Journal of Molecular Spectroscopy 278 (2012) 35–40

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Journal of Molecular Spectroscopy

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Fourier transform microwave spectroscopy of ScS (X2R+) and YS (X2R+)

G.R. Adande, D.T. Halfen, L.M. Ziurys ⇑Departments of Chemistry and Astronomy, Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 May 2012In revised form 5 July 2012Available online 22 July 2012

Keywords:FTMW spectroscopyScandium sulfide (ScS)Yttrium sulfide (YS)Hyperfine structureLaser ablation

0022-2852/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jms.2012.07.009

⇑ Corresponding author. Fax: +1 520 621 5554.E-mail address: lziurys@email.arizona.edu (L.M. Zi

The pure rotational spectra of the transition metal sulfide radicals ScS and YS in their 2R+ ground stateshave been measured in the range 8–48 GHz using Fourier transform microwave (FTMW) spectroscopy.The radicals were synthesized from the reaction of metal vapor, produced by laser ablation, and H2Sgas, heavily diluted in argon. A DC discharge was needed in the case of ScS. Four rotational transitionswere recorded for each molecule, in which multiple fine and hyperfine components were resolved. Thespectra were analyzed with a case (b) Hamiltonian, and rotational, fine, and hyperfine constants weredetermined for both molecules, improving the precision of previous parameters established from opticaland double resonance data. The quadrupole coupling constant eQq has been accurately established for ScSfor the first time, as well. From the rotational constants, the bond lengths were determined to be 2.1288 Åfor ScS and 2.2614 Å for YS. The hyperfine parameters suggest that, although ScS and YS are principallyionic molecules, they are more covalent than their oxygen analogs.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Transition metal sulfides play an important role in many areas ofscientific research. For example, such sulfides have numerous appli-cations in catalysis [1], as well as in the semiconductor industry [2].They also are relevant in biology, being linked to the primitive devel-opment of autotrophic life [3]. Because of their chemical importance,numerous theoretical investigations have been conducted in orderto understand the structural, electronic, and thermodynamic prop-erties of transition metal sulfides [4–6]. Such studies, however, canbe problematic because of the presence of low-lying electronicstates. High resolution spectroscopic data are therefore necessaryto benchmark and complement such calculations.

Two interesting transition metal sulfides are yttrium monosul-fide (YS) and scandium monosulfide (ScS). Their relatively simpleelectronic structure provides a good starting point for computa-tional models investigating transition metal bonding. Scandiumand yttrium both belong to the group III transition metals, whichhave the simplest open d shell configuration (s2d1). Both moleculeshave been studied spectroscopically at optical wavelengths, as wellas by Fourier transform infrared and optical double resonancemethods [7–13]. These studies have shown that both species have2R+ ground states, with the unpaired electron likely situated in a rhybridized molecular orbital, predominantly located on the metalatom [4]. From the experimental determination of magnetic hyper-fine parameters, it had been suggested that the unpaired electronin ScS occupies an orbital centered on the scandium atom with

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urys).

57% s character [10], while the equivalent orbital in YS is 53% sin composition [9].

Here we present the first measurements of the pure rotationalspectra of YS and ScS in their 2R+ ground states, recorded usingFourier transform microwave (FTMW) techniques. These radicalswere produced using a laser ablation source to generate metal va-por. For both molecules, the fine and hyperfine structures were re-solved in multiple rotational transitions, allowing for improveddetermination of the spectroscopic parameters, as well as an accu-rate measurement of the quadrupole coupling constant of ScS. Inthis paper we describe these results and their analysis, and givean interpretation of the fine and hyperfine constants for the tworadicals.

