Vibrational hot bands of linear C4 and C5 arising from a bending vibration with two quanta in the lowest bend: the (ν3 + 2ν5) − 2ν5 band of C4 and the (ν3 + 2ν7) − 2ν7 band of C5

We report the complete characterization of the hot band (ν3 + 2ν5) − 2ν5 for linear C4 and the analogous band (ν3 + 2ν7) − 2ν7 for linear C5 via gas phase, rotationally resolved spectra. C4 and C5 were generated using a flowing mixture of acetylene and helium subjected to a hollow cathode discharge. The spectra were recorded using a diode laser spectrometer. Σ–Σ and Δ–Δ transitions were observed for both molecular species. Analyses of these hot bands yielded the following lower state molecular parameters: ν0 = 1547.3164(5) cm−1, B = 5024.92(18) MHz, D = 0.924(52) kHz, xL = 0.5802(92) cm−1 and q5 = 10.8890(26) MHz for C4 and ν0 = 2166.2331(3) cm−1, B = 2576.524(87) MHz, D = 0.143(13) kHz, xL = 0.409(10) cm−1 and q7 = 3.9845(15) MHz for C5. The results were completely consistent with linear structures and support our previous conclusions with regard to semi-rigid linear nature of these two molecular species. This work represents the first conclusive study of a vibrational state with two quanta in the bend for any carbon cluster with four or more atoms.