irregular P- and R-branch intensities. u state has been observed in the 6300 cm Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The He2 molecule is known as the first Rydberg molecule, since its spectrum was reported in 1913. Many spectroscopic studies have been carried out as compiled in a book of Huber and Herzberg [1], and in a web site of Bernath [2]. Most of the spectra of He2 molecule have been observed in the visible and ultraviolet regions. Ginter et al. [3,4] compiled and analyzed the energy levels of Rydberg states originating from the electronic config2 urations ð1rg Þ2 ð1ru Þnpkð3 Pg ; 3 Rþ g Þ and ð1rg Þ ð1ru Þnsr; þ þ ndkð3 Ru ; 3 Ru ; 3 Pu ; 3 Du Þ by an eigenquantum defect theory, where n is the principal quantum number in the united atom molecular orbital designation. According to the energy levels listed in [3–5], many electronic transitions are expected in the infrared region. However, observations of the infrared spectra so far have been limited to the three band systems below 8000 cm1 : (1) b3 Pg – 1 a3 Rþ u with the 0–0 band origin of 4750 cm , studied by *
Corresponding author. Fax: +81-86-251-7853. E-mail address: [email protected] (K. Kawaguchi). 1 Present address: Taiho Pharmaceutical Co. Ltd., Nishiki-cho 1-27, Kanda, Tokyo 101-8444, Japan. 2 Permanent address: J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague, Czech Republic. 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.025
Hepner and Herman [6], Gloersen and Dieke [7], and Rogers et al. [8], (2) B1 Pg –A1 Rþ u with the 0–0 band origin at 3501 cm1 , studied by Solka et al. [9], and (3) the 4f–3d band in 5100–5800 cm1 spectral region, studied by Herzberg and Jungen [10]. The assignment of 4f–3d band was the first example concerning electronic states originating from the f orbital electron. Recently, we developed a time-resolved Fourier transform spectroscopic system (FTS) using a highresolution Bruker 120 HR and a microcontroller SX [11]. In the present Letter, we report an application of this system to the observations of He2 emission spectra produced by a pulsed discharge. This method has enabled us to observe many electronic transitions in the infrared region, including the previously reported bands. We report a spectroscopic analysis of newly observed three bands and their time profiles demonstrated on some selected transitions.
2. Experimental The spectra of He2 were observed in emission from a hollow cathode discharge plasma, with the time resolved Fourier transform high resolution interferometer [11]. The hollow cathode stainless steel tube was 20 cm long
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
with an inner diameter equal to 12 mm. The ac discharge was maintained by a high voltage transistor switch HTS 81 (Behlke electronic GmbH, Frankfurt, Germany) applied between the stainless steel anode and the grounded cathode. The emission of He2 has also been observed from a positive column, where lines from vibrationally excited states of b3 Pg were found to be more intense, compared with the case of the hollow cathode discharge. The present paper reports only the spectra obtained from the hollow cathode discharge, because of its higher efficiency in the production of the highly excited electronic states of He2 . The plasma made from a pure helium was cooled down by flowing water in outer jacket of the cell. The best conditions for the generation of He2 were found to be P(He) ¼ 1.33 kPa (10 Torr). The voltage drop across the discharge was 600 V, with pulse width of 20 ls and 0.5 A peak-to-peak current. The scanner velocity of FTS was set to produce a 10 kHz He–Ne laser fringe frequency which was used to trigger the discharge pulse. The recorded spectral range was 1800–8000 cm1 without an optical filter, at an unapodized resolution of 0.03 cm1 . The 32 scans were coadded so as to obtain a reasonable signal-to-noise ratio. The observed wavenumbers were calibrated using the b3 Pg –a3 Rþ u v ¼ 0–0 band observed by Rogers et al. [8]. The accuracy of the wavenumber measurement is estimated to be about 0.003 cm1 .
