High-resolution analysis of the S1-S0 transition of magnesium phthalocyanine: Rotational structure and electronic angular momentum.

We report a high-resolution spectroscopic analysis of the S1-S0 transition of magnesium phthalocyanine (MgPc), obtained by probing buffer-gas-cooled molecules with a narrow-linewidth laser. The observed spectrum exhibits a characteristic three-peak pattern, which is well reproduced by modeling MgPc as an oblate symmetric top with D4h symmetry. A key result of this work is that the spectrum is strongly influenced by electronic Coriolis coupling, which is associated with electronic angular momentum. The electronic Coriolis constant is determined to be ∼2, indicating an effective orbital angular momentum of about 2 in the excited S1 state, originating from the π-conjugated ring excitation. This provides a direct spectroscopic signature of electronic angular momentum in a large polyatomic molecule. The presence of nonzero electronic orbital angular momentum is qualitatively consistent with the perimeter model of phthalocyanines. The value lies within the range inferred from previous magnetic circular dichroism (MCD) studies. Compared with the previous MCD estimate, the present analysis provides a more state-specific and narrower constraint, demonstrating that high-resolution spectroscopy enables direct access to electronic and magnetic properties beyond conventional structural characterization.