Zeeman quantum beat observed by IR–UV double resonance LIF spectroscopy of acetylene in the Ã1Au 3ν3′+ν6′ and 3ν3′+ν4′ ungerade vibrational states

It has been found that rotational levels of the ungerade 3ν3′+ν6′ vibrational state of acetylene in the Ã1Au state split into two or more levels, while those of the 3ν3′+ν4′ state indicate no splitting. The level splitting is attributed to the S1–Tn coupling which has been proved by observation of the Zeeman quantum beat in the fluorescence decay from the split level under a weak magnetic field. From the present finding that the S1–Tn coupling occurs only in the combination state of the in-plane ν6′ mode with the 3ν3′ state, it is concluded that the S1 state couples with the Tn state in a C2h planar geometry.