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High-Resolution Laser Spectroscopy of YbCl: The B2Σ+–X2Σ+ Transition
Journal of Molecular Spectroscopy 206, 161–165 (2001) doi:10.1006/jmsp.2000.8281, available online at http://www.idealibrary.com on
High-Resolution Laser Spectroscopy of YbCl: The B2 Σ+ –X 2 Σ+ Transition C. Linton∗ and A. G. Adam† ∗ Physics Department and †Chemistry Department, University of New Brunswick, P. O. Box 4400, Fredericton, New Brunswick, Canada E3B 5A3 E-mail: [email protected], [email protected] Received November 2, 2000
High-resolution laser excitation spectra have been obtained for the 0–0, 1–1, and 0–1 bands of the B 2 6 + –X 2 6 + transition of YbCl and a rotational analysis has been performed on the 174 Yb35 Cl and 172 Yb35 Cl isotopomers. Comparison of the spin–rotation constant, γ , for the B 2 6 + state with the lambda-doubling constant of the A2 51/2 state (1) shows that the two excited states form a unique perturber pair arising from the 6 pσ and 6 pπ orbitals centered on the Yb+ ion. The principal ˚ γe = −2.1655(6) × 10−4 cm−1 , and 1G 1/2 = results for the B 2 6 + state are Be = 0.097552(5) cm−1 , Re = 2.43623(6) A, C 2001 Academic Press 313.111(2) cm−1 . ° Key Words: laser spectroscopy; YbCl. INTRODUCTION
In a recent publication (1), we described the first highresolution analysis of the A2 5–X 2 6 + transition in YbCl. This followed previous studies of the equivalent transition in YbF (2– 5). These experiments are all part of a study of the ytterbium halides with the aim of understanding in greater detail the electronic structure and bonding properties of molecules with both open and closed f shells. In parallel with the experimental studies, there have also been several theoretical investigations into the properties of the electronic states of ytterbium halides (6–9). The rotational analysis of the A–X transition (1) yielded rotational and spin–rotation constants for the ground state and rotational, spin–orbit, and lambda-doubling constants for the A2 5 state. Both the A and X states were unperturbed and the spin rotation in the X state was regular and positive and could be accounted for by interaction with the A2 51/2 state. The lambda doubling in A2 51/2 was large but regular and neither the ground state spin–rotation splitting nor the A-state lambda-doubling showed any significant variation with vibration. This is in stark contrast with YbF where (i) the spin rotation in the ground state is negligible at v = 0, becoming increasingly negative with vibration and (ii) the A2 51/2 state is heavily perturbed at v = 1 and the lambda doubling is large and irregular, showing a large variation with vibration. Previous low-resolution work on the Yb halides has shown transitions to the blue of the A–X system originating from higher electronic states of the molecules (10–12). Of particular interest is a transition assigned as B 2 6 + –X 2 6 + which is in the 500-nm region for both YbF and YbCl. An early partial rotational analysis (2) and a more recent investigation in our laboratory both
indicate that the B state of YbF is heavily perturbed. This work is in progress and will be published at a later date. This transition is of interest as both the A and B states are thought to arise from the same configuration, Yb+ (4 f 14 6 p)X − , where X represents a halogen. The 5 and 6 states would thus arise from the 6 pπ and 6 pσ orbitals and the interaction between them would be primarily responsible for the lambda doubling in the A state and the spin–rotation splitting in the B state. A high-resolution analysis of the B–X transition, in particular the spin–rotation splitting, would therefore be critical in confirming the configurational parentage of the A and B states and determining the extent of configurational mixing. This article describes the first high-resolution observation and analysis of the B 2 6 + –X 2 6 + transition in YbCl.
