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Infrared laser spectroscopy of the fundamental band of a 3π CO
Volume 136, number6
CHEMICAL PHYSICS LETTERS
22 May 1981
INFRARED LASER SPECTROSCOPY OF THE FUNDAMENTAL BAND OF a311 CO P.B. DAVIES and P.A. MARTIN Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP, UK
Received 20 February 1987; in final form 10 March 1987
Many rotational components of the fundamental band of metastable a 311CO have been measured in absorption using diode laser spectroscopy with concentration modulation detection. Line positions are in good agreement with predictions from optically derived parameters. Resolved or partially resolved splittings arising from lambda-doubling appear for the three 51components. Splittings in the 9= 1 and 2 spectra agree satisfactorily with molecular beam (t-f) and microwave results while those in the 9=0 fundamental deviate by several linewidths (up to 0.015 cm-‘) from calculated values.
1. Introduction The CO molecule in its a 3H state is one of the most widely studied metastable species, with a radiative lifetime of several milliseconds. Triplet-triplet transitions in which a 311CO is the lower state have been investigated while the formally forbidden transition to the ground ‘E + state has been measured in both emission and absorption [ 1,2]. The intercombination transitions greatly influence the dynamical behaviour of several excited states [ 31 as well as the spectroscopy of CO via perturbations. Ultraviolet and visible spectroscopy of CO has been complemented by several microwave and radiofrequency studies commencing with the classic work of Klemperer and co-workers [ 4,5 ] who measured radiofrequency lambda-doubling transitions for a= 1 and 2 a 3H CO in a molecular beam. Although confined to low Jlevels, the molecular-beam electron-bombardment source generated a-state molecules up to v= 7. Starting at v=4 a distinct perturbation was detected and attributed to interaction with the a’ ‘E + state [ 51. Lambda-doubling transitions in the a=0 manifold lie at much higher frequencies than for 8= 1 and 2, at least at low J. They appear as splittings of the J= 1+-Opure rotational spectra measured by Saykally [ 61 near 92 GHz (for v= O-4 levels). Three further v=O rotational transitions have been reported by Saykally et al. using far-infrared LMR [ 71. A re-appraisal of the high-resolution microwave
and radiofrequency results has shown that the perturbation of the a 3H spectra by the a’ state is more extensive than previously appreciated [ 6,8 1. While removing some of the inconsistencies between theory and experiment, this realisation also suggests that further high-resolution spectroscopy would help to clarify the full extent of the perturbation. The data available from microwave and rf experiments are essentially limited to low J values. No vibration-rotation spectra have been reported and, although Doppler-limited IR spectra cannot match the resolution of the lower frequency experiments, the IR spectrum is likely to produce a much more extensive data set. In addition, the relatively large Adoubling in the 3H0 component should be easily measurable. This report is a preliminary account of the measurement of the fundamental band of a3H CO at Doppler-limited resolution recorded with a diode laser spectrometer.
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental The experimental arrangement is shown schematically in fig. 1. The metastable molecule was generated in an ac discharge ( x 10 kHz) through mixtures of CO and helium. The 1 m long air-cooled discharge tube was the same as that used for measuring molecular ion spectra [ 91. Optimum conditions for 527
Volume 136, number 6
CHEMICAL PHYSICS LETTERS
Concentration Modulation Dmcharge Tube HgCdTe Detector
,
/
\ \+
r/
_
Dmde
22 May 1987
Table 1 Spectroscopic parameters (cm-‘) for a ‘lI CO [2] (figures in parentheses are one standard deviation [ 21)
Las.3
A
1O’A, B
lo60 10% c* 105Bo+ 103BB:
v=o
v=l
41.450(2) -20.6(7) 1.68162(3) 6.378 - 1.72(8) 0.8752(12) 2.90(6) 1.304(13)
41.290(3) - 19.9(8) 1.66265(l) 6.416 - 1.39(13) 0.8682(23) 3.04(10) 1.281(23)
w,= 1743.41(52), w.+x~=14.36(21), v,,= 1714.69. Amp 21
v
Laser Chart Recorder
Control Module
Fig. 1. Experimental arrangement for detection of a ‘Tl CO in an ac discharge using concentration modulation.
producing a 311spectra were CO:He about 1:5 at 350 mTorr total pressure. The laser beam, provided by one of three Spectra Physics Inc. diode lasers, was passed unidirectionally through the discharge and detected at 2J; i.e. 20 kHz (fig. 1). Concentration modulation of short-lived discharge products has already been shown to be extremely sensitive for detecting both neutral [ lo] and ionic transient species [9].
