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A New Band System of the YbF Molecule
JOURNAL OF MOLECULAR SPECTROSCOPY
174, 290–296 (1995)
A New Band System of the YbF Molecule K. N. UTTAM AND M. M. JOSHI Saha’s Spectroscopy Laboratory, Physics Department, Allahabad University, Allahabad 211002, India Received March 31, 1994 The thermal emission spectrum of the ytterbium monofluoride molecule has been photographed ˚ region at 23007C, at a reciprocal linear dispersion of 7.3 for the first time in the 3600–4600 A ˚ /mm. A total of 65 violet degraded bands have been recorded, out of which 55 are new. All A these bands have been classified and assigned to the two systems C 2P –X 2S / and D 2S / –X 2S / of YbF. The C 2P –X 2S / system has been classified for the first time. Vibrational constants for the relevant states have been evaluated. q 1995 Academic Press, Inc. 1. INTRODUCTION
The band spectrum of the ytterbium monofluoride (YbF) molecule has been investi˚ spectral regions (1–3) and has gated previously in the 4500–5900 and 3600–4000 A been classified into three systems, viz., A 2P –X 2S, B 2S –X 2S, and D 2S –X 2S. In addition, Lee and Zare (3) found evidence for two new systems C 2P –X 2S (4000– ˚ ) and A *?– X 2S (6000–6400 A ˚ ). However, due to the highly perturbed nature 4600 A of the upper state in both systems, Lee and Zare were unable to analyze the bands in these systems. Recently, Dolg et al. (4), on the basis of a theoretical study, assigned the ground state of YbF as 2S / . Since thermal emission is a low-energy excitation, the spectra involving much higher energy states generally do not appear. Thermally excited emission spectra are found to be almost free of atomic lines. In most cases, the spectrum obtained by this method involves the ground state and the lower electronic states of the system. We have been able to record for the first time the thermal emission spectrum of ˚ region, using a high temperature vacuum graphite furnace. YbF in the 3600–4600 A Here we report the vibrational analyses for the C–X and D–X systems. 2. EXPERIMENTAL DETAILS
The complete experimental setup has been described elsewhere (5). However, a few details are given here for completeness. A small quantity of spec-pure ytterbium fluoride (Johnson Mathey, purity 99.9%) was kept inside the experimental tube of the Saha’s high-temperature graphite furnace (6). After necessary routine adjustments were made and the furnace chamber was evacuated, it was filled with argon gas at a pressure of about 50 Torr to avoid the rapid effusion of molecular vapor from the open ends of the graphite tube. The tube was heated electrically by a 10-kVA primary tapped transformer to about 23007C and the spectrum of the resultant glow was recorded in the first order of a plane grating spectrograph equipped with a grating ˚ . An exposure time of about 4 min was found to be sufficient to blazed at 5600 A record a well-developed spectrum on ORWO 125 ASA black and white film. An 0022-2852/95 $12.00
290
Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
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NEW BAND SYSTEM OF YbF
˚ /mm. FIG . 1. Thermal emission spectrum of the YbF molecule at a reciprocal linear dispersion of 7.3 A
iron arc spectrum was recorded to provide calibration lines. The measurements were performed using a Carl-Zeiss Abbe Comparator. 3. RESULTS AND ANALYSES
˚ region, The thermal emission spectrum of YbF, photographed in the 3600–4600 A is reproduced in Fig. 1. The spectrum is almost free of atomic lines. A total of 65 violet degraded bands have been photographed, out of which 55 are entirely new. All ˚ region have the prominent bands reported by Lee and Zare (3) in the 3600–4000 A also appeared in the spectrum, in addition to the new bands. The observed bands have been analyzed into two systems viz. C–X and D–X. Contrary to the suggestion of Lee and Zare (3), no perturbation could be seen in the C–X system and bands up to the vibrational level £* Å 9 have been analyzed. The system C–X comprises two subsystems: C1 –X and C2 –X.