3. Observed spectra and analysis Fig. 1 shows a part of the observed time-resolved emission spectrum from a discharge in He. The dis-
257
charge was initiated at time zero and turned off at 20 ls. For AD-converter triggers, we used 3 ls for the zero offset and interval values, that is, AD conversion occurs every 3 ls from the start of the discharge and all together 30 pulses cover 87 ls. The strong line (5880 cm1 ) in Fig. 1 belongs to the He atomic line (4d–3p) and is observed as two intense peaks. It may be noted that the second peak appears after the discharge is off, that is, it is due to the afterglow plasma. The other spectral lines in Fig. 1 pertain to the 4f– 3d transitions of He2 which have been analyzed by Herzberg and Jungen [10]. It is noted that the intensity increases in the afterglow. The non-zero emission at around t ¼ 0 is produced by the He2 species that have survived after the discharge is turned off. This does not necessarily mean that the lifetime of the Rydberg states originating from the 4f electron is long, because the Rydberg states are thought to be produced by successive reactions of metastable He atoms, as discussed in a later section. Fig. 2 shows an observed spectrum in the 2750– 7200 cm1 region, where we averaged all 30 spectra obtained by the time-resolved method. As shown later, each line has a different time profile. We also found a Doppler shift of some spectral lines during the measuring period. In the averaging process there is a loss of information concerning time dependence and the Doppler shifts but the spectrum has a better signalto-noise ratio than a single spectrum, and it is more suitable for spectroscopic analysis. In the figure, the b3 Pg –a3 Rþ u v ¼ 0–0 band is strongly observed in the 4800 cm1 region. Most of spectral lines in the 5200– 5900 cm1 region could be attributed to the 4f–3d band [10], but there are some unidentified lines also observed
Fig. 1. A portion of the time-resolved spectrum observed by a pulsed discharge in He with a pressure of 1.33 kPa (10 Torr). The discharge was applied in the interval of 0–20 ls with a peak current of 0.5 A. The strongest peak belongs to the atomic He line (4d–3p). Other lines pertain to 4f–3d transitions of He2 .
258
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
Fig. 2. An observed spectrum of He2 in the 2750–7200 cm1 region, where 30 time-resolved spectra have been averaged. The discharge conditions are the same as given in the caption of Fig. 1.
in this range. In the 3300 cm1 region, the B1 Pg – A1 R þ u v ¼ 0–0 band was found to be weak. From the time profile, it appears that the population in the singlet B1 Pg state decreased during the discharge period and increased in the afterglow, similar to what was observed for high-energy triplet states. In addition to these already reported bands, the following new bands were observed. In the 3200 cm1 region, two series of lines have been observed with no Q-branch transitions. Rotational assignments are listed in Table 1 with the observed wavenumbers. The analysis using the standard energy level expressions gave the rotational and centrifugal distortion constants, and the band origin (term energy) as listed in Table 2. Spin splitTable 1 Observed transitions of He2 (cm1 )a N
P(N)
o.)c.
3 þ h3 Rþ u –g Rg 0 2 4 6 8 10 12 14
3177.5569 3134.9969 3107.2556 3080.1473 3053.7916 3028.3000 3003.7698 2980.2783
0.0006 )0.0005 )0.0001 )0.0010 0.0000 0.0012 )0.0002 )0.0002
3 þ g3 Rþ g –d Ru 1 3 5 7 9 11 13
3190.4064 3160.7770 3130.2548 3098.9306 3066.8917 3034.2243 3001.0169
)0.0019 )0.0003 0.0017 0.0021 )0.0002 )0.0031 0.0015
R(N)
o.)c.
3206.4133 3235.5447 3264.8626 3294.3028 3323.7108 3353.0067
0.0018 0.0000 )0.0064 0.0089 )0.0054 0.0013
3232.9664 3259.9345 3285.6522 3310.0069 3332.8839 3354.1697
)0.0013 0.0005 0.0016 0.0006 )0.0017 0.0005
a N denotes the rotational quantum number neglecting spin in lower electronic states.
