In centrosymmetric molecules such as acetylene, one expects one-photon and two-photon spectroscopy to be mutually exclusive by the g↔u electric dipole selection rule. An ultraviolet–ultraviolet double resonance (UVUVDR) spectrum of the 74 000–78 000 cm−1 region of acetylene, recorded using the Ã←X̃ transition for the first step in double resonance consists almost entirely of transitions terminating on levels of the Ẽ state identified in one-photon vacuum ultraviolet (vuv) spectroscopy. Evidence for a nonplanar, noncentrosymmetric structure of the Ẽ state obtained by rotationally resolved fluorescence dip detected UVUVDR is presented.
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Lundberg et al. (1992) studied this question.
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