Magnetic Hyperfine Structure in theX2Σ State of91ZrN

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

176, 219–221 (1996)

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NOTE Magnetic Hyperfine Structure in the X 2S State of Two band systems of ZrN, the ultraviolet and the yellow, have been observed and assigned as B2S –X2S and A2P – X2S transitions, respectively, by Bates and Dunn (1). Using matrix isolation spectroscopy, Bates and Gruen (2) have confirmed the lower state of those transitions is the ground state. Severe perturbations have been observed in the v Å 0 level of the 2P3/2 component in the A2P –X2S transition. In our experiment to record the A–X transition in high resolution using laser induced fluorescence spectroscopy to study the origin of these perturbations, we have observed the magnetic hyperfine structure of the ground state of 91ZrN. The hyperfine splitting conforms to the case (bbs) scheme, which is common to 2S state. We have produced ZrN molecules in an oven system (3) by chemical reaction of vaporized ZrCl4 , with a natural abundance of the five isotopes [90Zr (51.45%), 91Zr (11.23%), 92Zr (17.11%), 94Zr (17.40%), and 96Zr (2.8%)], and microwave discharged nitrogen gas. The laser induced fluorescence spectrum of the A–X (0, 0) band has been observed using a c.w. ring dye laser operating with rhodamine 560 and 590 dyes and pumped by an argon ion laser. The spectral resolution of such experiment is about 0.003 cm01, which is determined by the Doppler width of ZrN. The temperature of the molecule produced is estimated to be 550 K. Further experimental details can be found in previous

91

ZrN

publications ( 4, 5). Doppler limited spectra were recorded over the frequency regions of 17 030–17 190 cm01 and 17 600–17 760 cm01 by concatenating successive laser scans of 1 cm01. The laser induced fluorescence spectrum of the A2P –X2S ˚ and 5835 A ˚, (0, 0) transition shows strong heads at 5647 A 2 2 2 2 which correspond to the P3/2 – S and P1/2 – S subband transitions (1). Although the observed spectrum is complicated due to multiple isotopic species, transition lines from individual species of the 2P1/2 – 2S subband are easily identified at low N values. Figure 1 shows the R1 (6), R1 (7), and R1 (8) lines of this subband. The strongest line is due to the 90 ZrN, which is the most abundant isotope. As N increases in the R1 branch, the transitions corresponding to different isotopic species are resolved. The interesting feature of these spectra is the splitting into two components of the rotational lines belonging to the 91ZrN isotope. We have searched our spectra for branches of the 91ZrN that are subjected to minimum or no overlap. The result is listed in Table 1. The R1 , Q1 , and P1 branches can be followed to N £ 22 before overlapping of isotopic lines becomes serious. Because the low N levels of the 2P3/2 substate are severely perturbed, only a few members of the R21 branch were identified. This doublet structure is due to the magnetic nucleus of 91Zr atom which has nuclear spin I Å 52 and magnetic moment of

FIG. 1. Laser induced fluorescence spectrum of ZrN showing the R1(N) lines of the 2P1/2 – 2S subband. The 91ZrN (I Å 52) lines split into two compnents which are designated by the quantum number G.

219 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

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TABLE 1 Rotational Lines Assigned in the (0, 0) Band of the A2P –X2S transition of 91ZrN in cm01

01.303 (6). The splittings between the doublets are independent of the rotational quantum number N within our experimental uncertainty and the splitting is 0.198 { 0.003 cm01. Table 2 summarizes this result. The invariance of these separations with N indicates that the nuclear hyperfine interaction in the A2P state is negligible and the coupling of the nuclear spin angular moment with the X2S state conforms to case (bbs) scheme. This situation is similar to that of ScO and TiN molecules studied (7–9). If the magnitude of the hyperfine coupling is significant in the A2P state, the expected cou-

pling scheme can be either case (aa) or case (ab). Both these cases will produce a larger number of hyperfine components and in the case of (ab) the hyperfine splittings would decrease rapidly with N (10). Since none of these is observed in our spectra, we conclude that the hyperfine splitting in the A2P state is negligible. The observed hyperfine structure is that of the X2S state only. In the case (bbs) limit, the nuclear spin angular momentum, I, first couples to the electronic spin angular moment, S, to produce a resultant G which, in turn, couples to the rotational angular momentum, N, to yield the

TABLE 2 Separation between the G Å 2 and G Å 3 Hyperfine Levels of the X2S State of 91ZrN in cm01

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total angular momentum F. The nuclear hyperfine Hamiltonian for a 2S state consists of contributions from the Fermi contact and the spin–dipolar interactions. The value of the spin–dipolar parameter is estimated from the atomic parameter to be õ45 MHz (6), so its contribution to the observed hyperfine structure is negligible. With only the Fermi contact interaction, the first order correction to the energy is given by (9) E1(hfs.bbs) Å »NGFÉbF IO .SO ÉNGF …