EXPERIMENTAL METHOD
Gas-phase YbCl was produced in a Broida oven as previously described (1) by entraining the vapor from a heated mixture of YbCl3 and Al in argon at a pressure of ∼0.2 Torr. The molecules were excited using a Coherent 699-29 ring laser operating in single-frequency mode with Coumarin 480 dye and the resulting fluorescence was detected by a Jarrell Ash 0.5-m monochromator. The laser frequency was scanned using the Coherent “Autoscan” software which also measured the line positions in the excitation spectra. The spectrum was calibrated with an iodine fluorescence spectrum which was simultaneously recorded with the YbCl data and compared with the standard I2 atlas (13). The linewidths in the excitation spectrum were ∼0.02 cm−1 and the measurement accuracy was ∼0.004 cm−1 .
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RESULTS
Excitation spectra of the 0–0, 1–1, and 0–1 bands of the B 2 6 + –X 2 6 + transition of YbCl were obtained. The band structure is typical of 2 6–2 6 transitions (14), consisting of six branches, (P11 + Q 12 ) and R11 from the F1 component of the upper state and P22 and (R22 + Q 21 ) from the F2 component of the upper state. When the spin–rotation splitting in the lower state is not resolved, the branches inside parentheses are blended. If the upper state spin–rotation splitting is not resolved, then Q 12 + P22 and R11 + Q 21 will be blended. A spectrum of the head region of the 0–0 band is shown in Fig. 1a. It is observed that the P11 + Q 12 and P22 heads are clearly separated which indicates significant spin rotation splitting in the B 2 6 + state. While the region between the P11 + Q 12 and P22 heads contains only the P11 and Q 12 branches, the region to the high-frequency side of the P22 head consists of the other four branches. When this is combined with the fact that Yb has six stable isotopes of significant abundance and chlorine has two isotopes, it is evident that the overlapping in this region is so severe that assignment would be almost impossible without using very careful selective detection to try to isolate individual branches. Thus, the spectra were obtained in seg-
ments with the monochromator detecting a narrow region of one branch while the laser was scanned in another branch originating from the same upper state. In Fig. 1b the monochromator was set to detect in a region consisting of intermediate N lines of R22 + Q 21 and the higher N lines of the R11 branch, and the laser was scanned in the same region as in Fig. 1a. The slits were narrowed to limit the number of transitions detected and comparison of the two spectra shows that we have succeeded in isolating a limited number of transitions in each of the P11 + Q 12 and P22 branches. Similarly, by setting the monochromator to detect the P11 + Q 12 and P22 branches, we could obtain clear spectra of the R11 and R22 + Q 21 branches. In this way, we were able to obtain extensive branch-by-branch spectra of the 0–0, 1–1, and 0–1 bands and assign the lines. To isolate the 1–1 band, which was strongly overlapped with the R branches of the 0–0 band, we set the monochromator to detect the 1–0 band. The spin–rotation splitting in the ground state has been discussed in detail in the previous study of the A2 5–X 2 6 + transition (1). As N increased, this splitting was also resolved in the present B–X spectra and the spin–rotation doubling of the rotational lines in the “blended” branches was used to definitively assign the two heads to the correct spin–rotation component,
FIG. 1. The P11 + Q 12 and P22 branches in the 0–0 band of the B 2 6 + –X 2 6 + transition of YbCl: (a) Recorded with the monochromator detecting the head region of the 0–1 band with wide slits. The P11 and Q 12 heads are separated by the spin–rotation splitting in the lower state and the single P22 head is clearly seen at 19 940 cm−1 . (b) Recorded with the monochromator detecting in the R11 and R22 + Q 21 region with narrow slits. Because of the narrow detection range, only lines in the P11 + Q 12 from N = 45−56 and in P22 from N = 29−38 are recorded.
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Selective detection of the R11 and R22 + Q 21 branches in the 0–0 band of the B 2 6 + –X 2 6 + transition of YbCl. The “triplets” are transitions of the and 176 Yb35 Cl isotopomers. The numbers above the spectra are the N values of the lower state of the transitions. The top spectrum shows a portion of the R11 branch recorded with the monochromator detecting the P11 + Q 12 branch. The bottom spectrum, in the same region, shows the Q 21 and R22 branches, with the ground state spin–rotation splitting clearly resolved, recorded with the monochromator detecting the P22 branch. FIG. 2.
