Abstract
In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.
Highlights
Interstellar IR emission bands observed at wavelengths of 3.3, 6.2, 7.7, 8.6 and 11.2 lm are seen towards a variety of astronomical objects [1,2]
The experimental system consists of a Paul-type quadrupole ion trap (QIT) that is connected to a reflectron time-of-flight mass spectrometer
The mid-IR spectra of the three DBP isomers have been investigated by means of IRMPD spectroscopy
Summary
Interstellar IR emission bands observed at wavelengths of 3.3, 6.2, 7.7, 8.6 and 11.2 lm are seen towards a variety of astronomical objects [1,2]. Schlemmer et al [5] were the first to confirm that UV excitation of PAHs is the driving force for the appearance of the interstellar IR bands To achieve this, they conducted very sophisticated laboratory measurements that allowed for sensitive frequency resolved detection of the IR light emanating from laser excited neutral PAHs. Based on IR emission measurements of a number of small neutral PAHs, including a few methyl- and hetero atom substituted species, Schlemmer and coworkers concluded that small neutral PAHs can at most be minor contributors to the IR emission bands and that large PAHs and PAH cations are likely more relevant contributors [6,7]. They suggested that experimentally obtained absorption spectroscopic data and DFT
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