Quantum beat spectroscopic studies of Zeeman anticrossings in the Ã1Au state of the acetylene molecule (C2H2) Original Research Article

We present results obtained by quantum beat spectroscopy of the gaseous acetylene molecule (HCCH). Spectra are presented for the rotationless levels (J=K=N=I=0) of the Ã1Au ν3=0–2 states (ν3 is the trans-bending normal mode of the trans-bent excited state) and for the (J=K=N=1, I=0 level of the ν3=0 level with the goal of obtaining information about singlet ∼ triplet interactions. The energy range thus sampled was from 42200 to 44300 cm−1 above the rotationless zero-point level of the X̃1Σg+ state and the magnetic field range was from 0 to 8 T. The centers of anticrossings, the coupling matrix elements, the Landé g factors, and the triplet lifetimes were mainly derived by fitting the measurements to a two-level model. Additional parameters were also introduced to take into account more complex interactions (up to a four-level model). The crucial discussion concerns the gℓ factors (local Landé factors) obtained, which were rarely equal to 2.0023, the value for a free electron, and the evolution of these gℓ factors as a function of the excitation energy. We interpret the observed behavior as due to strong electronic inter-triplet couplings (assumed here to be T1 ∼ T2) and weaker singlet ∼ triplet couplings (mainly S1 ∼ T and secondarily S0 ∼ T). We relate the present observations to previous observations obtained from Zeeman anticrossing spectra, which showed an unusually rapid increase of both the coupling matrix elements and the density of coupled levels as a function of the excitation energy [P. Dupré et al., Chem. Phys. 152 (1991) 293]. From our observations and in relation to the cis-linear-transisomerization barrier on a triplet surface, as we previously proposed, we suggest that the interactions between acetylene electronic states may be ordered according to their strengths: (S0∼S1⪡S0∼T⪡S1∼T⪡T1∼T2).