Infrared diode laser spectroscopy of C4. The ν3 + ν4 − ν4 sequence band

The ν3 + ν4 − ν4 sequence band of the linear C4 radical in the gas phase has been studied using a tunable diode laser spectrometer. A flowing mixture of acetylene and helium was used to generate the C4 radicals. Seventy-three rovibrational transitions were measured in the 1530 to 1562 cm−1 spectral region. Transitions with N-values up to 60 were measured. The analysis yielded a band origin of 1548.0101(3) cm−1. In addition, rotational, centrifugal distortion, and 1-type doubling parameters for the degenerate bending states ν4 = 1 and (υ3 = 1, υ4 = 1) were determined. The l-type doubling parameter for υ4 = 1, q4 = 5.52(19) MHz, was used to estimate the ν4 frequency of gas-phase C4 to be 352(15) cm−1.