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A reinvestigation of the CBr+ chemiluminescence in a helium afterglow
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
20 September 1985
A R E I N V E S T I G A T I O N O F T H E CBr + C H E M I L U M I N E S C E N C E IN A H E L I U M A F T E R G L O W Masaharu TSUJI, Robert K U H N , John P. M A I E R Institut fftr Physikalische Chemie der Universiti~t Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland
Satoru N I S H I T A N I , Keiji S H I N O H A R A 1, Hiroshi OBASE and Yukio N I S H I M U R A Research Institute of Industrial Science and the Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shL Fukuoka 816, Japan Received 7 May 1985; in final form 21 June 1985
The emission spectrum produced from the flowing He afterglow reaction of CBr 4 has been re-examined. The a 31I 0 ~ ~ X 1~ + emission subsystem of CBr + is identified in addition to the a 3 I I l - ; X 1~1+ subsystem reported previously. The vibrational constants of the a3EIo + substate are estimated to be (in cm 1) T~ = 22449+_2, we - 906+2, o~exe = 7.8_+0.3, and the a3IIr state spin-orbit splitting constant of CBr + is 369+ 8 cm -1. A new ultraviolet emission system (2650 2900 ~.) is tentatively assigned to the A I I I ~ X i x + transition of CBr +.
1. Introduction
2. Experimental
Optical spectroscopy has been successfully used to study low-lying electronic states of group IV monohalide cations in a flowing afterglow [1-4]. This includes the CBr+(a 3111 - X 12;+) visible emission system which was observed and vibrationally analysed in a He afterglow reaction with CBr 4 [2]. By analogy with corresponding transition of the isoelectronic CSe and BBr molecules [5-7], a weaker a 3110+ - X 12;+ subsystem of CBr+ was expected to be observed about 100-350 cm-1 to the red of the dominant a 31I 1-X12; + system. However, it could not be identified under the spectral resolution used] In an attempt to locate the a 3II0+ - X 12;+ component, the CBr+ emission spectrum was reexamined under higher resolution. In the case of the isovalent CC1+ ion the A1H -+ X 1 Z+ UV emission system has been observed in addition to the a 3111 -+ X 12;+ system; the upper A 111 state arises from the same electronic configuration as the a 3IIr state [3]. In order to look for the corresponding A 111 -+ X 1N+ system of CBr+, the spectrum of the afterglow in the UV region was also examined.
The spectrum of CBr 4 in the UV region ( 2 0 0 350 nm) was measured by using the apparatus described in detail previously [2], whereas the CBr+(a-+ X) emission system in the visible region (420-480 nm) was re-examined with a low-pressure flowing-afterglow apparatus [8]. In the latter, the reaction cell, which consisted of a stainless-steel main flow tube (i.d. 102 mm) and a quartz discharge tube (i.d. 12 mm), was evacuated by a large capacity Roots pump (30000 £/min). Metastable He(2 3S) atoms and He + ions were produced by a 2.45 GHz microwave discharge of He gas at 0.4 Torr. A pair of grids was placed just at the exit of the discharge tube and by application of a positive potential the effect of ionic species on the observed emission was examined. The reagent CBr4, held at ~40° C, was bled through a 2 mm nozzle into the He flow. The white chemiluminescence was spectrally analysed with a Spex 1269 monochromator. The photons were detected by a cooled RCA C3 1034 photomultiplier operating in a photoncounting mode. The signals were recorded on-line with a LSI 11/23 microcomputer system which also controlled the grating step-motor of the monochromator. Emis-
1 Present address: SONY Corp., Atsugi 243, Japan.
0 009-2614/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
473
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
sion lines from concomitantly excited Br and He atoms, as well as from U atoms in a hollow cathode discharge lamp, were used for wavelength calibration.
