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Molecular Constants of PO (X2Π) from Microwave and Infrared Laser Spectroscopy
JOURNAL OF MOLECULAR SPECTROSCOPY
174, 599–602 (1995)
Molecular Constants of PO (X 2P) from Microwave and Infrared Laser Spectroscopy
The pure rotational spectrum of PO has been studied by Kawaguchi et al. (1) using microwave and far-infrared laser magnetic resonance spectroscopy. In addition, 27 R-branch vibration – rotation transitions of the fundamental band of PO in the X2P state have been observed by Butler et al. (2) in the reaction of solid red phosphorus with the products of a microwave discharge in a mixture of H2/He/O2. The oxidation of white phosphorus (P 4) is a well-known complex chain reaction which produces many oxides of phosphorus, including the free radical PO (3 – 6). The reaction is a sufficiently good source of PO that we have been able to record the fundamental band of this radical with very good S:N ratios. Approximately 50 new transitions of the fundamental band were recorded in both P and R branches. No Q-branch lines were detected, which is in accord with their low calculated intensities. The diode laser spectrometer and the measurement and calibration procedures have been described in detail in an earlier paper (6). The PO lines were calibrated against N 2O lines (7), which have a quoted accuracy of 0.0002 cm01. The absolute accuracy of the PO line frequencies was determined by their relative accuracy, which was about 0.0005 cm 01. Adding the newly observed lines to the microwave data in a global fit leads to a significant improvement in the constants of PO, in particular the constants for the £ Å 1 state reported earlier (2). In previous work (1, 2) the analysis was based on the R 2 effective Hamiltonian (8), which is not entirely satisfactory for calculating the energy of an isolated electronic state because R Å N 0 L has matrix elements in L connecting different electronic states. We have therefore used the N 2 effective Hamiltonian developed by Brown et al. (9). These two Hamiltonians lead to slightly different parameter values. The most significant effect of this difference is on the n0 and B values, which are related by (10 – 12),
n0(N2) 0 n0(R2) Å B0(N2) 0 B1(N2)
(1)
B£(N2) Å B£(R2) / 2D£(R2).
(2)
Some other parameters, for example, A £, p£, and q£, also have smaller differences which are Hamiltonian dependent. The program for the analysis was based on that of Brown and co-workers (9 – 11) for a 2P state in a Hund’s case (a) basis set, using the N2 effective Hamiltonian. In the final global fit the data sets were weighted according to their experimental uncertainties. The largest weight (10.05) was given to the microwave transitions of the 2P1/2 state. The infrared lines of the 2P1/2 state were given weights of unity. Because L -doubling was not resolved in the 2P3/2 state, the observed transitions of this state were used twice but with one-half the weight of those of the 2P 1/2 state. The (n obs 0 ncal) values listed in Table I for the 2P3/2 state are the average values. The R(1.5) transition of the 2P1/2 state and the R(2.5) and R(5.5) transitions of the 2P 3/2 state were weighted zero because of their poor signal-to-noise ratios. The molecular constants of PO in the X2P state are given in Table II. In the earlier study (1), since there are no cross transitions between the 2P1/2 and 2P3/2 components, the spin – orbit coupling constant A0 was fixed at the value 224.01 cm01 given by the ultraviolet spectrum (13). The hyperfine constants a, b, c, and d were refined from both microwave 599
0022-2852/95 $12.00 Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
600
NOTES TABLE I Newly Observed Infrared Transitions (cm01) of the Fundamental Band of PO
and far-infrared laser magnetic resonance spectra and do not directly relate to N and/or R angular momenta. Therefore these parameters were fixed in the final fit and their values were taken from Ref. (1). The fit gave a reasonable value of ( n obs 0 ncal) £ 2 1 10 06 cm 01 for the microwave transitions. The precision of the ground state constants was similar to those of the earlier studies (1, 2). In general the uncertainties in the molecular constants for the £ Å 1 state are reduced by at least a factor of 2 except for the distortion constant D1 and the band origin n0. Table III compares the present work and the earlier diode laser results (2). The differences in the molecular parameters from the N2 and R2 Hamiltonians, derived from columns 2 and 3 in Table III, are in excellent agreement with Eqs. (1) and (2). ACKNOWLEDGMENTS I thank Dr. Paul B. Davies for support and comments on an earlier draft of the manuscript, Dr. John M. Brown for kindly providing a copy of his computer program, Mr. Y. Liu for helpful discussion, and Churchill College, Cambridge for a Research Scholarship.
m4130$6856
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AP: Mole Spec
601
NOTES TABLE II 01
Molecular Constants (cm ) for Both £ Å 0 and £ Å 1 states of PO (1s Uncertainties given in Parentheses)
TABLE III Comparison of the Molecular Constants of the PO Radical in the 2P State
REFERENCES 1. 2. 3. 4. 5. 6.