D. Results and discussion Discharges in CO/He mixtures produced many lines attributable to transient species in the search regions between 1670-1730 and 1755-1780 cm-‘. Calibration lines for these regions were OCS [ 111 and NO [ 12,131 respectively. Most of the transient lines were present in absorption but some quite intense stimulated emission lines, so far unassigned, were also observed. Discharges in CO,/He mixtures produced similar spectra but with different relative intensity. In addition to the a ‘II CO spectra, hot band lines of ground-state CO were also prevalent and identified from their accurately calculated positions [ 141. They optimised in pure CO discharges at about 300 mTorr. Because of the many other lines present, however, an accurate prediction of the a 311 528
spectra was essential. This was provided using standard formula [ 151 and the parameters from the work of Field et al. [ 21 to calculate the centre of the lambda doublets. The relevant parameters are given in table 1 and the calculated and experimental positions in table 2. The agreement between these positions is satisfactory considering the level of theory used and leads to an unambiguous assignment. No Q-branch transition has been detected, partly due to their relatively low Intensity with respect to the P and R branches and to poor laser coverage in the appropriate regions. Confirmation of the assignment in the rR= 1 and 2 fundamentals was provided by lambda-doubling splittings which could be predicted from the more accurate radiofrequency [ 51 and microwave [ 61 results. Although these splittings are quite small and therefore poorly resolved in the infrared spectra, sometimes being less than the experimental linewidth, agreement with predictions was highly satisfactory in most cases (table 2). The relatively poor agreement for the P( 5) and P( 7) 3111 transitions is accounted for by experimental factors such as low laser power or inadequate mode coverage. The larger observed splitting of each transition in. the 8~0 spectra is much more amenable to accurate measurement and most can be measured to an accuracy of &7x 1O-4 cm-‘. Fig. 2 shows the well resolved pair of the P( 10) component. Since sZ=O lambda doublings have not been directly measured they were calculated for all the tine structure components using the matrix elements and parameters (table 1) of Field et al. [ 21 for a pure 3111 state. The results were in
Volume136,number6
CHEMICALPHYSICSLETTERS
22 May 1987
Table2 Fundamental band transitions and lambda-doubling splittings (cm- ’ ) in a ‘II CO (experimental measurement accuracy (1 u on several readings) is estimated to be > 7x 10m4cm-’ for the ‘l-l, P-branch and > 1 X lo-’ for all other lines) Component
_4-doubling splitting
Line position ‘) measured
calculated
measured
predicted
P(l1) P(l0) P(8) P(7) P(5) P(4) P(3) P(1) R(l3) Wl8) R(20)
1678.1367 1681.7180 1688.7309 1692.1483 1698.8258 1702.1056 1705.3271 1711.8956 1755.552 1768.929 1774.025
1678.15 1681.73 1688.73 1692.15 1698.84 1702.11 1705.32
0.035 1 0.0371 0.0381 0.0356 0.0251 0.0191 0.0101 (0.0081) b, 0.060 0.051 (0.048) b’
0.0493 0.0507 0.0509 0.0491 0.0401 0.0325 0.0229 0.001 0.049 0.034 0.029
‘f-L,
P(8) P(7) P(5) R(l5) Nl9)
1686.815 1690.488 1697.618 1763.029 1773.560
1686.83 1690.46 1697.58 1763.04 1773.62
0.058 0.039 0.032 0.020 0.008
0.0569 [ 61 0.057 0.051 0.019 0.008
‘ff*
P(l1) P(6) P(3) R(3) Nl4) Nl5) Nl8) R( 19)
1672.747 1691.941 1703.349 1728.691 1762.557 1765.277 1773.226 1775.801
1672.76 1691.97 1703.33 1728.71 1762.57 1765.32 1773.29 1775.86
0.014 0.004
0.0131 0.0052 0.0009 0.004 1 0.0120 0.0121 0.0118
‘fLl
1755.58 1768.97 1774.08
0.011 0.015 0.012 0.013
a) Mean of the lambda-doubled pair. ‘) Ambiguous measurement, possible interference from another transition.
