Laser Spectroscopic Studies of Tran si tion Metal Con taining Rad i cals

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1 Jour nal of the Chi nese Chem i cal So ci ety, 2001, 48, Laser Spectroscopic Studies of Tran si tion Metal Con taining Rad i cals Allan S-C. Cheung ( ) De part ment of Chem is try, The Uni ver sity of Hong Kong, Pokfulam Road, Hong Kong Our re cent work on the spec tro scopic stud ies of di atomic tran si tion metal rad i cals is briefly pre sented. Var i ous ex per i men tal meth ods to pro duce tran si tion metal rad i cals and ob tain elec tronic spec tra us ing la ser spec tro scopic tech niques are re viewed. Our lat est re sults on the A 2 -X 2 + tran si tion of TiN us ing po lar iza - tion spec tros copy and the C 4 - -X 4 - tran si tion of VS us ing la ser in duced flu o res cence spec tros copy are de - scribed and pre lim i nary re sults on these tran si tions are re ported. INTRODUCTION Tran si tion metal (TM) con tain ing di atomic rad i cals are of great in ter est in many branches of chem is try and phys ics. Their im por tance ranges from as tro phys ics, 1 high tem per a ture chem is try, 2 ca tal y sis 3 and sur face sci ence. 4 Astro - phys i cally, be cause of the high cos mic abun dances of the 3d tran si tion met als and ox y gen, and the large dis so ci a tion en er - gies of the di atomic ox ides, the band sys tems of com pounds such as TiO, VO, and ZrO com pletely dom i nate the spec tra of cooler (M-type) stars. 1 Re cently, tran si tion metal sul fides such as TiS, and ZrS have been ob served in stel lar spec tra in S-type stars. 5 De spite of this im por tance, only very lim ited knowl edge on the gas-phase spec tro scopic prop er ties and mo lec u lar struc ture of di atomic TM rad i cals has emerged from stud ies that have been pur sued to date. Di atomic TM rad i cals have as ton ish ingly com pli cated spec trum which even now are by no means fully un der stood. Al though a num - ber of these rad i cals have been gen er ated and their op ti cal spec tra have been ob tained with var i ous spec tro scopic tech - niques, the ex trac tion of mo lec u lar struc tural in for ma tion from the anal y sis of the spec tra is pro ceed ing at a much slower rate. The dif fi cul ties as so ci ated with an a lyz ing these spec tra come from the many un paired elec trons, which pro - duce large num ber of low-lying elec tronic states, many with high spin mul ti plic ity. Fur ther more, there are wide spread per tur ba tions be tween elec tronic states, which make anal y sis awk ward. In ad di tion, the pres ence of large mag netic hyper - fine struc tures caused by nu cleus with non-zero nu clear spin (I 0) and mul ti ple iso to pic spe cies with ap pre cia ble abun - dance would fur ther com pli cate the ob served spec tra. It should be ap pre ci ated that there are many dif fer ent ways of pro duc ing gas-phase di atomic TM rad i cals for la ser spec tro scopic stud ies. The com monly used meth ods are: (i) la ser va por iza tion/re ac tion su per sonic ex pan sion, 6 (ii) Broidatype (re sis tively-heated) oven, 7 (iii) re ac tion of chem i cal va - por with at oms gen er ated by mi cro wave dis charge 8 and (iv) chem i cal re ac tion in a d.c. metal arc dis charge. 