photoabsorption of ag clusters in he droplets: a transition from single- to multi-centered growth 67...
TRANSCRIPT
Photoabsorption of Ag Clusters in He Droplets: A Transition from Single- to Multi-Centered Growth
67th International Symposium on Molecular Spectroscopy
Luis F. Gomez1, Evgeny Loginov1, Naihao Chiang2, Avik Halder2, Nicholas Guggemos2, Vitaly V. Kresin2, and Andrey F. Vilesov1,2
1Department of Chemistry, University of Southern California, Los Angeles, USA2Department of Physics and Astronomy, University of Southern California, Los Angeles, USA
2
NHe ~ 104
Small Droplets vs. Larger Droplets
3
-2 0 2 4 60
5
10
Sig
nal a
t M=
8, m
V
Time, ms
Laser Pulse
Pulsed laser
Measuring the Spectra of Ag Clusters in Helium
4
Henne & Toennies, JCP (1998).Lewerenz et al, CPL (1993).Titration with Helium
He Droplet Sizes
Gomez, Loginov, Sliter, Vilesov, J. Chem. Phys. (2011)
5
PHe
He HeAg He
He Ag
P EN N
P E
HeN
EHe = 0.06 meV He-HeN binding energyEAg = 3 eV Ag binding energy
= avg. number of He atoms in droplet
Calorimetry in He Droplets
∆P ~ He flux
Ag[…4d105s1]
Loginov, Gomez, Chiang, Halder, Guggemos, Kresin, Vilesov, PRL (2011) 6
Spectra of Small Ag Clusters in Droplets
++ +
- --
++ +
- --++ +
- --++ +
- --++ +
- --++ +
- --
++ +
- --
++ +
- --
++ +
- --
++ +
- --++ +
- -- ++ +
- --++ +
- --
++ +
- --
++ +
- --++ •+
- --
Loginov, Gomez, Chiang, Halder, Guggemos, Kresin, Vilesov, PRL (2011) 7
Unexpected Strong IR Absorption in Large Ag Clusters
+++
-- -
++ +
- --
++
+
--
-++
+
--
-
≈ 0.5 eV
++
+
--
-
++
+
--
-
+++
-- -
+++
-- -
8
Origin of the Broadening in the Spectra of Metal Cluster-Cluster Aggregates: Dipole-Dipole Interaction of Excited Clusters
Recombination time of two Ag atoms
Time between pickup events
dropletrec
Ag rel
Vt
v
, 1TOF
n nAg
tt
N
9
Kinetics of Ag Cluster Growth in a Droplet
10Loginov, Gomez, Chiang, Halder, Guggemos, Kresin, Vilesov, PRL (2011)
Different Modes of Cluster Formation in the Droplets
• Large superfluid He droplets can be used for metal cluster
growth
• Small Ag clusters less than ~100 atoms are compact
• Larger Ag clusters are likely aggregates
• Found average sizes of very large droplets:
<NHe> ~ 108 – 1010, up to 1 µm in diameter
Conclusions