- Email: [email protected]
New Band Systems of the YbCl Molecule
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
185, 8–14 (1997)
MS977344
New Band Systems of the YbCl Molecule K. N. Uttam, R. Gopal, and M. M. Joshi Saha’s Spectroscopy Laboratory, Physics Department, Allahabad University, Allahabad 211002, India Received November 27, 1996; in revised form May 1, 1997
˚ region, has been photographed The thermal emission spectrum of the YbCl molecule, lying in the 3800 – 6000 A ˚ /mm. More than 196 violet degraded bands have been for the first time at a reciprocal linear dispersion of 7.3 A recorded on a 2-m plane grating spectrograph in first order and assigned to the vibrational schemes of six systems, viz. A , B1 , B 2 , C 1 , C2 , and D . While the systems A , C 1 , C2 , and D consist of single headed bands, the systems B1 and B2 consist of double headed bands. The vibrational analysis performed suggests that these systems arise from the ground state ( 2S / ) of YbCl molecule. The systems C1 – X , C2 – X , and D – X have been analyzed for the first time. More precise vibrational constants for the relevant states have been determined. q 1997 Academic Press
while a 2S state always possesses a single vibrational frequency. Since thermal emission is a low-energy excitation, the spectrum is always excited with ground state or low-lying electronic states of the molecule. Thermally excited spectrum is found to be almost free of atomic lines. In thermal emission, usually higher vibrational quanta are obtained and intensities are governed by Maxwellian distribution. The investigations of YbF ( 4 ) and YbI ( 5, 6 ) molecules in thermal emission have yielded fruitful results; we decided to examine the thermal emission spectrum of the YbCl molecule using a high-temperature graphite furnace. We have been able to record for the first time the thermal emission spectrum of the YbCl molecule in the 3800 – ˚ region. Here we report the first vibrational analy6000 A sis of C1 – X , C2 – X , and D – X systems along with the revised analysis of the known A – X and B – X systems.
1. INTRODUCTION
Several optical emission and absorption studies carried out previously on ytterbium monochloride ( YbCl ) were concerned with the ground and low-lying states of the YbCl molecule ( 1 – 3 ) , leading to information about energetics, vibrational frequencies, and dissociation energies. The electronic spectrum of the YbCl molecule was first studied by Gatterer et al. ( 1 ) in emission. The discrete ˚ region were attribbands obtained in the 4500 – 5800 A uted to two band systems, namely A – X and B – X . Initial vibrational constants for the ground state were obtained from the analysis of the A – X and B – X transition. Lee and Zare ( 2 ) photographed the chemiluminescent spectrum of ˚ region and rethe YbCl molecule in the 4700 – 5800 A 2 2 ported two systems, viz. A P – X S and B 2S – X 2S. Kramer ( 3 ) recorded the molecular emission of YbCl ˚ region from microwave discharge in the 4600 – 6000 A and confirmed that the system A – X involved the 2P – 2S transition. Apart from these two systems, Lee and Zare ( 2 ) identified the presence of new system in the 4000 – ˚ region and attributed to the C 2P – X 2S transition. 4600 A However because of the presence of a perturbed upper state they could not analyze these bands. A close scrutiny of the available references reveals that the spectroscopic information about the YbCl molecule is inadequate. While Kramer ( 3 ) reported only one system, Gatterer et al. ( 1 ) and Lee and Zare ( 2 ) analyzed two systems. The analyses proposed by Gatterer et al. ( 1 ) were limited to the first four vibrational levels of each state. Kramer ( 3 ) investigated the molecular emission from discharge through Hg, Yb, HgCl 2 , CsCl, and Ar vapors and was consequently confronted with a very large number of atomic lines due to Hg, Cs, and Yb, which rendered the measurements and classifications of the bands ambiguous. He further reported two values for the vibrational frequency of the 2S ground state of YbCl
2. EXPERIMENTAL DETAILS