ting was not observed in these bands. The magnitude of the rotational constants was useful for identification of the electronic state. The band origin frequencies 3204.9 cm1 and 3163.3 cm1 of the two bands were consistent 3 þ 3 þ with those of the g3 Rþ g –d Ru v ¼ 0–0 and h Ru – 3 þ g Rg v ¼ 0–0 transitions, respectively. Both the bands were identified for the first time in the infrared region, although the electronic states involved had been observed in other electronic transitions in the visible region. Molecular constants determined in the previous studies [1,12,13] are also listed in Table 2 for comparison. In the 3600 cm1 region, strong lines have been detected with an irregular spectral pattern. According to the energy levels compiled by Ginter et al. [3,4], we could identify some spectral lines as arising due to Table 2 3 þ 3 þ a Molecular constants of He2 in the h3 Rþ u , g Rg , and d Ru states Present
Previous
ðh3 Rþ uÞ B D 103 E
7.148 49(24) 0.505 3(24) 6368.1202(30)
7.149 0.524
ðg3 Rþ gÞ B D 103 E
7.096 423(94) 0.530 70(44) 3204.8589(11)
7.096 8(1) 0.538(7)
ðd3 Rþ uÞ B D 103 E
7.226 329(88) 0.519 91(37) 0.0
7.228 6(15) 0.532(3) 0.0
a
b
c
d
cm1 unit. Numbers in parentheses denote one standard deviation and apply to the last significant digits. b Ref. [1]. c Ref. [12]. d Ref. [13].
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
Fig. 3. Observed time profiles of emission intensities of He2 . Abscissa values show normalized intensities of the emission lines. The discharge conditions are as given in the caption of Fig. 1.
transitions from the 3 R state of 4d to the g3 Rþ g state, but many unidentified lines remain, presumably due to a strong l-uncoupling effect among the states. In the 6300 cm1 region, a new band was observed with a relatively simple P- and R-branch pattern, but the relative intensities of the branches are peculiar, as shown in Fig. 2. Combination differences suggest that the lower state is d3 Rþ g . The rotational constant for the upper state is determined to be 6.8529(4) cm1 , which is smaller than the usual value expected for the v ¼ 0, except for the c3 R þ g state, by the vibration rotation constant a (about 0.22 cm1 ). This indicates that the upper state may be assigned to a vibrationally excited state. However, no other transitions from vibrationally excited states of high-energy Rydberg states have been detected in the present hollow cathode discharge. In the 2600 cm1 region, many unidentified spectral lines are observed with ÔirregularÕ spectral pattern. These lines may be due to transitions between high-energy Rydberg states. The analysis will be reported with those of other infrared bands in future. Time profiles of observed spectral lines are depicted in Fig. 3 for several bands. Except for the transitions from the b3 P state, the high-energy Rydberg states are produced strongly in the afterglow plasma. This means these states are more fragile during the discharge period and may not be observed strongly in a normal DC discharge.
259
analysis may provide information about high-energy Rydberg states including states originating from f-orbital electrons. Fig. 4 shows the energy level diagram of He2 , where the energy values are relative to the a3 Rþ u v ¼ 0 state, which is located 144 952 cm1 (18 eV) above the repulsive ground X1 Rþ g state. The observed transitions are shown by arrows in the figure except for the 6300 cm1 band. In the present study, intense emissions from the h3 Rþ u , 4f and 4d states have been observed in the afterglow plasma. Since the spectra from these states are very weak during the discharge, the excitation may not be due to electron bombardment in the discharge. The metastable He atom in the 2s3 S and 2s1 S states with energies of 19.77 and 20.55 eV, respectively, are known to have long radiative lifetimes of 6 105 and 0.02 s. Since the energies of h3 Rþ u and 4f are higher by 21.3 eV (17 200 cm1 ) than the ground state of He2 , these electronic states cannot be produced by a reaction with a single metastable He stom. Therefore, the following reactions are thought to be responsible for the production of high-energy Rydberg states [14], Hem þ Hem ! Heþ þ He þ e
ð1Þ
Heþ þ He þ M ! Heþ 2 þM
ð2Þ
Heþ 2 þ e ! He2 þ hm;
ð3Þ
m
where He denotes a metastable He atom and M is a third body (He atom). Reaction (2) has a large contribution at a pressure higher than 1 Torr. The electron produced in reaction (1) has a relatively small energy compared to electrons produced in the electric discharge. The electron may be used for reaction (3) to produce excited electronic states of He2 .
