Rotationally resolved A3Σ―u―X3Σ―g spectrum of HC7H

The A 3 Σ ― u ―X 3 Σ ― g electronic spectrum of the linear carbon chain radical HC 7 H has been recorded fully rotationally resolved. Cavity ring-down spectroscopy is used to record the origin band transition in direct absorption through a supersonically expanding planar plasma, discharging a diluted gas mixture of acetylene in helium. Rotational resolution is obtained by operating a commercial pulsed dye laser system in a second grating order configuration, resulting in a narrower bandwidth of about 0.035 cm ―1 . In total, 39 resolved P- and R-branch transitions are included in a standard fit, yielding for the first time accurate rotational constants for both electronic states.