The complete experimental setup is reported elsewhere ( 7 ) . In brief, a small quantity of spec-pure ytterbium chloride ( Johnson Matthey, purity 99.9% ) was kept in the graphite tube of Saha’s high-temperature graphite furnace ( 8 ) . After making the necessary routine adjustments and evacuation, argon gas was admitted into the furnace chamber at a pressure of about 500 Torr of mercury with a view to minimize the rapid effusion of molecular vapors from the open ends of the graphite tube. A heavy current from the secondary terminals of a 10-kVA step-down transformer was passed through the graphite tube. The current could be varied with the help of control switch provided in the transformer. The strong bands were obtained at a temperature of 22007C, and the weak bands at 23007C. The spectra were recorded using a 2-m plane grating spectrograph with a grating blazed at 5600 8
0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
JMS 7344
/
6t1e$$$361
09-10-97 10:38:33
mspal
EMISSION SPECTRUM OF YbCl
9
˚ /mm. FIG. 1. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A
˚ ( inverse dispersion of 7.3 A ˚ /mm) . An exposure on the A order of 2 to 4 min was found sufficient to photograph a nice spectrum on ORWO 125 ASA black and white film. The iron arc spectrum was photographed to provide calibration lines. The spectra were measured on a C Z Abbe Comparator. 3. RESULTS AND DISCUSSION
The thermally excited emission spectrum of the YbCl ˚ region, has been molecule, lying in the 3800 – 6000 A photographed for the first time and is reproduced in Figs. 1, 2, and 3. The intense part of the spectrum lies in the ˚ region while the weaker part extends in 4500 – 6000 A
˚ region. The spectrum is well developed the 3800 – 4500 A and almost free of atomic lines. All the bands reported ˚ region, by earlier workers ( 1 – 3 ) , in the 4500 – 6000 A also appeared in our spectrograms along with new bands. A total of 196 single headed and double headed bands have been recorded and are classified into six systems, viz. A , B1 , B2 , C1 , C2 , and D . Out of the 196 observed bands the system A consists of 76 while the systems B1 , B2 , C1 , C2 , and D consist of 38, 46, 24, 9, and 3 bands, respectively. While the systems C1 , C2 , and D are entirely new, revised analysis of the systems A and B have been presented. Contrary to the suggestion of Lee and Zare ( 2 ) , we could not find perturbation in the C – X system and bands up to £ * ú 7 have been analyzed satisfactorily.
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$362
09-10-97 10:38:33
mspal
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
11
EMISSION SPECTRUM OF YbCl
TABLE 1 Band Head Data of the YbCl Molecule: A–X System
Note. Dn Å nobs 0 ncalc . * Bands reported by Lee and Zare (2).
The analysis of various band systems observed in the thermal emission spectrum of the YbCl molecule has yielded the following vibrational constants ( in cm01 ) : System
˚) Region (A
noo
ve*
ve*xe*
ve9
ve9xe9
A–X B1 –X B2 –X C1 –X C2 –X D–X
5300–5900 5000–5300 4750–5050 4350–4475 4250–4350 3900–4200
17888.8 19381.9(Q) 19939.9(Q) 22842.6 23385.5 24460.1
311.0 312.5 313.5 296.0 299.0 —
0.45 0.55 0.60 0.36 1.30 —
290.0 290.0 290.0 290.0 290.0 290.0
0.45 0.45 0.45 0.45 0.45 0.45
Tables 1 – 5 give the collection of band head data, visual relative intensities, and their vibrational assignments for the A – X , B1 – X , B2 – X , C1 – X , and C2 – X systems, respectively.
In our spectrogram two atomic lines of ytterbium at l Å ˚ and l Å 5558 A ˚ appear. These atomic lines arise 3988 A from the transitions 6s6p( 1 P1 ) –6s 2 ( 1 S0 ) and 6s6p( 3 P1 ) – 6s 2 ( 1 S0 ), respectively. It is reasonable to assume that in the formation of the ytterbium monochloride molecule these excited states of Yb combine with the normal state of p 5 ( 2 P3 / 2 ) of the chlorine atom. The resulting molecular states arising from separated atoms model taking Yb atom in excited states 1 P and 3 P and Cl in the ground state 2 P are given below. Yb( 1 P) / Cl( 2 P) Yb( 3 P) / Cl( 2 P)
Since the thermal emission is a low energy excitation, no
˚ /mm. FIG. 2. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A ˚ /mm. FIG. 3. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$362
09-10-97 10:38:33
S / (2), 2S 0 , 2P(2), 2D S / (2), 2S 0 , 2P(2), 2D, 4 / S (2), 4S 0 , 4P(2), 4D
2 2
mspal
12
UTTAM, GOPAL, AND JOSHI
TABLE 2 Band Head Data of the YbCl Molecule: B1 –X System
Note. Dn Å nobs 0 ncalc .