4. Discussion The pulsed discharge and multi-sampling system produce an interesting spectral feature of He2 in the infrared region. Especially, when the data sampling is carried out after turning off the discharge, intense emissions from many electronic bands are observed. The
Fig. 4. Energy level diagram of He2 . The transitions observed in the present study are shown by arrows. The energy values are represented relative to the a3 Rþ u v ¼ 0 state. n(>1) is the principal quantum number in a united atom molecular orbital designation. The ionization limit to 1 Heþ 2 is 34 316 cm .
260
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
Acknowledgements The present study is partially supported by a Grantin-Aid by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 13440181) and by the Grant Agency of the Czech Republic (Grant 203/01/0618). The authors are grateful to Dr. Romola DÕCunha for a careful and critical reading of the manuscript. References [1] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [2] P.F. Bernath, S. McLeod, J. Mol. Spectrosc. 207 (2001) 287.
[3] D.S. Ginter, M.L. Ginter, C.M. Brown, J. Chem. Phys. 81 (1984) 6013. [4] D.S. Ginter, M.L. Ginter, J. Chem. Phys. 88 (1988) 3761. [5] D.S. Ginter, M.L. Ginter, J. Mol. Spectrosc. 138 (1989) 29. [6] G. Hepner, L. Herman, C. r. hebd. Seanc. Acad. Sci. Paris 243 (1956) 1504. [7] P. Gloersen, G.H. Dieke, J. Mol. Spectrosc. 16 (1965) 191. [8] S.A. Rogers, C.R. Brazier, P.F. Bernath, J.W. Brault, Mol. Phys. 63 (1988) 901. [9] H. Solka, W. Zimmermann, A. Stahn, D. Reinert, W. Urban, Mol. Phys. 60 (1987) 1179. [10] G. Herzberg, Ch. Jungen, J. Chem. Phys. 84 (1984) 1181. [11] K. Kawaguchi, O. Baskakov, Y. Hosaki, Y. Hama, C. Kugimiya, Chem. Phys. Lett. 369 (2003) 293. [12] F.B. Orth, M.L. Ginter, J. Mol. Spectrosc. 61 (1976) 282. [13] M.L. Ginter, J. Mol. Spectrosc. 18 (1965) 321. [14] O. Motret, J.M. Pouvesle, J. Stevefelt, J. Chem. Phys. 83 (1985) 1095.
Time-resolved Fourier transform infrared emission spectroscopy of He2 produced by a pulsed discharge Yukio Hosaki 1, Svatopluk Civis 2, Kentarou Kawaguchi
*
Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan Received 12 September 2003 Published online:
Abstract Time-resolved Fourier transform spectroscopy was applied to the study of He2 infrared emission produced by a pulsed discharge in pure He. In the 1800–8000 cm1 region, new electronic bands have been observed in addition to the previously reported transitions. From the observed time profiles of emission spectra, Rydberg states with higher energy than the b3 P state produced 3 þ efficiently in afterglow plasma have been identified. Rotational assignments and a least-squares analysis for the h3 Rþ u –g Rg and 3 þ 1 g3 Rþ –d R bands in the 3200 cm region were carried out to determine the molecular constants. A new band from an unidentified g u 1 state to the d3 Rþ region with irregular P- and R-branch intensities. u state has been observed in the 6300 cm Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The He2 molecule is known as the first Rydberg molecule, since its spectrum was reported in 1913. Many spectroscopic studies have been carried out as compiled in a book of Huber and Herzberg [1], and in a web site of Bernath [2]. Most of the spectra of He2 molecule have been observed in the visible and ultraviolet regions. Ginter et al. [3,4] compiled and analyzed the energy levels of Rydberg states originating from the electronic config2 urations ð1rg Þ2 ð1ru Þnpkð3 Pg ; 3 Rþ g Þ and ð1rg Þ ð1ru Þnsr; þ þ ndkð3 Ru ; 3 Ru ; 3 Pu ; 3 Du Þ by an eigenquantum defect theory, where n is the principal quantum number in the united atom molecular orbital designation. According to the energy levels listed in [3–5], many electronic transitions are expected in the infrared region. However, observations of the infrared spectra so far have been limited to the three band systems below 8000 cm1 : (1) b3 Pg – 1 a3 Rþ u with the 0–0 band origin of 4750 cm , studied by *
Corresponding author. Fax: +81-86-251-7853. E-mail address: [email protected] (K. Kawaguchi). 1 Present address: Taiho Pharmaceutical Co. Ltd., Nishiki-cho 1-27, Kanda, Tokyo 101-8444, Japan. 2 Permanent address: J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague, Czech Republic. 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.025
