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We have recorded dispersed fluorescence (DF) spectra (18 cm−1 resolution) from the JKaKc=110 rotational level of six vibrational levels in the S1 (Ã 1Au) state of C213H2. Improvements in our methods of recording and calibrating DF spectra, that have enhanced the quality of our data sets, are briefly discussed. More than 50 fractionated bright state patterns associated with approximately conserved polyad quantum numbers have been extracted from our DF data sets using a spectroscopic pattern recognition technique, extended cross correlation. These polyads extend to internal energies as high as 20 000 cm−1 above the zero-point vibrational level. The polyad fractionation patterns observed at high energy are surprisingly simple relative to the corresponding patterns of C212H2. Comparison between the DF spectra of C213H2 and C212H2 reveals slower intramolecular vibration redistribution in C213H2, particularly with nonzero quanta of CC stretch excitation. More than 15 patterns were extracted above the energy at which acetylene↔vinylidene isomerization is predicted to be energetically feasible (∼15 200 cm−1) and the dynamical information encoded in these patterns is addressed. In particular, we have analyzed a subset of the C213H2 polyads, the pure bending polyads, those with zero quanta of excitation in each of the stretch modes, (Ns=v1+v2+v3=0). The observed pure bending levels are reproduced to a root-mean-square error of <1.5 cm−1 by two different effective Hamiltonian models: an 11 parameter normal-mode and a 13 parameter local-mode model.

Perturbations in the K = 1 and 3 levels of theà 1 A u ,4 n 3 state of acetylene are explained as interactions with levels of theà 1 A u ,3 1 B 4 (v 3 =1 ,v 4 +v 6 = 4) polyad. A satisfactory least-squares fit to the perturbed level structure has been obtained, treating the 4n 3 state as an asymmetric top perturbed by isolated K = 1 and 3 levels. Accurate deperturbed rotational constants for the interacting states are presented. . Nous obtenons un ajustement satisfaisant par moindres carrés pour la structure des niveaux perturbés, en traitant l'état 4n3 comme une toupie asymétrique perturbée par les niveaux isolés K =1e tK = 3. Nous présentons des valeurs précises libres de perturbation des constantes rotationnelles pour les niveaux en interaction.[Traduit par la Rédaction]