9 Amongst these meth ods, (i) is prob a bly the most pre ferred method of gen er at ing di atomic TM rad i cals be cause the gas-phase rad i - cals pro duced are at a lower tem per a ture, which is suit able for high res o lu tion stud ies. Since its dis cov ery, la ser in duced flu o res cence (LIF) spec tros copy 10 has been widely em ployed in ob tain ing high res o lu tion spec tra of TM rad i cals. Other tech niques such as po lar iza tion spec tros copy (PS), 10 cav ity ring-down la ser ab - sorp tion (CRLAS) spec tros copy 11 and, re cently, tran sient fre - quency mod u la tion ab sorp tion spec tros copy (TFMAS) 12 are also be com ing pop u lar be cause of their high sen si tiv ity. Our lab o ra tory has been en gaged in la ser spec tro scopic stud ies of di atomic TM rad i cals, in par tic u lar ni trides and sul fides, in the last few years. Ta ble 1 is a sum mary of the di atomic TM rad i cals that have been stud ied. Our re sults on MoN and TiS have been pub lished ear lier this year and work on other rad i - cals is in prog ress. Pre lim i nary re sults of our re cent ex per i - ments on TiN and VS are re ported here. Polarization spectroscopy of A 2 -X 2 + (0,0) band of TiN The emis sion spec trum of the red sys tem of TiN was first ob served by Dunn et al. 13,14 Ro ta tion anal y sis of the A 2 -X 2 + (0,0) band was per formed. Douglas and Veillete 15 ob tained the ab sorp tion spec tra of the same sys tem and re - ported mo lec u lar con stants for the A 2 state. Brabaharan et al., 16 us ing la ser in duced flu o res cence spec tros copy, stud ied the (0,0) (1,1) and (2,2) bands of A 2 -X 2 + sys tem and pub - lished im proved mo lec u lar con stants for the A 2 and X 2 + states. Var i ous small ro ta tional per tur ba tions have been noted Spe cial is sue for the Fourth Asian In ter na tional Sem i nars on Atomic and Mo lec u lar Phys ics

2 284 J. Chin. Chem. Soc., Vol. 48, No. 3, 2001 Cheung Table 1. Summary of Recent Studiesof Diatomic TM Radicals in Our Laboratory Molecule Production Method Spectroscopic techniques Transition Features Studied MoN MoCl 5 + N Intermodulated Fluorescence A 4 -X 4 - Hyperfine structure: MoN and 97 MoN ZrN ZrCl 4 + N Polarization A 2 -X 2 + Perturbation Laser induced fluorescence (LIF) B 2 -X ZrN hyperfine Perturbation TiN TiCl 4 + N Polarization A 2 -X 2 + Perturbation, LIF B 2 + -X 2 + B 2 + perturbed by 4 + TiS Ti + CS 2 LIF C 3 -X 3 LIF b 1 -X 3 Spin-forbidden transition VS V + CS 2 LIF C 4 - -X 4 - Hyperfine structure CrS Cr + CS 2 LIF A 5 -X 5 New systems HfS Hf + CS 2 LIF b 3 -X 1 Perturbation FeC Fe + CH 3 OH LIF 3 -X 3 i New systems by dif fer ent au thors, how ever, no at tempt has been made to study the or i gin of these per tur ba tions. In this work, we have re corded sub-doppler po lar iza tion spec trum of the A 2 -X 2 + tran si tion be tween 613 and 623 nm. Taking ad van tages of the nar row linewidth and the abil ity to dis tin guish the P and R branches from the Q branches in po lar iza tion spec tros copy, we have com pletely re solved all branches of the (0,0) band. The pre vi ously noted per tur ba tions were con firmed with better mea sure ments in this work. Our anal y sis shows that a 4 per turb ing state would likely be able to ac count for the per - tur ba tions ob served at 15.5 J 0.5 of the A 2 1/2-X 2 + subband. Po lar iza tion spec tros copy 10 has the ad van tages of high sen si tiv ity, high res o lu tion, and spec tral sim pli fi ca tion. Fig. 1 Fig. 1. Ex per i men tal ar range ment for po lar iza tion spec tros copy.