2. Experimental

The pure rotational spectra of YS and ScS were measured usingthe Balle–Flygare type Fourier transform microwave (FTMW) spec-trometer of the Ziurys group, described in detail elsewhere [14].Briefly, the instrument consists of a vacuum chamber containinga Fabry–Perot cavity with two spherical aluminum mirrors in anear confocal arrangement. The system is maintained at an un-loaded pressure of 10�8 Torr by a cryopump. Microwave radiationis launched into the cavity either through an antenna (4–40 GHz)or waveguide (40–60 GHz: see [15]) embedded in one mirror. Mol-ecules of interest are introduced into the chamber via a pulsedsupersonic nozzle. Molecular emission is collected by an antennaor waveguide embedded in the opposite mirror and detected as afunction of time with a low noise amplifier, the so-called FreeInduction Decay (FID). The time domain signal is digitized and

36 G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40

converted by Fast Fourier Transform (FFT) to generate a spectrum,which appears as Doppler doublets, resulting from the jet expan-sion of the mixture relative to the electric field in the cavity. Thetransition frequency is taken to be the average of the doublets.The resolution of the FTMW spectrometer is 4 kHz.

Both ScS and YS were created by the reaction of H2S with metalvapor produced by laser ablation. A gas mixture of 0.1% H2S in200 psi of argon was introduced into the cavity by a pulsed valve(General Valve, 0.8 mm nozzle orifice), to which the ablationsource was attached such that the gas mixture and the metal vaporwere injected approximately at the same time into the cavity. Apulsed Nd:YAG laser beam (200 mJ/pulse) was used to ablate themetal, contained in the form of a rotating, translating rod. In thecase of ScS, a DC discharge (0.6 kV, 20 mA) was also necessary formolecule production, applied to the metal/gas mixture immedi-ately following the ablation source. (Yttrium reacted with H2Sspontaneously without the need of a discharge.) The details ofthe discharge assisted laser ablation source, called DALAS, can befound in Sun et al. [16].

Fig. 1. Representative FTMW spectra recorded for YS (X2R+). In the upper panel,two hyperfine components of the N = 2 ? 1 rotational transition near 16.6 GHz aredisplayed, indicated by quantum number F, each arising from a different spindoublet, labeled by J. In the lower panel, two hyperfine lines from the N = 3 ? 2rotational transition near 24.9 GHz are shown, also originating in separate spin–rotation components. There is a frequency beak in each spectrum in order to showthe two spectral features. Doppler doublets are indicated by brackets. Each spectralfeature shown was measured in one 600 kHz wide scan, with 1000 pulses per scan.

3. Results

The rotational measurements were based on the constants ob-tained by Stringat et al. [13] and Azuma and Childs [8] for YSand Steimle et al. [10] for ScS. Because the magnetic moment of yt-trium is relatively small, the hyperfine pattern in YS follows a clas-sic bbJ coupling scheme, such that J = N + S and F = J + I. The nuclearspin of yttrium is I = 1/2. Four rotational transitions of YS(N = 1 ? 0 to N = 4 ? 3) were recorded over the range 8–34 GHz;see Table 1. Frequency predictions using the previous spectro-scopic parameters for YS were typically reliable to ±15 MHz. Fif-teen hyperfine components of this radical were recorded in total.

Representative spectra of YS are shown in Fig. 1. In the upperpanel, two hyperfine components in the N = 2 ? 1 rotational tran-sition near 16.6 GHz are displayed, labeled by quantum number F,with one from each spin–rotation doublet, indicated by J. Similarly,the lower panel shows two hyperfine components of the N = 3 ? 2transition neat 24.9 GHz, one from each spin–rotation component.All lines exhibit Doppler doublets, indicated by brackets.

In contrast to yttrium, scandium has a large magnetic momentand a nuclear spin of I = 7/2. As a consequence, the Fermi contactterm in ScS is very large relative to the spin–rotation interaction.The hyperfine structure is therefore of the same order of magni-tude as the fine structure, following a bbs coupling scheme as op-posed to bbJ, as for YS. Four rotational transitions were recordedfor ScS in the range 11–50 GHz, each consisting of numerous

Table 1Observed transition frequencies of YS (X2R+).

