Study of the singlet—triplet coupling in glyoxal by level-anticrossing spectroscopy. I. Experimental techniques and results

Level anticrossing and a new optical-radiofrequency double resonance technique were applied to a study of the singlet—triplet interactions for single rotational levels of the vibrationless ¹A_u state of glyoxal. The density and spectral distribution of triplet rotational levels virtually coupled to the (K = 6, J = 13) and (K = 7, J = 35) singlet states were determined. The values of the singlet—triplet coupling constants V_st for some selected level pairs were measured. The assignment to the weak-coupling limit is confirmed.