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Journal of Quantitative Spectroscopy &Radiative Transfer

Journal of Quantitative Spectroscopy & Radiative Transfer 147 (2014) 274–278

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Jet-cooled laser-induced fluorescence spectroscopyof ScH: Observation of an Ω0¼2�Ω″¼1 transition

Sheo Mukund a,b, Soumen Bhattacharyya a, S.G. Nakhate a,b,n

a Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, Indiab Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

Article history:Received 6 May 2014Received in revised form13 June 2014Accepted 16 June 2014Available online 25 June 2014

Keywords:Jet cooled spectra of ScHLaser-induced fluorescenceExcited electronic states

x.doi.org/10.1016/j.jqsrt.2014.06.01373/& 2014 Elsevier Ltd. All rights reserved.

esponding author at: Atomic and MolecuAtomic Research Centre, Mumbai 400085, I1 22 25592362; fax: þ91 22 25505151.ail address: nakhate@barc.gov.in (S.G. Nakha

a b s t r a c t

New bands of scandium monohydride at origins 17,914.5 and 17,942.3 cm�1 have beenobserved in a jet-cooled beam with laser-induced fluorescence spectroscopy. Mass-selectedresonant photoionization spectroscopy also confirmed the carrier of the band as ScH.The rotational analysis indicated that both transitions at 17,914.5 and 17,942.3 cm�1 areof Ω0 ¼2�Ω″¼1 type with vibrational assignments (0,0) and (1,1) respectively. The assignedg3Φ2–a

3Δ1 excitation is the first observed triplet–triplet transition in ScH.& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The electronic spectra of transition-metal-containingdiatomic molecules shed light on the role of d-electrons inthe chemical bond formation. The understanding of theirelectronic structure presents a serious challenge for boththeorist and experimentalist because of the complexelectronic structure arising from the partial occupation ofopen d orbitals. From the theorist point of view, evenqualitatively correct ab initio results for molecules contain-ing transition metal require incorporation of significantextent of electron correlation and relativistic effects com-pared to that of main group elements [1]. For this reason,experimental results on electronic states and the spectro-scopic properties of these molecules are of great impor-tance as they provide a critical test of adequacy of selectedmethods and level of theoretical treatment.

lar Physics Division,ndia.

te).

The spectra of ScH and ScD molecule were known since1973, when Smith [2] recorded a few complex bands inabsorption, mainly in the visible region. Bernard et al. [3]also observed ScH and ScD emission spectra in the nearinfrared and visible region. All these observed bandsremained largely unassigned because of limited resolution,poor signal-to-noise ratios and the complexity of thebands. Later, Ram and Bernath [4,5] studied the emissionspectra of ScH and ScD in the 3500–26,000 cm�1 range byFourier transform spectroscopy and observed several tran-sitions involving only singlet manifolds. They assigned theground state as 1Σþ based on ab initio predictions [6] andthe experimental results for isovalent YH [7], LaH [8] andalso ScF [9]. The observed bands were classified intoelectronic transitions between the ground X1Σþ , low-energy A1Δ and B1Π and high-energy C1Σþ , D1Π, E1Δ,F1Σ� , and G1Π states.

ScH is the simplest among transition-metal-containingmolecules. So, there have been large numbers of theore-tical studies on its electronic structure and spectroscopicproperties [6,10–15]. The ab initio calculations [6,11–13,15]predict the ground state of ScH as a 1Σþ state with a closelying 3Δ as first excited state. Chong et al. [14], in addition

17860 17880 17900 17920 17940 17960

32

1

52

4 3

7 6 5 4 3

5 72

65

43

21

R'(J)Q'(J)

P'(J)

R(J)Q(J)

P(J)

Wavenumber (cm−1)

Fig. 1. Rotational structure of the g3Φ2–a3Δ1 band of jet-cooled ScH

molecule. The PQR and P0Q0R0 represent (0,0) and (1,1) ScH bandsrespectively.

S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 147 (2014) 274–278 275

to the spectroscopic constants calculated dipole momentsof ground and low-lying states of first-row transitionmetal hydrides. Recently, Koseki et al. [15] calculatedpotential energy curves of low-lying spin-mixed statesfor group 3 and 5 hydrides and also reported transitionenergies and transition moments for few selected transi-tions and compared them with experiments.