3 La ser Spec tros copy of TiN and VS J. Chin. Chem. Soc., Vol. 48, No. 3, shows sche mat i cally the ar range ment of the op ti cal com po - nents and the re ac tion cham ber. In this ex per i ment, a sin gle mode tun able dye la ser (Co her ent ) out put was split into a strong pump beam (90%) and a weak probe (10%) beam. Af ter pass ing through a Fres nel rhomb quar ter wave plate, the cir cu larly po lar ized pump beam was di rected to the re ac tion cham ber and crossed with the lin early po lar ized probe beam at the cen tre of the re ac tion cham ber. Each end of the re ac tion cham ber was placed with a Glan-Thompson type lin ear po lar izer (P1 and P2). The ex tinc tion ra tio of the two crossed polarizers was smaller than Each beam was chopped at a dif fer ent fre quency. A sil i con de tec tor was placed be hind P2 to de tect the po lar iza tion sig nal, which was re trieved by a lock-in-amplifier set to the sum fre quency of the pump and probe mod u la tion fre quen cies. The dye la ser fre quency was mea sured by a wavemeter with an ac cu racy of 1 part in The ab so lute ac cu racy of the wavemeter is about cm -1, which was cal i brated us ing io dine lines. TiN mol e cules were pro duced in a flow sys tem where a mix ture of TiCl 4 and ar gon was al lowed to re act with mi cro wave dis - charged N2 gas. This re ac tion pro duced a light blue chem - iluminescence. The gas pres sure in side the flow sys tem was about 5 Torr and the chemiluminescence re gion was 7 to 8 cm long. The P and R branches were re corded with a cir cu larly po lar ized pump beam. How ever, for re cord ing the spec trum of the Q branches, the Fres nel rhomb in the beam path was re - placed by a po lar izer to ob tain a lin early po lar ized beam. The linewidth ob tained in this ex per i ment was about MHz, which de pended on the to tal pres sure in side the cham ber. The po lar iza tion spec trum of the A 2 -X 2 + (0,0) band shows strong heads at and nm, which cor re - spond to the A 2 3/2-X 2 + and A 2 1/2-X 2 subband tran si - tions. The ad van tage of po lar iza tion spec tros copy in dis tin - guish ing the P and R branches from the Q branch is par tic u - larly use ful here. Due to very small spin sep a ra tion be tween the F1 and F2 lev els in the ground state, the R 2 and Q21 lines are clouded to gether near cm -1 (about 35 lines in that 1 cm -1 ). With the use of dif fer ent po lar iza tion in the pump beam, we were able to sep a rate lines of the R 2 branch from those of Q21 branch. Fig. 2 shows a por tion of the Q21 branch ob tained with a lin early po lar ized pump beam. The per tur ba - tions noted by Athenour et al. 14 in the v = 0 level of the A 2 state, for J = 15.5 of the 2 3/2 substate and J = of the 2 1/2 substate have been ob served. The ef fec tive hamiltonian op er a tor suit able for the de - scrip tion of the 2 and 2 states can be found in Hougen 17. The ma trix el e ments used in this work for the cal cu la tion of ro ta tional en ergy lev els are same as those in Amiot et al. 18 The pa ram e ters in cluded in the de scrip tion of the 2 states are ro ta tion pa ram e ters B and D, and the spin ro ta tion pa ram e ter. For the 2 state seven pa ram e ters are in cluded: the band or - i gin o; the ro ta tion pa ram e ters B and D; the spin-orbit pa - ram e ter A, A D and the -dou bling pa ram e ters p and q. Be - cause of the per tur ba tions in the A 2 state, we have di vided our least squares fit ting into two steps. First, the ground state mo lec u lar con stants B, D and were ob tained by fit ting the Fig. 2. The band head re gion of the Q21 branch of the A 2 -X 2 (0,0) band of TiN.

4 286 J. Chin. Chem. Soc., Vol. 48, No. 3, 2001 Cheung com bi na tion dif fer ences, 2F1 (J) and 2F2 (J), and the spin sep a ra tion be tween the F 1 and F 2 com po nents. Sub se quently, the up per state term val ues were ob tained by add ing the tran - si tion fre quen cies to the ground state term val ues cal cu lated from those fit ted con stants. Be cause of strong per tur ba tions oc curs at the low J re gion, our least squares fit ting of the up - per state term val ues was lim ited to 13.5 J 40.5 of the A 2 1/2 and 1.5 J 35.5 of the A 2 3/2 ex clud ing J = The mo lec u lar con stants de ter mined are listed in Ta ble 1. For com par i son pur pose, the val ues ob tained by Athenour et al. 14 and Brabaharan et al. 16 are also listed in Ta ble 2. The agree - ment be tween these con stants is ex cel lent. How ever, our val - ues are mea sured with slightly higher ac cu racy. For the per tur ba tions found at the J = 15.5 of the 2 3/2 state, both -dou bling com po nents are shifted equally (~ cm -1 ) in the same di rec tion. This re quires a dou bly de - gen er ate per turb ing state to ac count for such ob ser va tion. Since only the J = 15.5 level is per turbed and no other nearby J level is per turbed, it is not pos si ble to work out in de tail the na ture of the per turb ing state at the pres ent mo ment. Fig. 3 is a plot of the term val ues of the A 2 state, the per tur ba tion at the 2 1/2 substate is shown, since the two -dou bling com po - nents are per turbed to a dif fer ent ex tend, a dou bly de gen er ate state would not pos si bly be the per turb ing state. 