172 Yb35 Cl, 174 Yb35 Cl,
F1 and F2 , in the upper state. We used the R branches, as the spectral congestion was considerably less than that in the headforming P branches and it was easier to detect the doubling of the rotational lines. The monochromator was set in each of the bandhead regions and the laser scanned at higher frequency to excite the corresponding R branches. With the monochromator on the lower frequency head at ∼19 935 cm−1 , each rotational transition in the corresponding R branch appeared as a triplet in which the three lines corresponded to the three most abundant isotopomers. However, when detected at the higher frequency head at ∼19 940 cm−1 , each rotational line appeared as a triplet of doubled lines at lower N or as two sets of triplets at higher N , each line having been doubled by the ground-state spin rotation. This is shown in Fig. 2. The single groups of lines in the upper spectrum must correspond to the R11 branch and therefore come from the F1 component of the B state and the doubled groups of lines in the lower spectrum belong to the R22 + Q 21 branches arising from the F2 component. Thus the 19 935 cm−1 head is from F1 (i.e., P11 + Q 12 ) and the 19 940 cm−1 head is from F2 (P22 ).
DISCUSSION
In Fig. 1a, the main heads are ∼5 cm−1 apart, which is very large for a 2 6–2 6 transition. It is probably for this reason that Uttam et al. (12), in their low-resolution photographic spectra, assigned them as two separate transitions, B1 –X and B2 –X . The rotational analysis described above confirms the transition as 2 + 2 + 6 – 6 and the 5 cm−1 separation of the heads is due partly to the large spin–rotation splitting in the B state plus the fact that, while the P22 head forms at very low N , the P11 head forms at N ∼ 40, where the ground state spin–rotation splitting is already resolved, so that we observe, in fact, two closely spaced heads, P11 and Q 12 . Once the transitions had been assigned, the analysis was straightforward, as there were no observed perturbations in any of the bands. This is in stark contrast to YbF where preliminary experiments in our laboratory have shown very large local perturbations in the B 2 6 + state. The state or states responsible for the perturbation in the B state in YbF have not yet been determined.
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TABLE 1 Molecular Constants (cm−1 ) for the X 2 Σ+ and B2 Σ+ States of YbCl
The lines were assigned the 0–0, 1–1, and 0–1 bands of the Yb35 Cl and 172 Yb35 Cl isotopomers with the aid of X 2 6 + state combination differences from the A2 5–X 2 6 + transition (1) and by checking that combination differences involving common levels were equal. A nonlinear least-squares fitting technique was used to fit the three bands individually using the standard expressions for 2 6 states (14): 174
F1 (N ) = B N (N + 1) − D{N (N + 1)}2 + 1/2γ N + 1/2γD N 2 (N + 1) F2 (N ) = B N (N + 1) − D{N (N + 1)}2 − 1/2γ (N + 1) + 1/2γD N (N + 1)2 . When a satisfactory fit had been obtained for each band, the bands were fit simultaneously to give a global set of molecular parameters. It was found that γ D , the distortion correction to the spin rotation constant, was well determined only for the B 2 6 + state and was thus fixed at zero for the ground state. The ground state constants were in excellent agreement with those obtained from the A2 5–X 2 6 + analysis. In the final fit, the ground state v = 0 constants were fixed at the A–X values and this made no significant difference to the results obtained from the fit. The v” = 1 parameters were not fixed as, in the A–X analysis, the transitions to states with v” > 0 were derived from measurements of dispersed fluorescence spectra, which are not as precise as those obtained from excitation spectra in the present experiments. The final molecular parameters are listed in Table 1 and the principal equilibrium constants are gathered in Table 2. For the ground state, Be and αe have been calculated from the present data rather than from the A–X data (1) as our v > 0 data are more precise. The isotopic ratios of vibrational and rotational constants (1G 1/2 and Be ) are also listed in Table 2 and can be seen to satisfy the isotope relations (14) for both states. Though the ratios of higher order constants, αe , De , etc., also satisfy the isotope relations, the uncertainties in the ratios are large enough to render comparison meaningless. The parameters of primary interest in the present work are the spin–rotation constants. As discussed in detail in Ref. (1), the ground state spin–rotation can be satisfactorily explained by the interaction of the Yb+ 6 pπ orbital in the A2 5 state with the small fraction of Yb+ 6 pσ mixed into the predominantly 6sσ orbital in the ground state. If, as predicted, the spin rotation of the B 2 6 + state and the 3 doubling in the A2 5 state are due to the “unique perturber” interaction between the B 2 6 + and A2 5 states arising from the Yb+ 6 pσ and 6 pπ configurations, then the 3-doubling constant p of the A state should be equal to γ , the spin–rotation constant of the B state. The values for 174 Yb35 Cl from Table 1 of γ = −0.21615(2) cm−1 for v = 0 and −0.21535(4) cm−1 for v = 1 compare very well with the A-state 3-doubling constants of p = −0.2175(1) cm−1 (v = 0) and −0.2168(1) cm−1 (v = 1). For 172 Yb35 Cl, the corresponding values for v = 0 and 1 are γ = −0.21662(2) and −0.21565(5)
1 2
Top entry 174 Yb35 Cl, lower entry 172 Yb35 Cl. Numbers in parentheses represent the standard error.