Ram and Bernath [4,5] assigned the observed emissionspectra to various transitions involving only the singletmanifold. They as well as Bernard et al. [3], also reportedthe observation of complex bands at 11,620 and 11,630 cm�1

assigned respectively to ScH and ScD. Although the bandscould not be rotationally analyzed due to the complexstructure or assigned to a particular transition, they specu-lated it to be analogous with the d3Φ–a3Δ transitions observedin isovalent YH [7] and LaH [8] in the same energy region.Bernard et al. [3] also observed a Q-form branch at 17,900cm�1 which also could not be analyzed. The origin of thisstructure was speculated due to analogous 3Φ–a3Δ transitionobserved in YH [16] and LaH [17] in the same energy region.The missing observation and assignment of transitions in thetriplet manifold of ScH, which otherwise are well character-ized in isovalent YH and LaH, motivated us to take up thepresent study.

2. Experiment

The ScH molecules were produced in a laser vaporiza-tion/supersonic molecular beam setup. The source and thefree-jet part with laser-induced fluorescence detectionwere described in detail previously [18]. In brief, the jet-cooled beam of ScH molecules were probed with a tunabledye laser and the resulting fluorescence was dispersed by amonochromator and detected by a photomultiplier tube.The monochromator was used as a broad band filter whilerecording the excitation spectra. However, it was used as ascanning monochromator with reduced slits while record-ing dispersed fluorescence (DF) spectra.

We have also employed the mass-selected resonantphotoionization spectroscopy technique in the presentexperiment. The free-jet was skimmed by a 5 mm dia-meter skimmer placed at a distance 200 mm from thepulsed valve nozzle. A resulting molecular beam (MB)entered the second vacuum chamber which was differen-tially pumped by a turbomolecular pump with pumpingspeed of 1100 L/s. The MB is then crossed with the counterpropagating resonant excitation tunable dye laser and XeClExcimer ionizing laser beams at a distance of 620 mmfrom the nozzle. The photoions generated were massanalysed by a Wiley–McLaren type time-of-flight massspectrometer equipped with a dual microchannel platedetector. An output signal from the microchannel platedetector was amplified by a 1 GHz bandwidth amplifier.The resonant photoionization spectrum was recorded byintegrating the signal corresponding to the ScHþ masspeak on a gated integrator and stored on a computer. Theexcitation spectra were obtained with a typical laser line-width of 0.1 cm�1. Transition wavenumbers were cali-brated with an absolute precision of �0.1 cm�1 by alaser wavelength calibration facility of the dye laser whichutilized Ne optogalvanic spectral lines.

3. Data analysis and results

3.1. Appearance of the spectra

We have used naturally abundant ammonia in our experi-ments and scandium has one naturally occurring isotope Sc-45. Thus excitation spectra of only the 45ScH isotoplogue wereobserved in our experiments. Two new violet degraded LIFexcitation bands with neighboring origins at 17,914.5 and17,942.3 cm�1 with rotational line spacing typical to ScHwere observed (Fig. 1). The dispersed fluorescence (DF) fromScH lines showed fluorescence only to the excitation wave-length and not to any of the vibrational levels of the lowerstate involved in the transition for further confirmation of thecarrier of the band to ScH. The carrier of these bands wasconfirmed by obtaining the mass-selected resonance photo-ionization excitation spectra of ScH which is identical to thatof the LIF excitation spectra. Both the bands at 17,914.5 and17,942.3 cm�1 showed intense single R- and Q- and a weaksingle P-branch indicating a ΔΛ¼þ1 transition. The band at17,942.3 cm�1 was 5 times weaker than the band at17,914.5 cm�1. The later band was observed up to J¼7 forP- and Q- and J¼6 for R-branch however, P-, Q-, and R-branches for 17,942.3 cm�1 band ran up to J¼4, 5, and 3respectively. The broad spectral feature at about 17,890 cm�1

does not belong to ScH. The dispersed fluorescence obtainedfrom this broad feature showed three lines with successivewavenumber separation of about 870. The carrier of thisspectral feature could not be identified.

3.2. Assignments of bands and molecular constants

The assignments of bands are based on the observationof the number of branches and their first line, combinationdifferences for the lower state, relative intensities of thebranches within the band and overall band intensity.Rotational quantum number assignments of the P- andR-branches were carried out using the combination differ-ences method. Distinctly different lower state combination

