Zicheng Duan

New vibrational assignments in the Ā1 Au-[Xtilde] 1Σ+ g electronic transition of acetylene, C2H2: the v′1 frequency

New laser-induced fluorescence spectra of supersonic jet cooled acetylene (C2H2) in the wavelength region 230–205 nm have led to an improved understanding of the vibrational structure of the A 1Au state. Among the new bands observed are two weak perturbed bands at 46008 cm−1 and 46116 cm−1. Rotational analyses of these bands, together with the corresponding ‘hot’ bands arising from the ground state v4 fundamental, have shown that the upper states have asymmetric top K structure that is unaffected by a axis Coriolis coupling; this means that they do not involve overtones of the low frequency bending vibrations and therefore must be combinations of ag vibrational normal modes. From their positions in the manifold, their vibrational assignments can only be 22 031 0; and 11 0;31 0. These assignments lead to values of x 22, x13, and a revised value for the symmetric CH stretching frequency, ν1 = 2880.5cm−1; this revised value is 160cm−1 lower than the previously accepted value, but consistent with new ab initio calculations that we performed at the EOM-CCSD level using a TZ2P (triple-zeta plus double polarization) basis set.

Unexpected simplicity in the S1–S0 dispersed fluorescence spectra of 13C2H2

We have recorded dispersed fluorescence (DF) spectra (18 cm−1 resolution) from the JKaKc=110 rotational level of six vibrational levels in the S1 (A 1Au) state of 13C2H2. 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 12C2H2. Comparison between the DF spectra of 13C2H2 and 12C2H2 reveals slower intramolecular vibration redistribution in 13C2H2, particularly with nonzero quanta of CC stretch excitation. More than 15 patterns were extracted above the energy at ...