and p = −0.2180(1) cm−1 and −0.2173(1) cm−1 . As discussed in Ref. (1), a theoretical value of −0.28 cm−1 was obtained by assuming that the states were in “pure precession” (15) arising from a pure 6 p orbital. This is about 30% higher than the experimental value, indicating that the states are not in pure precession but arise from mixed configurations that are predominantly 6 p. It has already been mentioned that, in YbF, the B state is heavily perturbed. This suggests that there is an interacting state from a different configuration very close in energy to the B state. This state must surely exist in YbCl, for which no perturbations are observed, but it must be sufficiently separated in energy so that the interaction is negligible. It was pointed out in Ref. (1) that, for the A2 51/2 state, the perturber is thought to be the Ä = 1/2 state from the Yb+ (4 f 13 6s) configuration. Ligand field theoretical calculations (6) place this state considerably higher in YbCl than in YbF. At present, the perturber of the B state in YbF is not known but it is expected that the same arguments
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TABLE 2 Principal Constants (cm−1 ) for the X2 Σ+ and B2 Σ+ States of YbCl
1 Numbers
in parentheses represent the standard error.
regarding the relative energies of the two states in YbF and YbCl would still apply. CONCLUSIONS
We have presented above the first high-resolution analysis of the B 2 6 + –X 2 6 + transition of YbCl. The results for the ground state correlate very well with those obtained from the A2 5–X 2 6 + transition (1). For the B state we have shown that the spin–rotation constant is almost the same as the A-state 3doubling constant confirming that both are caused by the interaction between the two states. Both states are well behaved and show none of the perturbations observed in YbF. Further work is required to gain a more complete understanding of the excited states and experiments on the transitions in the 450-nm region are planned together with an analysis of the hyperfine structure in the isotopomers containing 171 Yb and 173 Yb. ACKNOWLEDGMENTS We thank Joyce McBride, Cicely Brodie, David Matheson, and Michael Dick for their assistance at various stages of this project. The research was supported by grants from the Natural Science and Engineering Research Council of Canada.
