Anthony J. Merer

The C3-bending vibrational levels of the C3-Kr and C3-Xe van der Waals complexes studied by their Ã-X̃ electronic transitions and by ab initio calculations.

Fluorescence excitation spectra and wavelength-resolved emission spectra of the C(3)-Kr and C(3)-Xe van der Waals (vdW) complexes have been recorded near the 2(2-)(0), 2(2+)(0), 2(4-)(0), and 1(1)(0) bands of the Ã(1)Π(u)-X̃(1)Σ(g)(+) system of the C(3) molecule. In the excitation spectra, the spectral features of the two complexes are red-shifted relative to those of free C(3) by 21.9-38.2 and 34.3-36.1 cm(-1), respectively. The emission spectra from the à state of the Kr complex consist of progressions in the two C(3)-bending vibrations (ν(2), ν(4)), the vdW stretching (ν(3)), and bending vibrations (ν(6)), suggesting that the equilibrium geometry in the X̃ state is nonlinear. As in the Ar complex [Zhang et al., J. Chem. Phys. 120, 3189 (2004)], the C(3)-bending vibrational levels of the Kr complex shift progressively to lower energy with respect to those of free C(3) as the bending quantum number increases. Their vibrational structures could be modeled as perturbed harmonic oscillators, with the dipole-induced dipole terms of the Ar and Kr complexes scaled roughly by the polarizabilities of the Ar and Kr atoms. Emission spectra of the Xe complex, excited near the Ã, 2(2-) level of free C(3), consist only of progressions in even quanta of the C(3)-bending and vdW modes, implying that the geometry in the higher vibrational levels (υ(bend) ≥ 4, E(vib) ≥ 328 cm(-1)) of the X̃ state is (vibrationally averaged) linear. In this structure the Xe atom bonds to one of the terminal carbons nearly along the inertial a-axis of bent C(3). Our ab initio calculations of the Xe complex at the level of CCSD(T)∕aug-cc-pVTZ (C) and aug-cc-pVTZ-PP (Xe) predict that its equilibrium geometry is T-shaped (as in the Ar and Kr complexes), and also support the assignment of a stable linear isomer when the amplitude of the C(3) bending vibration is large (υ(4) ≥ 4).

Electronic bands of scandium monocarbide, ScC, in the region 14 140–16 000 cm−1

Scandium monocarbide molecules, ScC, have been prepared by the reaction of 532 nm laser-ablated Sc metal with acetylene or methane under supersonic jet-cooled conditions. Electronic spectra of Sc12C and Sc13C have been recorded in the region 14 140-16 000 cm-1 using laser-induced fluorescence, and about 40 bands of each isotopomer have been analyzed rotationally. Wavelength resolved emission spectra have been obtained for many of them. The results show that Sc12C has a 2Πi ground state, with a bond length of 1.952 Å. Its vibrational frequency and spin-orbit coupling constant are 648 cm-1 and -39.47 cm-1, respectively (631 cm-1 and -39.32 cm-1 in Sc13C). Lying 155.58 cm-1 above the X2Π3/2 level (154.72 cm-1 in Sc13C) is a 4Π5/2 level, the lowest spin-orbit component of a 4Πi state. The excited states at higher energy are very complicated. Bands from both the doublet and quartet spin manifolds are present, and there are strong doublet-quartet interactions which induce many nominally-forbidden bands violating the selection rule ΔS = 0. Eight excited electronic states have been recognized, including four 4Δ states. These 4Δ states represent four of the five 4Δ states from the electron configurations (C 2pσ)2 (C 2pπ)2 (Sc 3dδ)1 and (C 2pσ)1 (C 2pπ)2 (Sc 4sσ)1 (Sc 3dδ)1.

THE LOW-LYING ELECTRONIC STATES OF SCANDIUM MONOCARBIDE, ScC

Extensive wavelength-resolved fluorescence studies have been carried out for the electronic bands of ScC and ScC lying in the range 14000 16000 cm−1. Taken together with detailed rotational analyses of these bands, these studies have clarified the natures of the low-lying electronic states. The ground state is an Ω = 3/2 state, with a vibrational frequency of 648 cm−1, and the first excited electronic state is an Ω = 5/2 state, with a frequency of 712 cm−1, lying 155.54 cm−1 higher. These states are assigned as the lowest spin-orbit components of XΠi and aΠi, respectively. The quartet nature of the a state is confirmed by the observation of the Π3/2 component, 18.71 cm−1 above the Π5/2 component. The strongest bands in the region studied are two ∆7/2 Π5/2 transitions, where the upper states lie 14355 and 15445 cm−1 above XΠ3/2. Extensive doublet-quartet mixing occurs, which results in some complicated emission patterns. The energy order, aΠ above XΠ, is consistent with the ab initio calculations of Kalemos et al.,a but differs from that found by Simard et al in the isoelectronic YC molecule.b

Analysis Of Bands Of The 405 Nm Electronic Transition Of C3ar

Bands of the C3Ar complex can be observed near almost all the bands of the ÃΠu X̃Σg transition of C3. The strongest bands of C3Ar form close-lying pairs. Rotational analyses have been carried out for the bands at 25025 and 25029 cm−1(near the 02−0-000 band of C3) and 25426 and 25430 cm−1(near the 04−0-000 band). Each pair consists of a type A and a type C band of an asymmetric top, where the upper states interact by b-axis Coriolis coupling; this represents the lifting of the degeneracy of the Π state in the lower symmetry of the complex. Only K = even lower state levels are found, showing that C3Ar has the shape of a distorted letter T. The Ar atom lies 3.82 Å from the centre of mass of the C3 part. Emission spectra have been recorded and lifetimes measured for several C3Ar upper state levels. The assignment of the emission bands is complicated by significant intramolecular relaxation in the upper states, which populates mainly the lowest level of each local potential minimum of the upper state; however the variation of the upper state well depth (binding energy) with vibrational quantum number can then be determined.