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46574-UFS
Electronic Spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs) in the Gas Phase

Thomas D. Varberg, Macalester College

I spent the period August 24, 2007 to August 14, 2008 as a Visiting Professor of Chemistry at the University of Sydney, Australia, funded by my ACS-PRF UFS grant. My sabbatical hosts were Prof. Scott Kable and Dr. Tim Schmidt in the School of Chemistry.

The goal of our collaborative work, as described in my original proposal, was to measure electronic spectra of new, large polycyclic aromatic hydrocarbons (PAHs), which are proposed to exist in the interstellar medium and are potential candidates as carriers of the still unidentified "diffuse interstellar bands" first described in the 1920s. We used two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy as our preferred technique to make these measurements.

Our initial experiment upon my arrival in Sydney was to carry out REMPI measurements on the carrier of a strong electronic transition at 476.02 nm. I worked alongside of graduate student Damian Kokkin in this project. The carrier of this spectrum was not identified at the time of my writing the PRF proposal. In the intervening time, it has been identified to be the phenyl propargyl radical. The electronic spectrum of this radical was first measured in the Kable/Schmidt lab (by graduate student Neil Reilly) by recording its laser-induced fluorescence spectrum. Our contribution was to measure its REMPI spectrum, which will be included in a forthcoming publication describing the electronic structure of the molecule.

Following this initial experiment, we sought to measure REMPI spectra of several different organic free radicals that have not been measured in the gas phase. An example of these is the indenyl radical. We attempted to produce this free radical in the gas phase by expanding indenyl iodide in a molecular beam, which was passed through a pulsed electric discharge just downstream from the pulsed nozzle. We searched for the electronic spectrum of this radical, guided by a previous low-resolution spectrum in a solid matrix, but were unable to observe it.

Together with graduate student Tyler Troy, Kokkin and I then began an ultimately successful search for the electronic spectrum of the large polycyclic aromatic hydrocarbon, hexa-peri-hexabenzocoronene (HBC), C₄₂H₁₈. This compound was synthesized in collaboration with Dr. Nigel Lucas of the School of Chemistry. The targeted species was seeded in argon by ablating the solid HBC sample with laser pulses produced by a doubled (532 nm) Nd:YAG laser. Jet-cooled HBC molecules were cooperatively ionized by two laser pulses: the first was tuned to place HBC molecules into an excited state from where they could be ionized by the second, Þxed wavelength pulse.

The resonant 2-color 2-photon ionization spectrum of HBC is shown in Fig. 1. The spectrum is for the 522-amu (¹²C₄₂¹H₁₈) mass only, and as such it represents only the most abundant isotopomer (64%). The main features are positioned at 433.52 and 426.41 nm, labeled 'a' and 'b' in Fig. 1. We attribute the cluster of smaller peaks immediately to longer wavelength of these main features to "hot" bands, which are transitions arising from excited vibrational levels of the electronic ground state that remain populated by incomplete cooling in the supersonic expansion. To higher energy than these main features lie transitions involving quanta of totally symmetric vibrations built onto bands a and b.

Fig. 1: The excitation spectrum of hexa-peri-hexabenzocoronene (bottom) and a reconstruction of the diffuse interstellar band spectrum (top) from Jenniskens and Desert [1]. The arrow denotes an estimate of the position of the forbidden origin band, based on the solution phase spectra of Hendel, Khan and Schmidt [2].

This spectrum represents the Þrst optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium to be reported. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.

Towards the end of my time in Australia, I worked with Tyler Troy in searching for the REMPI gas-phase spectra of the two molecules shown below, one a simple gold-dithiol and the other a larger PAH derived from HBC. While we were initially unsuccessful, spectroscopic work on these two molecules is continuing.

Finally, we spent a few weeks fitting our recorded spectra of several vibrational bands of the A¹P–X¹S⁺, C¹P –X¹S⁺ and D¹S⁺–X¹S⁺ systems of the molecule gold fluoride (AuF). These electronic spectra were recorded in the summer of 2007 at Macalester College, and the analysis was undertaken with support from this PRF grant at the University of Sydney. We have completed our fitting of these bands, including a full analysis of the hyperfine structure of the
C–X system and are presently preparing a manuscript for publication.

[1]  Jenniskens, P.; Desert, F. X. Astron. Astrophys. Supp. Ser. 1994,106, 39.

[2]  Hendel, W.; Khan, Z. H.; Schmidt W. Tetrahedron 1986, 42, 1127.

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