Studies on intramolecular processes of excited polyatomic molecules by means of Zeeman quantum beat spectroscopy

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This work aims to elucidate the level structure of the rovibronic state of an electronically excited polyatomic molecule by means of the quantum beat spectroscopy. When molecules are excited coherently to more than one electronically excited levels and the fluorescence transitions from these levels occur to a common electronic ground level, a beat phenomenon appears in the fluorescence. The spectrum of the beat frequencies reflects the level structure of the excited molecule. Since this observation is made in the time domain, its resolution reaches as high as 1 MHz. Thus, it is possible to discuss a level splitting caused very weak perturbation.The above spectroscopic method has been applied to observe the level structures of CS_2 (V^1B_2) and C_2H_2 (A^^< wave >^4A_u) in supersonic jets. When CS_2 is excited to selected rovibronic levels of the V^1B_2 state under a weak magnetic field by pulsed laser excitation, the quantum beats appear due to Zeeman splitting of the relevant levels. This observation implies that excited CS_2 has a magnetic moment which attributable to interaction of the singlet^1B_2 state with the triplet ^3A_2.Quite similarly, most of rovibronic levels split into several levels each of which has its specific radiative lifetime and exhibits quantum beats due to Zeeman splitting as well as level anticrossing. Based on these quantum beat spectral data, the details of the coupling mechanism have been discussed.