Book of Abstracts Book of Abstracts - Universität Konstanz

B - 34
Relaxation Dynamics of Optically Excited States of Ag Cluster Anions
74
Markus Engelke, Marco Niemietz, Young Dok Kim, Gerd Ganteför
Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany
http://www.clusterphysik.uni-konstanz.de/
For bulk metals, optically excited electronic states can relax within few tens of
femtoseconds due to very fast Auger-like electron-electron scattering processes. As the size of a
particle decreases, changes in relaxation dynamics of optically excited states are observed [1,2].
Using Time-Resolved Two-Photon Photoemission (TR-PPE) spectroscopy, relaxation times of
optically excited states of Ag cluster anions consisting of less than 22 atoms (pump photon
energy of 1.5 eV) were determined. For Ag18 - , Ag19 - and Ag21 - , relaxation times amount to about
500 femtoseconds, which are significantly longer than those of d-metal clusters consisting of
less than 8 atoms [3]. The long life times of the excited states of these Ag cluster anions can be
explained by lower density of states above the upper levels of the non-excited electrons within
the pump photon energy in these clusters, caused by a large gap between 2s and 1f electronic
shells, i.e. electronic shell configurations significantly affect the electron relaxation dynamics.
-
Ag18 Intensity (a.u.)
References
0 fs
133 fs
266 fs
932 fs
1,9 ps
12,9 ps
48,6 ps
0 1 2
Binding Energy (eV)
Figure 1. TR-PPE spectra of Ag 18 - . A distinct peak can
be observed at a binding energy of about 1.3 eV,
accompanying broad states at higher binding energies.
The state at 1.3 eV can be attributed to the excitation of
an electron in the HOMO to an unoccupied state. As the
delay between pump and probe photons increases, the
signals between 1.2eV - 2.5 eV attenuate. By assuming
an exponential decay of the excited state, the lifetime is
estimated to be about 420 fs.
[1] J.R.R. Verlet, A.E. Bragg, A. Kammrath, O. Cheshnovsky, and D.M. Neumark,
J. Chem. Phys. 121, 10015 (2004).
[2] M. Niemietz, P. Gerhardt, G. Ganteför and Y. D. Kim, Chem. Phys. Lett. 380, 99 (2003).
[3] N. Pontius, M. Neeb, W. Eberhardt, G. Lüttgens, P. S. Bechthold, Phys. Rev. B 67, 354251 (2003)
and references therein