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The Rotational Structure of the (12,6) Band of theA2Πu–X2Σ+gSystem of N+2Studied by Velocity Modulation Laser Spectroscopy
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
183, 200–203 (1997)
MS967238
NOTE / The Rotational Structure of the (12,6) Band of the A 2Pu – X 2S / g System of N 2 Studied by Velocity Modulation Laser Spectroscopy
The velocity modulation technique has proven to be feasible in the study of higher vibrational levels of the A 2Pu state of N 2/ ( 1 – 5 ) because of the high vibrational temperature of N 2/ in the plasma ( 1, 3 ) and favorable Franck – Condon factors of the A 2Pu – X 2S g/ band system ( 6 ) in spectral regions readily accessible with single-mode lasers, as well as efficient suppression of the much stronger transitions in neutral N2 ( mainly the First Positive System) . By this technique the first rotational analyses and determinations of molecular constants of higher vibrational levels in the A state have been performed from direct observations of A – X transitions: the ( 7,3 ) band ( 2 ) , the ( 6,1 ) and ( 13,6 ) bands ( 3 ) , the ( 8,3 ) band ( 4 ) , and the ( 9,4 ) and ( 11,5 ) bands ( 5 ) . In this work the ( 12,6 ) band of the A 2Pu – X 2S g/ system is analyzed using velocity modulation laser spectroscopy in a He / N /2 gas discharge. The experimental setup has been described previously ( 4, 7 ) . In short, two laser beams obtained from the tunable Coherent 699-29 Autoscan single-mode dye laser system, with Rhodamine 6G dye, passed the 1-m-long discharge cell in opposite directions. The partial pressures of flowing He and N 2 in the cell were 9 and 0.5 Torr, respec-
tively, and the ac discharge was running at 50 mA and 1.4 kHz. The scanning speed of the laser was 100 MHz / sec. The absorption difference signal was demodulated by a lock-in amplifier with the time constant set at 3 sec and the spectrum between 16 830 and 17 030 cm01 was recorded. The absorption of the rotational lines was of the order 1 / 10 5 . Absorption of neutral species was well suppressed with the exception of the extremely strong He lines at 588 nm. The ( 7,2 ) band was also identified, but some observed lines could not be assigned. Most of them seem to belong to the ( 16,9 ) band or to the B – X system. In Fig. 1, a part of the spectrum is shown. As can be observed the intensity alternation is approximately 1 / 2 in accordance with a nuclear spin of 1 in the homonuclear N 2/ molecule ( 8 ) . In the rotational analysis, 139 assigned lines ( see Table 1 ) were used in the calculations of the molecular constants. The positions of the Doppler-broadened rotational lines were determined by using the line-fitting PC program Optimize 4.0 ( 9 ) ( cf. Fig. 2 ) , and the wavenumbers were obtained using iodine as a reference ( 10, 11 ) . Corrections were made to account for the 3-sec time constant during the
2 2 / 01 FIG. 1. Part of the N / . The Q R12 satellite lines 2 A Pu – X S g transition with three Q11 lines of the (12,6) band in the region 16 910–16 920 cm are weak and difficult to distinguish. The figure also shows the ‘‘staggering’’ effect; i.e., the line intensities are altering for a homonuclear molecule due to the spin Å 1 of the nucleus.
200 0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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NOTE
TABLE 1 Wavenumbers of the Observed Lines of the (12,6) A 2Pu – X 2S / g Transition
Copyright q 1997 by Academic Press
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FIG. 2. The figure shows the Q11 (8.5) line in high resolution; cf. Fig. 1. The weaker line is the Q R12 (7.5) satellite line sharing the same upper level. The solid line is the fitted curve obtained from the Optimize program ( 9). The splitting is 0.074 cm01 .
