- Rotationally resolved spectra of two bands in the S1 → S0 transition of the p-difluorobenzene molecule and its van der Waals complex with Ar have been measured by mass-selective resonance-enhanced two-photon ionization. The rotational structure of the 000 and the 2710 bands in the monomer as well as in the complex differ from each other. They can be theoretically reproduced assuming a transition moment oriented along the short in-plane axis of the molecule in the case of the 000 transition and the long in-plane axis in the case of the 2710 transition. Since the magnitude of the moments of inertia is changed in the complex by adding an Ar atom, complexation leads to a change of rotational structures of the same band. The analysis of the rotational structure points to Herzberg–Teller coupling by vibronic interaction with the S2 (1B1u) state as the mechanism responsible for the appearance of the 2710 band. The rotational constants determined from a fit of the spectra yield an effectiveRotationally resolved spectra of two bands in the S1 → S0 transition of the p-difluorobenzene molecule and its van der Waals complex with Ar have been measured by mass-selective resonance-enhanced two-photon ionization. The rotational structure of the 000 and the 2710 bands in the monomer as well as in the complex differ from each other. They can be theoretically reproduced assuming a transition moment oriented along the short in-plane axis of the molecule in the case of the 000 transition and the long in-plane axis in the case of the 2710 transition. Since the magnitude of the moments of inertia is changed in the complex by adding an Ar atom, complexation leads to a change of rotational structures of the same band. The analysis of the rotational structure points to Herzberg–Teller coupling by vibronic interaction with the S2 (1B1u) state as the mechanism responsible for the appearance of the 2710 band. The rotational constants determined from a fit of the spectra yield an effective van der Waals distance of 3.55 (2) Å (1 Å = 10−10 m) of the Ar atom from the p-difluorobenzene plane, which decreases by 0.06 Å on electronic excitation to the S1 state.…