3. Results and analysis The recording of the spectral region of the CBr+ a 3II 1 --> X 1 ~+ transition at a resolution of 0.6 A fwhm shows a new weak molecular emission system consisting of 10 bands. These are located to the red and to the blue of the known origin band of the a 3 ii 1 X 1 N+ system [2] as shown in fig. 1. The wavenumbers of the band maxima are given in table l. As in the case for the latter system the new weak bands are redshaded but the rotational contours of the individual bands differ somewhat. Only the bands below 22200 cm-1 show distinct heads. This new system disappears completely, as do the bands of the a 3 i l l "+ X t ~+ system, when a positive potential is applied to the ion collecting grids. This indicates that the emitting state is produced by a reaction of He + ions rather than He(2 3S) metastables with CBr 4 . Fig. 2a shows this spectral region recorded (using the apparatus described in ref. [2]) 5 cm, and fig. 2b 15 cm downstream from the CBr 4 gas inlet. It is apparent that the intensity ratio of'the bands at 22790 cm - I (i.e. a 3 I I 1 -+ X I ~ + (0,0)) and at 22423 c m - 1 remains constant in both spectra, within the limits of error. This means that the latter band belongs to a transition of the new system involving the lowest vibrational level of the excited state; vibrationally excited states would be relaxed
20 September 1985
Table 1 Wavenumbersof the band maxima of the bands attributed to the a 3r10+~ X 1~ +transition of CBr+. The accuracy is -+1cna-1 (v', v")
Vvac(Cm-1)
Uobs. - Vcalc.(cm-1)
(3,2) (4,3) (0,0) (2,2) (3,3) (4,4) (5,5) (4,5) (5,6) (6,7)
23162 23076 22423 22302 22236 22162 22077 21249 21181 21097
0 -2 0 -1 2 3 -1 -1 4 -2
by collisions with He atoms at this pressure of ~ 1 Torr [2]. Using the molecular constants of the X 1~+ state of CBr+, a vibrational numbering is readily found and a least-squares fit of the measured positions of the band maxima yields the vibrational constants of the emitting state. The values of the constants are
CBr + -XI";*
(1,0)(3,2) (2jr) (,:~(54)
(a)
(1,1)(3,3) (0,0)(2,2)(4,4.) HeI
I I
I (o.o) _ ~ ib-x,:E.
(b)
cO
d , - " (-,i e'f
aalr0.- Xr:~ * (0,0)
Brl J
I
440
L
i
450
i
1
460
q
I
,
470 n rn
Fig. L The a alIo+ 1 ~ X 1Z+ emission system of CBr+ obtained from the He++ Cl~r4 reaction at thermal energy, recorded with a resolution of 0.6 A fwhm. Lines marked with a dot are BrI, BrlI and HeI emission lines. 474
,
420
i
430
,
&40
i
i
1.50
,
nm
Fig. 2. The a 31"10+ 1 "+X 1~+ emission system of CBr+recorded (a) 5 cm and (b) 1'5 cm downstream from the CBr4 sample gas inlet. The spectrum was obtained with the apparatus described in ref. [2] with a resolution of 3 A fwhm.