REFERENCES 1. T. C. Melville, J. A. Coxon, and C. Linton, J. Mol. Spectrosc. 200, 229–234 (2000). 2. R. F. Barrow and A. H. Chojnicki, J. Chem. Soc. Faraday Trans. 2 71, 728–735 (1975). 3. 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). 4. B. E. Sauer, J. Wang, and E. A. Hinds, Phys. Rev. Lett. 74, 1554–1557 (1995). 5. B. E. Sauer, J. Wang, and E. A. Hinds, J. Chem. Phys. 105, 7412–7420 (1996). 6. A. L. Kaledin, M. C. Heaven, R. W. Field, and L. A. Kaledin, J. Mol. Spectrosc. 179, 310–319 (1996). 7. M. Dolg, H. Stoll, and H. Preuss, Chem. Phys. 165, 21–30 (1992). 8. S. G. Wang and W. H. E. Schwarz, J. Phys. Chem. 99, 11,687 (1995). 9. W. Lu, M. Dolg, and L. Li, J. Chem. Phys. 108, 2886–2895 (1998). 10. H. U. Lee and R. N. Zare, J. Mol. Spectrosc. 64, 233–243 (1977). 11. J. Kramer, J. Chem. Phys. 68, 5370–5377 (1978). 12. K. N. Uttam, R. Gopal, and M. M. Joshi, J. Mol. Spectrosc. 185, 8–14 (1997). 13. S. Gerstenkorn and P. Luc, “Atlas du Spectre d’Absorption de la Molecule d’Iode,” Laboratoire Aim´e-Cotton, CNRS II, Orsay, 1978. 14. G. Herzberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” Van Nostrand, Princeton, NJ, 1950. 15. H. Lefebvre-Brion and R. W. Field, “Perturbations in the Spectra of Diatomic Molecules,” Academic Press, Orlando, FL, 1986.
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High-Resolution Laser Spectroscopy of YbCl: The B2 Σ+ –X 2 Σ+ Transition C. Linton∗ and A. G. Adam† ∗ Physics Department and †Chemistry Department, University of New Brunswick, P. O. Box 4400, Fredericton, New Brunswick, Canada E3B 5A3 E-mail: [email protected], [email protected] Received November 2, 2000
High-resolution laser excitation spectra have been obtained for the 0–0, 1–1, and 0–1 bands of the B 2 6 + –X 2 6 + transition of YbCl and a rotational analysis has been performed on the 174 Yb35 Cl and 172 Yb35 Cl isotopomers. Comparison of the spin–rotation constant, γ , for the B 2 6 + state with the lambda-doubling constant of the A2 51/2 state (1) shows that the two excited states form a unique perturber pair arising from the 6 pσ and 6 pπ orbitals centered on the Yb+ ion. The principal ˚ γe = −2.1655(6) × 10−4 cm−1 , and 1G 1/2 = results for the B 2 6 + state are Be = 0.097552(5) cm−1 , Re = 2.43623(6) A, C 2001 Academic Press 313.111(2) cm−1 . ° Key Words: laser spectroscopy; YbCl. INTRODUCTION
In a recent publication (1), we described the first highresolution analysis of the A2 5–X 2 6 + transition in YbCl. This followed previous studies of the equivalent transition in YbF (2– 5). These experiments are all part of a study of the ytterbium halides with the aim of understanding in greater detail the electronic structure and bonding properties of molecules with both open and closed f shells. In parallel with the experimental studies, there have also been several theoretical investigations into the properties of the electronic states of ytterbium halides (6–9). The rotational analysis of the A–X transition (1) yielded rotational and spin–rotation constants for the ground state and rotational, spin–orbit, and lambda-doubling constants for the A2 5 state. Both the A and X states were unperturbed and the spin rotation in the X state was regular and positive and could be accounted for by interaction with the A2 51/2 state. The lambda doubling in A2 51/2 was large but regular and neither the ground state spin–rotation splitting nor the A-state lambda-doubling showed any significant variation with vibration. This is in stark contrast with YbF where (i) the spin rotation in the ground state is negligible at v = 0, becoming increasingly negative with vibration and (ii) the A2 51/2 state is heavily perturbed at v = 1 and the lambda doubling is large and irregular, showing a large variation with vibration. Previous low-resolution work on the Yb halides has shown transitions to the blue of the A–X system originating from higher electronic states of the molecules (10–12). Of particular interest is a transition assigned as B 2 6 + –X 2 6 + which is in the 500-nm region for both YbF and YbCl. An early partial rotational analysis (2) and a more recent investigation in our laboratory both
indicate that the B state of YbF is heavily perturbed. This work is in progress and will be published at a later date. This transition is of interest as both the A and B states are thought to arise from the same configuration, Yb+ (4 f 14 6 p)X − , where X represents a halogen. The 5 and 6 states would thus arise from the 6 pπ and 6 pσ orbitals and the interaction between them would be primarily responsible for the lambda doubling in the A state and the spin–rotation splitting in the B state. A high-resolution analysis of the B–X transition, in particular the spin–rotation splitting, would therefore be critical in confirming the configurational parentage of the A and B states and determining the extent of configurational mixing. This article describes the first high-resolution observation and analysis of the B 2 6 + –X 2 6 + transition in YbCl.