N0 ? N00 J0 ? J00 F0 ? F00

1 ? 0 1/2 ? 1/2 1 ? 13/2 ? 1/2 1 ? 03/2 ? 1/2 2 ? 1

2 ? 1 3/2 ? 1/2 1 ? 03/2 ? 1/2 2 ? 15/2 ? 3/2 2 ? 15/2 ? 3/2 3 ? 2

3 ? 2 5/2 ? 3/2 2 ? 15/2 ? 3/2 3 ? 27/2 ? 5/2 3 ? 27/2 ? 5/2 4 ? 3

4 ? 3 7/2 ? 5/2 3 ? 27/2 ? 5/2 4 ? 39/2 ? 7/2 4 ? 39/2 ? 7/2 5 ? 4

a In MHz.

hyperfine components: see Table 2. For comparison with YS, thebbJ notation was used for labeling the transitions. The transitionfrequencies were typically ±3 MHz away from predictions, basedon the previous constants.

mobsa mobs � mcalc

a

8296.712 0.0038327.470 �0.0038348.735 0.001

16624.146 �0.00316649.810 0.00216654.901 0.00116674.080 �0.006

24955.916 0.00624974.668 0.00224982.239 0.00324999.912 �0.002

33282.711 �0.00933299.468 0.00133309.440 �0.00233325.797 0.004

Table 2Observed transition frequencies of ScS (X2R+).

N0 ? N00 J0 ? J00 F0 ? F00 mobsa mobs � mcalc

a

1 ? 0 3/2 ? 1/2 3 ? 3 11805.146 0.0013/2 ? 1/2 5 ? 4 11849.830 �0.0053/2 ? 1/2 4 ? 4 11898.629 0.006

2 ? 1 3/2 ? 3/2 3 ? 4 23504.279 �0.0053/2 ? 1/2 2 ? 3 23576.110 �0.0093/2 ? 3/2 4 ? 4 23607.576 �0.0023/2 ? 1/2 5 ? 4 23629.356 0.0015/2 ? 1/2 4 ? 4 23646.380 0.0025/2 ? 3/2 2 ? 2 23664.364 �0.0015/2 ? 3/2 5 ? 4 23669.405 0.0003/2 ? 1/2 3 ? 3 23686.733 �0.0095/2 ? 3/2 6 ? 5 23696.680 0.0015/2 ? 1/2 3 ? 4 23699.674 �0.0055/2 ? 3/2 5 ? 5 23718.195 0.0023/2 ? 1/2 4 ? 3 23790.044 �0.002

3 ? 2 5/2 ? 3/2 1 ? 2 35416.208 0.0015/2 ? 3/2 4 ? 4 35442.239 0.0015/2 ? 3/2 3 ? 3 35454.586 �0.0035/2 ? 3/2 6 ? 5 35457.035 �0.0017/2 ? 5/2 5 ? 5 35461.236 �0.0047/2 ? 5/2 3 ? 2 35470.531 0.0045/2 ? 5/2 5 ? 4 35471.275 0.0007/2 ? 5/2 4 ? 3 35472.333 0.0025/2 ? 3/2 2 ? 2 35480.732 �0.0087/2 ? 5/2 6 ? 5 35512.429 0.0007/2 ? 3/2 5 ? 4 35523.071 0.0037/2 ? 5/2 4 ? 4 35525.635 0.0037/2 ? 5/2 2 ? 2 35530.564 �0.0087/2 ? 5/2 7 ? 6 35532.415 0.0027/2 ? 5/2 6 ? 6 35533.948 0.0057/2 ? 5/2 3 ? 3 35535.677 0.0025/2 ? 3/2 4 ? 3 35545.540 �0.001

4 ? 3 7/2 ? 5/2 4 ? 4 47259.004 �0.0017/2 ? 5/2 7 ? 6 47285.741 �0.0137/2 ? 5/2 6 ? 5 47298.714 �0.0049/2 ? 7/2 5 ? 4 47305.885 0.0029/2 ? 7/2 4 ? 3 47311.688 0.0039/2 ? 7/2 6 ? 5 47345.299 �0.0027/2 ? 5/2 5 ? 4 47347.847 0.0049/2 ? 7/2 7 ? 6 47348.429 0.0077/2 ? 5/2 4 ? 3 47349.957 �0.0019/2 ? 7/2 8 ? 7 47364.951 0.005

a In MHz.

