Journal of Molecular Spectroscopy 289 (2013) 1–3

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Journal of Molecular Spectroscopy

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Note

Observation of a new X = 1 excited state in LaH

0022-2852/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jms.2013.05.003

⇑ Corresponding author at: Homi Bhabha National Institute, Bhabha AtomicResearch Centre, Mumbai 400 085, India. Fax: +91 22 25505151.

E-mail address: nakhate@barc.gov.in (S.G. Nakhate).

Suresh Yarlagadda a, Sheo Mukund a,b, Soumen Bhattacharyya b, S.G. Nakhate a,b,⇑a Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

a r t i c l e i n f o

Article history:Received 4 April 2013In revised form 10 May 2013Available online 23 May 2013

Keywords:Electronic state of LaHLaser-induced fluorescence spectroscopyJet-cooled spectra

a b s t r a c t

We report the observation of an X = 1 electronic state with T0 = 20041.96(1) and T1 = 21359.1(5) cm�1.This state was identified by analyzing the laser-induced fluorescence spectra of jet-cooled LaH molecule.The spectra showed two new bands having P, R and strong Q branch structures with band origin at19972.00(2) originated from v = 1 and 20041.96(1) cm�1 from v = 0 of the X1R+ ground state. The molec-ular constants of the excited state were determined. The dispersed fluorescence study also lead to theobservation of X = 2 spin-orbit component of the low-energy b3P state, which remain unobserved in ear-lier studies.

� 2013 Elsevier Inc. All rights reserved.

The spectrum of LaH in visible region was first observed by Ba-cis and Bernard [1]. These bands were classified [2] into severalelectronic transitions, assigned as 3U4–3D3, 3U3–3D2, 1D–1P, and1R–1P with 3D1 component unobserved. On the basis of primitiveab initio calculations on isovalent ScH [3], they suspected theground state of LaH as 3D. Later, Ram and Bernath [4] observedseveral bands in infrared region using Fourier transform emissionspectroscopy, which were classified into two new systems, A1P–X1R+ and d3U–a3D. Although, they had no direct experimentalproof, they assigned 1R+ rather than 3D as the ground state ofLaH on the basis of high level ab initio calculations of Das and Bal-asubramanian [5]. In light of the observation of X = 1 componentof the a3D1 state, they also reclassified the 1D–1P, and 1R–1P tran-sitions reported by Bernard and Bacis [2] to 3U2–a3D1 and 0+–a3D1.More recently, Bernard and Chevillard [6] have observed a C1R+

state at 10150.9 cm�1. In our recent work [7], we confirmed 1R+

symmetry of the ground state by observation of several bands inred-yellow region involving transition from the X1R+ to new ex-cited D1, E1 and 0+(3R�) electronic states. The dispersed fluores-cence from these new excited states established the missingenergy link between the singlet and triplet manifolds in LaH byobservation of a3D1,2, b3P0,1 and B1D2 low-energy states [8]. In thisnote, we report the observation of new high-energy excited elec-tronic state X = 1. The X = 2 spin-orbit component of low-energyb3P state, which was unobserved in our previous work [8] is alsoidentified.

The LaH molecule was produced in a laser vaporization/super-sonic molecular beam setup described in detail previously else-where [9]. The jet-cooled beam of LaH molecules was probed

with tunable dye laser and the resulting fluorescence was dis-persed by a monochromator and detected by photomultiplier tube.The monochromator was used as a broad band filter while record-ing the excitation spectra. However, it was used as a scanningmonochromator with reduced slit while recording dispersed fluo-rescence (DF) spectra.

The spectra due to LaH were observed in the range 19,850–20,140 cm�1 (Fig. 1). A strong and a weak band with origins respec-tively at 20,042 and 19,972 cm�1 are attributed to LaH. The bandswith origins respectively at 20,014 and 19,921 cm�1 are (3, 0) and(4, 1) bands of B2R+–X2R+ system of LaO. The excitation bands dueto LaH were observed to be very weak while the monochromatorwas parked at the excitation laser wavelength. However, the dis-persed fluorescence spectra from these bands showed a strongfluorescence at 1646 displacement wavenumber to the red fromthe excitation laser (Fig. 2). This line was assigned as fluorescenceto the v = 0 of a3D2 state in our earlier work [8]. The excitationspectrum shown in Fig. 1 was recorded by parking the monochro-mator at 1660 cm�1 red to the laser wavelength. The ground statevibration, xe of LaO is 817.26 cm�1. The fluorescence from LaO at1616 displacement wavenumber from excitation laser, terminatingto v = 2 of the ground state, appeared in the excitation spectrumsince the monochromator was used as broad band pass filter. Therotational structure in the new bands at 20,042 and 19,972 cm�1