excellent agreement with the reported Q= 1 and 2 doublings from the molecular beam work [2,5]and the spacings predicted for the J= 1+Cl microwave transitions [ 71 and gave confidence in the O= 0 calculations. The calculated splittings for the IR spectra are given in table 2. Relatively poor agreement is found for the IR=O spectrum, with calculated splittings consistently 0.0 12-0.015 cm- ’ larger than measured for the P branch. Nevertheless, the J dependence of the splittings ib well predicted by the calculations, rising to a maximum around P(9) in the Q=O P branch. The absolute deviation corresponds to over two linewidths and is outside the estimated experimental accuracy. Since the calculated values are highly reliable for Q= 1 and 2 it is concluded that this is possible evidence for perturbation
of the lower vibrational levels of a 311CO as alluded to by Saykally et al. [ 7,8]. Further measurements would help us to confirm this; it is clear from these preliminary results, however, that a large experimental data base can be accumulated from the IR spectra for a thorough investigation of the effects of perturbations. The observation of spectra of CO a 311 at room temperature contrasts with the microwave study of CO dc discharges, which required liquid-nitrogen cooling. Linewidths were typically 5 x 1O-3 cm- ’ (fwhm) compared with a room-temperature Doppler width of 4 x 1O- 3 cm- ‘, suggesting a low translational temperature. However, a quantitative deduction is not possible as the line profiles show some asymmetry at their base (fig. 2), possibly 529
Volume 136, number 6
CHEMICAL PHYSICS LETTERS
CO a%,
22 May 1987
frequencies from 1 to 100 kHz are available with this apparatus.
P(10) F
Acknowledgement
ocs
-dt
We thank the SERC and the Royal Society for equipment grants and the SERC and Spectra Physics UK for a CASE Studentship for PAM.
References
I
I 1681.700 cm-l
,750
Fig. 2. Lambda doublet components of the P( 10) transition in the fundamental band of a ‘II, CO. The reference gas line position is 1681.7725 cm-’ [ 1 I] and was used to calibrate the CO lines with the aid of the germanium etalon pattern (free spectral range 0.01616f0.0004 cm-‘) shown in the bottom trace.
caused by distortion in the discharge waveform. No hot band lines have yet been positively identified in searches based on predictions from the optically derived parameters. In addition to a more extensive study of the fundamental band, two further aspects of this work are worth investigating. Firstly, the detection of “crossstack” transitions which will yield a more direct measurement of the spin-orbit constant than previously available. Secondly, the use of an ac discharge should enable the dynamics of the decay of the metastable to be followed. Variable modulation
530
[ 1 ] S.J. Tilford and J.D. Simmons, J. Phys. Chem. Ref. Data 1 (1972) 147. [ 21 R.W. Field, S.G. Tilford, R.A. Howard and J.D. Simmons, J. Mol. Spectry. 44 (1972) 347. [ 3 ] M.A.A. Clyne and M.C. Heaven, J. Chem. Sot. Faraday Trans. II 79 (1981) 1375. [ 41 R.S. Freund and W. Klemperer, J. Chem. Phys. 43 (1965) 2422. [5] R.C. Stem, R.H. Gammon, M.E. Lesk, R.S. Freund and W.A. Klemperer, J. Chem. Phys. 52 (1970) 3467. [ 61 R.J. Saykally, Ph.D. thesis, University of Wisconsin (1977). [ 71 R.J. Saykally, KM. Evenson, E.R. Comben and J.M. Brown, Mol. Phys. 58 (1986) 735. [ 81 R.C. Wood, unpublished work. [9] P.B. Davies, S.A. Johnson, P.A. Hamilton andT.J. Sears, J: Chem. Phys. 108 (1986) 335. [lo] P.B. Davies, P.R. Brown and S.A. Johnson, Chem. Phys. Letters 133 (1987) 239. [ 111 A.G. Maki, W.B. Olsen and R.L. Sams, J. Mol. Spectry. 8 1 (1980) 122. [ 121 A. Valentin, A. Henry, Ph. Cardinet, M.F. LeMoal, Da-Wun Chen and R.N. Rao, J. Mol. Spectry. 70 (1978) 9. [ 131 A. Hinz, J.S. Wells and A.G. Maki, J. Mol. Spectry. 119 (1986) 120. [ 141 A.W. Mantz, E.R. Nichols, B.D. Alpert and R.N. Rao, J. Mol. Spectry. 35 (1970) 325. [ 15 ] G. He&erg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950).