Hepner and Herman [6], Gloersen and Dieke [7], and Rogers et al. [8], (2) B1 Pg –A1 Rþ u with the 0–0 band origin at 3501 cm1 , studied by Solka et al. [9], and (3) the 4f–3d band in 5100–5800 cm1 spectral region, studied by Herzberg and Jungen [10]. The assignment of 4f–3d band was the first example concerning electronic states originating from the f orbital electron. Recently, we developed a time-resolved Fourier transform spectroscopic system (FTS) using a highresolution Bruker 120 HR and a microcontroller SX [11]. In the present Letter, we report an application of this system to the observations of He2 emission spectra produced by a pulsed discharge. This method has enabled us to observe many electronic transitions in the infrared region, including the previously reported bands. We report a spectroscopic analysis of newly observed three bands and their time profiles demonstrated on some selected transitions.
2. Experimental The spectra of He2 were observed in emission from a hollow cathode discharge plasma, with the time resolved Fourier transform high resolution interferometer [11]. The hollow cathode stainless steel tube was 20 cm long
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
with an inner diameter equal to 12 mm. The ac discharge was maintained by a high voltage transistor switch HTS 81 (Behlke electronic GmbH, Frankfurt, Germany) applied between the stainless steel anode and the grounded cathode. The emission of He2 has also been observed from a positive column, where lines from vibrationally excited states of b3 Pg were found to be more intense, compared with the case of the hollow cathode discharge. The present paper reports only the spectra obtained from the hollow cathode discharge, because of its higher efficiency in the production of the highly excited electronic states of He2 . The plasma made from a pure helium was cooled down by flowing water in outer jacket of the cell. The best conditions for the generation of He2 were found to be P(He) ¼ 1.33 kPa (10 Torr). The voltage drop across the discharge was 600 V, with pulse width of 20 ls and 0.5 A peak-to-peak current. The scanner velocity of FTS was set to produce a 10 kHz He–Ne laser fringe frequency which was used to trigger the discharge pulse. The recorded spectral range was 1800–8000 cm1 without an optical filter, at an unapodized resolution of 0.03 cm1 . The 32 scans were coadded so as to obtain a reasonable signal-to-noise ratio. The observed wavenumbers were calibrated using the b3 Pg –a3 Rþ u v ¼ 0–0 band observed by Rogers et al. [8]. The accuracy of the wavenumber measurement is estimated to be about 0.003 cm1 .
3. Observed spectra and analysis Fig. 1 shows a part of the observed time-resolved emission spectrum from a discharge in He. The dis-
257
charge was initiated at time zero and turned off at 20 ls. For AD-converter triggers, we used 3 ls for the zero offset and interval values, that is, AD conversion occurs every 3 ls from the start of the discharge and all together 30 pulses cover 87 ls. The strong line (5880 cm1 ) in Fig. 1 belongs to the He atomic line (4d–3p) and is observed as two intense peaks. It may be noted that the second peak appears after the discharge is off, that is, it is due to the afterglow plasma. The other spectral lines in Fig. 1 pertain to the 4f– 3d transitions of He2 which have been analyzed by Herzberg and Jungen [10]. It is noted that the intensity increases in the afterglow. The non-zero emission at around t ¼ 0 is produced by the He2 species that have survived after the discharge is turned off. This does not necessarily mean that the lifetime of the Rydberg states originating from the 4f electron is long, because the Rydberg states are thought to be produced by successive reactions of metastable He atoms, as discussed in a later section. Fig. 2 shows an observed spectrum in the 2750– 7200 cm1 region, where we averaged all 30 spectra obtained by the time-resolved method. As shown later, each line has a different time profile. We also found a Doppler shift of some spectral lines during the measuring period. In the averaging process there is a loss of information concerning time dependence and the Doppler shifts but the spectrum has a better signalto-noise ratio than a single spectrum, and it is more suitable for spectroscopic analysis. In the figure, the b3 Pg –a3 Rþ u v ¼ 0–0 band is strongly observed in the 4800 cm1 region. Most of spectral lines in the 5200– 5900 cm1 region could be attributed to the 4f–3d band [10], but there are some unidentified lines also observed
Fig. 1. A portion of the time-resolved spectrum observed by a pulsed discharge in He with a pressure of 1.33 kPa (10 Torr). The discharge was applied in the interval of 0–20 ls with a peak current of 0.5 A. The strongest peak belongs to the atomic He line (4d–3p). Other lines pertain to 4f–3d transitions of He2 .