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UTTAM AND JOSHI TABLE I Bandhead Data of the YbF Molecule: The C1 –X Subsystem
a. The C1 –X (4000–4600 A˚ ) Subsystem This subsystem is entirely new. It consists of double-headed and violet degraded bands. About 35 bands could be identified. Each band has a P head and a stronger Q head lying on the lower wavelength side. The stronger bands in the D£ Å 0 sequence
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NEW BAND SYSTEM OF YbF TABLE I—Continued
are clearly noticeable in Fig. 1. The wavenumbers of the observed bandheads could be represented by the equations: nP Å 23 018.7 / 523.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2
nQ Å 23 026.6 / 523.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
The relevant bandhead data together with visual estimates of their intensities and vibrational assignments have been listed in Table I. b. The C2 –X (4000–4600 A˚ ) Subsystem ˚ region. It consists of 20 This relatively weaker subsystem lies in the 4000–4600 A double-headed bands degraded to the violet. Table II gives the band head data, their assignments, and visually estimated intensities. The following expressions represent the observed data: nP Å 23 241.6 / 507.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2
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UTTAM AND JOSHI TABLE II Bandhead Data of the YbF Molecule: The C2 –X Subsystem
nQ Å 23 255.3 / 507.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
c. The D–X (3600–4000 A˚ ) System The stronger bands in the D£ Å 0, {1, {2 sequence of this system may be seen in Fig. 1. The following equation represents the observed data, which are summarized in Table III: n Å25 980.5 / 574.6 ( £* / 1/2) 0 2.8 ( £* / 1/2) 2 0505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
d. Vibrational Isotope Effect Natural ytterbium consists of six stable isotopes with atomic masses 170, 171, 172, 174, and 176. The last five have comparable abundances of 14.3, 21.8, 16.1, 31.8, and 12.7%, respectively. Fluorine has only one isotope ( 19F). The vibrational isotope shifts calculated for the five main Yb isotopes using standard expressions (7) vary
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NEW BAND SYSTEM OF YbF TABLE III Bandhead Data of the YbF Molecule: The D–X System
from 01.1 to 2.2 cm01 , which is comparable to the experimental uncertainties. Nevertheless, the presence of the isotopic molecules did result in a perceptible broadening of the bandheads. 4. DISCUSSION
The thermal emission spectrum, attributable to YbF, has resulted in the observation ˚ region, for which satisfactory analyses of a large number of bands in the 3600–4600 A have been proposed. The vibrational constants (in cm01 ) obtained are collected below: The lower state vibrational constants determined here agree with those reported by System C1 –X C2 –X D–X
Region 4000–4600 4000–4600 3600–4000
˚ A ˚ A ˚ A
v*e
v*e x*e
v9e
v9e x9e
n00
523.0 507.0 574.6
2.0 2.0 2.8
505.5 505.5 505.5
1.9 1.9 1.9
23 035.3 (Q) 23 256.0 (Q) 26 014.8 (P)
Barrow and Chojnicki (1) and Lee and Zare (3) for the ground state of YbF. This confirms that the present systems of bands are indeed due to the YbF molecule, and that the transitions terminate on the ground state. The two subsystems C1 –X and C2 –X appear in the relatively stronger part of the spectrum. The fact that corresponding bands in the two systems occur with a nearly constant separation of 220.7 cm01 suggests that these subsystems are, very likely due to a transition involving a doublet electronic state which is, presumably, a 2P state. The lower state of these systems probably has negligible electronic splitting and may therefore be 2S. The normal state electronic configurations of ytterbium and fluorine atoms are 70
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Yb: KLMN5s 25p 66s 2 — 1 S
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UTTAM AND JOSHI 9
F: K2s 22p 5 — 2 P.
Considering the separated atoms model, we have Yb in 1 S and F in 2 P as their ground atomic state. The combination will give rise to 2P and 2S / molecular states. On the basis of a rotational structure study (1) and recent theoretical work (4), it is now well established that the ground state of the YbF molecule is 2S / . Our work on the isovalent molecule YbI has led to a similar conclusion regarding its ground state (8). With 2S / as the lower state, the plausible optical transitions are 2P – 2S / and 2S / – 2S / . The observed features of the spectra support the assignment of the two subsystems C1 –X and C2 –X (both consisting of double-headed bands with intense Q heads) to the transition 2P – 2S / . The D–X system comprising sharp and singleheaded bands might involve a 2S / – 2S / transition. In the present work we have been able to identify a single atomic line of ytterbium ˚ , arising from the transition 6s6p ( 1 P1 ) r 6s 2 ( 1 S0 ). It is therefore at l Å 3988 A possible to suppose that the excited state of YbF responsible for these transitions arises from this excited state of ytterbium. The Yb atom in this state together with F in the ground state leads to the molecular states 2S / (2), 2S 0 , 2P (2), and 2D. Thus, the transitions responsible for the spectrum under consideration would be 2S / – 2S / and 2P – 2S / , taking 2S / as the ground state, which is in accordance with the observed features of the band spectrum. Similar results have been reported for these systems in Ref. (3). ACKNOWLEDGMENTS The authors are grateful to Dr. T. K. Balsubramanian, BARC, Bombay for fruitful discussions. K. N. Uttam is also thankful to CSIR, New Delhi for financial support. REFERENCES 1. 2. 3. 4. 5.