EXPERIMENTAL METHOD
Gas-phase YbCl was produced in a Broida oven as previously described (1) by entraining the vapor from a heated mixture of YbCl3 and Al in argon at a pressure of ∼0.2 Torr. The molecules were excited using a Coherent 699-29 ring laser operating in single-frequency mode with Coumarin 480 dye and the resulting fluorescence was detected by a Jarrell Ash 0.5-m monochromator. The laser frequency was scanned using the Coherent “Autoscan” software which also measured the line positions in the excitation spectra. The spectrum was calibrated with an iodine fluorescence spectrum which was simultaneously recorded with the YbCl data and compared with the standard I2 atlas (13). The linewidths in the excitation spectrum were ∼0.02 cm−1 and the measurement accuracy was ∼0.004 cm−1 .
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RESULTS
Excitation spectra of the 0–0, 1–1, and 0–1 bands of the B 2 6 + –X 2 6 + transition of YbCl were obtained. The band structure is typical of 2 6–2 6 transitions (14), consisting of six branches, (P11 + Q 12 ) and R11 from the F1 component of the upper state and P22 and (R22 + Q 21 ) from the F2 component of the upper state. When the spin–rotation splitting in the lower state is not resolved, the branches inside parentheses are blended. If the upper state spin–rotation splitting is not resolved, then Q 12 + P22 and R11 + Q 21 will be blended. A spectrum of the head region of the 0–0 band is shown in Fig. 1a. It is observed that the P11 + Q 12 and P22 heads are clearly separated which indicates significant spin rotation splitting in the B 2 6 + state. While the region between the P11 + Q 12 and P22 heads contains only the P11 and Q 12 branches, the region to the high-frequency side of the P22 head consists of the other four branches. When this is combined with the fact that Yb has six stable isotopes of significant abundance and chlorine has two isotopes, it is evident that the overlapping in this region is so severe that assignment would be almost impossible without using very careful selective detection to try to isolate individual branches. Thus, the spectra were obtained in seg-
ments with the monochromator detecting a narrow region of one branch while the laser was scanned in another branch originating from the same upper state. In Fig. 1b the monochromator was set to detect in a region consisting of intermediate N lines of R22 + Q 21 and the higher N lines of the R11 branch, and the laser was scanned in the same region as in Fig. 1a. The slits were narrowed to limit the number of transitions detected and comparison of the two spectra shows that we have succeeded in isolating a limited number of transitions in each of the P11 + Q 12 and P22 branches. Similarly, by setting the monochromator to detect the P11 + Q 12 and P22 branches, we could obtain clear spectra of the R11 and R22 + Q 21 branches. In this way, we were able to obtain extensive branch-by-branch spectra of the 0–0, 1–1, and 0–1 bands and assign the lines. To isolate the 1–1 band, which was strongly overlapped with the R branches of the 0–0 band, we set the monochromator to detect the 1–0 band. The spin–rotation splitting in the ground state has been discussed in detail in the previous study of the A2 5–X 2 6 + transition (1). As N increased, this splitting was also resolved in the present B–X spectra and the spin–rotation doubling of the rotational lines in the “blended” branches was used to definitively assign the two heads to the correct spin–rotation component,
FIG. 1. The P11 + Q 12 and P22 branches in the 0–0 band of the B 2 6 + –X 2 6 + transition of YbCl: (a) Recorded with the monochromator detecting the head region of the 0–1 band with wide slits. The P11 and Q 12 heads are separated by the spin–rotation splitting in the lower state and the single P22 head is clearly seen at 19 940 cm−1 . (b) Recorded with the monochromator detecting in the R11 and R22 + Q 21 region with narrow slits. Because of the narrow detection range, only lines in the P11 + Q 12 from N = 45−56 and in P22 from N = 29−38 are recorded.
