Λ-type Doublets and Lifetime Broadening in the B²Π–X²Π Electronic Origin Band of C₆H

We present a high-resolution study on the B²Π–X²Π electronic origin band spectrum of the C₆H radical with its center at ~18990 cm⁻¹. The C₆H radicals are produced in a pulsed supersonic slit-jet discharge source and detected in direct absorption by sensitive cavity ring-down spectroscopy. By using a home-made narrow linewidth nanosecond pulsed laser source, the experimental spectrum allows to resolve the Λ-type doublet fine structures in high-J rotational lines. A set of improved B²Π state constants, including the Λ-type doubling parameters of p'=−1.16(9)×10⁻³ cm⁻¹, q'=−1.22(7)×10⁻⁴ cm⁻¹, are determined from analysis of the experimental spectrum. An analysis on the spectral line profile has resulted in a significant linewidth broadening of ~0.05±0.01 cm⁻¹, corresponding to a B²Π state lifetime of ~100±20 ps. The short lifetime of the excited B²Π state is proposed due to fast internal vibronic couplings to high-lying vibronic levels of lower electronic states.