TABLE 2 Obtained Rotational Constants in cm01 of the X 2S / g (£9 Å 6) Ground State
Note. The numbers in parentheses are one standard deviation units of the last significant digits given. recordings. In the line fitting, deconvolution of close-lying lines was possible. The rather poor S / N ratio makes some line positions uncertain, but, with some exceptions, the lines were determined to better than 0.01 cm 01 ( i.e., obs 0 calc ) . In the fitting procedure the same standard models of the lower and upper states were used as in the analysis of the ( 8,3 ) band ( 4 ) and of the ( 9,4 ) band ( 5 ) . The X 2S g/ ground state was described by a Hund’s case ( b ) Hamiltonian and the least-squares fit gave rotational constants for the ground state and term values for the upper state, as described ˚ slund ( 12 ) . The derived molecular constants are given in Table by A 2. The RMS error of fit was 0.0064 cm01 . With values fixed to the previously determined sets of lower state constants ( see Table 2 ) , there were noticeable increases in the RMS error of fits, which is why the constants of the present work were kept in the derivation of the term values of the upper A 2Pu state. As was expected, our observations show no evidence of strong perturbations from the B state, since, according to ( 14 ) , for £ * Å 12 strong interaction with this state occurs around J Ç 50. The term values were fitted to a Hund’s case ( a ) Hamiltonian. The derived molecular constants are given in Table 3. The RMS error of fit was 0.0076 cm01 . Naturally, the precision in the derived constants is higher in the present work than in the case of indirect observation of the A state in the perturbation
TABLE 3 Rotational Constants in cm01 of the A 2Pu (£* Å 12) State
Note. The numbers in parentheses are one standard deviation units of the last significant digits given.
Copyright q 1997 by Academic Press
AID
JMS 7238
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NOTE analysis performed by Klynning and Page` s ( 14 ) . The agreement however is good.
REFERENCES 1. C. S. Gudeman, C. C. Martner, and R. J. Saykally, Chem. Phys. Lett. 146, 108–112 (1985). 2. M. B. Radunsky and R. J. Saykally, J. Chem. Phys. 87, 898–901 (1987). 3. D. T. Cramb, A. G. Adam, D. M. Steunenberg, A. J. Merer, and M. C. L. Gerry, J. Mol. Spectrosc. 141, 281–289 (1990). 4. B. Lindgren, P. Royen, and M. Zackrisson, J. Mol. Spectrosc. 146, 343–350 (1991). 5. B. Lindgren, P. Royen, and M. Zackrisson, J. Mol. Spectrosc. 156, 319–326 (1992). 6. W. B. Maier II and R. F. Holland, J. Chem. Phys. 59, 4501–4534 (1973). 7. P. Royen and M. Zackrisson, J. Mol. Spectrosc. 155, 427–429 (1992). 8. R. C. Weast (Ed.), ‘‘Handbook of Chemistry and Physics,’’ CRC Press, Cleveland, 1985.
9. V. Tu¨rck and O. Stier, ‘‘Scientific Tool, Optimize 4.0,’’ Technische Universita¨t Berlin, Fachbereich Physik, 1992. 10. S. Gerstenkorn and P. Luc, ‘‘Atlas du spectre d’absorption de la molecules d’iode 14800–20000 cm01 , Complement,’’ Ed. du CNRS, Paris, 1978. 11. S. Gerstenkorn and P. Luc, Rev. Phys. Appl. 14, 791–794 (1979). ˚ slund, J. Mol. Spectrosc. 50, 424–434 (1974). 12. N. A 13. R. A. Gottscho, R. W. Field, K. A. Dick, and W. Benesch, J. Mol. Spectrosc. 74, 435–455 (1979). 14. L. Klynning and P. Page`s, Phys. Scr. 25, 543–560 (1982).