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
20 September 1985
Table 3 Wavenumbers of the bandheads that are tentatively attributed to the A 1II ~ X 1~+ emission system of CBr+. All values ± 6 cm-1
Table 2 Spectroscopic constants of CBr÷ (cm -I ). See text for a discussion of the values. The uncertainty represents 2 standard deviations State
Te
toe
WeXe
Label
Vvac(Cm-1 )
aali1 a) a 3IIo+ X ~ +a)
22818_+6 22449 ± 2 0
903 ± 6 906 ± 2 956 ± 1
11 ± 1 7.8 ± 0.3 4.7 ± 0.4
1 2 3 4 5 6 7 8
37279 36841 36556 36304 35887 35634 35363 34705
a) From ref. [2]. gathered in table 2 together with the corresponding values for the a 3II 1 and X 1 ~+ states ofCBr + reported in ref. [2]. All the positions of t h e observed band maxima could be reproduced using these constants to within 4 cm - 1 or better (cf. table 1). The harmonic terms for b o t h excited states are practically identical and the s p i n - o r b i t splitting constant for the a 3 i i r state is found to be 369 + 8 cm - 1 . This value is comparable to that of CSe (327 cm - 1 [5]), but is larger than those of BBr (178 cm - 1 [6,7]), SiC1+ (115 cm - 1 [1]) and SiBr+ (194 cm - 1 [4]). These observations lead to the conclusion that the new observed emission bands belong to the a 3110+ -+ X 1 ]g+ transition of CBr + ion. The large difference in the first anharmonic term reflects the fact that this also contains contributions due to the mixing of the components o f the a 3II r states with neighbouring singlet states (cf. ref. [2], and references therein). As a result of this mixing the spin-forbidden t r i p l e t singlet transition becomes allowed [9]. The flowing He afterglow spectrum o f CBr 4 in the UV region exhibits several new red-degraded bands
1
2
I
250
260
partially superimposed u p o n a broad emission band due to electronically excited bromine molecules (fig. 3). When ions are prevented from entering the collision region by a potential on the grids, these new bands are completely suppressed, indicating that the excitation source is thermal He + ions. In table 3 are given the bandhead positions o f the eight dominant bands labelled in fig. 3. The separations between the bands numbered 2 and 5, 3 and 6, 4 and 7 are close to the vibrational frequency o f CBr + in the X 1 ]~+ state and the spectral features are similar to those o f the CC1+ A 11-I~ X 1~+ transition [3]. It seems therefore reasonable to assume that most o f these bands are associated with the A 1 II ~ X 1 ~+ transition of CBr +. This assignment is supported by a reasonable agreement o f the relative energies E( 3 IIr)/E(1 II) o f the 311r and l l I states between CBr + (0.62) and the isoelectronic CSe (0.69 [5] ) and BBr (0.55 [6,7]) molecules. A preliminary vibrational analysis, based on
270
4
5 6
280
8
290
nm
Fig. 3. A portion of the emission spectrum obtained from the He(2 aS), He+ + CBr4 reaction at thermal energy. The bands labelled 1-8 are tentatively assigned to the A llI ~ X 1~+ transition of CBr+. The spectrum was recorded with the apparatus described in ref. [2] with a resolution of 0.9 A fwhm. 475
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
the assumption that bands 2, 3, 4 and 5, 6, 7 are the Ap = 0 and - 1 sequences with u' = 0 - 2 , respectively, provides a rough vibrational frequency o f the A 111 state ( ~ 670 c m - 1 ) . A higher-resolution study, including a rotational analysis, is required to make an unambiguous assignment.
Acknowledgement This work is part o f Project No. 2.429-0.84 o f the Schweizerischer Nationalfonds zur F/Srderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz SA, and F. Hoffmann-La Roche & Cie SA, Basel are also thanked for financial support.
476
20 September 1985
References [1] M. Tsuji, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 59 (198t) 985. [2] M. Tsuji, K. Shinohara, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 61 (1983) 251. [3] M. Tsuji, T. Mizuguchi, K. Shinohara and Y. Nishimura, Can. J. Phys. 61 (1983) 838. [4] M. Tsuji, K. Shinohara, S. Nishitani, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 62 (1984) 353. [5] J. Lebreton, G. Bosser and L. Marsigny, J. Phys. B6 (1973) L226. [6] J. Lebreton, L. Marsigny and G. Bosser, Compt. Rend. Acad. Sci. (Paris)C271 (1970) 1113. [7] J. Lebreton, J. Chim. Phys. Phys.-Chim. Biol. (Paris) 70 (1973) 738. [8] M. Tsuji and J.P. Maier, Chem. Phys. 97 (1985) 397. [9] I. Kovacs, Rotational structure in the spectra of diatomic molecules (Van Nostrand, Princeton, 1969).
