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

Abstract

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.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products

“…However, IR excitation can lead to a large change in the absorption cross section even for broad bands. [47][48][49][50][51] In addition, absorption of further IR photons may cause fragmentation and thus induce a dip in the REMPI spectrum. 50 As absorption of the first IR photon is the bottleneck for this process, linear IR spectra are recorded.…”

Infrared bands of neutral gas-phase carbon clusters in a broad spectral range

“…The observation of signal depletion can be the consequence of infrared multiple photon dissociation (IRMPD), 45 in which each cluster resonantly absorbs multiple infrared photons until the dissociation threshold is overcome, or due to a change in the ionization efficiency of the clusters upon resonant vibrational excitation. 46 Given that we do not see a clear indication of signal ingrowth upon resonant vibrational excitation, the latter explanation seems more likely. Defining the signal intensity of a cluster with and without FELIX influence by I and I 0 , respectively, and the FELIX power as P , the infrared yield is written as Y IR = −ln( I / I 0 )/ P .…”

Polycyclic aromatic hydrocarbon growth in a benzene discharge explored by IR-UV action spectroscopy

“…Structural information is obtained by matching the measured IR absorption spectra to calculated spectra of candidate compounds. [69][70][71][72] As will be discussed below, this approach enables us to identify many new species resulting from the complex network of PAH formation pathways starting with benzene that were, as of yet, not observable.…”