Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands

Abstract

The so-called aromatic infrared bands (AIBs) are attributed to emission of polycyclic aromatic hydrocarbons (PAHs). The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, that is to say with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared (IR) spectra of three different PAH dications, naphthalene 2+ , anthracene 2+ , and phenanthrene 2+ , in the vibrational fingerprint region 500-1700 cm −1. The dications were produced by electron impact ionization (EI) of the vapors with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH 2+ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory (DFT) calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10-15 µm region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs.

Multiply charged naphthalene and its C10H8 isomers: bonding, spectroscopy and implications in AGN environments

“…Furthermore, even for highly valence-excited states of the naphthalene cation, relaxation pathways to the ground state due to vibronic coupling and conical intersections are present (Marciniak et al 2015;Reitsma et al 2019). This trend, which is also observed for larger PAHs (Wenzel et al 2020), corroborates the hypothesis that multiply charged PAHs (Tielens 2008;Zhen et al 2017Zhen et al , 2018Banhatti et al 2021), together with their nitrogendoped, phosphorus-doped, and protonated counterparts (Mattioda et al 2008;Ricks et al 2009;Álvaro Galué & Díaz Leines 2017;Meyer et al 2021;Oliveira et al 2021), are also viable candidates as carriers of the unidentified infrared (UIR) bands.…”

Stable excited dication: trapping on the S1 state of formaldehyde dication after strong field ionization

“…18 Metastable molecules in their excited states can be important intermediates where the stored electronic energy can be used to facilitate other reactions. 10,19 For example, in atmospheric chemistry or intergalactic chemistry, the long lifetime of excited metastable states enables the initiation of reactions through collision with other species. 20 Metastable excited states can also be used for population inversion, which is important for lasing.…”

“…In that case, the gap between the ground state and excited state may be larger than what one would intuitively expect. 9,10 Nevertheless, the gaps are still smaller than for neutral molecules, and a long-lived excited state is not intuitively expected.…”

Multiply charged naphthalene and its C10H8 isomers: bonding, spectroscopy, and implications in AGN environments

“…Furthermore, even for highly valence-excited states of the naphthalene cation, relaxation pathways to the ground state due to vibronic coupling and conical intersections are present (Marciniak et al 2015;Reitsma et al 2019). This trend, which is also observed for larger PAHs (Wenzel et al 2020), corroborates the hypothesis that multiply charged PAHs (Tielens 2008;Zhen 2 Santos et al et al 2017, 2018Banhatti et al 2021), together with their nitrogendoped, phosphorus-doped, and protonated counterparts (Mattioda et al 2008;Ricks et al 2009;Álvaro Galué & Díaz Leines 2017;Meyer et al 2021;Oliveira et al 2021), are also viable candidates as carriers of the unidentified infrared (UIR) bands.…”

“…Moreover, combination bands of moderate intensity in the ∼1400-1600 cm −1 (6.3-7.1 𝜇m) range considerably change the profile of the spectrum in comparison to the harmonic approximation. In fact, anharmonic corrections seem to play an important role in the IR spectra of doubly charged naphthalene (1 2+ ) (Banhatti et al 2021): the average and maximum frequency errors of the anharmonic spectra were, respectively, 3.9 and 8.3 cm −1 -∼20% and ∼41% less than the ones of the scaled-harmonic spectrum. These errors are consistent throughout the entire frequency range of groups 2 and 3 (2500 cm −1 > 𝜈), and are independent of the mode symmetry.…”