258
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
Fig. 2. An observed spectrum of He2 in the 2750–7200 cm1 region, where 30 time-resolved spectra have been averaged. The discharge conditions are the same as given in the caption of Fig. 1.
in this range. In the 3300 cm1 region, the B1 Pg – A1 R þ u v ¼ 0–0 band was found to be weak. From the time profile, it appears that the population in the singlet B1 Pg state decreased during the discharge period and increased in the afterglow, similar to what was observed for high-energy triplet states. In addition to these already reported bands, the following new bands were observed. In the 3200 cm1 region, two series of lines have been observed with no Q-branch transitions. Rotational assignments are listed in Table 1 with the observed wavenumbers. The analysis using the standard energy level expressions gave the rotational and centrifugal distortion constants, and the band origin (term energy) as listed in Table 2. Spin splitTable 1 Observed transitions of He2 (cm1 )a N
P(N)
o.)c.
3 þ h3 Rþ u –g Rg 0 2 4 6 8 10 12 14
3177.5569 3134.9969 3107.2556 3080.1473 3053.7916 3028.3000 3003.7698 2980.2783
0.0006 )0.0005 )0.0001 )0.0010 0.0000 0.0012 )0.0002 )0.0002
3 þ g3 Rþ g –d Ru 1 3 5 7 9 11 13
3190.4064 3160.7770 3130.2548 3098.9306 3066.8917 3034.2243 3001.0169
)0.0019 )0.0003 0.0017 0.0021 )0.0002 )0.0031 0.0015
R(N)
o.)c.
3206.4133 3235.5447 3264.8626 3294.3028 3323.7108 3353.0067
0.0018 0.0000 )0.0064 0.0089 )0.0054 0.0013
3232.9664 3259.9345 3285.6522 3310.0069 3332.8839 3354.1697
)0.0013 0.0005 0.0016 0.0006 )0.0017 0.0005
a N denotes the rotational quantum number neglecting spin in lower electronic states.
ting was not observed in these bands. The magnitude of the rotational constants was useful for identification of the electronic state. The band origin frequencies 3204.9 cm1 and 3163.3 cm1 of the two bands were consistent 3 þ 3 þ with those of the g3 Rþ g –d Ru v ¼ 0–0 and h Ru – 3 þ g Rg v ¼ 0–0 transitions, respectively. Both the bands were identified for the first time in the infrared region, although the electronic states involved had been observed in other electronic transitions in the visible region. Molecular constants determined in the previous studies [1,12,13] are also listed in Table 2 for comparison. In the 3600 cm1 region, strong lines have been detected with an irregular spectral pattern. According to the energy levels compiled by Ginter et al. [3,4], we could identify some spectral lines as arising due to Table 2 3 þ 3 þ a Molecular constants of He2 in the h3 Rþ u , g Rg , and d Ru states Present
Previous
ðh3 Rþ uÞ B D 103 E
7.148 49(24) 0.505 3(24) 6368.1202(30)
7.149 0.524
ðg3 Rþ gÞ B D 103 E
7.096 423(94) 0.530 70(44) 3204.8589(11)
7.096 8(1) 0.538(7)
ðd3 Rþ uÞ B D 103 E
7.226 329(88) 0.519 91(37) 0.0
7.228 6(15) 0.532(3) 0.0
a
b
c
d
cm1 unit. Numbers in parentheses denote one standard deviation and apply to the last significant digits. b Ref. [1]. c Ref. [12]. d Ref. [13].