S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 147 (2014) 274–278276

differences for the 17,914.5 and 17,942.3 cm�1 bands fromthose of ground X1Σþ state [4] indicated that thesetransitions originate from low-lying excited states. Ourpast experiments on LaH showed that the a3Δ1 state at1260 cm�1 was populated in the jet-cooled beam [19,20].The ab initio calculations of ScH [6] predict a 3Δ state asthe first excited state at 2581 cm�1. No Ω-splitting wasobserved in rotational lines observed up to maximum of J″¼7 in P- and Q-, and J″¼6 for R-branch. This is expectedin the spectra obtained at a laser linewidth of 0.1 cm�1

and for a small Ω-doubling constant qv of the order of 1–2�10�3 typical for the a3Δ1 state in YH [7] and LaH [8].With no Ω-splitting seen at the employed experimentalresolution, we have assigned rotational quantum numberto the Q branch by comparing the separation Δ1F(J) of thesuccessive rotational levels obtained from correspondingPQ and RQ pair of lines with that of Δ2F(J) obtained fromcorresponding RP pair of lines [21]. Fig. 1 shows therotational quantum number assignment of the P-, Q- andR-branches for both the bands. The unassigned lines in theexcitation spectra also fluoresce 1600 cm�1 to the red ofexcitation wavelength. The excitation spectra recordedwhile parking the monochromator at 1600 cm�1 to thered of excitation wavelength showed mostly unassignedlines. As is evident from Fig. 1, the first line in P-, Q- and R-branches for both the bands are R(1), Q(2), and P(3) whichindicate that the transitions are Ω0 ¼2�Ω″¼1. The ab initiocalculations [6] predict a 3Δ state as the first excited stateat 2581 cm�1. Bernard and Basis [16,17] reported theviolet degraded 3Φ–a3Δ transitions respectively at 17,899and 16,007 cm�1 for isovalent YD and LaH. As mentionedearlier, Bernard et al. [3] also observed only a Q-formbranch at 17,900 cm�1 belonging to ScH which could not

Table 1Molecular constants (cm�1) for the g3Φ2–a

3Δ1 system and radiative lifetimes (n

Constants a3Δ1

v¼0 v¼1

Tv a bBv 3.9391(69) 3.863(9)Dv�103 �4.33(27) �4.11(36)Hv�105 �1.90(30) –

τ – –

Table 2Observed line positions (cm�1) of the g3Φ2–a

3Δ1 system of ScH.

J (0,0) band of g3Φ2–a3Δ1

R(J) O�C Q(J) O�C P(J) O�C

1 17,919.30 0.002 17,929.78 0.05 17,903.44 0.003 17,940.77 �0.14 17,905.77 0.07 17,879.38 �0.034 17,952.63 �0.03 17,908.56 0.10 17,873.21 �0.055 17,964.80 0.06 17,911.55 0.03 17,867.31 �0.016 17,976.86 �0.01 17,914.61 �0.06 17,861.46 0.017 17,917.75 0.01 17,855.56 0.01

Note: O�C are observed minus calculated line positions.

be analyzed. At 17,903 cm�1, we also observed a Q-branch.The above observations indicate that the neighboringbands at 17,914.5 and 17,942.3 cm�1 can be plausiblyassigned as (0,0) and (1,1) band of the g3Φ2–a

3Δ1 transitionin ScH. The nomenclature for the newly observed 3Φ stateas the ‘g’ state is taken from the equivalent 3Φ state assig-ned by Ram and Bernath in YH [7]. Due to the unavail-ability of deuterated ammonia (ND3) in our laboratory,isotope shift studies for vibrational quantum numberassignments of the electronic states could not be carriedout. However, our assignment of the v¼0 levels for theupper and lower electronic state was based on the obser-vation of bands on the lowest wavenumber side. Despiteour best efforts we were unable to observe sub-bandscorresponding to the other spin–orbit g3Φ3–a

3Δ2 andg3Φ4–a

3Δ3 transitions.In the absence of the observation of transitions among

other spin–orbit components of the a3Δ and g3Φ states, theobserved wavenumbers of the (0,0) and (1,1) g3Φ2–a

3Δ1

sub-bands were fitted by treating each spin–orbit compo-nent as a separate Hund's case (c) state. The term energyexpression for Hund's case (c) is given by

FvðJÞð ef Þ ¼ TvþBv½ðJþ1Þ�Ω2��Dv½JðJþ1Þ�Ω2�2

þHv½JðJþ1Þ�Ω2�37 ð1=2ÞqvJðJþ1Þ ð1Þ

Here Bv, Dv, and Hv are the rotational constant and its higher-order corrections and qv is a Ω-doubling constant. In theabsence of observation of Ω-splitting, the value of qv wastaken as zero. The molecular constants for the v¼0 and 1level of the a3Δ1 and g3Φ2 states were determined in the fitand are given in Table 1. The observed line positions are listedin Table 2. Equilibrium rotational constants, Be, αe and re

s) of ScH.

