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Journal of Quantitative Spectroscopy &Radiative Transfer

Journal of Quantitative Spectroscopy & Radiative Transfer 145 (2014) 17–19

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Notes

Observation of two new Ω¼0þ electronic states in LaH

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

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

a r t i c l e i n f o

Article history:Received 10 March 2014Received in revised form9 April 2014Accepted 20 April 2014Available online 28 April 2014

Keywords:Gas phase lanthanum hydride spectraExcited electronic statesLaser-induced fluorescence spectroscopyFree-jet molecular beam

x.doi.org/10.1016/j.jqsrt.2014.04.02373/& 2014 Elsevier Ltd. All rights reserved.

esponding author at: Atomic and MolecuAtomic Research Centre, Mumbai 400085,62;1 22 25505151.ail address: nakhate@barc.gov.in (S.G. Nakha

a b s t r a c t

Two new excited electronic states of LaH were identified at T0¼21970.7 and 22100.3 cm�1

in rotationally resolved laser-induced fluorescence jet-cooled excitation spectra. Theobservation of a single P- and R-branch structure in both the bands, originating fromν¼0 of X1Σþ ground state confirms Ω¼0þ for both the states. While the rotational linesup to P(7) and R(6) were observed in the 21970.7 cm�1 band, lines up to P(12) and R(10)were observed in the 22100.3 cm�1 band. The molecular constants for both the excitedstates were determined.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Bacis and Bernard first reported the electronic spectraof LaH in the visible spectrum [1] and identified 3Φ4–

3Δ3,3Φ3–

3Δ2, 1Δ–1Π, and 1Σ–1Π electronic transitions [2]. Theysuspected 3Δ as the ground state based on the available abinitio calculations on isovalent ScH [3]. Later, Ram andBernath [4] observed A1Π–X1Σþ and d3Φ–a3Δ bands in theinfrared region using Fourier transform emission spectro-scopy and assigned 1Σþ as the ground state of LaH on thebasis of high level ab initio calculations of Das andBalasubramanian [5]. The observation of Ω¼1 componentof the a3Δ1 state helped them to reclassify the previouslyreported 1Δ–1Π, and 1Σ–1Π transitions [2] to 3Φ2–a

3Δ1 and0þ–a3Δ1. More recently, Bernard and Chevillard [6] haveobserved a C1Σþ state at 10150.9 cm�1. In our recentwork, we confirmed 1Σþ symmetry of the ground stateby observing bands originating from the X1Σþ state to

lar Physics Division,India. Tel.: þ91 22

te).

several singlet and triplet excited states [7,8]. The dis-persed fluorescence from these excited to the low-energya3Δ, b3Π and A1Δ states helped us to establish the missingenergy link between the singlet and triplet manifoldsin LaH [9]. In this note, we report the observation oftwo new Ω¼0þ excited electronic states at 21970.7 and22100.3 cm�1.

2. Experiment

The LaH molecule was produced in a laser vaporization/supersonic molecular beam setup described previously indetail elsewhere [10]. A rotating and translating lantha-num metal rod was ablated by �10 mJ/pulse, 355 nmradiation from a Q-switched Nd:YAG laser focused to an�1 mm2 spot. The resulting lanthanum plasma reactedwith 2% ammonia seeded in He emerging from a pulsevalve under a backing pressure of 275 kPa. Approximately5 cm downstream of the nozzle, the free-jet was probed bya tunable dye laser at right angles to the beam expansionaxis. The resulting laser-induced fluorescence (LIF) wascollected orthogonal to both free-jet and dye laser beamaxes, dispersed by a monochromator and subsequently

Table 1Molecular constants (cm�1) and radiative lifetimes (ns) for the Ω¼0þ

excited states of LaH. The number in parentheses denotes one standarddeviation in the last quoted digits.

Constants Ω¼0þ Ω¼0þ

Tv 21970.71 (2) 22100.31 (4)Bv 3.5070 (20) 3.2902 (49)Dv�103 �0.446 (37) �5.18 (18)Hv�105 – �3.75 (21)Lv�107 – 1.11 (8)τ 37 (4) 39 (2)

S. Yarlagadda et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 145 (2014) 17–1918

detected by a Peltier-cooled photomultiplier tube. Themonochromator was used as a broad band filter whilerecording the excitation spectra. An output signal from thephotomultiplier was amplified by a 1 GHz bandwidthamplifier, integrated by a gated integrator, and stored ona computer.

The LaH excitation bands were weak in intensity whilethe monochromator was parked at the excitation laserwavelength. However, the dispersed fluorescence spectrafrom both bands showed a strong fluorescence to v¼1 ofthe b3Π1 state displaced by about 5066 cm�1 from thelaser line [9]. The excitation spectrum shown in Fig. 1 wasrecorded by parking the monochromator at 5066 cm�1 tothe red of the laser wavelength. Atomic Ne lines [11]recorded by optogalvanic spectroscopy and atomic La linesappearing in the LIF spectra were used for wavelengthcalibration.

