Electronic spectrum of 9-methylanthracenium radical cation

Area of Science:

• Physical Chemistry
• Spectroscopy
• Astrochemistry

Background:

• Polycyclic Aromatic Hydrocarbon (PAH) radical cations are abundant in space.
• Their electronic spectra are crucial for understanding interstellar chemistry.
• The diffuse interstellar bands (DIBs) remain largely unassigned.

Purpose of the Study:

• To report the predissociation spectrum of the cold, argon-tagged, 9-methylanthracenium radical cation.
• To assign electronic transitions and understand vibronic activity.
• To discuss the relevance to diffuse interstellar bands.

Main Methods:

• Predissociation spectroscopy of cold, argon-tagged 9-methylanthracenium radical cation.
• Comparison with ab initio calculations for electronic transition energies and intensities.
• Time-Dependent Density Functional Theory (TD-B3LYP) for excited state frequencies and intensities.
• Modelling of vibronic interactions.

Main Results:

• The spectrum spans 8000–44,500 cm⁻¹, revealing at least eight electronic transitions.
• Infrared D1←D0 transitions show significant vibronic activity.
• Dissociation via CH2 or CH3 loss observed at high visible photon energies.

Conclusions:

• The reported spectrum provides a detailed characterization of 9-methylanthracenium radical cation electronic transitions.
• Vibronic analysis aids in understanding spectral features.
• The findings contribute to the ongoing effort to assign diffuse interstellar bands.