R. F. BARROW AND A. H. CHOJNICKI, J. Chem. Soc. Faraday Trans. 2 71, 728–735 (1975). A. YOKOZEKI AND M. MENZINGER, Chem. Phys. 14, 427–439 (1976). H. U. LEE AND R. N. ZARE, J. Mol. Spectrosc. 64, 233–243 (1977). M. DOLG, H. STOLL, AND H. PRESS, Chem. Phys. 165, 21–30 (1992). K. N. UTTAM, ‘‘Investigations of Spectra of the Diatomic Molecules in Thermal Emission’’ Ph.D. Thesis, University of Allahabad, Allahabad, India, 1993. 6. M. N. SAHA, N. K. SUR, AND K. MAJUMDAR, Z. Phys. 40, 648–651 (1927). 7. G. HERZBERG, ‘‘Molecular Spectra and Molecular Structure, Vol. I Spectra of Diatomic Molecules,’’ p. 318. Van Nostrand, New York, 1950. 8. K. N. UTTAM AND M. M. JOSHI, Pramana 42, 239–243 (1994).
m4130$6838
10-26-95 19:41:26
mspea
AP: Mole Spec
174, 290–296 (1995)
A New Band System of the YbF Molecule K. N. UTTAM AND M. M. JOSHI Saha’s Spectroscopy Laboratory, Physics Department, Allahabad University, Allahabad 211002, India Received March 31, 1994 The thermal emission spectrum of the ytterbium monofluoride molecule has been photographed ˚ region at 23007C, at a reciprocal linear dispersion of 7.3 for the first time in the 3600–4600 A ˚ /mm. A total of 65 violet degraded bands have been recorded, out of which 55 are new. All A these bands have been classified and assigned to the two systems C 2P –X 2S / and D 2S / –X 2S / of YbF. The C 2P –X 2S / system has been classified for the first time. Vibrational constants for the relevant states have been evaluated. q 1995 Academic Press, Inc. 1. INTRODUCTION
The band spectrum of the ytterbium monofluoride (YbF) molecule has been investi˚ spectral regions (1–3) and has gated previously in the 4500–5900 and 3600–4000 A been classified into three systems, viz., A 2P –X 2S, B 2S –X 2S, and D 2S –X 2S. In addition, Lee and Zare (3) found evidence for two new systems C 2P –X 2S (4000– ˚ ) and A *?– X 2S (6000–6400 A ˚ ). However, due to the highly perturbed nature 4600 A of the upper state in both systems, Lee and Zare were unable to analyze the bands in these systems. Recently, Dolg et al. (4), on the basis of a theoretical study, assigned the ground state of YbF as 2S / . Since thermal emission is a low-energy excitation, the spectra involving much higher energy states generally do not appear. Thermally excited emission spectra are found to be almost free of atomic lines. In most cases, the spectrum obtained by this method involves the ground state and the lower electronic states of the system. We have been able to record for the first time the thermal emission spectrum of ˚ region, using a high temperature vacuum graphite furnace. YbF in the 3600–4600 A Here we report the vibrational analyses for the C–X and D–X systems. 2. EXPERIMENTAL DETAILS
The complete experimental setup has been described elsewhere (5). However, a few details are given here for completeness. A small quantity of spec-pure ytterbium fluoride (Johnson Mathey, purity 99.9%) was kept inside the experimental tube of the Saha’s high-temperature graphite furnace (6). After necessary routine adjustments were made and the furnace chamber was evacuated, it was filled with argon gas at a pressure of about 50 Torr to avoid the rapid effusion of molecular vapor from the open ends of the graphite tube. The tube was heated electrically by a 10-kVA primary tapped transformer to about 23007C and the spectrum of the resultant glow was recorded in the first order of a plane grating spectrograph equipped with a grating ˚ . An exposure time of about 4 min was found to be sufficient to blazed at 5600 A record a well-developed spectrum on ORWO 125 ASA black and white film. An 0022-2852/95 $12.00