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Selective detection of the R11 and R22 + Q 21 branches in the 0–0 band of the B 2 6 + –X 2 6 + transition of YbCl. The “triplets” are transitions of the and 176 Yb35 Cl isotopomers. The numbers above the spectra are the N values of the lower state of the transitions. The top spectrum shows a portion of the R11 branch recorded with the monochromator detecting the P11 + Q 12 branch. The bottom spectrum, in the same region, shows the Q 21 and R22 branches, with the ground state spin–rotation splitting clearly resolved, recorded with the monochromator detecting the P22 branch. FIG. 2.
172 Yb35 Cl, 174 Yb35 Cl,
F1 and F2 , in the upper state. We used the R branches, as the spectral congestion was considerably less than that in the headforming P branches and it was easier to detect the doubling of the rotational lines. The monochromator was set in each of the bandhead regions and the laser scanned at higher frequency to excite the corresponding R branches. With the monochromator on the lower frequency head at ∼19 935 cm−1 , each rotational transition in the corresponding R branch appeared as a triplet in which the three lines corresponded to the three most abundant isotopomers. However, when detected at the higher frequency head at ∼19 940 cm−1 , each rotational line appeared as a triplet of doubled lines at lower N or as two sets of triplets at higher N , each line having been doubled by the ground-state spin rotation. This is shown in Fig. 2. The single groups of lines in the upper spectrum must correspond to the R11 branch and therefore come from the F1 component of the B state and the doubled groups of lines in the lower spectrum belong to the R22 + Q 21 branches arising from the F2 component. Thus the 19 935 cm−1 head is from F1 (i.e., P11 + Q 12 ) and the 19 940 cm−1 head is from F2 (P22 ).
DISCUSSION
In Fig. 1a, the main heads are ∼5 cm−1 apart, which is very large for a 2 6–2 6 transition. It is probably for this reason that Uttam et al. (12), in their low-resolution photographic spectra, assigned them as two separate transitions, B1 –X and B2 –X . The rotational analysis described above confirms the transition as 2 + 2 + 6 – 6 and the 5 cm−1 separation of the heads is due partly to the large spin–rotation splitting in the B state plus the fact that, while the P22 head forms at very low N , the P11 head forms at N ∼ 40, where the ground state spin–rotation splitting is already resolved, so that we observe, in fact, two closely spaced heads, P11 and Q 12 . Once the transitions had been assigned, the analysis was straightforward, as there were no observed perturbations in any of the bands. This is in stark contrast to YbF where preliminary experiments in our laboratory have shown very large local perturbations in the B 2 6 + state. The state or states responsible for the perturbation in the B state in YbF have not yet been determined.
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TABLE 1 Molecular Constants (cm−1 ) for the X 2 Σ+ and B2 Σ+ States of YbCl
The lines were assigned the 0–0, 1–1, and 0–1 bands of the Yb35 Cl and 172 Yb35 Cl isotopomers with the aid of X 2 6 + state combination differences from the A2 5–X 2 6 + transition (1) and by checking that combination differences involving common levels were equal. A nonlinear least-squares fitting technique was used to fit the three bands individually using the standard expressions for 2 6 states (14): 174
F1 (N ) = B N (N + 1) − D{N (N + 1)}2 + 1/2γ N + 1/2γD N 2 (N + 1) F2 (N ) = B N (N + 1) − D{N (N + 1)}2 − 1/2γ (N + 1) + 1/2γD N (N + 1)2 . When a satisfactory fit had been obtained for each band, the bands were fit simultaneously to give a global set of molecular parameters. It was found that γ D , the distortion correction to the spin rotation constant, was well determined only for the B 2 6 + state and was thus fixed at zero for the ground state. The ground state constants were in excellent agreement with those obtained from the A2 5–X 2 6 + analysis. In the final fit, the ground state v = 0 constants were fixed at the A–X values and this made no significant difference to the results obtained from the fit. The v” = 1 parameters were not fixed as, in the A–X analysis, the transitions to states with v” > 0 were derived from