JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 14
8 APRIL 2002
2
2
¿
The permanent electric dipole moments for the A ⌸ and B ⌺ states
2
and the hyperfine interactions in the A ⌸ state of lanthanum
monoxide, LaO
T. C. Steimlea) and Wilton Virgo
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604
͑
Received 3 December 2001; accepted 17 January 2002͒
2
2
ϩ
2
2
ϩ
2
ϩ
2
ϩ
The optical Stark effect in the ͑0,0͒ A ⌸3 –X ⌺ , A ⌸ –X ⌺ , and B ⌺ –X ⌺ band
/2
1/2
systems of a lanthanum monoxide, LaO, supersonic molecular beam sample have been analyzed to
2
2
2
ϩ
produce permanent dipole moments, , for the A ⌸3 , A ⌸ , and B ⌺ states of 1.89͑6͒ D,
/2
1/2
2
2
ϩ
2
.44͑2͒ D, and 0.2͑1͒ D, respectively. Fine structure splitting in the field free A ⌸ –X ⌺ and
A ⌸ –X ⌺ spectra were analyzed to produce the magnetic hyperfine spectroscopic parameters
3/2
2
2
ϩ
1
/2
aϭ233͑4͒ MHz, cϭϪ261͑12͒ MHz, and dϭ410͑4͒ MHz. A comparison with other group IIIA
monoxides is made. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1458245͔
I. INTRODUCTION
(2p )ϩ metal (n/s(nϪ1)d ) covalent bond. The 1 orbital
0 0
is an O (2pϮ1) ϩ metal (nϪ1)dϮ1 covalent bond which
facilitates -electron back donation. The variation of the per-
manent electric dipole moment amongst the four low-lying
states is primarily a reflection of the effect of variation in the
polarizability of the singly occupied orbital.
The group IIIA diatomic monoxides ScO, YO, and LaO,
are convenient systems for experimental studies of system-
atic trends in chemical bonding because of the ease of pro-
duction and the existence of intense, well characterized, vis-
ible electronic band systems which exhibit large magnetic
hyperfine structure. The limited number of valence electrons
for these simple molecules also makes quantitative theoreti-
cal predictions possible. An effective gauge of the accuracy
of an electronic structure calculation is its ability to ad-
equately reproduce values for the permanent electric dipole
moment, , which provides the most direct probe of the
charge distribution of the metal–oxide bond. In addition, the
permanent electric dipole moment enters into the description
of numerous physical phenomena including the interaction
with light. Here we report on the experimental determination
of in the low-lying excited states of LaO and comment on
the variation in the charge distribution amongst these and the
ground electronic state. In addition, the magnetic hyperfine
The most extensive theoretical prediction for the ground
and excited state properties of LaO was made by Schamps
1
et al. in which they compared electronic state distributions
and orbital compositions predicted by ligand field theory
͑
LFT͒ with those from an ab initio multiconfiguration self-
consistent-field-multireference configuration interaction
MCSCF-MRCI͒ calculation. Interestingly, the LFT, which
considers the electrostatic relaxation of the valence electrons
͑
ϩ2
Ϫ2
when the La and O point charges are placed at the equi-
librium internuclear distances, predicted the vertical excita-
tion energies for the AЈ ⌬ , A ⌸, and B ⌺ states some-
2
2
2
ϩ
r
what more accurately than the MCSCF-MRCI prediction.
The Mulliken population analysis of the MCSCF-MRCI pre-
ϩ0.74 Ϫ0.74
2
ϩ
2
diction set the net charge as La
state. This point charge corresponds to a dipole moment of
.5 D at the equilibrium bond distance of 1.825 Å. This is far
O
for the X ⌺
interaction in the A ⌸ state is analyzed and used to glean
insight into the nature of the unfilled molecular orbital asso-
ciated with this state.
6
2
ϩ
greater than the experimentally determined values of
The electronic wave functions for the X ⌺ ,
2
2
2
2
ϩ
3.207͑11͒ D, because it does not account for the opposing
AЈ ⌬ , A ⌸, and B ⌺ states are given to a first approxi-
r
moments due to the distortion of the singly occupied 3
orbital nor the distortion of the core orbitals on the oxygen
center. Neither a Mulliken population analysis nor values of
mation by a single Slater determinant derived from the mo-
lecular orbital configurations,
2
2
4
2
ϩ
͑
͑
͑
͑
core͒1 2 1 3→X ⌺ ,
͑1͒
͑2͒
͑3͒
͑4͒
for the excited states were given in Ref. 1. There have
2
2
4
2
been at least three other theoretical calculations that have
core͒1 2 1 1␦→AЈ ⌬r,
included predictions for the ground state dipole moment,
2
2
4
2
2
ϩ
3–5
3,4
core͒1 2 1 2→A ⌸ ,
(X ⌺ ),
The earlier predications produced values of
r
about 3.8 D and the more recent calculation produced values
ranging from 2.18 D to 2.37 D depending upon the compu-
2
2
4
2
ϩ
core͒1 2 1 4→B ⌺ ,
5
where the 3, 1␦, 2, and 4 orbital are various ligand field
induced mixtures of the metal centered ns and (nϪ1)d and
to a lesser extent the np orbitals. The 2 orbital is an O
tational method employed. The earlier predictions are in
slightly better agreement with the experimentally determined
value. None of these calculations included a prediction of
for the excited electronic states.
a͒Author to whom correspondence should be addressed. Telephone ͑480͒
A review of the extensive spectroscopy of LaO can be
found in the recently published analyses of the high tempera-
9
0021-9606/2002/116(14)/6012/9/$19.00
6012
© 2002 American Institute of Physics
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1
Steimle
