Ionization of large molecules with short laser pulses
The decay dynamics of fullerenes and polyaromatic hydrocarbons (PAH) after femtosecond-laser excitation have been studied experimentally by electron momentum mapping and mass spectrometry. The electron spectra of fullerenes and large PAH-molecules were found to be exponentially decreasing with energy without any distinct peaks outside the photon energy. This is interpreted as a statistical electron emission from a transient hot, highly excited, electron system where vibrations are still cold. The smooth electron distributions observed were shown not to be due to tunneling ionization. The photoelectron distributions showed a prolongation along the laser eld polarization axis, which was interpreted as eld-assisted thermal ionization. For the smallest molecule in the study, anthracene, a transition to atomic like, above threshold ionization, was observed. Molecular fusion of clusters of fullerenes, formed at liquid nitrogen temperature, after fs-laser excitation has also been studied. Products of up to ve fused molecules were observed. The mass spectra are the results of several processes including multiple ionization, evaporation of intact fullerenes and post-fusion C2 evaporation.
Parts of work
I. Momentum-map-imaging photoelectron spectroscopy of fullerenes with femtosecond laser pulses. M. Kjellberg, O. Johansson, F. Jonsson, A. V. Bulgakov, C. Bordas, E. E. B. Campbell and K. Hansen, Phys. Rev. A, 81, 023202 (2010). ::doi::10.1103/PhysRevA.81.023202II. Femtosecond electron spectroscopy of coronene, benzo[GHI]perylene and anthracene. M. Kjellberg, A. V. Bulgakov, M. Goto, O. Johansson and K. Hansen (2010). Unpublished.III. Molecular fusion within fullerene clusters induced by femtosecond laser excitation. M. Hedén, M. Kjellberg, A. V. Bulgakov, K. Hansen and E. E. B. Campbell. Eur. Phys. D, 43, 255 (2007). ::doi::10.1140/epjd/e2007-00121-xIV. Competition between fission and intra-cluster fusion in highly excited fullerene clusters. K. Hansen, M. Kjellberg, A. V. Bulgakov and E. E. B. Campbell. Israel J. Chem., 47, 43, (2007). ::doi::10.1560/IJC.47.1.43V. Ionisation of fullerenes and fullerene clusters using ultrashort laser pulses. E. E. B. Campbell, K. Hansen, M. Hedén, M. Kjellberg and A. V. Bulgakov. Photochem. Photobiol. Sci., 5, 1183, (2006). ::doi::10.1039/b612749e
Doctor of Philosophy
Göteborgs universitet. Naturvetenskapliga fakulteten
Department of Physics ; Institutionen för fysik
Fredagen den 26 mars 2010, kl. 10.00, Kollektorn, MC2, Kemivägen 9".
Date of defence
Photoelectron spectroscopy, Momentum mapping imaging, thermal ionization, above threshold ionization, molecular fusion, fullerenes, polyaromatic hydrocarbons, femtosecond-laser.