The C̃2B3u ← X̃2B2g electronic absorption spectrum of butatriene cation in a neon matrix

The 2B3u ← 2B2g electronic absorption of butatriene cation (BT+) has been observed in a 6 K neon matrix. The origin band lies at 511.9 nm. The electronic transition assignment is based on comparison with the photoelectron spectrum of butatriene and the vibrational frequencies of BT+ calculated with the CASPT2 (5, 6) method. Three vibrational modes of energy 207, 511 and 813 cm−1, with their overtones and combinations, are active in the 2B3u state of BT+. It is shown that the emission observed from a glow discharge of 2-butyne at 491 nm and attributed to the origin band of this electronic transition [1] of BT+ is due to another species, because the difference of 850 cm−1 to the absorption spectrum is too large. No fluorescence of BT+ was detected in the matrix and it is expected that the 2B3u electronic state relaxes non-radiatively on a fs time scale.