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Absorption and dispersed fluorescence spectra are reported for a variety of 1-phenylalkynes cooled in a supersonic free jet. The 1B2(ππ)←1A1 spectra of these compounds is characterized by a near perfect cancellation of transition strength sources for the totally symmetric vibronic bands including the origin. Two skeletal distortion modes are found to dominate the spectra, inducing strong transitions by vibronic coupling to a higher state of 1A1 symmetry. These two modes are rotated in the 1B2 state relative to their configuration in the ground state. The nature of these modes is found to be largely unaffected by changes in the length of the alkyl chain, but vibrational relaxation into the chain still appears to proceed on a subnanosecond time scale.
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Powers et al. (1981) studied this question.
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