Far-IR Absorption of Neutral Polycyclic Aromatic Hydrocarbons (PAHs): Light on the Mechanism of IR–UV Ion Dip Spectroscopy

Gas-phase
IR–UV double-resonance laser spectroscopy is an
IR absorption technique that bridges the gap between experimental
IR spectroscopy and theory. The IR experiments are used to directly
evaluate predicted frequencies and potential energy surfaces as well
as to probe the structure of isolated molecules. However, a detailed
understanding of the underlying mechanisms is, especially in the far-IR
regime, still far from complete, even though this is crucial for properly
interpreting the recorded IR absorption spectra. Here, events occurring
upon excitation to vibrational levels of polycyclic aromatic hydrocarbons
by far-IR radiation from the FELIX free electron laser are followed
using resonance-enhanced multiphoton ionization spectroscopy. These
studies provide detailed insight into how ladder climbing and anharmonicity
influence IR–UV spectroscopy and therefore the resulting IR
signatures in the far-IR region. Moreover, the potential energy surfaces
of these low-frequency delocalized modes are investigated and shown
to have a strong harmonic character.