Sub-Doppler laser spectroscopy of ZrN

He Jiang, Chensheng Ma, G.S.-M. Tong, A.S.-C. Cheung*

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong

Abstract

The (1,1) band of the A2P-X2S1 transition of ZrN has been studied using intermodulated laser induced fluorescencespectroscopy. The sub-Doppler spectrum obtained was at a resolution of about 150 MHz. The magnetic hyperfine structurecaused by the91ZrN nuclear (I� 5/2) is partially resolved, and the coupling of the nuclear spin angular momentum with X2S1

state conforms to case bb s scheme. This hyperfine interaction splits a rotational line into 12 hyperfine components which aredivided into two groups with G� 2 (5 components) and G� 3 (7 components). The ground state molecular constants neededfor explaining the observed hyperfine structures are: B� 0.479809 cm21, D� 4.515x1027 cm21. g � 2 0.002445 cm21, b�2 0.0645 cm21, c� 2 0.0045 cm21. q 1999 Elsevier Science B.V. All rights reserved.

Keywords:Magnetic hyperfine structure; Case bb s; Molecular constants

1. Introduction

In our experiment to record the A2P-X2S1 transi-tion in high resolution using laser induced fluores-cence (LIF) spectroscopy to study the origin of theperturbations in the A2P state, we have observedthe magnetic hyperfine structure of the91ZrN [1,2].The magnetic hyperfine splitting conforms to the casebb s scheme, which is common to2S state [3] In ourDoppler limited LIF spectrum, the rotational lines ofthe 91ZrN split into two components which corre-sponding to the group of G� 2 and G� 3 hyperfinestructure [2]. In this paper, we report our study of thepartially resolved hyperfine structure using the sub-Doppler technique of intermodulated fluorescence(IMF) spectroscopy.

2. Experimental

ZrN molecules were produced in an oven systemwhere white powder of ZrCl4, with natural abundanceof the five major Zr isotopes [90Zr (51.45%), 91Zr(11.23%), 92Zr (17.11%), 94Zr (17.40%) and96Zr(2.8%)], was vaporized and allowed to react withmicrowave discharged N2 gas. We found that afterthe introduction of Ar as a carrier gas to bring theZrCl4 vapor out of the oven, the production of ZrNhas increased roughly 3 times than that without usingthe carrier gas [4]. The reaction produced a pale bluechemiluminescence. The temperature of the heatedregion was estimated to be about 140oC and thetotal pressure was around 8 Torr.

Spectra of ZrN were recorded at sub-Doppler reso-lution by intermodulated fluorescence spectroscopy[5]. Fluorescence signal was excited by a c.w. ringdye laser operating with rhodamine 560 and 590dyes and pumped by an Ar1 laser. In these experi-ments the chopper frequencies were 1458 and1755 Hz, the laser power was about 100 mW each

Journal of Molecular Structure 480–481 (1999) 277–281

0022-2860/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00699-1

* Corresponding author. Tel.: (852) 2859 2150; fax: (852) 28571586.

E-mail address:hrsccsc@hkucc.hku.hk (A.S.-C. Cheung)

beam, and the fluorescence signal was recordedthrough a 0.75 m monochromator equipped with aphotomultiplier tube. A narrow-band electrical filterselected the sum of the chopping frequencies, and theintermodulated signal was extracted with a lock-inamplifier. Frequency calibration was obtained byinserting two 5% beam splitters in the path of thelaser beam to produce two extra beams; one of thesewas sent to a 300 MHz Fabry-Perot etalon to generatea series of equally spaced fringes and the other to an I2

cell where I2 fluorescence spectrum was recorded. The

wave numbers of the I2 lines are listed in Gersterkornand Luc [6].

For intensity reasons we were not able to run theoven system such that the total pressure inside wasless than 1 Torr when recording intermodulated fluor-escence signal. As a result the linewidths in ourspectra were pressure broadened and were never lessthan about 150 MHz even though the laser linewidthis 1–2 MHz. For the weaker G� 2 groups we had toincrease the total pressure; the linewidths rose to250 MHz.

H. Jiang et al. / Journal of Molecular Structure 480–481 (1999) 277–281278

Fig. 1. Doppler limited and sub-Doppler spectra of R21 (14.5) lines of A2P-X2S1 (1,1) band.