CHEMICAL PHYSICS LETTERS
20 September 1985
A R E I N V E S T I G A T I O N O F T H E CBr + C H E M I L U M I N E S C E N C E IN A H E L I U M A F T E R G L O W Masaharu TSUJI, Robert K U H N , John P. M A I E R Institut fftr Physikalische Chemie der Universiti~t Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland
Satoru N I S H I T A N I , Keiji S H I N O H A R A 1, Hiroshi OBASE and Yukio N I S H I M U R A Research Institute of Industrial Science and the Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shL Fukuoka 816, Japan Received 7 May 1985; in final form 21 June 1985
The emission spectrum produced from the flowing He afterglow reaction of CBr 4 has been re-examined. The a 31I 0 ~ ~ X 1~ + emission subsystem of CBr + is identified in addition to the a 3 I I l - ; X 1~1+ subsystem reported previously. The vibrational constants of the a3EIo + substate are estimated to be (in cm 1) T~ = 22449+_2, we - 906+2, o~exe = 7.8_+0.3, and the a3IIr state spin-orbit splitting constant of CBr + is 369+ 8 cm -1. A new ultraviolet emission system (2650 2900 ~.) is tentatively assigned to the A I I I ~ X i x + transition of CBr +.
1. Introduction
2. Experimental
Optical spectroscopy has been successfully used to study low-lying electronic states of group IV monohalide cations in a flowing afterglow [1-4]. This includes the CBr+(a 3111 - X 12;+) visible emission system which was observed and vibrationally analysed in a He afterglow reaction with CBr 4 [2]. By analogy with corresponding transition of the isoelectronic CSe and BBr molecules [5-7], a weaker a 3110+ - X 12;+ subsystem of CBr+ was expected to be observed about 100-350 cm-1 to the red of the dominant a 31I 1-X12; + system. However, it could not be identified under the spectral resolution used] In an attempt to locate the a 3II0+ - X 12;+ component, the CBr+ emission spectrum was reexamined under higher resolution. In the case of the isovalent CC1+ ion the A1H -+ X 1 Z+ UV emission system has been observed in addition to the a 3111 -+ X 12;+ system; the upper A 111 state arises from the same electronic configuration as the a 3IIr state [3]. In order to look for the corresponding A 111 -+ X 1N+ system of CBr+, the spectrum of the afterglow in the UV region was also examined.
The spectrum of CBr 4 in the UV region ( 2 0 0 350 nm) was measured by using the apparatus described in detail previously [2], whereas the CBr+(a-+ X) emission system in the visible region (420-480 nm) was re-examined with a low-pressure flowing-afterglow apparatus [8]. In the latter, the reaction cell, which consisted of a stainless-steel main flow tube (i.d. 102 mm) and a quartz discharge tube (i.d. 12 mm), was evacuated by a large capacity Roots pump (30000 £/min). Metastable He(2 3S) atoms and He + ions were produced by a 2.45 GHz microwave discharge of He gas at 0.4 Torr. A pair of grids was placed just at the exit of the discharge tube and by application of a positive potential the effect of ionic species on the observed emission was examined. The reagent CBr4, held at ~40° C, was bled through a 2 mm nozzle into the He flow. The white chemiluminescence was spectrally analysed with a Spex 1269 monochromator. The photons were detected by a cooled RCA C3 1034 photomultiplier operating in a photoncounting mode. The signals were recorded on-line with a LSI 11/23 microcomputer system which also controlled the grating step-motor of the monochromator. Emis-
1 Present address: SONY Corp., Atsugi 243, Japan.
0 009-2614/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
473
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
sion lines from concomitantly excited Br and He atoms, as well as from U atoms in a hollow cathode discharge lamp, were used for wavelength calibration.
