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... Show moreGas-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. Show less
The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation(C16H10+; Py+ and C16H8+; ddPy+) are presented. Infrared predissociation spectroscopy is employed... Show moreThe vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation(C16H10+; Py+ and C16H8+; ddPy+) are presented. Infrared predissociation spectroscopy is employed tomeasure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. Thespectrum of Py+ allows a detailed comparison with harmonic and anharmonic density functional theory(DFT) calculated normal mode frequencies. The spectrum of ddPy+ is dominated by absorption featuresfrom two isomers (4,5-ddPy+ and 1,2-ddPy+) with, at most, minor contributions from other isomers.These findings can be extended to explore the release of hydrogen from interstellar PAH species. Ourresults suggest that this process favours the loss of adjacent hydrogen atoms. Show less