TABLE 3 Band Head Data of the YbCl Molecule: B2 –X System
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
EMISSION SPECTRUM OF YbCl
TABLE 4 Band Head Data of the YbCl Molecule: C1 –X System
TABLE 5 Band Head Data of the YbCl Molecule: C2 –X System
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
13
14
UTTAM, GOPAL, AND JOSHI
intercombination system will appear. Recently Kaledin et al. (9), on the basis of ionic bonding model and ligand field theory, calculated the molecular electronic states of lanthanide monohalides. The rotational and laser spectroscopic studies (10, 11) on the YbF molecule confirmed the A 2P –X 2S / and B 2S / –X 2S / transition. Lee and Zare (2) deduced similar results for the A–X and B–X systems of the YbCl molecule. However, the nature of the transition associated with the new band systems observed in the thermal emission could not be decided unambiguously without rotational studies which are in progress in our laboratory. ACKNOWLEDGMENTS One of us (K. N. Uttam) is grateful to DST, New Delhi and CSIR, New Delhi for the financial support.
REFERENCES 1. A. Gatterer, G. Piccardi and F. Vincenzi, Ricerchi Spttroc. Lab. Astrofil Specola Vaticana 1, 181 (1942). 2. H. U. Lee and R. N. Zare, J. Mol. Spectrosc. 64, 233–243 (1977). 3. J. Kramer, J. Chem. Phys. 64, 5370–5377 (1978). 4. K. N. Uttam and M. M. Joshi, J. Mol. Spectrosc. 174, 290–296 (1995). 5. K. N. Uttam and M. M. Joshi, Pramana 42, 239–243 (1994). 6. K. N. Uttam and M. M. Joshi, Indian J. Phys. B 69, 261–265 (1995). 7. K. N. Uttam, ‘‘Investigations of Spectra of the Diatomic Molecules in Thermal Emission,’’ Ph.D. thesis, University of Allahabad, Allahabad, India, 1993. 8. M. N. Saha, N. K. Sur, and K. Majumdar, Z. Phys. 40, 648–651 (1927). 9. A. L. Kaledin, M. C. Heaven, R. W. Field, and L. A. Kaledin, J. Mol. Spectrosc. 179, 310–319 (1996). 10. R. F. Barrow and A. H. Chojnicki, J. Chem. Soc. Faraday Trans. 271, 728–735 (1975). 11. K. L. Dunfield, C. Linton, T. E. Clarke, J. McBride, A. G. Adam, and J. R. D. Peers, J. Mol. Spectrosc. 174, 433–445 (1995).
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$363
09-10-97 10:38:33
mspal
185, 8–14 (1997)
MS977344
New Band Systems of the YbCl Molecule K. N. Uttam, R. Gopal, and M. M. Joshi Saha’s Spectroscopy Laboratory, Physics Department, Allahabad University, Allahabad 211002, India Received November 27, 1996; in revised form May 1, 1997
˚ region, has been photographed The thermal emission spectrum of the YbCl molecule, lying in the 3800 – 6000 A ˚ /mm. More than 196 violet degraded bands have been for the first time at a reciprocal linear dispersion of 7.3 A recorded on a 2-m plane grating spectrograph in first order and assigned to the vibrational schemes of six systems, viz. A , B1 , B 2 , C 1 , C2 , and D . While the systems A , C 1 , C2 , and D consist of single headed bands, the systems B1 and B2 consist of double headed bands. The vibrational analysis performed suggests that these systems arise from the ground state ( 2S / ) of YbCl molecule. The systems C1 – X , C2 – X , and D – X have been analyzed for the first time. More precise vibrational constants for the relevant states have been determined. q 1997 Academic Press
while a 2S state always possesses a single vibrational frequency. Since thermal emission is a low-energy excitation, the spectrum is always excited with ground state or low-lying electronic states of the molecule. Thermally excited spectrum is found to be almost free of atomic lines. In thermal emission, usually higher vibrational quanta are obtained and intensities are governed by Maxwellian distribution. The investigations of YbF ( 4 ) and YbI ( 5, 6 ) molecules in thermal emission have yielded fruitful results; we decided to examine the thermal emission spectrum of the YbCl molecule using a high-temperature graphite furnace. We have been able to record for the first time the thermal emission spectrum of the YbCl molecule in the 3800 – ˚ region. Here we report the first vibrational analy6000 A sis of C1 – X , C2 – X , and D – X systems along with the revised analysis of the known A – X and B – X systems.