3. Results and analysis The recording of the spectral region of the CBr+ a 3II 1 --> X 1 ~+ transition at a resolution of 0.6 A fwhm shows a new weak molecular emission system consisting of 10 bands. These are located to the red and to the blue of the known origin band of the a 3 ii 1 X 1 N+ system [2] as shown in fig. 1. The wavenumbers of the band maxima are given in table l. As in the case for the latter system the new weak bands are redshaded but the rotational contours of the individual bands differ somewhat. Only the bands below 22200 cm-1 show distinct heads. This new system disappears completely, as do the bands of the a 3 i l l "+ X t ~+ system, when a positive potential is applied to the ion collecting grids. This indicates that the emitting state is produced by a reaction of He + ions rather than He(2 3S) metastables with CBr 4 . Fig. 2a shows this spectral region recorded (using the apparatus described in ref. [2]) 5 cm, and fig. 2b 15 cm downstream from the CBr 4 gas inlet. It is apparent that the intensity ratio of'the bands at 22790 cm - I (i.e. a 3 I I 1 -+ X I ~ + (0,0)) and at 22423 c m - 1 remains constant in both spectra, within the limits of error. This means that the latter band belongs to a transition of the new system involving the lowest vibrational level of the excited state; vibrationally excited states would be relaxed
20 September 1985
Table 1 Wavenumbersof the band maxima of the bands attributed to the a 3r10+~ X 1~ +transition of CBr+. The accuracy is -+1cna-1 (v', v")
Vvac(Cm-1)
Uobs. - Vcalc.(cm-1)
(3,2) (4,3) (0,0) (2,2) (3,3) (4,4) (5,5) (4,5) (5,6) (6,7)
23162 23076 22423 22302 22236 22162 22077 21249 21181 21097
0 -2 0 -1 2 3 -1 -1 4 -2
by collisions with He atoms at this pressure of ~ 1 Torr [2]. Using the molecular constants of the X 1~+ state of CBr+, a vibrational numbering is readily found and a least-squares fit of the measured positions of the band maxima yields the vibrational constants of the emitting state. The values of the constants are
CBr + -XI";*
(1,0)(3,2) (2jr) (,:~(54)
(a)
(1,1)(3,3) (0,0)(2,2)(4,4.) HeI
I I
I (o.o) _ ~ ib-x,:E.
(b)
cO
d , - " (-,i e'f
aalr0.- Xr:~ * (0,0)
Brl J
I
440
L
i
450
i
1
460
q
I
,
470 n rn
Fig. L The a alIo+ 1 ~ X 1Z+ emission system of CBr+ obtained from the He++ Cl~r4 reaction at thermal energy, recorded with a resolution of 0.6 A fwhm. Lines marked with a dot are BrI, BrlI and HeI emission lines. 474
,
420
i
430
,
&40
i
i
1.50
,
nm
Fig. 2. The a 31"10+ 1 "+X 1~+ emission system of CBr+recorded (a) 5 cm and (b) 1'5 cm downstream from the CBr4 sample gas inlet. The spectrum was obtained with the apparatus described in ref. [2] with a resolution of 3 A fwhm.