CHEMICAL PHYSICS LETTERS
22 May 1981
INFRARED LASER SPECTROSCOPY OF THE FUNDAMENTAL BAND OF a311 CO P.B. DAVIES and P.A. MARTIN Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP, UK
Received 20 February 1987; in final form 10 March 1987
Many rotational components of the fundamental band of metastable a 311CO have been measured in absorption using diode laser spectroscopy with concentration modulation detection. Line positions are in good agreement with predictions from optically derived parameters. Resolved or partially resolved splittings arising from lambda-doubling appear for the three 51components. Splittings in the 9= 1 and 2 spectra agree satisfactorily with molecular beam (t-f) and microwave results while those in the 9=0 fundamental deviate by several linewidths (up to 0.015 cm-‘) from calculated values.
1. Introduction The CO molecule in its a 3H state is one of the most widely studied metastable species, with a radiative lifetime of several milliseconds. Triplet-triplet transitions in which a 311CO is the lower state have been investigated while the formally forbidden transition to the ground ‘E + state has been measured in both emission and absorption [ 1,2]. The intercombination transitions greatly influence the dynamical behaviour of several excited states [ 31 as well as the spectroscopy of CO via perturbations. Ultraviolet and visible spectroscopy of CO has been complemented by several microwave and radiofrequency studies commencing with the classic work of Klemperer and co-workers [ 4,5 ] who measured radiofrequency lambda-doubling transitions for a= 1 and 2 a 3H CO in a molecular beam. Although confined to low Jlevels, the molecular-beam electron-bombardment source generated a-state molecules up to v= 7. Starting at v=4 a distinct perturbation was detected and attributed to interaction with the a’ ‘E + state [ 51. Lambda-doubling transitions in the a=0 manifold lie at much higher frequencies than for 8= 1 and 2, at least at low J. They appear as splittings of the J= 1+-Opure rotational spectra measured by Saykally [ 61 near 92 GHz (for v= O-4 levels). Three further v=O rotational transitions have been reported by Saykally et al. using far-infrared LMR [ 71. A re-appraisal of the high-resolution microwave
and radiofrequency results has shown that the perturbation of the a 3H spectra by the a’ state is more extensive than previously appreciated [ 6,8 1. While removing some of the inconsistencies between theory and experiment, this realisation also suggests that further high-resolution spectroscopy would help to clarify the full extent of the perturbation. The data available from microwave and rf experiments are essentially limited to low J values. No vibration-rotation spectra have been reported and, although Doppler-limited IR spectra cannot match the resolution of the lower frequency experiments, the IR spectrum is likely to produce a much more extensive data set. In addition, the relatively large Adoubling in the 3H0 component should be easily measurable. This report is a preliminary account of the measurement of the fundamental band of a3H CO at Doppler-limited resolution recorded with a diode laser spectrometer.
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental The experimental arrangement is shown schematically in fig. 1. The metastable molecule was generated in an ac discharge ( x 10 kHz) through mixtures of CO and helium. The 1 m long air-cooled discharge tube was the same as that used for measuring molecular ion spectra [ 91. Optimum conditions for 527
Volume 136, number 6
CHEMICAL PHYSICS LETTERS
Concentration Modulation Dmcharge Tube HgCdTe Detector
,
/
\ \+
r/
_
Dmde
22 May 1987
Table 1 Spectroscopic parameters (cm-‘) for a ‘lI CO [2] (figures in parentheses are one standard deviation [ 21)
Las.3
A
1O’A, B
lo60 10% c* 105Bo+ 103BB:
v=o
v=l
41.450(2) -20.6(7) 1.68162(3) 6.378 - 1.72(8) 0.8752(12) 2.90(6) 1.304(13)
41.290(3) - 19.9(8) 1.66265(l) 6.416 - 1.39(13) 0.8682(23) 3.04(10) 1.281(23)
w,= 1743.41(52), w.+x~=14.36(21), v,,= 1714.69. Amp 21
v
Laser Chart Recorder
Control Module
Fig. 1. Experimental arrangement for detection of a ‘Tl CO in an ac discharge using concentration modulation.
producing a 311spectra were CO:He about 1:5 at 350 mTorr total pressure. The laser beam, provided by one of three Spectra Physics Inc. diode lasers, was passed unidirectionally through the discharge and detected at 2J; i.e. 20 kHz (fig. 1). Concentration modulation of short-lived discharge products has already been shown to be extremely sensitive for detecting both neutral [ lo] and ionic transient species [9].