measurements of dispersed fluorescence spectra, which are not as precise as those obtained from excitation spectra in the present experiments. The final molecular parameters are listed in Table 1 and the principal equilibrium constants are gathered in Table 2. For the ground state, Be and αe have been calculated from the present data rather than from the A–X data (1) as our v > 0 data are more precise. The isotopic ratios of vibrational and rotational constants (1G 1/2 and Be ) are also listed in Table 2 and can be seen to satisfy the isotope relations (14) for both states. Though the ratios of higher order constants, αe , De , etc., also satisfy the isotope relations, the uncertainties in the ratios are large enough to render comparison meaningless. The parameters of primary interest in the present work are the spin–rotation constants. As discussed in detail in Ref. (1), the ground state spin–rotation can be satisfactorily explained by the interaction of the Yb+ 6 pπ orbital in the A2 5 state with the small fraction of Yb+ 6 pσ mixed into the predominantly 6sσ orbital in the ground state. If, as predicted, the spin rotation of the B 2 6 + state and the 3 doubling in the A2 5 state are due to the “unique perturber” interaction between the B 2 6 + and A2 5 states arising from the Yb+ 6 pσ and 6 pπ configurations, then the 3-doubling constant p of the A state should be equal to γ , the spin–rotation constant of the B state. The values for 174 Yb35 Cl from Table 1 of γ = −0.21615(2) cm−1 for v = 0 and −0.21535(4) cm−1 for v = 1 compare very well with the A-state 3-doubling constants of p = −0.2175(1) cm−1 (v = 0) and −0.2168(1) cm−1 (v = 1). For 172 Yb35 Cl, the corresponding values for v = 0 and 1 are γ = −0.21662(2) and −0.21565(5)
1 2
Top entry 174 Yb35 Cl, lower entry 172 Yb35 Cl. Numbers in parentheses represent the standard error.
and p = −0.2180(1) cm−1 and −0.2173(1) cm−1 . As discussed in Ref. (1), a theoretical value of −0.28 cm−1 was obtained by assuming that the states were in “pure precession” (15) arising from a pure 6 p orbital. This is about 30% higher than the experimental value, indicating that the states are not in pure precession but arise from mixed configurations that are predominantly 6 p. It has already been mentioned that, in YbF, the B state is heavily perturbed. This suggests that there is an interacting state from a different configuration very close in energy to the B state. This state must surely exist in YbCl, for which no perturbations are observed, but it must be sufficiently separated in energy so that the interaction is negligible. It was pointed out in Ref. (1) that, for the A2 51/2 state, the perturber is thought to be the Ä = 1/2 state from the Yb+ (4 f 13 6s) configuration. Ligand field theoretical calculations (6) place this state considerably higher in YbCl than in YbF. At present, the perturber of the B state in YbF is not known but it is expected that the same arguments
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THE B 2 6 + –X 2 6 + TRANSITION OF YbCl
165
TABLE 2 Principal Constants (cm−1 ) for the X2 Σ+ and B2 Σ+ States of YbCl
1 Numbers
in parentheses represent the standard error.
regarding the relative energies of the two states in YbF and YbCl would still apply. CONCLUSIONS
We have presented above the first high-resolution analysis of the B 2 6 + –X 2 6 + transition of YbCl. The results for the ground state correlate very well with those obtained from the A2 5–X 2 6 + transition (1). For the B state we have shown that the spin–rotation constant is almost the same as the A-state 3doubling constant confirming that both are caused by the interaction between the two states. Both states are well behaved and show none of the perturbations observed in YbF. Further work is required to gain a more complete understanding of the excited states and experiments on the transitions in the 450-nm region are planned together with an analysis of the hyperfine structure in the isotopomers containing 171 Yb and 173 Yb. ACKNOWLEDGMENTS We thank Joyce McBride, Cicely Brodie, David Matheson, and Michael Dick for their assistance at various stages of this project. The research was supported by grants from the Natural Science and Engineering Research Council of Canada.
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