Renner-Teller Coupling Research Articles

Excited state dynamics and bimolecular quenching processes for NH 2(Ã 2A 1)

Radiative lifetimes and quenching rate constants have been measured for the à 2A 1 state of NH 2, formed through ArF excimer laser photolysis of NH 3 at 193 nm. The NH 2(à 2A 1 → X̃ 2B 1) emission is discrete, allowing single vibrational and rotational levels to be isolated. An average radiative lifetime of 31 ± 4 μs and an NH 3 quenching rate constant of (6.1 ± 0.2) × 10 −10 cm 3 molecule −1 s −1 is found to be independent of the K″ a rotational level and ν′ 2 vibrational level for K″ a = 1, 4, 5 and 6 and ν′ 2 = 6, 7 and 8. The 050 Σ level ( K′ a = 0) lifetime is slightly longer (46 ± 6 μs) and its quenching rate constant somewhat lower [(5.0 ± 0.2) × 10 −10 cm 3 molecule −1 s −1]. These lifetimes are interpreted as radiative lifetimes of levels of the à 2A 1 state, unperturbed by Renner—Teller coupling with the ground state. The quenching by NH 3 is shown to be predominantely electronic quenching of the à 2A 1 state, with vibrational and rotational relaxation within the excited state being at least ten times slower. A second, previously unobserved, long-lived component is found in the decay of broad-band emission, comprising about 15% of the fluorescence intensity between 620 and 890 nm. Its lifetime is ⩾ 100 μs, coupling between some levels of the à 2A 1 and X̃ 2B 1 states.

Theory of resonance raman scattering by degenerate modes due to renner-teller interactionin porphyrins

Resonance Raman spectra and excitation profiles associated with Renner-Teller coupling in square molecules are analysed theoretically and an experiment is proposed to search for this effect in mettalloporphyrins.

Vibronic coupling in linear molecules and linear-to-bent transitions: HCN

The hamiltonian describing vibronic coupling between Π and Σ electronic states of linear molecules is derived. In contrast to the well-known Renner-Teller coupling of the two components of a Π electronic state, the Σ-Π vibronic coupling term is linear in the bending coordinate. The symmetry properties of the hamiltonian are analyzed. A numerical procedure which allows the calculation of the spectrum (e.g. the absorption or the photoelectron spectrum) for arbitrary values of the coupling constant is presented. The adiabatic potential energy curves are investigated. A nonlinear equilibrium geometry results for the lowest of the interacting states if the vibronic coupling is strong enough. Intensity borrowing effects and non-adiabatic effects due to the vibronic coupling are qualitatively discussed and the importance of the simultaneous coupling to totally symmetric vibrational modes is pointed out. The inclusion of the totally symmetric modes influences the spectrum in a nontrivial way and is essential for descr in the photoelectron spectrum of HCN and DCN. The energies and coupling constants entering the hamiltonian are calculated by ab initio methods including the effects of electron correlation. The ab initio calculation predicts strong vibronic interaction between the closely spaced 2Π and 2Σ states responsible for the first band in the photoelectron spectrum. By slightly readjusting some of the parameters excellent agreement between the computed and the experimental spectrum is obtained, confirming that the very complex structure of the spectrum can be fully understood in terms of Σ-Π vibronic coupling and excitation of the CN stretching mode. The second example considered is the 9 eV band in the absorption spectrum of HCN and DCN. The vibronic coupling mechanism explains the experimentally observed strong excitation of the bending mode and the complexity of the vibrational structure of this band.

The effective Hamiltonian for the Renner-Teller effect

A derivation of the effective vibronic Hamiltonian for a linear triatomic molecule in a Π electronic state within the harmonic approximation is presented. In addition to the well-established vibrational and Renner-Teller coupling terms, there is a small term which has been overlooked before. The form of this term is derived and the parameter involved, g K , is related to other observable quantities. Several experimental examples are presented which demonstrate that the extra term is needed to reproduce the vibronic levels in Π electronic states, the contribution being of the order of a few cm −1.

On the Jahn-Teller effect in ReF6

New experimental and theoretical work has led to a re-evaluation of the Jahn-Teller effect in ReF6. Low temperature neat and mixed crystal Raman and absorption spectra have been taken. The theory applicable to two Jahn-Teller active vibrations and to a Jahn-Teller molecule in a low symmetry crystalline field is discussed. The main conclusions of the study are that (1) the standard linear Jahn-Teller treatment of vibronically active modes as independent motions characterized by a given vibronic angular momentum is in general inadequate ; (2) Renner-Teller (quadratic) or higher-order vibronic coupling terms are important ; (3) crystal-field splitting of the vibronic levels is not observed ; and (4) vibrational exciton band widths of the Jahn-Teller active vibrations are substantially reduced.

Study of Nonadiabatic Transitions in Triatomic Molecules

Nonadiabatic coupling between electronic states in triatomic molecules has been studied with special consideration given to the possibility of internal conversion. Approximate calculations of electronic and vibrational coupling elements and density of states, which determine the transition probability, have been performed for H2O and some other molecules. The results suggest that the statistical model mechanism for large molecules should not be applied to triatomics. Under favorable conditions, e.g., sufficiently different shape or position of the two potential curves involved, strong coupling might occur which is large enough to explain anomalously long fluorescence lifetimes in small molecules. However, observations of lifetime lengthening of the first excited states of NO2 and SO2 cannot easily be explained because internal conversion through vibrational modes is symmetry forbidden. In this case, the role and relative importance of intersystem crossing and Renner—Teller coupling need further investigation.