D. Results and discussion Discharges in CO/He mixtures produced many lines attributable to transient species in the search regions between 1670-1730 and 1755-1780 cm-‘. Calibration lines for these regions were OCS [ 111 and NO [ 12,131 respectively. Most of the transient lines were present in absorption but some quite intense stimulated emission lines, so far unassigned, were also observed. Discharges in CO,/He mixtures produced similar spectra but with different relative intensity. In addition to the a ‘II CO spectra, hot band lines of ground-state CO were also prevalent and identified from their accurately calculated positions [ 141. They optimised in pure CO discharges at about 300 mTorr. Because of the many other lines present, however, an accurate prediction of the a 311 528
spectra was essential. This was provided using standard formula [ 151 and the parameters from the work of Field et al. [ 21 to calculate the centre of the lambda doublets. The relevant parameters are given in table 1 and the calculated and experimental positions in table 2. The agreement between these positions is satisfactory considering the level of theory used and leads to an unambiguous assignment. No Q-branch transition has been detected, partly due to their relatively low Intensity with respect to the P and R branches and to poor laser coverage in the appropriate regions. Confirmation of the assignment in the rR= 1 and 2 fundamentals was provided by lambda-doubling splittings which could be predicted from the more accurate radiofrequency [ 51 and microwave [ 61 results. Although these splittings are quite small and therefore poorly resolved in the infrared spectra, sometimes being less than the experimental linewidth, agreement with predictions was highly satisfactory in most cases (table 2). The relatively poor agreement for the P( 5) and P( 7) 3111 transitions is accounted for by experimental factors such as low laser power or inadequate mode coverage. The larger observed splitting of each transition in. the 8~0 spectra is much more amenable to accurate measurement and most can be measured to an accuracy of &7x 1O-4 cm-‘. Fig. 2 shows the well resolved pair of the P( 10) component. Since sZ=O lambda doublings have not been directly measured they were calculated for all the tine structure components using the matrix elements and parameters (table 1) of Field et al. [ 21 for a pure 3111 state. The results were in
Volume136,number6
CHEMICALPHYSICSLETTERS
22 May 1987
Table2 Fundamental band transitions and lambda-doubling splittings (cm- ’ ) in a ‘II CO (experimental measurement accuracy (1 u on several readings) is estimated to be > 7x 10m4cm-’ for the ‘l-l, P-branch and > 1 X lo-’ for all other lines) Component
_4-doubling splitting
Line position ‘) measured
calculated
measured
predicted
P(l1) P(l0) P(8) P(7) P(5) P(4) P(3) P(1) R(l3) Wl8) R(20)
1678.1367 1681.7180 1688.7309 1692.1483 1698.8258 1702.1056 1705.3271 1711.8956 1755.552 1768.929 1774.025
1678.15 1681.73 1688.73 1692.15 1698.84 1702.11 1705.32
0.035 1 0.0371 0.0381 0.0356 0.0251 0.0191 0.0101 (0.0081) b, 0.060 0.051 (0.048) b’
0.0493 0.0507 0.0509 0.0491 0.0401 0.0325 0.0229 0.001 0.049 0.034 0.029
‘f-L,
P(8) P(7) P(5) R(l5) Nl9)
1686.815 1690.488 1697.618 1763.029 1773.560
1686.83 1690.46 1697.58 1763.04 1773.62
0.058 0.039 0.032 0.020 0.008
0.0569 [ 61 0.057 0.051 0.019 0.008
‘ff*
P(l1) P(6) P(3) R(3) Nl4) Nl5) Nl8) R( 19)
1672.747 1691.941 1703.349 1728.691 1762.557 1765.277 1773.226 1775.801
1672.76 1691.97 1703.33 1728.71 1762.57 1765.32 1773.29 1775.86
0.014 0.004
0.0131 0.0052 0.0009 0.004 1 0.0120 0.0121 0.0118
‘fLl
1755.58 1768.97 1774.08
0.011 0.015 0.012 0.013
a) Mean of the lambda-doubled pair. ‘) Ambiguous measurement, possible interference from another transition.
