New vibronic bands of the laser-vaporized C3 cluster. Determination of the ν3 fundamental in the Ã1Πu state

The laser induced fluorescence (LIF) spectrum for the Ã1Πu− X̃1Σg+ band of laser-vaporized C3 was measured under jet-cooled conditions. By careful inspection of the rotationally resolved LIF spectrum, eight vibronically allowed transitions exhibiting a 1Σu+−1Σg+ type rotational structure were identified. This finding led to the determination the vibrational energy for the (ν1, ν2, ν3) = (0, 1+, 1) level of the à state, from which the fundamental wavenumber of the ν3 anti-symmetric mode was estimated to be 2~539 cm−1, which is considerably smaller than any of the previous estimates, 650, 808 and 890 cm−1. In the high-resolution (2~0.08 cm−1) measurements for the hot-band region, 1Πg−1Πu+ type bands exhibiting a characteristic feature for the transitions from the X̃(0, 0, 1) level were identified. By assigning the Ã(0, 0, 1)−X̃(0, 0, 1) band, the ν3 fundamental was directly determined to be 541.7(1) cm−1, in excellent agreement with the above prediction from the vibronically allowed 1Σu−1Σg+ transitions.