UV action spectroscopy of protonated PAH derivatives

Methyl substituted quinolines

¹ Department of Physics and AstronomyAarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: lha@phys.au.dk
² Institute for Storage Ring Facilities, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 30 May 2011
Accepted: 4 July 2011

Abstract

Aims. We investigate the production of molecular photofragments upon UV excitation of PAH derivatives, relevant for the interstellar medium.

Methods. The action absorption spectra of protonated gas-phase methyl-substituted quinolines (CH₃ − C₉H₇NH⁺) have been recorded in the 215–338 nm spectral range using the electrostatic storage ring ELISA, an electrospray ion source and 3 ns UV laser pulses.

Results. It is shown that the absorption profile is both redshifted and broadened when moving the methyl group from the heterocycle containing nitrogen to the homoatomic ring. The absorption profiles are explained by TD-DFT calculations. The dissociation time of the studied molecules is found to be of several milliseconds at 230 nm and it is shown that after redistribution of the absorbed energy the molecules dissociate in several channels. The dissociation time found is an order of magnitude faster than the estimated IR relaxation time. Photophysical properties of both nitrogen containing and methyl-substituted PAHs are interesting in an astrophysical context in connection with identifying the aromatic component of the interstellar medium.