g3Φ2

v¼0 v¼1

aþ17,914.50(4) bþ17,942.29(4)4.3669(71) 4.289(9)�1.62(30) �1.15(44)�1.15(36) –

42(2)

(1,1) band of g3Φ2–a3Δ1

R(J) O�C Q(J) O�C P(J) O�C

17,946.94 �0.0617,957.37 0.11 17,931.44 �0.0117,968.19 �0.02 17,933.66 �0.02 17,907.92 0.04

17,936.28 �0.04 17,901.79 0.0017,939.11 0.02

S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 147 (2014) 274–278 277

respectively of 3.977(11), 0.076(11) cm�1 and 2.0737(14) Å forthe a3Δ1 state and 4.406(11), 0.078(11) cm�1 and 1.9701(12)Å for the g3Φ2 state were determined.

4. Discussion

The observation of two new bands led to the assignmentof two new electronic states of ScH with Ω¼2 and 1 in v¼0and 1 levels. Based on the nature of the experiment whereonly ground and low-energy states are populated, similaritiesin transition energies and rotational structure of the observedbands with those observed in isovalent YH and LaHmoleculesand also ab initio calculations [6], the newly observed statesare assigned as a3Δ1 and g3Φ2. The term energies of thesetriplet electronic states with respect to the ground state couldnot be determined due to the absence of observation of theintercombination transition. However, ab initio calculations ofAnglada et al. [6] predict the a3Δ state at 2581 cm�1. Thesecalculations predict transition energies with an uncertainty ofabout 72000 cm�1 and transition moments for many elec-tric dipole allowed transitions. The observed g3Φ2–a

3Δ1 tran-sition at 17,914.5 cm�1 in the present work is in goodagreement with their calculated 23Φ–13Δ transition at19,200 cm�1 with a substantial transition moment of 0.99 a.u. While g3Φ2–a

3Δ1 (0,0) and (1,1) bands were observed withconsiderable intensity, transitions to the other spin–orbitg3Φ3–a

3Δ2 and g3Φ4–a3Δ3 components were not observed.

One of the reasons for lack of observation of other spincomponents of g3Φ state could be predissociation. This hypo-thesis may be likely as it does not contradict the experimen-tally estimated dissociation energy D0 of 16,6157725 cm�1

[22] and also ab initio De value of 18,850 cm�1 [6]. Theobservation of only one g3Φ4�a3Δ3 out of the three possiblesub-bands in YH by Bernard and Bacis [16] was conjecturedon the basis of predissociation of g3Φ2 and g3Φ3 spincomponents.

The negative sign of the centrifugal distortion constantD for both the states, especially in the lower a3Δ1 state(Table 1) is quite unusual and indicates perturbation. Taheret al. [23] and Xia et al. [24] observed perturbationsrespectively in v¼1 and 2 of the ground X1Σþ state inScI. They explained this perturbation due to L-uncouplingand spin–orbit interaction between X1Σþ and a3Δ1 statesvia 1Π and 3Π states. The ΔG1/2 of the X1Σþ state is1547 cm�1 and thus energy-wise v¼2 of this state canperturb the a3Δ1 state through above interaction viapredicted b3Π [6] and observed B1Π states [5]. FollowingTaher et al., we attempted a deperturbation analysis byadding a H12[J(Jþ1)]1/2 interaction term in a 2�2 matrixmodel to fit the lower state term energies, determined bycombination differences. However, we did not get mean-ingful parameters which reproduce lower state termenergies. The parameters indicated e/f parity level separa-tions of few cm�1 which was not observed experimentally.The fact that Ram and Bernath [4] did not observe anyperturbations in B1Π (v¼0–2) indicates that this state doesnot interact with a3Δ1 state. The direct interaction of theb3Π with the a3Δ1 state were not considered as no extralines originating from the b3Π state were observed. Abetter perturbation analysis could be possible with theobservation of high J lines, which may help to obtain true

rotational constants for a3Δ1 state from the unperturbedportion of the spectra. The narrow energy window offeredby jet-cooled spectra restricts the observation only to afew low J lines, which makes analysis difficult.

5. Conclusion

The present work reports the first observation of anelectronic transition among the triplet manifolds in ScH.Laser-induced fluorescence and mass-selected resonancephotoionization spectroscopy of jet-cooled ScH moleculesled to the observation of a (0,0) and (1,1) bands of Ω0 ¼2�Ω″¼1 transition now assigned as g3Φ2–a

3Δ1. Rotationalconstants determined from the fit indicated perturbationsin both states.

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