2 and 4 are pos si ble non-degenerate states worth con sid er ing. We have cal cu lated the en ergy of higher vi bra tional lev els of the X 2 + Table 2. Molecular Constants for the A 2 -X 2 (0,0) Band (cm -1 ) This work Ref. 16 Ref. 14 X 2 + B (3) D 8.71 (2) A 2 o (1) A (1) B (4) D 9.95 (2) p (5) q (6) A D 0.10 (2) 0.4 RMS us ing pub lished con stants, how ever, even the clos est level misses the A 2 by 100 cm -1, which is also not likely to be the per turb ing state. Work is in prog ress to fit the per turbed lev - els with a 4 state. Near Infrared Laser Spectroscopy of the C 4 - -X 4 - of VS There is only very lim ited in for ma tion avail able on the spec tro scopic prop er ties of VS. Bauschlicher and Langhoff 19 us ing cou pled-pair func tional (CPF) cal cu la tion stud ied the low-lying elec tronic states of VS and shown that the ground Fig. 3. Re duced en ergy lev els of the v = 0 level of the A 2 ties of the lev els. state of TiN plot ted against J(J+1). Let ters e and f re fer to the par i -

5 La ser Spec tros copy of TiN and VS J. Chin. Chem. Soc., Vol. 48, No. 3, state is a 4 - state. Re cently, Bauschlicher and Maitre 20 using com plete ac tive space self-consistent field (CASSCF) cal cu - la tions, fur ther con firmed that the ground state is a 4 - state and also con cluded that the bond ing mech a nism in the tran si - tion metal sul fides and ox ides is very sim i lar. Even thought ex ten sive ex per i men tal and the o ret i cal stud ies have been per - formed to VO (which is isovalent with VS), there has not been any ex per i men tal work on VS. We re port here the first ex per i men tal in ves ti ga tion of gas-phase VS in the near in fra - red re gion us ing la ser va por iza tion/re ac tion su per sonic ex - pan sion and la ser in duced flu o res cence spec tros copy. The ex per i men tal ap pa ra tus for ob tain ing the spec trum of VS has been de scribed in our ear lier pub li ca tion, 21 a brief dis cus sion of the ex per i men tal con di tions is given here. Fig. 4 is a sche - matic di a gram of the ex per i men tal setup. Pulses of 532 nm, 5-6 mj from a Nd:YAG la ser were fo cused onto the sur face of a va na dium metal rod to gen er ate V at oms in a plasma. A pulsed valve was syn chro nized with an ap pro pri ate de lay time to re lease a gas mix ture of 4% CS2 in Ar to al low re ac - tion with V at oms. VS mol e cules formed were cooled by su - per sonic ex pan sion into the vac uum cham ber. The Nd:YAG la ser-valve sys tem was op er ated at 10 Hz. The jet cooled mol - e cules were ex cited by c.w. ring Ti:sap phire (Co her ent ) la ser pumped by an Ar + la ser. La ser in duced flu o res - cence sig nal was col lected by means of a lens sys tem, di - rected through a cut off fil ter, and de tected by a Hamamatsu R super red photomultiplier tube. The sig nal was in put to a box car in te gra tor for av er ag ing. The c.w. Ti:sap phire la - ser was scanned at a rate about 0.8 cm -1 per 9 min utes. The linewidth of the VS spec trum was mea sured to be MHz, which de pended upon the back ing pres sure of the jet and the back ground pres sure in the vac uum cham ber. The wave length of the Ti:sap phire la ser was mea sured by a wavemeter with ab so lute ac cu racy about cm -1. High res o lu tion la ser in duced flu o res cence spec tra be - tween and 846.l nm have been re corded, which con sists of five vi bra tional bands: nm (11824 cm -1 ) (0, l) band nm (12366 cm -1 ) (0, 0) nm (12813 cm -1 ) (l, 0) nm (13247 cm -1 ) (2, 0) nm (13695 cm -1 ) (3, 0) De tailed rotationally anal y sis of the band sys tems in this re gion con firmed that they are the vi bra tional bands of the C 4 - -X 4 - tran si tion. Fig. 5 shows the branch struc ture (24 branches) of an ex pected 4-4 tran si tion. The ground Fig. 4. Ex per i men tal ar range ment for la ser va por iza tion/re ac tion la ser in duced flu o res cence spec tros copy.

