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Notes: The rotational band structure of a magnetic dipole vibronic transition is exactly the same as that of an electric dipole vibronic transition. It is often assumed that molecular vibronic transitions are electric dipole, introducing an ambiguity about the change in symmetry if the transition is weak enough to be an allowed magnetic dipole transition and the symmetry of either vibronic state is unknown. In atomic spectroscopy, it is known that either the Zeeman effect or the polarization dependence of wide angle fluorescence interference can be used to distinguish among electric-dipole, magnetic-dipole, electric-quadrupole, enforced-dipole transitions, etc. This note points out that rotationally resolved double resonance using polarized light can be used to determine the type of an unknown multipole transition if the type (magnetic or electric) of the other double resonance transition is known. Stark and Zeeman effects are valuable special cases since the type of the Stark (or Zeeman) transition moment is known a priori to be electric (or magnetic) dipole. Published Stark spectra of formaldehyde-d2 experimentally prove Innes' assignment of a magnetic dipole transition.

Notes: We report here the first measurement of the electric dipole moment of the A˜ 1A‘ state of HCN. Fluorescence excitation spectra of the A˜←X˜ 210K10 band were recorded using narrow band (〈0.1 cm−1) vacuum ultraviolet (VUV) produced by four-wave sum mixing in an improved strontium heat pipe. Accurate (±0.007 cm−1) term values for this band are reported. Surprisingly, we found that the asymmetry doubling of the upper state does not extrapolate to zero at J=0. We suggest that the most plausible explanation for this anomalous asymmetry doubling is a spin–orbit perturbation by a nearly degenerate level of the 1 3A' state. The a component of the dipole moment has been determined by the Stark splitting of the Q(1) transition as a function of electric field in a novel Stark cell capable of sustaining high electric fields (41 kV/cm) at pressures of 50 mTorr. The observed value of the dipole moment μa=0.99(10) D is in agreement with simple molecular orbital expectations and a semiempirical study.

Notes: Using a combination of low resolution dispersed A˜→X˜ fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the first stages of vibrational energy flow in the highly vibrationally excited acetylene prepared by A˜→X˜ emission over the energy range 5 000–18 000 cm−1. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the EVIB=7000 cm−1 region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling–Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck–Condon bright CC stretch and trans-bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling–Dennison coupling between the cis and trans bends in IVR [J. Chem. Phys. 95, 6336 (1991)].We prove that the symmetric CH stretch vibration, previously thought to play a crucial role in the redistribution of vibrational energy, is Franck–Condon inactive. We have also shown that vibrational-l-resonance among the states with excitation of both degenerate bending modes, when combined with a Fermi resonance which couples CC stretch/trans/cis-bend excited states to the antisymmetric CH stretch, determines the subsequent flow of vibrational energy after the Darling–Dennison bending resonance. These resonances all scale with vibrational excitation in nearly the simple manner expected for the lowest order anharmonic terms in the Hamiltonian, which allows the prediction of the fastest processes at high energy from a detailed study of the high resolution spectrum at lower energy. We find some interesting rules for vibrational energy flow in the short time dynamics: (i) CC stretch excitation is necessary for stretch–bend coupling; (ii) if V2‘ and V4‘ are the quantum numbers of the initially excited bright state, and vb‘ = v4‘ + v5‘ is the total bending quantum number of a state coupled to that bright state, then V4‘ ≥ vb‘ ≥ (V4‘–2V2‘); (iii) the total stretch quantum number ns‘ = (v1‘ + v2‘ + v3‘) is also conserved by the short time dynamics.These are severe and well characterized restrictions on the range of quantum numbers accessible to the initial bright state during the first stages of intramolecular vibrational redistribution of energy.