1. Electronic bands of ScC in the region 620 – 720 nm
Chiao-Wei Chen, Anthony J. Merer and
Yen-Chu Hsu
Institute of Atomic and Molecular Sciences, Taipei, Taiwan

2. Left-hand side of the Periodic Table
Li Be
Na Mg
Transition metals
K Ca Sc Ti V ….
Rb Sr Y Zr Nb ….
Cs Ba La …. (Rare earths)

3. Motivation
Scandium oxide, ScO, gives prominent bands in the spectra of M-type stars, along with TiO, VO and ZrO. Since C has roughly the same cosmic abundance as O, the carbides of these metals should also be astrophysically important.
Diatomic carbide spectra have been observed for all the early 3d and 4d transition metals with the exception of Sc.
The observed spectra are immensely complicated and in many cases “only the surface has been scratched”. Will ScC, with the fewest electrons, help to clarify these spectra?

4. Experimental details
A rotating scandium rod is ablated by 532 nm radiation from a pulsed Nd:YAG laser.
The resulting plume of metal vapour is entrained in a supersonic jet-cooled puff of gas, typically 0.08% acetylene in argon at a backing pressure of 3 atm.
Reaction generates ScC molecules, which are observed by laser-induced fluorescence. So far only medium resolution spectra have been recorded, using a pulsed dye laser.

6. v'
= 3 2 1
Studied so far

7. (Scandium plus acetylene)

9. Ω = 5/2 – 3/2
7/2 – 5/2
7/2 – 5/2
v′ = 1
v′ = 0
v′ = 0
? 5/2 – 3/2

10. Ground state constants of ScC
Ω″ = 5/2
Ω″ = 3/2
Sc12C
B (10) (6) cm-1
104 D (9) (43) s
Sc13C
B (8) (9)
104 D (6) (8) s
1.0619
B(12C)/B(13C)
1.0597
m13/m12 =

11. X4Pi or X2Di ground state?
The observed W″ = 5/2 and 3/2 lower states, with
B(3/2) > B(5/2), are consistent with X4Pi or 2Di.
We cannot distinguish these from the spectrum
, but
The isoelectronic YC has a 4Pi ground state
(Simard et al, Chem. Phys. Lett. 230, 103 (1994)).
The low-lying molecular orbitals and electron
configurations do not give rise to any 2Di states.

12. Molecular orbital scheme for ScC
Ground state: π3 σ σ 4Πi

13. Ground state bond length and spin-orbit coupling
The B value of the hypothetical middle of the X4P state is
cm-1 (12C). This gives the bond length as
r0 = Å.
From Mulliken’s formula for the B values of the spin-orbit
components of a degenerate state,
Beff = B(1 + 2BS/AL)
together with the value of B(3/2) – B(5/2), we obtain
AL (Sc12C) ≈ -52 cm-1
This is surprisingly high!

14. Ground state bond lengths in the “early” 3d and 4d monocarbidesSc Y Ti Mo Nb Zr V Cr
π3 σ σ
π4 σ σ
π4 σ2 δ
π4 σ2 δ2
45/2
3+
3
23/2
The actual values can be got from the Sigmaplot data file: 3d , 1.695, 1.617, d , 1.807, 1.700, 1.676 14

15. Potential energy curves for ScC
MRDCI
G.H. Jeung and J. Koutecký,
J. Chem. Phys. 88, 3747 (1988)
Two ab initio calculations have been done:-
MRCI
E/h
A. Kalemos, A. Mavridis and J.F. Harrison,
J. Phys. Chem. A, 105, 755 (2001)
LIF region
X2P
X4P

16. These upper states appear to be 4Δ
Ω = 7/2 – 5/2 bands of ScC
Sc12C Sc13C
T B T B
Δν (12C – 13C) B/13B
v′= *
v′= *
v′=
v′= *
v′= *
These upper states appear to be 4Δ

17. Vibrational assignments1 3 3 1 1 2 2 1 1 2

18. The next step: Higher resolution
Summary
ScC has a 4Πi ground state, with r0 = Å. The ground state
symmetries of the early 3d and 4d monocarbides are the same.
Because the Sc, 4sσ m.o. is almost degenerate with the C, 2p and 2s m.o.s, there are large numbers of low-lying electronic states.
At least eight case (a)-coupled electronic states contribute to the
LIF spectrum in the red and near IR regions.
Detailed assignment of the upper electronic states has not been possible so far, though at least three of the states are probably 4Δ.
No Sc hyperfine structure is observable at medium resolution.
The next step: Higher resolution