excellent agreement with the reported Q= 1 and 2 doublings from the molecular beam work [2,5]and the spacings predicted for the J= 1+Cl microwave transitions [ 71 and gave confidence in the O= 0 calculations. The calculated splittings for the IR spectra are given in table 2. Relatively poor agreement is found for the IR=O spectrum, with calculated splittings consistently 0.0 12-0.015 cm- ’ larger than measured for the P branch. Nevertheless, the J dependence of the splittings ib well predicted by the calculations, rising to a maximum around P(9) in the Q=O P branch. The absolute deviation corresponds to over two linewidths and is outside the estimated experimental accuracy. Since the calculated values are highly reliable for Q= 1 and 2 it is concluded that this is possible evidence for perturbation
of the lower vibrational levels of a 311CO as alluded to by Saykally et al. [ 7,8]. Further measurements would help us to confirm this; it is clear from these preliminary results, however, that a large experimental data base can be accumulated from the IR spectra for a thorough investigation of the effects of perturbations. The observation of spectra of CO a 311 at room temperature contrasts with the microwave study of CO dc discharges, which required liquid-nitrogen cooling. Linewidths were typically 5 x 1O-3 cm- ’ (fwhm) compared with a room-temperature Doppler width of 4 x 1O- 3 cm- ‘, suggesting a low translational temperature. However, a quantitative deduction is not possible as the line profiles show some asymmetry at their base (fig. 2), possibly 529
Volume 136, number 6
CHEMICAL PHYSICS LETTERS
CO a%,
22 May 1987
frequencies from 1 to 100 kHz are available with this apparatus.
P(10) F
Acknowledgement
ocs
-dt
We thank the SERC and the Royal Society for equipment grants and the SERC and Spectra Physics UK for a CASE Studentship for PAM.
References
I
I 1681.700 cm-l
,750
Fig. 2. Lambda doublet components of the P( 10) transition in the fundamental band of a ‘II, CO. The reference gas line position is 1681.7725 cm-’ [ 1 I] and was used to calibrate the CO lines with the aid of the germanium etalon pattern (free spectral range 0.01616f0.0004 cm-‘) shown in the bottom trace.
caused by distortion in the discharge waveform. No hot band lines have yet been positively identified in searches based on predictions from the optically derived parameters. In addition to a more extensive study of the fundamental band, two further aspects of this work are worth investigating. Firstly, the detection of “crossstack” transitions which will yield a more direct measurement of the spin-orbit constant than previously available. Secondly, the use of an ac discharge should enable the dynamics of the decay of the metastable to be followed. Variable modulation
530
[ 1 ] S.J. Tilford and J.D. Simmons, J. Phys. Chem. Ref. Data 1 (1972) 147. [ 21 R.W. Field, S.G. Tilford, R.A. Howard and J.D. Simmons, J. Mol. Spectry. 44 (1972) 347. [ 3 ] M.A.A. Clyne and M.C. Heaven, J. Chem. Sot. Faraday Trans. II 79 (1981) 1375. [ 41 R.S. Freund and W. Klemperer, J. Chem. Phys. 43 (1965) 2422. [5] R.C. Stem, R.H. Gammon, M.E. Lesk, R.S. Freund and W.A. Klemperer, J. Chem. Phys. 52 (1970) 3467. [ 61 R.J. Saykally, Ph.D. thesis, University of Wisconsin (1977). [ 71 R.J. Saykally, KM. Evenson, E.R. Comben and J.M. Brown, Mol. Phys. 58 (1986) 735. [ 81 R.C. Wood, unpublished work. [9] P.B. Davies, S.A. Johnson, P.A. Hamilton andT.J. Sears, J: Chem. Phys. 108 (1986) 335. [lo] P.B. Davies, P.R. Brown and S.A. Johnson, Chem. Phys. Letters 133 (1987) 239. [ 111 A.G. Maki, W.B. Olsen and R.L. Sams, J. Mol. Spectry. 8 1 (1980) 122. [ 121 A. Valentin, A. Henry, Ph. Cardinet, M.F. LeMoal, Da-Wun Chen and R.N. Rao, J. Mol. Spectry. 70 (1978) 9. [ 131 A. Hinz, J.S. Wells and A.G. Maki, J. Mol. Spectry. 119 (1986) 120. [ 141 A.W. Mantz, E.R. Nichols, B.D. Alpert and R.N. Rao, J. Mol. Spectry. 35 (1970) 325. [ 15 ] G. He&erg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950).