Paper SS1-MoA7
Photochemistry on Metal Nanoparticles: Comparison of Nanosecond and Femtosecond Laser Induced NO Photodesorption from NO Dimer Layers on Supported Ag Nanoparticles

Monday, October 18, 2010, 4:00 pm, Room Picuris

Metal nanoparticles (MNP) have special electronic and optical properties due to their dimensions being smaller than many length parameters of electrons and photons with solids; this leads to drastic changes of surface photochemistry [1]. For oxide-supported Ag nanoparticles (AgNPs), we have reported before on strong influences of the initial excitation (2.3 – 4.7 eV, in particular of Mie plasmons at 3.5 eV), and of the mean particle size (2 to 12 nm), on the photodesorption cross sections (PCSs) of NO from NO dimer layers [2,3], using nanosecond laser pulses. However, the mechanism of desorption remains the same as on Ag(111) [2-4], as indicated by detailed measurements of final state energy distributions (translational, rotational, and vibrational) of the desorbing molecules [5], except for the smallest particles at the highest excitation energy [2] where much higher final state energies are found. These results are compatible with desorption via a transient ion, negative (TNI) in most cases [4], and positive (TPI) in the latter case [2]. In the entire range of nanosecond laser excitation, strictly linear behavior is found.

We have now done similar measurements using femtosecond laser excitation (3.1 eV photons only), increasing the excitation density by more than 10⁴ for the same photon fluences. No change is seen on Ag(111), while for AgNPs femtosecond plasmon excitation drastically increases the PCSs as well as changes the dynamics, as indicated by different final state energies of the desorbed NO. We interpret this behavior as due to the confinement of excitation in the NPs which then leads to multiple pump-up of hot electrons during the laser pulse.

After a general survey of the basics of photochemistry on nanoparticles as compared to surfaces of bulk crystals, and of the indicated previous results, the recent measurements will be described and discussed, aiming at conclusions about their relevance for the understanding of excitations and photochemistry on nanoparticles.

[1] K. Watanabe, D. Menzel, N. Nilius, and H.-J. Freund, Chem. Rev. 106, 4301 (2006)

[2] D. Mulugeta, K. H. Kim, K. Watanabe, D. Menzel, and H.-J. Freund, Phys. Rev. Lett. 101, 14613 (2008)..

[3] K. H. Kim, K. Watanabe, D. Menzel, and H.-J. Freund, J. Am. Chem. Soc., 131, 1660 (2009).

[4] F.M. Zimmermann and W. Ho, Surf. Sci. Rep. 22, 127 (1995), and references therein.

[5] D. Mulugeta, Ph.D. Thesis, TU Berlin 2010; and D. Mulugeta, K. Watanabe, D. Menzel, and H.-J. Freund, to be published.