A. Al-Khalili H. Ludwigs P. Royen Department of Physics Stockholm University P.O. Box 6730 S-113 85 Stockholm, Sweden Received August 30, 1996; in revised form November 12, 1996
Copyright q 1997 by Academic Press
AID
JMS 7238
/
6t18$$$$41
04-02-97 15:49:22
mspas
183, 200–203 (1997)
MS967238
NOTE / The Rotational Structure of the (12,6) Band of the A 2Pu – X 2S / g System of N 2 Studied by Velocity Modulation Laser Spectroscopy
The velocity modulation technique has proven to be feasible in the study of higher vibrational levels of the A 2Pu state of N 2/ ( 1 – 5 ) because of the high vibrational temperature of N 2/ in the plasma ( 1, 3 ) and favorable Franck – Condon factors of the A 2Pu – X 2S g/ band system ( 6 ) in spectral regions readily accessible with single-mode lasers, as well as efficient suppression of the much stronger transitions in neutral N2 ( mainly the First Positive System) . By this technique the first rotational analyses and determinations of molecular constants of higher vibrational levels in the A state have been performed from direct observations of A – X transitions: the ( 7,3 ) band ( 2 ) , the ( 6,1 ) and ( 13,6 ) bands ( 3 ) , the ( 8,3 ) band ( 4 ) , and the ( 9,4 ) and ( 11,5 ) bands ( 5 ) . In this work the ( 12,6 ) band of the A 2Pu – X 2S g/ system is analyzed using velocity modulation laser spectroscopy in a He / N /2 gas discharge. The experimental setup has been described previously ( 4, 7 ) . In short, two laser beams obtained from the tunable Coherent 699-29 Autoscan single-mode dye laser system, with Rhodamine 6G dye, passed the 1-m-long discharge cell in opposite directions. The partial pressures of flowing He and N 2 in the cell were 9 and 0.5 Torr, respec-
tively, and the ac discharge was running at 50 mA and 1.4 kHz. The scanning speed of the laser was 100 MHz / sec. The absorption difference signal was demodulated by a lock-in amplifier with the time constant set at 3 sec and the spectrum between 16 830 and 17 030 cm01 was recorded. The absorption of the rotational lines was of the order 1 / 10 5 . Absorption of neutral species was well suppressed with the exception of the extremely strong He lines at 588 nm. The ( 7,2 ) band was also identified, but some observed lines could not be assigned. Most of them seem to belong to the ( 16,9 ) band or to the B – X system. In Fig. 1, a part of the spectrum is shown. As can be observed the intensity alternation is approximately 1 / 2 in accordance with a nuclear spin of 1 in the homonuclear N 2/ molecule ( 8 ) . In the rotational analysis, 139 assigned lines ( see Table 1 ) were used in the calculations of the molecular constants. The positions of the Doppler-broadened rotational lines were determined by using the line-fitting PC program Optimize 4.0 ( 9 ) ( cf. Fig. 2 ) , and the wavenumbers were obtained using iodine as a reference ( 10, 11 ) . Corrections were made to account for the 3-sec time constant during the
2 2 / 01 FIG. 1. Part of the N / . The Q R12 satellite lines 2 A Pu – X S g transition with three Q11 lines of the (12,6) band in the region 16 910–16 920 cm are weak and difficult to distinguish. The figure also shows the ‘‘staggering’’ effect; i.e., the line intensities are altering for a homonuclear molecule due to the spin Å 1 of the nucleus.
200 0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
JMS 7238
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6t18$$$$41
04-02-97 15:49:22
mspas
NOTE
TABLE 1 Wavenumbers of the Observed Lines of the (12,6) A 2Pu – X 2S / g Transition
Copyright q 1997 by Academic Press
AID
JMS 7238
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201
202
NOTE
FIG. 2. The figure shows the Q11 (8.5) line in high resolution; cf. Fig. 1. The weaker line is the Q R12 (7.5) satellite line sharing the same upper level. The solid line is the fitted curve obtained from the Optimize program ( 9). The splitting is 0.074 cm01 .
