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

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)

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?

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.

v' = 3 2 1 Studied so far

(Scandium plus acetylene)

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

Ground state constants of ScC Ω″ = 5/2 Ω″ = 3/2 Sc12C B 0.4534 (10) 0.4619 (6) cm-1 104 D -0.21 (9) 0.004 (43) s 0.019 0.019 Sc13C B 0.4279 (8) 0.4349 (9) 104 D -0.16 (6) -0.03 (8) s 0.016 0.022 1.0619 B(12C)/B(13C) 1.0597 m13/m12 = 1.0649

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.

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

Ground state bond length and spin-orbit coupling The B value of the hypothetical middle of the X4P state is 0.4660 cm-1 (12C). This gives the bond length as r0 = 1.954 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!

Ground state bond lengths in the “early” 3d and 4d monocarbides Sc 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 1.954, 1.695, 1.617, 1.619 4d 2.050, 1.807, 1.700, 1.676 14

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

These upper states appear to be 4Δ Ω = 7/2 – 5/2 bands of ScC Sc12C Sc13C T0 B T0 B Δν (12C – 13C) 12B/13B 15875.14 0.3093 15802.59 0.3555 15809.55 0.4117 15784.51 0.4024 15427.97 0.4266 15399.97 0.4023 28.00 v′=2 1.060 15381.69 0.3219 * 15289.00 0.4596 15293.33 0.4268 -4.33 v′=0 1.077 * 15270.57 0.4577 15274.55 0.4261 -3.98 v′=0 1.074 14520.92 0.4611 14512.97 0.4288 7.95 v′=1 1.075 14485.65 0.4247 14326.68 0.4343 * 14199.34 0.4454 14201.91 0.4161 -2.57 v′=0 1.070 * These upper states appear to be 4Δ

Vibrational assignments 1 3 3 1 1 2 2 1 1 2

The next step: Higher resolution Summary ScC has a 4Πi ground state, with r0 = 1.954 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