show a single P, a single R and a single but intense Q branches.The rotational quantum number assignments for the P and Rbranches in both the LaH bands were straight forward (Fig. 1).The lower level combination differences for the 20,042 and19,972 cm�1 bands coincided within the experimental precisionrespectively with those of v = 0 and v = 1 levels of the X1R+ state re-ported earlier [4]. Due to unavailability of deuterated ammonia(ND3), our assignment of vibrational quantum number of v0 = 0 ofthe upper state is based on the observation of band on the lowest

19855 19910 19965 20020 20075 20130

LaO

*

P'(J)

R'(J)

Q'(J)

Q(J)

P(J)

R(J)0

24

68 10

15

24

68

1012

1416

110

16

110

02

52

57

*

3x

LaO

Wavenumber (cm-1)

Fig. 1. Rotational structure of the X = 1–X1R+ bands of jet-cooled LaH molecule.The PQR and P0Q0R0 represents (0,0) and (1,1) LaH bands respectively. Theoverlapping LaO bands are marked in the spectrum along with La atomic linesmarked with ‘�’.

(v=0)b3Π

0 2000 4000 6000

X1Σ+(v=0)

Displacement (cm-1)

1

v=0a3Δ

2

0 1 2

B1Δ2

(v=0)

4x

Fig. 2. Dispersed fluorescence spectrum of LaH observed by exciting the R(2) line ofthe (0,0) band of X = 1–X1R+ system.

Table 1Molecular constants (in cm�1) and radiative lifetimes for the X = 1 excited state ofLaH.

Constants v = 0 v = 1

Tv 20041.96(1) 21359.1(5)Bv 3.93916(28) 3.8468(15)Dv � 104 2.154(15) 2.38(18)qv � 102 �5.845(42) �3.5(1)qDv � 105 7.10(24) –Variance of fit in cm�1 (No. of lines) 0.14 (43) 0.21 (22)s (ns) 19.7 (16) 23.0 (8)

2 S. Yarlagadda et al. / Journal of Molecular Spectroscopy 289 (2013) 1–3

wavenumber side. The presence of P, Q, and R branch structure andthe first line assignment of P(2), Q(1) and R(0) in 20,042 and19,972 cm�1 bands confirm that the upper state has X = 1.Rotational perturbations were observed for the few lines in the20,042 cm�1 band. The Q(7) line showed irregular spacing and Qlines beyond J = 14 were untraceable due to interference fromLaO band (Fig. 1). The R lines beyond J = 15 could not be identifieddue to perturbations. The corresponding P lines viz. P(17) and P(18)were also perturbed and higher P lines were not traceable. Thisband also showed few extra lines, belonging to LaH that couldnot be grouped as members of the band. The combination

differences from the P and R branches for the 20,042 cm�1 bandindicated perturbations in the e-parity levels of the upper stateat least at J = 16 and 17. Also the irregular spacing in Q(7) line indi-cated perturbation in f-parity level at J = 7. As the 19,972 cm�1

band is a hot band, the rotational lines only up to R(5), P(9) andQ(9) were observed. No rotational perturbations were observedfor this band. The molecular constants for both the bands weredetermined by fitting the observed set of transition wavenumbersof the excitation bands to the rotational energy level expression ofthe form:

FvðJÞ ¼ Tv þ BvJðJ þ 1Þ � Dv½JðJ þ 1Þ�2 þ Hv½JðJ þ 1Þ�3

for the X1R+ ground state and

FvðJÞe

f

� �¼ Tv þ BvJðJ þ 1Þ � Dv½JðJ þ 1Þ�2 � 1=2fqvJðJ þ 1Þ þ qDv

½JðJ þ 1Þ�2g

for the excited state with X = 1. Here Bv, Dv, and Hv are the rota-tional constant and its higher-order corrections and qv and qD arethe X-doubling constant and its higher order corrections. Each rota-tional line was given an appropriate weight depending on the sig-nal-to-noise ratio and the extent of blending. The molecularconstants for the lower state of the 20,042 and 19,972 cm�1 bandswere constrained in the fit at the more precise values obtained byRam and Bernath [4] respectively for v = 0 and 1 level of the X1R+

ground state and the molecular constants for the upper states weretreated as adjustable parameters. The obviously perturbed lines inthe 20,042 cm�1 band were not considered in the fit. The molecularconstants obtained for the upper states are given in Table 1. Theterm energy of the excited state was determined as 21359.1(5)cm�1 by adding the energy of v = 1 level of the X1R+ state [8] tothe band origin of the 19,972 cm�1 band. The rotational constantfor the 21359.1 cm�1 state is smaller by �2% to that of20,042 cm�1 state. In addition, both the excited states have similarradiative lifetimes (Table 1), which suggests that the 21359.1 cm�1

state is v = 1 of the 20,042 cm�1 state. The DG1/2 value of1317.1(5) cm�1 for the X = 1 upper state is comparable to that oflow-lying states; viz. 1326.1 cm�1 for a3D1, 1312 cm�1 for b3P1

and 1283.6 cm�1 for A1P1 state [8].The DF spectrum from the R(2) line of the 20,042 cm�1 band

(Fig. 2) showed the transitions mainly to the triplet states, viz.a3D2 (v = 0 and 1) and v = 0 of b3P0,1,2 apart from the weak fluores-cence back to v = 0 level of the X1R+ ground state and v = 0 level ofthe B1D2 state. We have identified the X = 2 spin-orbit componentof b3P state at T0 = 4048(2) cm�1 and v = 1 level of a3D2 at 2966(9)cm�1 in the present work, whereas the observation of v = 0 of a3D2

and b3P0,1 states were reported earlier [8]. Thus contrary to theprediction of ab initio studies [5] the spin-orbit splitting of theb3P state is observed as regular and the spin-orbit separationrespectively of 190 cm�1 between X = 0 to 1 and 294 cm�1 be-tween X = 1 to 2 components of b3P state are in poor agreementwith the ab initio values respectively of �331 and �351 cm�1.

The DF spectrum from the 20,042 cm�1 state showed transi-tions mainly to the triplet states. This indicates that the state hastriplet multiplicity. The appearance of strong Q-branch in the spec-tra as well strong fluorescence to a3D2 state suggests the 3P ratherthan 3R� parent term of the state. However, the short radiativelifetime of �20 ns of this state contradicts with the triplet charac-ter. This probably indicates that X = 1 assignment in Hund’s case(c), usual for heavy molecules, is more appropriate for this state.

Acknowledgments

The authors are thankful to Dr. B.N. Jagatap, Head, Atomic andMolecular Physics Division, Bhabha Atomic Research Centre forhis constant encouragement and support.

S. Yarlagadda et al. / Journal of Molecular Spectroscopy 289 (2013) 1–3 3

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jms.2013.05.003.

References

[1] R. Bacis, A. Bernard, A. Zgainski, C.R. Acad. Sci. B 280 (1975) 77–78.

[2] A. Bernard, R. Bacis, Can. J. Phys. 54 (1976) 1509–1517.[3] P.R. Scott, W.G. Richards, J. Phys. B 7 (1974) 1679–1682.[4] R.S. Ram, P.F. Bernath, J. Chem. Phys. 104 (1996) 6444–6451.[5] K.K. Das, K. Balasubramanian, Chem. Phys. Lett. 172 (1990) 372–378.[6] A. Bernard, J. Chevillard, J. Mol. Spectrosc. 208 (2001) 150–151.[7] S. Yarlagadda, S. Mukund, S.G. Nakhate, Chem. Phys. Lett. 537 (2012) 1–5.[8] S. Mukund, S. Yarlagadda, S. Bhattacharyya, S.G. Nakhate, J. Chem. Phys. 137

(2012) 234309–(1–5).[9] S.G. Nakhate, S. Mukund, Chem. Phys. Letts. 496 (2010) 243–247.