[1]

bF Å [G(G / 1) 0 S(S / 1) 0 I(I / 1)], 2 where bF is the Fermi contact parameter. For I Å 52 and S Å 21, G can take the values of 2 and 3, and each G value has (2G / 1) F components. When the hyperfine splitting is much larger than the spin – rotation splitting, the energy levels of the X2S are those of nearly a pure case (bbs) molecule with G being the approximately good quantum number. Furthermore, the hyperfine splitting is independent of the rotational quantum number N. By examining the splitting of these doublets with respect to the unsplit line position, it is possible to assign, in 91ZrN, the line with lower frequency as G Å 2 and the higher frequency as G Å 3. This is because the 12 hyperfine components are distributed as 5 and 7 among the G Å 2 and G Å 3 levels, respectively; if the center of mass of these lines is to be preserved the group of line with more components should lie closer to the unsplit position. Figure 1 shows such a pattern and it is also clear that G Å 3 components are slightly broader than G Å 2 components. This difference in halfwidth is due to the unresolved F components; since G Å 3 has more components and should be somewhat broader when g is not zero. The separation between G Å 2 and G Å 3 hyperfine levels of 91ZrN is 3bF . Since the splitting is 0.198 cm01, then bF Å 00.066 { 0.003 cm01 (01980 { 90 MHz). ZrN is isoelectronic with YO (11) and the electron occupation of molecular valence orbitals is similar to TiN (12), X2S/rrr11s2 5p412s1

contact parameter not only confirms the occupation of the ss orbital, it also allows estimation of the contribution of the 5ss atomic orbital to the 12s molecular orbital. Assuming that the 12s orbital is a linear combination of the Zr 5s and 5p orbitals, i.e., 12s Å c15s(Zr) / c25p(Zr)

where c1 and c2 are mixing coefficients. A comparison of the currently determined bF Å 01980 MHz for ZrN (X2S/) to bFatomic Å 02753 MHz (6) yields c1 Å 0.85 and c2 Å 0.52. Ab initio calculation with Mulliken population analysis (13) can yield information on the electron density distribution in a molecular orbital, which can be compared with our estimate in this work. ACKNOWLEDGMENTS The authors gratefully acknowledge the technical assistance of Dr. N.S-K. Sze in this work. A.S-C.C. and H.L. thank the Hong Kong Research Grant Council for support. C.M-T.C. thanks the University of Hong Kong for the studentship. We also thank the referee for useful comments.

REFERENCES 1. J. K. Bates and T. M. Dunn, Can. J. Phys. 54, 1216–1223 (1976). 2. J. K. Bates and D. M. Gruen, High Temp. Sci. 18, 27–43 (1978). 3. J. B. West, R. S. Bradford, J. D. Eversole, and C. R. Jones, Rev. Sci. Instrum. 46, 164–168 (1975). 4. N. S.-K. Sze and A. S-C. Cheung, J. Quant. Spectrosc. Radiat. Transfer 52, 145–150 (1994). 5. N. S.-K. Sze and A. S-C. Cheung, J. Mol. Spectrosc. 173, 194–204 (1995). 6. W. Weltner Jr., ‘‘Magnetic Atoms and Molecules,’’ scientific and academic eds. Dover, New York, 1983. 7. A. Adams, W. Klemperer, and T. M. Dunn, Can. J. Phys. 46, 2213– 2220 (1968). 8. W. J. Childs and T. C. Steimle, J. Chem. Phys. 88, 6168–6174 (1988). 9. B. Simard, H. Niki, and P. A. Hackett, J. Chem. Phys. 92, 7012–7020 (1990). 10. T. M. Dunn in ‘‘Molecular Spectroscopy: Modern Research’’ (K. Narahari Rao, Ed.), Chap. 4-4, p. 231. Academic Press, New York, 1972. 11. W. J. Childs, O. Poulsen, and T. C. Steimle, J. Chem. Phys. 88, 598– 606 (1988). 12. C. W. Bauschlicher, Jr., Chem. Phys. Lett. 100, 515–518 (1983). 13. R. S. Mulliken and W. C. Ermler. ‘‘Diatomic Molecules: Results of ab Initio Calculations.’’ Academic Press, New York, 1977.

A2P rrr11s2 5p4 6p1, where the 12s is constructed mainly from 5ss (Zr) and 5ps (Zr) atomic orbitals, and the 6p is from 4dp (Zr) and 5pp (Zr) orbitals. Therefore the transition from X2S to A2P state corresponds to promotion of an electron from a nonbonding 5s5p hybrid orbital to a 5p4d hybrid orbital centered on zirconium (1). The fact that the explanation of the hyperfine structure of 91ZrN requires only the ground state hyperfine interaction confirms that hyperfine splitting in the A2P state is small. This observation is consistent with the occupation of the p orbital in the A state. The large size of the Fermi

HAIYANG LI C. M-T. CHAN A. S-C. CHEUNG1 Department of Chemistry The University of Hong Kong Pokfulam Road Hong Kong Received November 6, 1995; in revised form December 13, 1995; accepted December 26, 1995 1

To whom correspondence should be addressed.

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