3. Results and discussion

The (1,1) band of the A2P-X2S transition of ZrNhave been recorded with Doppler limited resolution inearlier work [1–3]. The branches of the91ZrN that aresubjected to minimum overlapping were chosen forintermodulated fluorescence work. Fig. 1 shows anexample of the Doppler limited and intermodulatedspectrum of the R21 (14.5) lines. The91ZrN transitionslines clearly split into two components. This isbecause of the hyperfine structure which arises fromthe interaction between nuclear spin, I� 5/2, and theelectron spin angular momentum in the ground state.These two lines are labelled by the quantum numberG � 2 and 3. The Fermi contact parameter is moder-ately large because the observed splitting between thetwo G components is about 0.2 cm21. The IMFspectra of the R21 branch of (1,1) band with selectedJ values between 7.5 to 18.5 were recorded. Fig. 2shows the partially resolved hyperfine structure ofthe G� 2 and 3 components of the R21 (13.5) and

R21 (15.5). As already discussed in our earlier paper[2], the invariance of the separation between the G�2 and G� 3 groups with N indicates that the nuclearhyperfine interaction in the A2P state is negligible andthe coupling of the nuclear spin angular momentumwith X2S1 state conforms to case bb s scheme.

The suitable choice of basis set of a2S1 state withlarger Fermi contact interaction is case bb s, becausethe Fermi contact operator, bI ·S, couples the vectorSto I more strongly than the spin-rotation interactioncouples it to N. In case bb s coupling scheme:

G � S1 I and F� N 1 G

The ground state of91ZrN is of this type:91Zr has aspin I� 5/2, which means that all the rotational levelsare split into G� 2 and G� 3 components. The fullrotational and hyperfine Hamiltonian for the descrip-tion of a 2S state is given by [7]

H � BN2 2 DN4 1 gN·S1 bI ·S1 cI zSz

1 eT2 �Q� ·T2�E�

The terms included in the description of a2S1

states are: B and D are the rotational constant andits centrifugal distortion,g is the spin-rotation para-meters, b is the Fermi contact parameter, and c is thedipolar interaction, while the last team is the nuclearelectric quadruple coupling. The matrix elements intensor expression of case bb s coupling have beenlisted in Barnes et al. [7]. Matrix elements in algebraicform including diagonal and off-diagonal in N, andG � 2 and 3 have been worked out. A matrix with36 × 36 elements, which is divided into N-2, N andN 1 2 blocks each of 12× 12 in size, was set up tocalculate the energy levels of the ground state ofZrN.

Since we could only resolve partially the hyperfinestructure in our IMF spectra, we have chosen toperform our analysis by comparing computed separa-tions between individual hyperfine components tothose observed. Fig. 3 shows the hyperfine energylevels of the X2S1, v � 1 level of 91ZrN versus N.Levels are calculated from the molecular constantsbelow:

B 0.479809 cm21

D 4.515x 1027 cm21

H. Jiang et al. / Journal of Molecular Structure 480–481 (1999) 277–281 279

Fig. 2. The partially resolved G� 2 and 3 components of R21

(13.5) and R21 (15.5) lines of A2P-X2S1 (1,1) band.

g 2 0.002445 cm21

b 2 0.0645 cm21

c 2 0.0045 cm21

B, D andg were obtained from the isotopic relation-ship using molecular parameters of90ZrN, and b and cwere adjusted in our calculation until the computedenergy levels resemble that of the observed hyperfinestructure. For instance, from the partially resolved R21

(15.5) lines, the separation between adjacent peaks isabout 0.005 cm21, which is close to the separation

between adjacent levels as shown in Fig. 3. In thiswork, the dipolar parameter, c, is marginallydetermined, however, we were not able to determinethe electric quadruple parameter.

The hyperfine structure of the G� 3 group of the(0,0) band is much better resolved in our IMF spectra,the separation between individual hyperfine transi-tions is twice as large as the separation observed inthev � 1 level. Further work is currently in progressto identify the cause of such widening in hyperfinestructure in thev � 0 level.

H. Jiang et al. / Journal of Molecular Structure 480–481 (1999) 277–281280

Fig. 3. Hyperfine energy levels of the X2S1 v � 1 level of 91ZrN.

Acknowledgements

ASCC thanks Prof. A.J. Merer for helpful discus-sion regarding the matrix elements in case bb s scheme.The work reported was supported by the Hong KongResearch Grants Council under Grant HKU 253/95P.H.J. and G.S-M.T. thank the University of Hong Kongfor the award of studentships.

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