K. KAWAGUCHI, S. SAITO, AND E. HIROTA, J. Chem. Phys. 79, 629–634 (1983). J. E. BUTLER, K. KAWAGUCHI, AND E. HIROTA, J. Mol. Spectrosc. 101, 161–166 (1983). L. ANDREWS AND R. WITHNALL, J. Am. Chem. Soc. 110, 5605–5611 (1988). Z. MIELKE, M. MCCLUSKEY, AND L. ANDREWS, Chem. Phys. Lett. 165, 146–154 (1990). H. B. QIAN, P. B. DAVIES, I. K. AHMAD, AND P. A. HAMILTON, Chem. Phys. Lett. 235, 255–259 (1995). H. B. QIAN, P. A. HAMILTON, AND P. B. DAVIES, J. Chem. Soc. Faraday Trans., in press.
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
602
NOTES
7. A. G. MAKI AND J. S. WELLS, ‘‘Wavenumber Calibration Tables From Heterodyne Frequency Measurements,’’ U.S. Govt. Printing Office, Washington, DC, 1991. 8. J. M. BROWN, M. KAISE, C. M. L. KERR, AND D. J. MILTON, Mol. Phys. 36, 553–582 (1978). 9. J. M. BROWN, E. A. COLBOURN, J. K. G. WATSON, AND F. D. WAYNE, J. Mol. Spectrosc. 74, 294– 318 (1979). 10. J. M. BROWN, J. E. SCHUBERT, C. E. BROWN, J. S. GEIGER, AND D. R. SMITH, J. Mol. Spectrosc. 114, 185–196 (1985). 11. J. M. BROWN, J. E. SCHUBERT, R. J. SAYKALLY, AND K. M. EVENSON, J. Mol. Spectrosc. 120, 421– 434 (1986). 12. Y. LIU, Z. LIU, AND P. B. DAVIES, J. Mol. Spectrosc. 171, 402–419 (1995). 13. R. D. VERMA AND S. R. SINGHAL, Can. J. Phys. 53, 411–419 (1975). HAI-BO QIAN Department of Chemistry University of Cambridge Lensfield Road Cambridge CB2 1EW United Kingdom Received June 3, 1995; in revised form July 31, 1995
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
174, 599–602 (1995)
Molecular Constants of PO (X 2P) from Microwave and Infrared Laser Spectroscopy
The pure rotational spectrum of PO has been studied by Kawaguchi et al. (1) using microwave and far-infrared laser magnetic resonance spectroscopy. In addition, 27 R-branch vibration – rotation transitions of the fundamental band of PO in the X2P state have been observed by Butler et al. (2) in the reaction of solid red phosphorus with the products of a microwave discharge in a mixture of H2/He/O2. The oxidation of white phosphorus (P 4) is a well-known complex chain reaction which produces many oxides of phosphorus, including the free radical PO (3 – 6). The reaction is a sufficiently good source of PO that we have been able to record the fundamental band of this radical with very good S:N ratios. Approximately 50 new transitions of the fundamental band were recorded in both P and R branches. No Q-branch lines were detected, which is in accord with their low calculated intensities. The diode laser spectrometer and the measurement and calibration procedures have been described in detail in an earlier paper (6). The PO lines were calibrated against N 2O lines (7), which have a quoted accuracy of 0.0002 cm01. The absolute accuracy of the PO line frequencies was determined by their relative accuracy, which was about 0.0005 cm 01. Adding the newly observed lines to the microwave data in a global fit leads to a significant improvement in the constants of PO, in particular the constants for the £ Å 1 state reported earlier (2). In previous work (1, 2) the analysis was based on the R 2 effective Hamiltonian (8), which is not entirely satisfactory for calculating the energy of an isolated electronic state because R Å N 0 L has matrix elements in L connecting different electronic states. We have therefore used the N 2 effective Hamiltonian developed by Brown et al. (9). These two Hamiltonians lead to slightly different parameter values. The most significant effect of this difference is on the n0 and B values, which are related by (10 – 12),
n0(N2) 0 n0(R2) Å B0(N2) 0 B1(N2)
(1)
B£(N2) Å B£(R2) / 2D£(R2).