290
Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
m4130$6838
10-26-95 19:41:26
mspea
AP: Mole Spec
291
NEW BAND SYSTEM OF YbF
˚ /mm. FIG . 1. Thermal emission spectrum of the YbF molecule at a reciprocal linear dispersion of 7.3 A
iron arc spectrum was recorded to provide calibration lines. The measurements were performed using a Carl-Zeiss Abbe Comparator. 3. RESULTS AND ANALYSES
˚ region, The thermal emission spectrum of YbF, photographed in the 3600–4600 A is reproduced in Fig. 1. The spectrum is almost free of atomic lines. A total of 65 violet degraded bands have been photographed, out of which 55 are entirely new. All ˚ region have the prominent bands reported by Lee and Zare (3) in the 3600–4000 A also appeared in the spectrum, in addition to the new bands. The observed bands have been analyzed into two systems viz. C–X and D–X. Contrary to the suggestion of Lee and Zare (3), no perturbation could be seen in the C–X system and bands up to the vibrational level £* Å 9 have been analyzed. The system C–X comprises two subsystems: C1 –X and C2 –X.
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AP: Mole Spec
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UTTAM AND JOSHI TABLE I Bandhead Data of the YbF Molecule: The C1 –X Subsystem
a. The C1 –X (4000–4600 A˚ ) Subsystem This subsystem is entirely new. It consists of double-headed and violet degraded bands. About 35 bands could be identified. Each band has a P head and a stronger Q head lying on the lower wavelength side. The stronger bands in the D£ Å 0 sequence
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293
NEW BAND SYSTEM OF YbF TABLE I—Continued
are clearly noticeable in Fig. 1. The wavenumbers of the observed bandheads could be represented by the equations: nP Å 23 018.7 / 523.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2
nQ Å 23 026.6 / 523.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
The relevant bandhead data together with visual estimates of their intensities and vibrational assignments have been listed in Table I. b. The C2 –X (4000–4600 A˚ ) Subsystem ˚ region. It consists of 20 This relatively weaker subsystem lies in the 4000–4600 A double-headed bands degraded to the violet. Table II gives the band head data, their assignments, and visually estimated intensities. The following expressions represent the observed data: nP Å 23 241.6 / 507.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2
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AP: Mole Spec
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UTTAM AND JOSHI TABLE II Bandhead Data of the YbF Molecule: The C2 –X Subsystem
nQ Å 23 255.3 / 507.0 ( £* / 1/2) 0 2.0 ( £* / 1/2) 2 0 505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
c. The D–X (3600–4000 A˚ ) System The stronger bands in the D£ Å 0, {1, {2 sequence of this system may be seen in Fig. 1. The following equation represents the observed data, which are summarized in Table III: n Å25 980.5 / 574.6 ( £* / 1/2) 0 2.8 ( £* / 1/2) 2 0505.5 ( £9 / 1/2) / 1.9 ( £9 / 1/2) 2 .