Y. Hosaki et al. / Chemical Physics Letters 383 (2004) 256–260
Fig. 3. Observed time profiles of emission intensities of He2 . Abscissa values show normalized intensities of the emission lines. The discharge conditions are as given in the caption of Fig. 1.
transitions from the 3 R state of 4d to the g3 Rþ g state, but many unidentified lines remain, presumably due to a strong l-uncoupling effect among the states. In the 6300 cm1 region, a new band was observed with a relatively simple P- and R-branch pattern, but the relative intensities of the branches are peculiar, as shown in Fig. 2. Combination differences suggest that the lower state is d3 Rþ g . The rotational constant for the upper state is determined to be 6.8529(4) cm1 , which is smaller than the usual value expected for the v ¼ 0, except for the c3 R þ g state, by the vibration rotation constant a (about 0.22 cm1 ). This indicates that the upper state may be assigned to a vibrationally excited state. However, no other transitions from vibrationally excited states of high-energy Rydberg states have been detected in the present hollow cathode discharge. In the 2600 cm1 region, many unidentified spectral lines are observed with ÔirregularÕ spectral pattern. These lines may be due to transitions between high-energy Rydberg states. The analysis will be reported with those of other infrared bands in future. Time profiles of observed spectral lines are depicted in Fig. 3 for several bands. Except for the transitions from the b3 P state, the high-energy Rydberg states are produced strongly in the afterglow plasma. This means these states are more fragile during the discharge period and may not be observed strongly in a normal DC discharge.
259
analysis may provide information about high-energy Rydberg states including states originating from f-orbital electrons. Fig. 4 shows the energy level diagram of He2 , where the energy values are relative to the a3 Rþ u v ¼ 0 state, which is located 144 952 cm1 (18 eV) above the repulsive ground X1 Rþ g state. The observed transitions are shown by arrows in the figure except for the 6300 cm1 band. In the present study, intense emissions from the h3 Rþ u , 4f and 4d states have been observed in the afterglow plasma. Since the spectra from these states are very weak during the discharge, the excitation may not be due to electron bombardment in the discharge. The metastable He atom in the 2s3 S and 2s1 S states with energies of 19.77 and 20.55 eV, respectively, are known to have long radiative lifetimes of 6 105 and 0.02 s. Since the energies of h3 Rþ u and 4f are higher by 21.3 eV (17 200 cm1 ) than the ground state of He2 , these electronic states cannot be produced by a reaction with a single metastable He stom. Therefore, the following reactions are thought to be responsible for the production of high-energy Rydberg states [14], Hem þ Hem ! Heþ þ He þ e
ð1Þ
Heþ þ He þ M ! Heþ 2 þM
ð2Þ
Heþ 2 þ e ! He2 þ hm;
ð3Þ
m
where He denotes a metastable He atom and M is a third body (He atom). Reaction (2) has a large contribution at a pressure higher than 1 Torr. The electron produced in reaction (1) has a relatively small energy compared to electrons produced in the electric discharge. The electron may be used for reaction (3) to produce excited electronic states of He2 .
4. Discussion The pulsed discharge and multi-sampling system produce an interesting spectral feature of He2 in the infrared region. Especially, when the data sampling is carried out after turning off the discharge, intense emissions from many electronic bands are observed. The
Fig. 4. Energy level diagram of He2 . The transitions observed in the present study are shown by arrows. The energy values are represented relative to the a3 Rþ u v ¼ 0 state. n(>1) is the principal quantum number in a united atom molecular orbital designation. The ionization limit to 1 Heþ 2 is 34 316 cm .
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Acknowledgements The present study is partially supported by a Grantin-Aid by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 13440181) and by the Grant Agency of the Czech Republic (Grant 203/01/0618). The authors are grateful to Dr. Romola DÕCunha for a careful and critical reading of the manuscript. References [1] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [2] P.F. Bernath, S. McLeod, J. Mol. Spectrosc. 207 (2001) 287.
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