Photophysical Properties of Protonated Aromatic Hydrocarbons

Citation

Kapinus, Vadym A. (2005) Photophysical Properties of Protonated Aromatic Hydrocarbons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3A85-DC20. https://resolver.caltech.edu/CaltechETD:etd-01252005-123917

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and their derivatives are among the likely candidates for diffuse interstellar band (DIB) carriers. In diffuse interstellar clouds molecules are expected to be ionized, and PAHs in particular are predicted to be in protonated form. Protonated PAHs are closed-shell ions with electronic transitions that are red-shifted compared to neutral PAHs.

This thesis presents an experimental and theoretical analysis of two-, three-, and four-ring protonated PAHs. The energetics of protonated PAH isomers in their ground electronic state and the proton tunneling barriers were calculated using density functional theory. The loss of an H atom or H2 molecule from a protonation site were identified as the most favorable dissociation channels for protonated PAHs. They were found to be within a few kcal/mol of each other in energy, and about 45-60 kcal/mol above the ground state.

The Configuration Interaction Singles (CIS) method was used to estimate the positions of the electronic transitions for protonated PAHs. Red shifts up to 150 nm were predicted for protonated naphthalene, anthracene, phenanthrene and pyrene. This places the S1 ← S0 transitions of protonated PAHs well into the DIB wavelength region.

Experimentally, a robust hydrogen discharge source was designed to produce protonated PAHs. Laser photodissociation of protonated PAHs was studied with an excimer laser/reflectron time-of-flight mass spectrometer. Small protonated PAHs were found to be very photostable. Nanosecond pulse length photodissociation was multiphoton even at short wavelengths (193 nm). For protonated anthracene, the dissociation threshold was estimated to be 13 -- 15 eV, much higher than the predicted thermodynamic threshold of 2.5 -- 3.0 eV. This was attributed to intramolecular vibrational relaxation (IVR) upon electronic excitation.

Cluster photodissociation was used to locate the electronic transitions of protonated anthracene between 420 and 540 nm. Clusters with water molecules were produced in a two-valve mixing discharge source. Their photodissociation spectrum was recorded using a novel hybrid cavity optical parametric oscillator. The observed absorption bands (20 nm FWHM) are too wide to account for the DIBs. This spectral broadening most likely results from rapid IVR induced by the high density of states in protonated PAHs.

Thesis Files