d. Vibrational Isotope Effect Natural ytterbium consists of six stable isotopes with atomic masses 170, 171, 172, 174, and 176. The last five have comparable abundances of 14.3, 21.8, 16.1, 31.8, and 12.7%, respectively. Fluorine has only one isotope ( 19F). The vibrational isotope shifts calculated for the five main Yb isotopes using standard expressions (7) vary
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AP: Mole Spec
295
NEW BAND SYSTEM OF YbF TABLE III Bandhead Data of the YbF Molecule: The D–X System
from 01.1 to 2.2 cm01 , which is comparable to the experimental uncertainties. Nevertheless, the presence of the isotopic molecules did result in a perceptible broadening of the bandheads. 4. DISCUSSION
The thermal emission spectrum, attributable to YbF, has resulted in the observation ˚ region, for which satisfactory analyses of a large number of bands in the 3600–4600 A have been proposed. The vibrational constants (in cm01 ) obtained are collected below: The lower state vibrational constants determined here agree with those reported by System C1 –X C2 –X D–X
Region 4000–4600 4000–4600 3600–4000
˚ A ˚ A ˚ A
v*e
v*e x*e
v9e
v9e x9e
n00
523.0 507.0 574.6
2.0 2.0 2.8
505.5 505.5 505.5
1.9 1.9 1.9
23 035.3 (Q) 23 256.0 (Q) 26 014.8 (P)
Barrow and Chojnicki (1) and Lee and Zare (3) for the ground state of YbF. This confirms that the present systems of bands are indeed due to the YbF molecule, and that the transitions terminate on the ground state. The two subsystems C1 –X and C2 –X appear in the relatively stronger part of the spectrum. The fact that corresponding bands in the two systems occur with a nearly constant separation of 220.7 cm01 suggests that these subsystems are, very likely due to a transition involving a doublet electronic state which is, presumably, a 2P state. The lower state of these systems probably has negligible electronic splitting and may therefore be 2S. The normal state electronic configurations of ytterbium and fluorine atoms are 70
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Yb: KLMN5s 25p 66s 2 — 1 S
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AP: Mole Spec
296
UTTAM AND JOSHI 9
F: K2s 22p 5 — 2 P.
Considering the separated atoms model, we have Yb in 1 S and F in 2 P as their ground atomic state. The combination will give rise to 2P and 2S / molecular states. On the basis of a rotational structure study (1) and recent theoretical work (4), it is now well established that the ground state of the YbF molecule is 2S / . Our work on the isovalent molecule YbI has led to a similar conclusion regarding its ground state (8). With 2S / as the lower state, the plausible optical transitions are 2P – 2S / and 2S / – 2S / . The observed features of the spectra support the assignment of the two subsystems C1 –X and C2 –X (both consisting of double-headed bands with intense Q heads) to the transition 2P – 2S / . The D–X system comprising sharp and singleheaded bands might involve a 2S / – 2S / transition. In the present work we have been able to identify a single atomic line of ytterbium ˚ , arising from the transition 6s6p ( 1 P1 ) r 6s 2 ( 1 S0 ). It is therefore at l Å 3988 A possible to suppose that the excited state of YbF responsible for these transitions arises from this excited state of ytterbium. The Yb atom in this state together with F in the ground state leads to the molecular states 2S / (2), 2S 0 , 2P (2), and 2D. Thus, the transitions responsible for the spectrum under consideration would be 2S / – 2S / and 2P – 2S / , taking 2S / as the ground state, which is in accordance with the observed features of the band spectrum. Similar results have been reported for these systems in Ref. (3). ACKNOWLEDGMENTS The authors are grateful to Dr. T. K. Balsubramanian, BARC, Bombay for fruitful discussions. K. N. Uttam is also thankful to CSIR, New Delhi for financial support. REFERENCES 1. 2. 3. 4. 5.
R. F. BARROW AND A. H. CHOJNICKI, J. Chem. Soc. Faraday Trans. 2 71, 728–735 (1975). A. YOKOZEKI AND M. MENZINGER, Chem. Phys. 14, 427–439 (1976). H. U. LEE AND R. N. ZARE, J. Mol. Spectrosc. 64, 233–243 (1977). M. DOLG, H. STOLL, AND H. PRESS, Chem. Phys. 165, 21–30 (1992). K. N. UTTAM, ‘‘Investigations of Spectra of the Diatomic Molecules in Thermal Emission’’ Ph.D. Thesis, University of Allahabad, Allahabad, India, 1993. 6. M. N. SAHA, N. K. SUR, AND K. MAJUMDAR, Z. Phys. 40, 648–651 (1927). 7. G. HERZBERG, ‘‘Molecular Spectra and Molecular Structure, Vol. I Spectra of Diatomic Molecules,’’ p. 318. Van Nostrand, New York, 1950. 8. K. N. UTTAM AND M. M. JOSHI, Pramana 42, 239–243 (1994).
m4130$6838
10-26-95 19:41:26
mspea
AP: Mole Spec