G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40 37

In case bbs, S couples with I instead of N to give the intermediatequantum number G, taking the place of J, i.e. I + S = G. G then cou-ples with N to create F. In this notation, all the transitions reportedin Table 2 would either be assigned to G = 3 or G = 4. Adding N0 orN00 to the respective G generates the F0 or F00, as given in the table.

Representative spectra of ScS are shown in Fig. 2. The top paneldisplays two hyperfine components of the N = 2 ? 1 rotational tran-sition near 23.6 GHz, both arising in the J = 5/2 ? 3/2 fine structuredoublet (or G = 3, F = 3 ? 2 and G = 4, F = 6 ? 5). Three hyperfinelines of the N = 3 ? 2 transition near 35.5 GHz are shown in the low-er panel, one arising from the J = 5/2 ? 3/2 doublet and the othertwo from the 7/2 ? 5/2 doublet (all G = 4). Hyperfine transitionsare labeled by F, and the Doppler doublets are indicated by brackets.

4. Analysis

Both molecules were fit with the non-linear least-squares anal-ysis program SPFIT [17]. The following bbJ effective Hamiltonianwas used [18]:

Heff ¼ Hrot þ Hsr þ Hmhf þ HeQq þ Hnsr ð1Þ

Rotational, spin–rotation, magnetic hyperfine, electric quadru-pole and nuclear spin–rotation interactions were considered inthe analyses.

The fitted spectroscopic parameters for YS and ScS are pre-sented in Table 3. Also given in the table are the constants previ-ously obtained from the optical and double resonance data. Inthe analysis of YS, only five parameters were necessary to obtainan rms of 4 kHz, the experimental precision. In contrast, in thedouble resonance study of YS [8], both CI and cD were additionallyused in the spectral fitting. Both parameters did not significantlyimprove the fit in this work and therefore were not included. AsTable 3 shows, the FTMW study has improved the precision ofthe rotational constants, and the fine and hyperfine parametersare consistent with the past double resonance work. In the caseof ScS, the FTMW data has increased the accuracy of the spectro-scopic constants by factors of 10–100, including the first reliabledetermination of the quadrupole coupling constant eQq. In thisanalysis, CI was necessary to achieve an rms comparable to theexperimental precision (4 kHz).

5. Discussion

5.1. Hyperfine and fine structure interactions

The electronic configuration of ScS is postulated to be: (core)10r2 4p4 11r1 [4,10,19]. The unpaired electron in ScS thus lies inthe 11r orbital, which is thought to be centered on the scandium

Fig. 2. Representative FTMW spectra recorded for ScS (X2R+). In the upper panel,two hyperfine components of the N = 2 ? 1 rotational transition near 23.6 GHz arepresented, both arising in the J = 5/2 ? 3/2 fine structure doublet and labeled by theF quantum number. In the lower panel, three hyperfine lines of the N = 3 ? 2transition near 35.5 GHz are displayed, one arising from the J = 5/2 ? 3/2 doubletand the others from the 7/2 ? 5/2 component. There are frequency breaks in eachspectrum in order to display multiple hyperfine transitions. Doppler doublets areindicated by brackets. Each spectral feature shown was measured in one 600 kHzwide scan, with 1500–2000 pulses per scan.

38 G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40

nucleus [10]. The orbital composition is primarily a mixture of me-tal 4s and 3d character, because the 4p orbitals of scandium liemuch higher in energy (20000 cm�1 above the 4s level) [19]. Inanalogy to the metal oxides, the 11r orbital is thought to be mostlynon-bonding, with slight bonding character due to a small admix-ture of the sulfur 3p orbital [19]. Because the Fermi contact termonly arises from the contribution of electrons in s orbitals, it can

Table 3Spectroscopic constants for ScS (X2R+) and YS (X2R+).a

Parameter ScS ScS (optical)b

B 5915.2294(12) 5914.72(39)D 0.002873(50) 0.002893(23)c 96.3356(73) 92.8(2.2)cD – –bF 1671.2(2.4) 1673.9(6.2)c 112.558(12) 116(41)CI 0.01665(98) –eQq 55.709(54) 63(159)

