Far-Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbons: Zero Kinetic Energy Photoelectron Spectroscopy of Pentacene Vaporized From Laser Desorption

Abstract

Far-IR spectroscopy of pentacene cation, Zhang, et al 2 ABSTRACT The distinctive set of infrared (IR) emission bands at 3. 3, 6.2, 7.7, 8.6, and 11.3 µm are ubiquitously seen in a wide variety of astrophysical environments. They are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, not a single PAH species has yet been identified in space as the mid-IR vibrational bands are mostly representative of functional groups and thus do not allow one to fingerprint individual PAH molecules. In contrast, the far-IR (FIR) bands are sensitive to the skeletal characteristics of a molecule, hence they are important for chemical identification of unknown species. With an aim to offer laboratory astrophysical data for the Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy, and similar future space missions, in this work we report neutral and cation FIR spectroscopy of pentacene (C 22 H 14 ), a five-ring PAH molecule. We report three IR active modes

Zero kinetic energy photoelectron spectroscopy of jet cooled benzo[a]pyrene from resonantly enhanced multiphoton ionization

“…The most striking feature of all the ZEKE spectra is the dominance of one vibrational band corresponding to the same vibrational excitation of the intermediate state. Unlike all the other PAH molecules we have studied, [16][17][18] a clear propensity of preserving the vibrational excitation of the intermediate state can be identified in the ZEKE spectra of BaP, with negligible vibrational relaxation or mode mixing. [35][36][37][38][39] When ambiguities arise in the assignment of combination bands, we therefore believe that the combination containing the original vibrational excitation is the most probable.…”

“…The experimental apparatus is a differentially pumped high vacuum molecular beam machine, with the detection chamber enclosed inside the source chamber. [16][17][18] The sample BaP (Aldrich) was housed and heated to 170 • C in the pulsed valve located in the source chamber to achieve sufficient vapor pressure. The vapor was seeded in 1600 torr of argon and co-expanded into vacuum through a pulsed valve with a 1 mm orifice.…”

“…As for the simulation of the S 2 ← S 0 transition, the disagreement further confirms our assignment of the S 1 electronic state and the observed vibrational bands. It is worth noting that among the three PAHs that we have investigated, [16][17][18] BaP is the first case when we used the newly available vibronic coupling feature of GAUSSIAN 09 to calculate the REMPI spectrum. The remarkable agreement in this case and the disagreements of our previous work on tetracene, pentacene, or pyrene testify to the importance of vibronic coupling in PAHs.…”

“…The technique of zero kinetic energy (ZEKE) photoelectron spectroscopy serves as an ideal tool, albeit indirect, for the study of low frequency vibrational modes of cations. [16][17][18] ZEKE is known for its high resolution of cation rovibrational spectroscopy. The high Rydberg states in ZEKE are longer lived when they are associated with lower vibronic states of the cation, and this feature makes ZEKE particularly suitable for studies of lower frequency vibrational modes.…”

Search for far-IR PAH bands with Herschel: modelling and observational approaches

“…These features involve the whole carbon skeleton of the molecule and are therefore more specific to the exact molecular identity than the ones in the mid-IR, which instead probe functional groups. Experimental data in the far-IR range are still scarce and await for further progress in the coming years (Zhang et al 2010;Mattioda et al 2009;Pirali et al 2006). On the other hand, more and more data are available from quantumchemistry calculations using density functional theory (DFT; cf.…”