(2)
Some other parameters, for example, A £, p£, and q£, also have smaller differences which are Hamiltonian dependent. The program for the analysis was based on that of Brown and co-workers (9 – 11) for a 2P state in a Hund’s case (a) basis set, using the N2 effective Hamiltonian. In the final global fit the data sets were weighted according to their experimental uncertainties. The largest weight (10.05) was given to the microwave transitions of the 2P1/2 state. The infrared lines of the 2P1/2 state were given weights of unity. Because L -doubling was not resolved in the 2P3/2 state, the observed transitions of this state were used twice but with one-half the weight of those of the 2P 1/2 state. The (n obs 0 ncal) values listed in Table I for the 2P3/2 state are the average values. The R(1.5) transition of the 2P1/2 state and the R(2.5) and R(5.5) transitions of the 2P 3/2 state were weighted zero because of their poor signal-to-noise ratios. The molecular constants of PO in the X2P state are given in Table II. In the earlier study (1), since there are no cross transitions between the 2P1/2 and 2P3/2 components, the spin – orbit coupling constant A0 was fixed at the value 224.01 cm01 given by the ultraviolet spectrum (13). The hyperfine constants a, b, c, and d were refined from both microwave 599
0022-2852/95 $12.00 Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
600
NOTES TABLE I Newly Observed Infrared Transitions (cm01) of the Fundamental Band of PO
and far-infrared laser magnetic resonance spectra and do not directly relate to N and/or R angular momenta. Therefore these parameters were fixed in the final fit and their values were taken from Ref. (1). The fit gave a reasonable value of ( n obs 0 ncal) £ 2 1 10 06 cm 01 for the microwave transitions. The precision of the ground state constants was similar to those of the earlier studies (1, 2). In general the uncertainties in the molecular constants for the £ Å 1 state are reduced by at least a factor of 2 except for the distortion constant D1 and the band origin n0. Table III compares the present work and the earlier diode laser results (2). The differences in the molecular parameters from the N2 and R2 Hamiltonians, derived from columns 2 and 3 in Table III, are in excellent agreement with Eqs. (1) and (2). ACKNOWLEDGMENTS I thank Dr. Paul B. Davies for support and comments on an earlier draft of the manuscript, Dr. John M. Brown for kindly providing a copy of his computer program, Mr. Y. Liu for helpful discussion, and Churchill College, Cambridge for a Research Scholarship.
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
601
NOTES TABLE II 01
Molecular Constants (cm ) for Both £ Å 0 and £ Å 1 states of PO (1s Uncertainties given in Parentheses)
TABLE III Comparison of the Molecular Constants of the PO Radical in the 2P State
REFERENCES 1. 2. 3. 4. 5. 6.
K. KAWAGUCHI, S. SAITO, AND E. HIROTA, J. Chem. Phys. 79, 629–634 (1983). J. E. BUTLER, K. KAWAGUCHI, AND E. HIROTA, J. Mol. Spectrosc. 101, 161–166 (1983). L. ANDREWS AND R. WITHNALL, J. Am. Chem. Soc. 110, 5605–5611 (1988). Z. MIELKE, M. MCCLUSKEY, AND L. ANDREWS, Chem. Phys. Lett. 165, 146–154 (1990). H. B. QIAN, P. B. DAVIES, I. K. AHMAD, AND P. A. HAMILTON, Chem. Phys. Lett. 235, 255–259 (1995). H. B. QIAN, P. A. HAMILTON, AND P. B. DAVIES, J. Chem. Soc. Faraday Trans., in press.
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec
602
NOTES
7. A. G. MAKI AND J. S. WELLS, ‘‘Wavenumber Calibration Tables From Heterodyne Frequency Measurements,’’ U.S. Govt. Printing Office, Washington, DC, 1991. 8. J. M. BROWN, M. KAISE, C. M. L. KERR, AND D. J. MILTON, Mol. Phys. 36, 553–582 (1978). 9. J. M. BROWN, E. A. COLBOURN, J. K. G. WATSON, AND F. D. WAYNE, J. Mol. Spectrosc. 74, 294– 318 (1979). 10. J. M. BROWN, J. E. SCHUBERT, C. E. BROWN, J. S. GEIGER, AND D. R. SMITH, J. Mol. Spectrosc. 114, 185–196 (1985). 11. J. M. BROWN, J. E. SCHUBERT, R. J. SAYKALLY, AND K. M. EVENSON, J. Mol. Spectrosc. 120, 421– 434 (1986). 12. Y. LIU, Z. LIU, AND P. B. DAVIES, J. Mol. Spectrosc. 171, 402–419 (1995). 13. R. D. VERMA AND S. R. SINGHAL, Can. J. Phys. 53, 411–419 (1975). HAI-BO QIAN Department of Chemistry University of Cambridge Lensfield Road Cambridge CB2 1EW United Kingdom Received June 3, 1995; in revised form July 31, 1995
m4130$6856
11-08-95 01:11:01
mspeas
AP: Mole Spec