TABLE 2 Obtained Rotational Constants in cm01 of the X 2S / g (£9 Å 6) Ground State
Note. The numbers in parentheses are one standard deviation units of the last significant digits given. recordings. In the line fitting, deconvolution of close-lying lines was possible. The rather poor S / N ratio makes some line positions uncertain, but, with some exceptions, the lines were determined to better than 0.01 cm 01 ( i.e., obs 0 calc ) . In the fitting procedure the same standard models of the lower and upper states were used as in the analysis of the ( 8,3 ) band ( 4 ) and of the ( 9,4 ) band ( 5 ) . The X 2S g/ ground state was described by a Hund’s case ( b ) Hamiltonian and the least-squares fit gave rotational constants for the ground state and term values for the upper state, as described ˚ slund ( 12 ) . The derived molecular constants are given in Table by A 2. The RMS error of fit was 0.0064 cm01 . With values fixed to the previously determined sets of lower state constants ( see Table 2 ) , there were noticeable increases in the RMS error of fits, which is why the constants of the present work were kept in the derivation of the term values of the upper A 2Pu state. As was expected, our observations show no evidence of strong perturbations from the B state, since, according to ( 14 ) , for £ * Å 12 strong interaction with this state occurs around J Ç 50. The term values were fitted to a Hund’s case ( a ) Hamiltonian. The derived molecular constants are given in Table 3. The RMS error of fit was 0.0076 cm01 . Naturally, the precision in the derived constants is higher in the present work than in the case of indirect observation of the A state in the perturbation
TABLE 3 Rotational Constants in cm01 of the A 2Pu (£* Å 12) State
Note. The numbers in parentheses are one standard deviation units of the last significant digits given.
Copyright q 1997 by Academic Press
AID
JMS 7238
/
6t18$$$$41
04-02-97 15:49:22
mspas
203
NOTE analysis performed by Klynning and Page` s ( 14 ) . The agreement however is good.
REFERENCES 1. C. S. Gudeman, C. C. Martner, and R. J. Saykally, Chem. Phys. Lett. 146, 108–112 (1985). 2. M. B. Radunsky and R. J. Saykally, J. Chem. Phys. 87, 898–901 (1987). 3. D. T. Cramb, A. G. Adam, D. M. Steunenberg, A. J. Merer, and M. C. L. Gerry, J. Mol. Spectrosc. 141, 281–289 (1990). 4. B. Lindgren, P. Royen, and M. Zackrisson, J. Mol. Spectrosc. 146, 343–350 (1991). 5. B. Lindgren, P. Royen, and M. Zackrisson, J. Mol. Spectrosc. 156, 319–326 (1992). 6. W. B. Maier II and R. F. Holland, J. Chem. Phys. 59, 4501–4534 (1973). 7. P. Royen and M. Zackrisson, J. Mol. Spectrosc. 155, 427–429 (1992). 8. R. C. Weast (Ed.), ‘‘Handbook of Chemistry and Physics,’’ CRC Press, Cleveland, 1985.
9. V. Tu¨rck and O. Stier, ‘‘Scientific Tool, Optimize 4.0,’’ Technische Universita¨t Berlin, Fachbereich Physik, 1992. 10. S. Gerstenkorn and P. Luc, ‘‘Atlas du spectre d’absorption de la molecules d’iode 14800–20000 cm01 , Complement,’’ Ed. du CNRS, Paris, 1978. 11. S. Gerstenkorn and P. Luc, Rev. Phys. Appl. 14, 791–794 (1979). ˚ slund, J. Mol. Spectrosc. 50, 424–434 (1974). 12. N. A 13. R. A. Gottscho, R. W. Field, K. A. Dick, and W. Benesch, J. Mol. Spectrosc. 74, 435–455 (1979). 14. L. Klynning and P. Page`s, Phys. Scr. 25, 543–560 (1982).
A. Al-Khalili H. Ludwigs P. Royen Department of Physics Stockholm University P.O. Box 6730 S-113 85 Stockholm, Sweden Received August 30, 1996; in revised form November 12, 1996
Copyright q 1997 by Academic Press
AID
JMS 7238
/
6t18$$$$41
04-02-97 15:49:22
mspas