6 288 J. Chin. Chem. Soc., Vol. 48, No. 3, 2001 Cheung Fig. 5. En ergy level di a gram for the 4-4 tran si tion. state, X 4 -, has large spin-spin in ter ac tion, the cou pling case con form to case (a) scheme, how ever, the up per state, C 4 -, is closer to case (b). Be cause of the dif fer ence in cou pling scheme of the up per and lower states, all the 24 branches ex - pected have been ob served. The hyperfine struc ture caused by 51 V nu cleus (I = 7/2) is mostly re solved in our spec trum, which dis plays the beau ti ful pat tern of eight lines as a group. Fig. 6 is a por tion of the (0, l) band with the R 1 branch form - ing a head at cm -1. Be fore per form ing de tail anal y sis to the hyperfine struc tures we have car ried out a least squares fit to all branches to con firm our as sign ments. In this fit, the cen - ter po si tions of the hyperfine pat terns were used. Since the spec trum was re corded un der low tem per a ture, mo lec u lar tran si tion lines up to J = 15 were ob served. The hamiltonian used for a 4 state con sists of the band or i gin, o, ro ta tional con stant B, spin spin-rotation con stant and spin-spin con - stant. The re sults of our pre lim i nary anal y sis are listed in Ta ble 3. The de ter mined ground state vi bra tional sep a ra tion and bond length are com pared with cal cu lated val ues as fol - lows: Bond length (Å) e (cm -1 ) Ex per i men tal value Bauschlicher & Langhoff (1986) Bauschlicher & Maitre (1995) The cal cu lated bond length is larger by about 0.02 Å, which is ex pected from us ing ab in itio cal cu la tions. The Table 3. Molecular Constants for the C 4 - and X 4 - States of VS (cm -1 ) Band Parameter C 4 - states X 4 - states (0, 0) o (3) 0 B (1) (1) (4) (5) (2) (1) RMS (0, l) o (3) 0 B (1) (1) (3) (5) (2) (1) RMS 0.024

7 La ser Spec tros copy of TiN and VS J. Chin. Chem. Soc., Vol. 48, No. 3, Fig. 6. The band head re gion of the C 4 -X 4 (0,1) band of VS. agree ment be tween the cal cu lated and ex per i men tal value of e is good. Fur ther work to an a lyze the hyperfine struc ture is in prog ress. ACKNOWLEDGEMENTS I thank my co-workers, whose ef forts con trib uted to the ma te rial pre sented here: Qin Ran, W. S. Tam, Hongbing Chen and Yongfang Li. I also like to thank Pro fes sor A. J. Merer for help ful dis cus sion and the anal y sis of the VS spec tra. The work de scribed here was sup ported by a grant from the Re - search Grants Coun cil of the Hong Kong Spe cial Ad min is tra - tive Re gion, China (Pro ject no. HKU 7094/98P). Re ceived De cem ber 31, Key Words La ser spec tros copy; Ti ta nium nitride; Vanadium sul fide. REFERENCES 1. White, N. H.; Wing, R. F. Astrophys. J. 1978, 222, Carlson, K. D.; Claydon, C. R. Adv. High Temp. Sci ence 1967, 1, Rao, C. N. R. Annu. Rev. Phys. Chem. 1989, 40, Wojciechowska, M.; Haber, J.; Lomnicki, S.; Stoch, J. J. Mol. Catal. A 1999, 141, Joyce, R. R.; Hinkle, K. H.; Wallace, L.; Dulick, M.; Lam - bert, D. L. Astrophys. J. 1998, 116, Simard, B.; Mitch ell, S. A.; Humphires, M. R.; Hackett, P. A. J. Mol. Spectrosc. 1988, 129, West, J. B.; Brad ford, R. S.; Eversole, J. D.; Jones, C. R. Rev. Scient. Instrum. 1975, 46, Cheung, A. S-C.; Hansen, R. C.; Merer, A. J. J. Mol. Spectrosc. 1982, 91, Chen, Y.; Jin, J.; Hu, C.; Yang, Xi; Ma, X.; Chen, C. J. Mol. Spectrosc. 2000, 203, Demtröder, W. La ser Spec tros copy Ba sic Con cepts and In stru men ta tion; 2 nd Edi tion; Springer-Verlag: Berlin, Sherer, J. J.; Paul, J. B.; Col lier, C. P.; Saykally, R. J. J.

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