4D13
Photodissociation and photon-trap spectroscopy of metal cluster ions(Genesis Research Institute, Inc.1, Tokyo University of Science2, Toyota Technological Institute3)
Bartels Christof1, Kasai Chiharu1,2, Egashira Kazuhiro1, !Terasaki Akira1,3, Kondow Tamotsu1,3"#$ A quantitative understanding of the electronic structure of clusters is of fundamental
importance for the development of nanoscience. Although most anticipated applications areexpected to rely on deposited clusters with less well-defined size distribution, experiments onvery small clusters in the gas phase are essential for establishing and testing new models for thisdifficult size regime [1]. Our approach is to investigate size-selected cluster ions by laser
spectroscopy. In order to achieve sufficient ion densities, clusters are accumulated in a trap.
As many features of the electronic structure appear broadened and are thus more difficult tounderstand at room temperature, we use buffer-gas cooling to lower the thermal energy of theclusters. At a temperature of 10 K, the thermal energy per degree of freedom is less than 1 meVor 8 cm-1, and consequently vibrational excitation can be neglected in small clusters. Moreover,
we present a novel technique for the direct measurement of the absorption of clusters by
combining the ion trap with cavity ring-down or photon-trap spectroscopy [2,3]. The extinction oflight is converted to a characteristic change of the photon lifetime in the cavity, which provides
sensitivity of up to 1 ppm absorbance per round-trip, depending on the reflectivity of the cavity
mirrors. All experiments can also be performed under magnetic fields of up to 5 T, created by asuperconducting magnet around the trap, which makes it possible to probe magnetic properties
by magneto-optical effects [2].
"%&$ The experimental setup is shown in Figure 1. Cluster ions are produced in amagnetron sputter source and then guided through the machine using RF octopole ion guides.
A quadrupole mass filter is used to select a single cluster size. The ion trap itself is surroundedby two stages of cooling cells, which can be cooled by liquid nitrogen and liquid helium,respectively. Pre-cooled helium buffer gas is injected into the inner cooling cell, and clusters arethermalized by buffer-gas cooling. The laser system consists of a frequency-tripled 10 Hz!
!
Figure 1: Schematic of the experimental setup.

Nd:YAG and an optical parametric oscillator/amplifier that can be tuned over a broad range ofwavelengths. Typical pulse energies are in the range of 1 to 10 !J, and pulse duration is around
7 ns. For photodissociation spectroscopy, the cluster ions are irradiated by a fixed number oflaser pulses, extracted from the trap and mass-analyzed in a second quadrupole mass filter.
For photon-trap spectroscopy, the ring-down signal is measured for alternating cycles of empty
and loaded ion trap, and the difference is taken to be proportional to the clusters’ absorption.
" ' ( ) * + $ We investigated the absorption of the silver dimer cation Ag2+ byphotodissociation spectroscopy. A strong absorption peak at around 3.0 eV shows apronounced temperature dependence (Figure 2). For the room-temperature data, this peak isvery broad with FWHM > 0.1 eV and clearly asymmetric with a longer tail towards the low-energy
side. While this shape seems to suggest to be a reflection of the thermal population ofro-vibrational states, the width is not consistent with a temperature of 300 K. The data taken at atemperature of 10 K show considerable narrowing, but part of the asymmetry persists, and nodetailed structure can be observed. Theoretical calculations predict the vibrational spacing of
the Ag2+ ground state to be 128.2 cm-1 [4], so we expect any occupation of higher vibrational
states to be completely negligible at 10 K. Analysis of the spectra is in progress forunderstanding the temperature dependence of the absorption profile by calculating
Franck-Condon overlap factors between the vibrational levels of the ground and excitedelectronic states.

2Cross section (Å )

2.0

1.5

10 K
300 K

1.0

0.5

0.0
2.80

2.85

2.90

2.95

3.00

3.05

3.10

Photon energy (eV)!
Figure 2: Absorption of Ag2+ at 10 K (solid symbols) and 300 K (open symbols).

References:[1] A. Terasaki, J. Phys. Chem. A (Feature Article) 111, 7671 (2007)[2] A. Terasaki, T. Majima and T. Kondow, J. Chem. Phys. (Communication) 127, 231101 (2007)[3] A. Terasaki, T. Majima, C. Kasai and T. Kondow, Eur. Phys. J. D 52, 43 (2009)[4] V. Bona!i"-Koutecký, L. #ešpiva, P. Fantucci and J. Koutecký, J. Chem. Phys. 98, 7981 (1993)