56(27)e

rms of fit 0.004r0 (Å) 2.128824(2) 2.13750(6)

a Constants in MHz unlesss specified. Errors quoted are 3r.b From [12], unless otherwise specified.c From [13].d From [8].e From [10].

be used to evaluate the amount of s character in a given orbitalby comparing it to the atomic value of scandium, 2823 MHz [20].The ratio is then [bF(ScS)/bF(Sc)] = |c1|2 � 0.58, such that the un-paired electron in ScS is in an orbital with 58% s character. This va-lue is smaller than that found for ScO, where the analogouselectron has 69% s character [20]. The decrease in s character uponreplacement of O by S suggests that the atomic 3d orbital is betterstabilized by the less electronegative sulfur atom.

Similarly, the YS electronic configuration is thought to be (core)13r2 6p4 14r1 [11]. As with scandium, the 5p orbitals in yttrium liemore than 16000 cm�1 above the 5s level [21], and are unlikely tocontribute appreciably to the 14r orbital. This orbital is thus likelyto be sd hybridized, with a small sulfur 3p contribution. The atomicvalue of the Fermi contact term for the yttrium atom is �1250 MHz[8]. Therefore, the unpaired electron in YS is 53% s in character, asopposed to 62% for YO [22]. Again, the amount of s characterdecreases with replacement of the oxygen atom with sulfur.

The dipolar hyperfine constant c is defined as [23]:

c ¼ 32

gslBgNlN1n

X ð3 cos2 hi � 1Þr3

i

� �s

ð2Þ

Assuming a negligible role for the sulfur 3p orbital, contribu-tions to c must come principally from the metal d orbital that ishybridized with the metal s orbital, as the angular expectation va-lue for an s electron is zero. Assuming pure sd hybridization, thedipolar hyperfine parameter reduces to

c ¼ 32

gslBgNlN3 cos2 h� 1

r3

� �dr

ð3Þ

For dr electrons, h3 cos2 hi � 1i = 4/7. Using this factor, h1/r3i canbe calculated for the unpaired electron in both scandium andyttrium sulfide. For ScS, h1/r3i � 1.013 a.u�3 for the unpaired elec-tron. This value can be compared to the atomic value for the scan-dium d electron of h1/r3i � 0.911 a.u�3 [10] and for the ion Sc+:h1/r3i � 1.851 a.u�3 [24]. The magnitude of h1/r3i of the unpairedelectron is significantly closer to the neutral value, suggesting aconsiderable degree of covalency in the Sc–S bond. For YS, theunpaired electron has h1/r3i � 1.887 a.u�3. The analogous atomicvalues are h1/r3i � 2.373 for the Y+ ion [24] and h1/r3i � 1.711 a.u�3

for the neutral atom [25]. The trend found in scandium for h1/r3i isrepeated in yttrium, indicating some fraction of covalent characteralso in this molecule.

In this work, the quadrupole coupling constant for ScS has beenaccurately determined for the first time. In the context of aTownes–Dailey analysis, the quadrupole coupling constant eQq0

YS YS (optical)

4163.0992(21) 4160.2(2.7)c

0.001331(79) 0.0011(6)c

42.252(15) 42.2382(6)d

– 1.8243(21) � 10�4d

�667.8(1.2) �667.479(60)d

�42.470(94) �42.684(54)d

�0.0046(6)d

0.0042.261416(1)

Table 4Quadrupole coupling constants of scandium species.

Species eQq (MHz)

ScS 55.709(54)ScO 72.240(15)ScCl 68.2067(90)ScF 74.09(15)

Fig. 3. A comparison of experimentally-determined and theoretical [4] ground statebond lengths for the 3d transition metal sulfides and oxides. The experimental bondlength values are r0 and the theoretical ones are re. The experimental and theoryvalues are in relatively good agreement. The sulfides and oxide bond lengths showsubtle variations across the periodic table, with notable differences at scandium andzinc, and from iron to nickel.

