Knowledge Bank

Laser-induced fluorescence(LIF) spectra of $C2H2,C2D2$, and $C2HD$ were measured with $0.05cm-1$ resolution to assign respective rovibronic levels in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma u$ state. In $C2H2$, the $3\nu 3$ and $4\nu 3$ vibronic states are perturbed strongly since each rotational level splits into a number of levels, while no level splitting occurs for the $2\nu 3$ state. Most of levels exhibit Zeeman quantum beats(ZQB) in the fluorescence when a weak magnetic field is applied indicating that excited $C2H2$ has large g-values characteristic to respective rovibronic levels which couple with a triplet state. Besides ZQB, quantum beats ascribed to level anticrossing between the A singlet level and a non-fluorescent level were found for a number of excited levels. These anticrossings are due to interaction with its strength of the order of MHz. In the LIF spectra of $C2D2$ and $C2HD$, the observed bands could be assigned to the transitions to the $\nu 2+m\nu 3+n\nu 6$ states. Contrary to the case of excited $C2H2$, no level splitting as well as no ZQB were found for respective rovibronic levels which could be assigned unambigiously. This result indicates that the deuteration results in reduction of the coupling strength with triplet states, though the anticrossings observed for $C2H2$ are found also for excited levels of $C2D2$ and $C2HD$.