Portion of the millimeter-wave spectrum recorded with RF discharge on (in red) and off (in black) showing the K_a series in the ^qR(18) (J′←J″ = 19 ← 18) branch of syn vinyl mercaptan. For the lowest K_a value visible on the plot, the asymmetry splitting (K_a + K_c = J or J + 1) is resolved. Transitions in black most likely arise from the precursor while transitions in red labeled with an asterisk remain unidentified (see text).