G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40 39

can be expressed in terms of eQq320, the quadrupole coupling cre-ated by a 3d orbital of scandium [26]:

eQq0 ¼ eQq320 � ndr þ12

ndp � ndd

� �ð4Þ

Here ni are the orbital populations. Assuming the only contribu-tion to eQq in ScS is the unpaired electron in the hybridized sd orbi-tal, ndr = 1 and the other populations are zero. The term eQq320 canbe evaluated using the formula [26]:

eQq320 ¼ �2:3532lðlþ 1Þ

ð2lþ 3Þð2l� 1ÞQa0

r

� �3� �

ð5Þ

For a scandium nucleus, Q = �23.1 fm2 [26], l = 2 for a d elec-tron, and h1/r3i can be obtained from c, as discussed. The couplingconstant eQq is then calculated to be 31.46 MHz. This value isabout 56% of the experimental constant of 55.709 MHz, which sug-gests that core electrons in scandium sulfide also contribute to eQq.

Some insight into the bonding in ScS can be gleamed from acomparison of quadrupole parameters among scandium species,as listed in Table 4. For ScO, eQq = 72.24 MHz [20], while therespective values are 74.09 MHz and 68.21 MHz for ScF and ScCl.[26]. For ScS, the eQq = 55.709 MHz. Thus, while the quadrupoleconstants indicate that the electronic distribution is similar inthese molecules, there are some differences. ScS is apparently themost covalent of the four molecules, while ScF is the more ionic.This result is perhaps expected as oxygen and the halogens aremore electronegative than sulfur.

Finally, the values of the spin–rotation parameters should benoted. These constants are c = 96.3356 MHz and 42.252 MHz for

ScS and YS, respectively. The values of the spin–rotation parame-ters in the corresponding oxides are c = 3.2175 MHz (ScO) and�9.2254 MHz (YO). Very small or negative values of the spin–rota-tion constant in the oxides have been attributed to second-orderspin–orbit coupling from unobserved low-lying electronic 2Pstates, arising from the promotion of one electron from the HOMOp shell [22]. In the metal sulfides, other electronic states must becontributing to the second-order spin–orbit interaction, generatingnet positive spin–rotation constants.

5.2. Periodic trends in 3D-transition metal monosulfides

From the measured rotational constant of ScS in its groundstate, a r0 bond length of 2.1288 Å has been determined. In Fig. 3,the experimentally-determined bond lengths for the 3d transitionmetal oxides and sulfides are plotted, as well as the valuesobtained from theory using DFT methods [4]. The agreementbetween theory and experiment is rather good for ScS and ScO.Nonetheless, there are two notable differences between the oxideand sulfide series. First, the bond length of ScS is greater than thatof ZnS by almost 0.1 Å. In contrast, that of ScO is smaller than thebond distance of ZnO by about 0.03 Å. Secondly, while FeO, CoOand NiO have similar bond lengths, the bond lengths decreasesteadily from FeS to NiS.

This effect can be qualitatively explained by comparing theatomic orbitals. The energy separation between the 4s and 3d orbi-tals of the transition metals and the 2p orbital of oxygen is gener-ally larger than the separation with the 3p orbital of sulfur byabout 34,000 cm�1 [27]. Consequently, there is more valence orbi-tal overlap between the atoms in the monosulfides (i.e. increasedbonding character), while in the oxides, these orbitals are predom-inantly non-bonding. Therefore, addition of electrons into the va-lence orbitals partly stabilizes the monosulfide molecules,shortening the bond lengths, as noted by Bridgeman and Rothery[4]; this stabilization is not as significant for the monoxide species.

6. Conclusion

The pure rotational spectra of ScS and YS in their 2R+ groundstates have been measured using FTMW spectroscopy, in combina-tion with laser ablation. Spectroscopic constants have been im-proved for both radical species. Analysis of the hyperfineparameters indicates that both YS and ScS are somewhat morecovalent than their oxygen analogs. In addition, YS is slightly lessionic than ScS. These data support the theoretical prediction thattransition-metal bonds to sulfur are different than those to oxygen.

Acknowledgment

This work was supported by NSF Grant CHE-1057924.

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