Photoinduced dissociation of anionic and electron detachment of dianionic gold clusters by use of a laser pointer

doi:10.1016/S1387-3806(01)00529-2

International Journal of Mass Spectrometry

Volume 213, Issues 2–3, 1 February 2002, Pages 157-161

Dedicated to H.-J. Kluge who has continuously supported the Cluster Trap (up to the supply of his laser pointer for the present work) on the occasion of his 60th birthday.

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Abstract

Size-selected anionic and dianionic gold clusters have been stored in a Penning trap and irradiated with the green light of a laser pointer. As examples of special interest, the systems Au₇⁻ and Au₂₉²⁻ have been chosen. In particular, Au₇⁻, a small gold cluster with closed electron shell, is observed to decay into Au₆⁻ and Au₅⁻ with a decay pathway branching ratio similar to that of Au₉⁺. The dianionic cluster Au₂₉²⁻ shows electron detachment upon photoexcitation. This observation is in agreement with independent experiments [Stoermer et al., Int. J. Mass Spectrom. 201 (2001) 63], where Au₂₉²⁻ is found to be the smallest dianion produced by neutral monomer evaporation of hot dianionic gold clusters.

Introduction

When atomic clusters are heated up they can release their excess energy through various competing decay channels such as evaporation of neutral constituents, fission into charged fragments, thermionic emission of electrons or radiative cooling, i.e. emission of blackbody-like radiation (see e.g. [1], [2], [3]). During the last few years, several collision-induced-dissociation experiments with transition metal cluster anions have been reported [4], [5], [6], [7], [8] which resulted in mostly qualitative knowledge of the systems’ decay pathways. In particular, the fragmentation behavior of the anions M_n−1⁻ (where “M” stands for the element of interest and the subscript describes the number of atoms of the cluster) has been found to be similar to that of the cations M_n+1⁺. This is a strong indication that the corresponding number of atomic valence electrons n is responsible for the systems’ properties.

Recently, cationic gold clusters have been investigated by photoexcitation in order to quantitatively study not only their fragmentation pathway branching ratios as a function of cluster size [9], but also to investigate these branching ratios as a function of the excitation energy [10]. In the present experiments, these investigations have been extended to anionic clusters. Instead of applying a pulsed dye laser system as before [11], [12], a much simpler device has been used for photoexcitation. In the following, we present experiments on the activation of stored anionic and dianionic gold clusters with the green light of a laser pointer. This procedure does not allow to follow the processes in a time-resolved manner as in earlier studies of delayed electron emission of tungsten clusters, W_n⁻ [13], [14]. However, the emphasis of the present studies is not on the time structure of the decay, but on the possible competition between the decay pathways, i.e. electron emission and evaporation of neutral atoms and dimers.

Two particularly interesting systems have been chosen for these studies: (1) Au₇⁻ with a shell closure at eight atomic valence electrons, which can be compared to the isoelectronic system of Au₉⁺ [9] and (2) Au₂₉²⁻, which has been reported to be the smallest dianionic gold cluster produced from hot larger dianionic precursors [15]. Thus, whereas larger clusters are expected to show a significant propensity toward neutral monomer evaporation upon excitation, Au_n²⁻→Au_n−1²⁻+Au, the cluster Au₂₉²⁻ should exclusively show electron emission Au₂₉²⁻→Au₂₉⁻+e⁻ [15].

Section snippets

Experimental procedure

The experimental setup has been described in detail elsewhere [12], [16], [17], [18]. In the present case of the cluster anions, the experimental sequence consists of the following steps (see Fig. 1): (1) production of cluster anions in a Smalley-type laser vaporization source [19], [20]; (2) transfer to the Penning trap and capture in flight [21]; (3) accumulation of several cluster-ion bunches by multiple application of steps (1) and (2) [22]; (4) size selection by resonant radial ejection

Anion Au₇⁻

Fig. 2 shows time-of-flight spectra of Au₇⁻ without (top) and with 5 s irradiation (bottom) with the laser-pointer light. The excitation leads to fragments Au₆⁻ and Au₅⁻ with an intensity ratio of I(Au₅⁻)/I(Au₆⁻)=2.54(74). By comparison with similar experiments on cationic gold clusters [9], [10], [27] it can be assumed (1) that the decay occurs only upon absorption of at least two photons and (2) that the sequential decay Au₇⁻→Au₆⁻→Au₅⁻ is energetically highly suppressed, i.e. (most of) the Au

Conclusion

The light of a green laser pointer has been used for the photoexcitation of stored singly and doubly charged anionic gold clusters. In the case of Au₇⁻ agreement has been found between the observed and the expected fragmentation pathway branching ratios for dimer and monomer decay. This indicates that the details of the fragmentation process are governed by the electronic properties of the systems under investigation. The measurements have been extended to the case of the dianion Au₂₉²⁻. For

Acknowledgements

This work has been supported by the DFG, the “Materialwissenschaftliches Forschungszentrum Mainz” the “Fonds der Chemischen Industrie” and the EU networks “Eurotraps” and “Cluster Cooling.”

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Regular articlePhotoinduced dissociation of anionic and electron detachment of dianionic gold clusters by use of a laser pointer

Abstract

Introduction

Section snippets

Experimental procedure

Anion Au7−

Conclusion

Acknowledgements

Phys. Rep.

Chem. Phys.

Chem. Phys. Lett.

Int. J. Mass Spectrom.

Nucl. Instr. and Meth. A

Int. J. Mass Spectrom. Ion Processes

Philos. Mag. B

Annu. Rev. Phys. Chem.

Eur. Phys. J. D

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

Eur. Phys. J. D

Eur. Phys. J. D

Appl. Phys. B

Regular article
Photoinduced dissociation of anionic and electron detachment of dianionic gold clusters by use of a laser pointer

Anion Au₇⁻