1. INTRODUCTION
Several optical emission and absorption studies carried out previously on ytterbium monochloride ( YbCl ) were concerned with the ground and low-lying states of the YbCl molecule ( 1 – 3 ) , leading to information about energetics, vibrational frequencies, and dissociation energies. The electronic spectrum of the YbCl molecule was first studied by Gatterer et al. ( 1 ) in emission. The discrete ˚ region were attribbands obtained in the 4500 – 5800 A uted to two band systems, namely A – X and B – X . Initial vibrational constants for the ground state were obtained from the analysis of the A – X and B – X transition. Lee and Zare ( 2 ) photographed the chemiluminescent spectrum of ˚ region and rethe YbCl molecule in the 4700 – 5800 A 2 2 ported two systems, viz. A P – X S and B 2S – X 2S. Kramer ( 3 ) recorded the molecular emission of YbCl ˚ region from microwave discharge in the 4600 – 6000 A and confirmed that the system A – X involved the 2P – 2S transition. Apart from these two systems, Lee and Zare ( 2 ) identified the presence of new system in the 4000 – ˚ region and attributed to the C 2P – X 2S transition. 4600 A However because of the presence of a perturbed upper state they could not analyze these bands. A close scrutiny of the available references reveals that the spectroscopic information about the YbCl molecule is inadequate. While Kramer ( 3 ) reported only one system, Gatterer et al. ( 1 ) and Lee and Zare ( 2 ) analyzed two systems. The analyses proposed by Gatterer et al. ( 1 ) were limited to the first four vibrational levels of each state. Kramer ( 3 ) investigated the molecular emission from discharge through Hg, Yb, HgCl 2 , CsCl, and Ar vapors and was consequently confronted with a very large number of atomic lines due to Hg, Cs, and Yb, which rendered the measurements and classifications of the bands ambiguous. He further reported two values for the vibrational frequency of the 2S ground state of YbCl
2. EXPERIMENTAL DETAILS
The complete experimental setup is reported elsewhere ( 7 ) . In brief, a small quantity of spec-pure ytterbium chloride ( Johnson Matthey, purity 99.9% ) was kept in the graphite tube of Saha’s high-temperature graphite furnace ( 8 ) . After making the necessary routine adjustments and evacuation, argon gas was admitted into the furnace chamber at a pressure of about 500 Torr of mercury with a view to minimize the rapid effusion of molecular vapors from the open ends of the graphite tube. A heavy current from the secondary terminals of a 10-kVA step-down transformer was passed through the graphite tube. The current could be varied with the help of control switch provided in the transformer. The strong bands were obtained at a temperature of 22007C, and the weak bands at 23007C. The spectra were recorded using a 2-m plane grating spectrograph with a grating blazed at 5600 8
0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
JMS 7344
/
6t1e$$$361
09-10-97 10:38:33
mspal
EMISSION SPECTRUM OF YbCl
9
˚ /mm. FIG. 1. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A
˚ ( inverse dispersion of 7.3 A ˚ /mm) . An exposure on the A order of 2 to 4 min was found sufficient to photograph a nice spectrum on ORWO 125 ASA black and white film. The iron arc spectrum was photographed to provide calibration lines. The spectra were measured on a C Z Abbe Comparator. 3. RESULTS AND DISCUSSION
The thermally excited emission spectrum of the YbCl ˚ region, has been molecule, lying in the 3800 – 6000 A photographed for the first time and is reproduced in Figs. 1, 2, and 3. The intense part of the spectrum lies in the ˚ region while the weaker part extends in 4500 – 6000 A
˚ region. The spectrum is well developed the 3800 – 4500 A and almost free of atomic lines. All the bands reported ˚ region, by earlier workers ( 1 – 3 ) , in the 4500 – 6000 A also appeared in our spectrograms along with new bands. A total of 196 single headed and double headed bands have been recorded and are classified into six systems, viz. A , B1 , B2 , C1 , C2 , and D . Out of the 196 observed bands the system A consists of 76 while the systems B1 , B2 , C1 , C2 , and D consist of 38, 46, 24, 9, and 3 bands, respectively. While the systems C1 , C2 , and D are entirely new, revised analysis of the systems A and B have been presented. Contrary to the suggestion of Lee and Zare ( 2 ) , we could not find perturbation in the C – X system and bands up to £ * ú 7 have been analyzed satisfactorily.
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$362
09-10-97 10:38:33
mspal
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
11
EMISSION SPECTRUM OF YbCl
TABLE 1 Band Head Data of the YbCl Molecule: A–X System
Note. Dn Å nobs 0 ncalc . * Bands reported by Lee and Zare (2).