3. Data analysis and results

Since lanthanum has only one naturally occurring isotopeand naturally abundant ammonia was used, two bandsobserved with origins at 21970.7 and 22100.3 cm�1 belongedto 139La1H. Both bands originated from the v¼0 of X1Σþ state.The rotational structure in these two bands shows a single P-and a single R-branch, which confirmsΩ of the upper states as0þ . The rotational quantum number assignments for the P-and R-branches in both the LaH bands were straightforward(Fig. 1). The lower level combination differences for thesebands coincided, within the experimental precision, withthose of v¼0 level of the X1Σþ state [4]. Due to theunavailability of isotopically enriched deuterated ammonia,our assignment of vibrational quantum number v0 ¼0 of the

21880 21920 21960 22000

6

*

P Branch R6207 5 4 3 2 1

9101112

*

Wavenumber

Fig. 1. Rotational structure of the (0,0) 0þ–X1Σþ bands at 21,971 and 22,100asterisks. The simulated spectrum (inverted) was generated with derived moestimated rotational temperature of 120 K.

upper state is solely based on the observation of this band onthe lowest wavenumber side.

The molecular constants for both bands were deter-mined by fitting the spectra using Pgopher program [12]with Hamiltonian of the form

FvðJÞ ¼ TvþBvJðJþ1Þ�Dv½JðJþ1Þ�2þHv½JðJþ1Þ�3þLv½JðJþ1Þ�4;

for the upper and lower states. Here Bv, Dv, Hv, and Lv, arethe rotational constants. While the molecular constants forthe lower state of these bands were constrained in the fitto the more precise values [4], parameters for the upperstates were adjusted and are given in Table 1. Both theexcited states have similar radiative lifetimes. The negativeDv, value in both the excited states is unusual. Theinteraction of the observed states with other levels, whichcould be a dark state in our spectra, can cause thisapparent centrifugal distortion effect. No local perturba-tion in term energies was observed in the reduced termenergy plotted against J(Jþ1) for both the electronic states.However, intensity perturbation was observed in both the

22040 22080 22120

**

Branch

P Branch

0 2 5 10

1234567

R Branch

(cm-1)

cm�1 of jet-cooled LaH molecule. The La atomic lines are marked withlecular constants in Table 1, a spectral line width of 0.15 cm�1 and an

S. Yarlagadda et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 145 (2014) 17–19 19

bands. As seen from Fig. 1, the R(6) and P(8) rotational lineswere not observed in the 22,100 cm�1 band while R(5), R(6) and P(7) line intensities in the 21,971 cm�1 band weredrastically reduced.

The DF spectrum recorded from isolated rotationallines of both bands showed transitions to mainly tripletstates. Both states mainly fluoresce to v¼1 of b3Π1. Inaddition, a weak fluorescence was observed from the22,000 cm�1 state to v¼0–1 of a3Δ1, v¼1 of b3Π0 andv¼0 of A1Π and from 21,971 cm-1 state to v¼1 of a3Δ1

state. The transition to the low-lying triplet states indi-cates that both states have triplet multiplicity. However,the short radiative lifetime of �40 ns of these statescontradicts the triplet character. This probably indicatesan Ω¼0þ assignment in Hund's case (c), usual for heavymolecules, and is more appropriate for this state. This wasalso the case for an Ω¼1 excited state at 20,042 cm�1

reported in our earlier work [8]. No theoretical predictionsare available for the electronic states reported in thepresent work.

References

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[2] Bernard A, Bacis R. Analysis of the 3Φ-3Δ, 1Δ-1Π, and 1Σ-1Πsystems of lanthanum hydride, LaH and LaD. Can J Phys 1976;54:1509–17.

[3] Scott PR, Richards WG. On the low-lying electronic states of ScH. JPhys B 1974;7:1679–82.

[4] Ram RS, Bernath PF. Fourier transform emission spectroscopy of newinfrared systems of LaH and LaD. J Chem Phys 1996;104:6444–51.

[5] Das KK, Balasubramanian K. Spectroscopic constants and potentialenergy curves of electronic states of LaH. Chem Phys Lett 1990;172:372–8.

[6] Bernard A, Chevillard J. On the C1Σþ state of LaH. J Mol Spectrosc2001;208:150–1.

[7] Yarlagadda S, Mukund S, Nakhate SG. Jet-cooled laser-inducedfluorescence spectroscopy of LaH: observation of new excitedelectronic states. Chem Phys Lett 2012;537:1–5.

[8] Yarlagadda S, Mukund S, Bhattacharyya S, Nakhate SG. Observationof a new Ω¼1 excited state in LaH. J Mol Spectrosc 2013;289:1–3.

[9] Mukund S, Yarlagadda S, Bhattacharyya S, Nakhate SG. Energylinkage between the singlet and triplet manifolds in LaH, andobservation of new low-energy states. J Chem Phys 2012;137:1–5 (234309).

[10] Nakhate SG, Mukund S. Electronic structure of ScN: jet-cooled laser-induced fluorescence spectroscopy. Chem Phys Lett 2010;496:243–7.

[11] Saloman EB, Sansonetti CJ. Wavelengths, energy level classifications,and energy levels for the spectrum of neutral neon. J Phys Chem RefData 2004;33:1113–58.

[12] C.M. Western, PGOPHER, a program for simulating rotational struc-ture, University of Bristol; 2010 ⟨http://pgopher.chm.bris.ac.uk⟩.