Sub-Doppler electronic spectrum of the benzene-D₂ complex

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Van der Waals complexes provide insights into intermolecular forces.
• Understanding the dynamics of small molecules like H2 interacting with aromatic systems is crucial.

Purpose of the Study:

• To investigate the excitation spectrum of the benzene-D2 van der Waals complex.
• To resolve and assign vibronic bands for both para and ortho D2 spin isomers.
• To determine vibrational frequencies and rotational barriers of van der Waals modes.

Main Methods:

• Sub-Doppler resolution spectroscopy using mass-selective two-color resonance-enhanced two-photon ionization.
• Analysis of vibronic bands and rotational constants.
• Determination of upper-state lifetimes through homogeneous line broadening.

Main Results:

• Both para (j=1) and ortho (j=0) D2 spin isomers were identified in the benzene-D2 complex.
• Vibrational frequencies for three van der Waals modes were determined for benzene-H2 and benzene-D2.
• Intermolecular distances and rotational barriers for H2/D2 internal rotation were evaluated (72 cm-1 for H2, 66 cm-1 for D2).
• Upper-state lifetimes were found to be in the range of 0.3-0.7 ns.

Conclusions:

• The study provides a detailed characterization of the benzene-D2 van der Waals complex, including its vibrational and rotational dynamics.
• Comparison with benzene-H2 reveals subtle differences in intermolecular interactions and rotational barriers.
• The findings contribute to a deeper understanding of non-covalent interactions and energy transfer in molecular systems.