The analysis of various band systems observed in the thermal emission spectrum of the YbCl molecule has yielded the following vibrational constants ( in cm01 ) : System
˚) Region (A
noo
ve*
ve*xe*
ve9
ve9xe9
A–X B1 –X B2 –X C1 –X C2 –X D–X
5300–5900 5000–5300 4750–5050 4350–4475 4250–4350 3900–4200
17888.8 19381.9(Q) 19939.9(Q) 22842.6 23385.5 24460.1
311.0 312.5 313.5 296.0 299.0 —
0.45 0.55 0.60 0.36 1.30 —
290.0 290.0 290.0 290.0 290.0 290.0
0.45 0.45 0.45 0.45 0.45 0.45
Tables 1 – 5 give the collection of band head data, visual relative intensities, and their vibrational assignments for the A – X , B1 – X , B2 – X , C1 – X , and C2 – X systems, respectively.
In our spectrogram two atomic lines of ytterbium at l Å ˚ and l Å 5558 A ˚ appear. These atomic lines arise 3988 A from the transitions 6s6p( 1 P1 ) –6s 2 ( 1 S0 ) and 6s6p( 3 P1 ) – 6s 2 ( 1 S0 ), respectively. It is reasonable to assume that in the formation of the ytterbium monochloride molecule these excited states of Yb combine with the normal state of p 5 ( 2 P3 / 2 ) of the chlorine atom. The resulting molecular states arising from separated atoms model taking Yb atom in excited states 1 P and 3 P and Cl in the ground state 2 P are given below. Yb( 1 P) / Cl( 2 P) Yb( 3 P) / Cl( 2 P)
Since the thermal emission is a low energy excitation, no
˚ /mm. FIG. 2. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A ˚ /mm. FIG. 3. Thermal emission spectrum of the YbCl molecule at a reciprocal linear dispersion of 7.3 A Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$362
09-10-97 10:38:33
S / (2), 2S 0 , 2P(2), 2D S / (2), 2S 0 , 2P(2), 2D, 4 / S (2), 4S 0 , 4P(2), 4D
2 2
mspal
12
UTTAM, GOPAL, AND JOSHI
TABLE 2 Band Head Data of the YbCl Molecule: B1 –X System
Note. Dn Å nobs 0 ncalc .
TABLE 3 Band Head Data of the YbCl Molecule: B2 –X System
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
EMISSION SPECTRUM OF YbCl
TABLE 4 Band Head Data of the YbCl Molecule: C1 –X System
TABLE 5 Band Head Data of the YbCl Molecule: C2 –X System
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$7344
09-10-97 10:38:33
mspal
13
14
UTTAM, GOPAL, AND JOSHI
intercombination system will appear. Recently Kaledin et al. (9), on the basis of ionic bonding model and ligand field theory, calculated the molecular electronic states of lanthanide monohalides. The rotational and laser spectroscopic studies (10, 11) on the YbF molecule confirmed the A 2P –X 2S / and B 2S / –X 2S / transition. Lee and Zare (2) deduced similar results for the A–X and B–X systems of the YbCl molecule. However, the nature of the transition associated with the new band systems observed in the thermal emission could not be decided unambiguously without rotational studies which are in progress in our laboratory. ACKNOWLEDGMENTS One of us (K. N. Uttam) is grateful to DST, New Delhi and CSIR, New Delhi for the financial support.
REFERENCES 1. A. Gatterer, G. Piccardi and F. Vincenzi, Ricerchi Spttroc. Lab. Astrofil Specola Vaticana 1, 181 (1942). 2. H. U. Lee and R. N. Zare, J. Mol. Spectrosc. 64, 233–243 (1977). 3. J. Kramer, J. Chem. Phys. 64, 5370–5377 (1978). 4. K. N. Uttam and M. M. Joshi, J. Mol. Spectrosc. 174, 290–296 (1995). 5. K. N. Uttam and M. M. Joshi, Pramana 42, 239–243 (1994). 6. K. N. Uttam and M. M. Joshi, Indian J. Phys. B 69, 261–265 (1995). 7. K. N. Uttam, ‘‘Investigations of Spectra of the Diatomic Molecules in Thermal Emission,’’ Ph.D. thesis, University of Allahabad, Allahabad, India, 1993. 8. M. N. Saha, N. K. Sur, and K. Majumdar, Z. Phys. 40, 648–651 (1927). 9. A. L. Kaledin, M. C. Heaven, R. W. Field, and L. A. Kaledin, J. Mol. Spectrosc. 179, 310–319 (1996). 10. R. F. Barrow and A. H. Chojnicki, J. Chem. Soc. Faraday Trans. 271, 728–735 (1975). 11. K. L. Dunfield, C. Linton, T. E. Clarke, J. McBride, A. G. Adam, and J. R. D. Peers, J. Mol. Spectrosc. 174, 433–445 (1995).
Copyright q 1997 by Academic Press
AID
JMS 7344
/
6t1e$$$363
09-10-97 10:38:33
mspal