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
20 September 1985
Table 3 Wavenumbers of the bandheads that are tentatively attributed to the A 1II ~ X 1~+ emission system of CBr+. All values ± 6 cm-1
Table 2 Spectroscopic constants of CBr÷ (cm -I ). See text for a discussion of the values. The uncertainty represents 2 standard deviations State
Te
toe
WeXe
Label
Vvac(Cm-1 )
aali1 a) a 3IIo+ X ~ +a)
22818_+6 22449 ± 2 0
903 ± 6 906 ± 2 956 ± 1
11 ± 1 7.8 ± 0.3 4.7 ± 0.4
1 2 3 4 5 6 7 8
37279 36841 36556 36304 35887 35634 35363 34705
a) From ref. [2]. gathered in table 2 together with the corresponding values for the a 3II 1 and X 1 ~+ states ofCBr + reported in ref. [2]. All the positions of t h e observed band maxima could be reproduced using these constants to within 4 cm - 1 or better (cf. table 1). The harmonic terms for b o t h excited states are practically identical and the s p i n - o r b i t splitting constant for the a 3 i i r state is found to be 369 + 8 cm - 1 . This value is comparable to that of CSe (327 cm - 1 [5]), but is larger than those of BBr (178 cm - 1 [6,7]), SiC1+ (115 cm - 1 [1]) and SiBr+ (194 cm - 1 [4]). These observations lead to the conclusion that the new observed emission bands belong to the a 3110+ -+ X 1 ]g+ transition of CBr + ion. The large difference in the first anharmonic term reflects the fact that this also contains contributions due to the mixing of the components o f the a 3II r states with neighbouring singlet states (cf. ref. [2], and references therein). As a result of this mixing the spin-forbidden t r i p l e t singlet transition becomes allowed [9]. The flowing He afterglow spectrum o f CBr 4 in the UV region exhibits several new red-degraded bands
1
2
I
250
260
partially superimposed u p o n a broad emission band due to electronically excited bromine molecules (fig. 3). When ions are prevented from entering the collision region by a potential on the grids, these new bands are completely suppressed, indicating that the excitation source is thermal He + ions. In table 3 are given the bandhead positions o f the eight dominant bands labelled in fig. 3. The separations between the bands numbered 2 and 5, 3 and 6, 4 and 7 are close to the vibrational frequency o f CBr + in the X 1 ]~+ state and the spectral features are similar to those o f the CC1+ A 11-I~ X 1~+ transition [3]. It seems therefore reasonable to assume that most o f these bands are associated with the A 1 II ~ X 1 ~+ transition of CBr +. This assignment is supported by a reasonable agreement o f the relative energies E( 3 IIr)/E(1 II) o f the 311r and l l I states between CBr + (0.62) and the isoelectronic CSe (0.69 [5] ) and BBr (0.55 [6,7]) molecules. A preliminary vibrational analysis, based on
270
4
5 6
280
8
290
nm
Fig. 3. A portion of the emission spectrum obtained from the He(2 aS), He+ + CBr4 reaction at thermal energy. The bands labelled 1-8 are tentatively assigned to the A llI ~ X 1~+ transition of CBr+. The spectrum was recorded with the apparatus described in ref. [2] with a resolution of 0.9 A fwhm. 475
Volume 119, number 6
CHEMICAL PHYSICS LETTERS
the assumption that bands 2, 3, 4 and 5, 6, 7 are the Ap = 0 and - 1 sequences with u' = 0 - 2 , respectively, provides a rough vibrational frequency o f the A 111 state ( ~ 670 c m - 1 ) . A higher-resolution study, including a rotational analysis, is required to make an unambiguous assignment.
Acknowledgement This work is part o f Project No. 2.429-0.84 o f the Schweizerischer Nationalfonds zur F/Srderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz SA, and F. Hoffmann-La Roche & Cie SA, Basel are also thanked for financial support.
476
20 September 1985
References [1] M. Tsuji, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 59 (198t) 985. [2] M. Tsuji, K. Shinohara, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 61 (1983) 251. [3] M. Tsuji, T. Mizuguchi, K. Shinohara and Y. Nishimura, Can. J. Phys. 61 (1983) 838. [4] M. Tsuji, K. Shinohara, S. Nishitani, T. Mizuguchi and Y. Nishimura, Can. J. Phys. 62 (1984) 353. [5] J. Lebreton, G. Bosser and L. Marsigny, J. Phys. B6 (1973) L226. [6] J. Lebreton, L. Marsigny and G. Bosser, Compt. Rend. Acad. Sci. (Paris)C271 (1970) 1113. [7] J. Lebreton, J. Chim. Phys. Phys.-Chim. Biol. (Paris) 70 (1973) 738. [8] M. Tsuji and J.P. Maier, Chem. Phys. 97 (1985) 397. [9] I. Kovacs, Rotational structure in the spectra of diatomic molecules (Van Nostrand, Princeton, 1969).