Electronic structure of bali: The (2)2∑+ state

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Electronic structure of BaLi: the (2) 2Sigma+ state

View the table of contents for this issue, or go to the journal homepage for more 1994 J. Phys. B: At. Mol. Opt. Phys. 27 L153

(http://iopscience.iop.org/0953-4075/27/8/004)

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J. Phys. B: AI. Mol. OpL Phys. 27 (1994) L153-Ll55. Prinred in the UK

LETTER TO THE EDITOR

Electronic structure of BaLi: the (2)% state

J

Verghst, J d’Incan1, C Effantint, A Bernards, G Fabrell, R Stringat11 and A BoulezbarB

t L a b m o i r e Aim6 Covon. CNRS I1,91405 Onay, France

t

Labratoire de Spectrom6trie lonique el Mol6culaire, CNRS et Univenitd Claude Bernard.

Lyon I, 43 Bd du 11 Novembre 1918,69622 Villeurbanne, France

6 Observaloirc de Lyon. CNRS et Universit6 Claude Bernard. _~. . , Lyon 1.69561 Saint-Genis-

.

Laval. France

11 Laborafoire d’Optique Alomique el Mol€ceulaire, Facult6 des Sciences UNSA. 06108 Nice,

FIUW2

1

Labraloire de Physique Atomique el MoI6culaire. Univenil6 Hassan 11, Facultd des Sciences, Ain Chok, BP 5366. M b i f . Casablanca, Morocco

Received 18 February 1994

Abstract. A System of the BaLi molecule observed in ^the infrared at 55006300 cm-’ is ascribed lo a transition between a ’E+ slate and the ground stale X ’E+. The (00) band is mlationally analysed. The upper elecvonic state at an energy of about 5800 cm-’ is identified with the previously predicled (2)’X+ state, Values of constvlu (in cm-‘, for v = 0, Ba’Li) are as follows: Tm = 5719.57, Bo = 0.21683, IO6& = 0.8W. yo = 0.156.

The thermal emission at high temperature of the BaLi molecule has been spectroscopically investigated at high resolution with the Fourier transform spectrometer of Laboratoire Aim6 Cotton at Orsay. In a recent paper (d’Incan

el ol 1994)

details have been given concerning the experimental conditions, and the analysis of the (0-0) band of the (2)2n + X *E+

system, which lies in the region

7550-8400

cm-I, has been reported for both isotopomers, Ba6Li and Ba7Li. The energy found for the level

U

= 0 of the 211 state (approximately 7823 cm-’ above X

’E+(u =

0)) allowed

us

to identify this state with the

(2)’il

state predicted by AIlouche and Aubert-Frhon

(1994).

It bas come to light that the fainter molecular emission observed in both Ba6Li and Ba7Li spectra at longer wavelengths could be attributed to bands of a new system. On

the basis of the known rotational constants in the ground state

^U

= 0 level, the rotational

struchue

of the strongest band could be interpreted in terms of a ‘Z’+-X 2C+(u = 0) transition. However, the relative intensities of the branches indicate that the transition is not a pure parallel one since the only apparent branches are PI and RZ of almost equal intensities and, relatively weaker, the satellite branches

‘Qlz

and

R Q z l

(figure

1).

The band could be rotationally analysed up to N

R

75 in the case of Ba’Li and N

95

in the case of Ba6Li without observing any peturbation effects and a fit of the wavenumbers of all the lines observed was performed. Standard term values for

*C+

states were used to derive the rotational constants in the upper level, the parameters in the ground state being held fixed at the values obtained from the treatment of the

(2)’Il --f

X

2C+(0-0)

band (d’Incan

et al 1994).

Under these conditions no ambiguity remains on the positive value of the spin splitting

y

parameter in the upper state.

0953-4W5/94/080153CO3s1950 @

1994

IOP Publishing Ltd

L153

L154 Letter to the Editor

P, 20.5 30.5

C ' I I

rr, 15.5

20.5 30.5 'QL2

I I

5785 57% "1

(11.5 45.5 P,

20.5 30.5

R.

1

I I

25.5 30.5

'

^{Q 2 1}

I I

5805 5B15 SB25

Figure 1. part of the (2)*Zf -X 2Z+ emission specfrum for Ba6Li.

wing the (MI n _SY em

The vibrational assignment

(M))

of the band was justified

a

posteriori from the calculation of Franck-Condon factors in agreement with the observed band intensity distribution.

Among the predicted

'C+

electronic states (Allouche and Aubert-Fr6con 1994) only the

(2)*C+

state is expected to lie about 6000 cm-l above the ground state so that it may be identified unambiguously as the upper state of the present transition.

The following values (in cm-') of the rotational parameters are found in the ( 2 ) 2 Z + ( ~

=

0) level of Ba'Li (first entry) and of Ba6Li (second entry) with, between brackets, an uncertainty of 2u in units of the last digit.

Table 1.

T = 5779.570(6) 5780.800(2)

B=O.216825(7) ldD=O.799(3) 10i2H=0.7(3)

0.251 0260) 1.0&13(5) 0.88(3)

y = 0.1561(2) I @ Y N = -0.16(1) I O ' O ~ N N = -0.W)

0.18025(6) -0.1%(3) -0.97(3)

This set of constants allows the observations to be reproduced to within the experimental error of the measurements, the standard deviation of the fits being

0.004

cm-'.

The validity of the model predictions (Allouche and Aubert-Fricon 1994) was verified with the (2)'iI state and is now confirmed with the

(2)*C+

state for which the theoretical values, Tw = 6090 cm-' and BO = 0.213 cm-' corresponding to Ba'Li, agree well with the experimental results.

Worthy of note are the following two points.

(i) The relative intensities between the branches of the (2)'CC-X

'Cc

transition, e.g.

respectively 1.1, 0.0,

0.0, 1.0, 0.3.0.4

in P I , Pz, RI, R2,

'Ql2. R Q z l

at N = 3 0 for Ba6Li,

do not correspond to the ones expected for a pure parallel transition between two case b

states; these appear rather compatible with values of the transition moments jq

^%

0.6 and

a

-0.4 (estimated from branch intensity expressions given by Kopp and Hougen (1967)

Letter to the Editor L155 for $-4 transitions in which the $ states are

not

affected by states having a different value of a).

(ii) The fact that the values of

y

in (2)2Z+ and

p

in (Z)Zil are close together suggests that the two states are in a unique perturber relationship (Lefebvre-Bnon and Field 1976);

however, a tentative attempt to interpret them simultaneously in this model has shown it to be too simple to represent adequately the observations.

These features and some others, e.g. the difficulty already mentioned (d'Incan

et a1

1994) in modelling the spin-orbit interaction in (2)Zll, are obviously due to mixing effects between the low-lying states. More. work is now envisaged in order to characterize the states which may interact with

(2)2C+ and (2)'Il and thus

try

to explain the peculiarities

of the spectra.

We are grateful to Dr R F Barrow of Oxford University for valuable comments

References

Allouche A R and Aubert-Fdcon

M

1994 3. Chem Phys. 100 9 3 8 4

d'lncan 1, Effantin

C

Bernard A, Fabre 0 , Stringat R, B o u l e z h A and Ver@s J 1994 J. Chem Phys. 100 945-9 Kopp I and Hougen 1 T 1961 Can. J, Phys. 45 2581-96

Lefebvre-Brio" H and Field R W 1986 Perfurbations in fhe Specfro of Diatomic Moleerrlcs (Orlmdo.

n:

Academic)