Probing several structures of Fe(H2O)n+ and Co(H2O)n+ (n=1,…,10) cluster ions
Introduction
Marinelli and Squires [1] and Magnera et al. [2] were the first groups to report binding energies of a water molecule in M(H2O)n+ clusters, where M is a transition metal. The accuracy of these early measurements has been improved in a series of collision induced dissociation (CID) experiments performed in the group of Armentrout and co-workers [3], [4]. Moreover, ab initio calculations have been performed by Bauschlicher and co-workers, providing the necessary information on the structure of these clusters [5], [6], [7]. Taken together, these works have shown that the most stable configuration of the M(H2O)n≤4+ clusters have the water molecules directly attached to the metal ion.
Our recent work on CID of the Fe(H2O)2+, Co(H2O)2+ and Au(H2O)2+ clusters by helium has shown that an ion source which associates laser ablation and supersonic expansion can generate metastable clusters where one of the water molecules lies in the second solvation shell [8]. The existence of such a species, where a water molecule is present in the second shell, even though the first shell is not closed, is actually not a surprise. For example, photofragmentation spectra were reported by the group of Fuke for the Mg(H2O)1−5+ ions [9]. The most stable structure of these ions has up to three water molecules in the first solvation shell, additional water molecules being in outer shells. This corresponds to the dominant isomer responsible for the experimental spectra. Nevertheless shoulders and weak peaks in the spectra have recently been assigned to less stable isomers that are also present in the cluster ion beam [10]. Interestingly, these cluster ions were produced in a source that is comparable to our. Structural isomers of the Cs(H2O)4+ cluster have been reported also [11], [12].
The present work aims at investigating conformations of the M(H2O)n+ cluster ions (M=Co, Fe), where one or several water molecules are located beyond the first solvation shell, although the first solvation shell is not completed. Four families of clusters labeled I1, I2, I3 and I≥4 will be considered throughout the present paper. They correspond a different number of water molecules in the first shell, respectively, one–four or more. Hence, each family will correspond to a different local environment of the metal ion. Of course, only the isomer family I1 has to be considered for the M(H2O)+ cluster ions. Two kinds of isomers I2 (the most stable one) and I1 are to be anticipated for the M(H2O)2+ cluster ions as observed in our former work when M=Fe, Co and Au [8]. It is useful to recall also that the most stable isomer of the M(H2O)1+, M(H2O)2+, M(H2O)3+, and M(H2O)4+ cluster ions, has all the water molecules directly bonded to the metal ion, and consequently corresponds to the isomer family I1, I2, I3 and I≥4, respectively.
The Smalley type source used in our previous work is again used to generate Co(H2O)1≤n≤10+ and Fe(H2O)1≤n≤10+ clusters [8]. Two different experiments are performed to interrogate the structure of the cluster ions produced by this source:
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the first type of experiments is CID, using helium as the target gas. Recent work in our group has shown that molecular dynamics simulations describing the energy transfer between helium and the cluster can be used to extract quantitative information on the water binding energy from CID measurements [13]. The same technique is used here for the data analysis;
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the second type of experiments takes advantage that the visible and close UV electronic excitation of the cluster ions is due to an electronic transition of the core ion. Hence photofragmentation of the clusters at 532, 355 and 266 nm is used to document the water environment about the metal ion. Preliminary results of this type have been reported for the Fe(H2O)1≤n≤9+ cluster ions [14] and a full discussion will be given here.
Section snippets
Apparatus
The apparatus is drawn in Fig. 1. Details can be found in [8], [14], [15]. Briefly, the cluster beam is produced in a Smalley source where an ablation laser is focused on a metal rod. The ion source is coupled to a pulsed helium/water jet, in order to carry the ions into a supersonic expansion zone. The gaseous mixture cooled by the expansion contains helium (the carrier gas), water which was seeded into helium prior to the expansion, plus neutral atoms and positively charged ions from the
The Fe(H2O)1,2+ and Co(H2O)1,2+ cluster ions
The CID cross-section of the Fe(H2O)1,2+ and Co(H2O)1,2++He collisions have been measured as a function of the center of mass collision energy. The corresponding results are shown in Fig. 5. A preliminary version of these cross-sections appeared already in [8]. However, the present results benefit from an improved procedure for the data acquisition, which yield higher accuracy as those shown for the Au(H2O)1,2+ ions in [8]. Hence the present Fe(H2O)1,2+ and Co(H2O)1,2+ cross-sections deserve
Co(H2O)1,2+ and Fe(H2O)1,2+ clusters
The energy dependence of the CID cross-sections shown in Fig. 5 have been fully interpreted in Section 3.1. When passing through the experimental points, the curve predicts the absolute value and the energy dependence of the CID cross-section. The output was in full consistency with the experimental data of Armentrout and co-workers both for Co(H2O)+ and Fe(H2O)+, and for the compact isomer I2 of Co(H2O)2+ and Fe(H2O)2+ [20]. The new data brought by the present work is the binding energy of the
Concluding remarks
A laser ablation source, coupled to a supersonic expansion, has been used in the present work to form Co(H2O)n+ and Fe(H2O)n+ cluster ions with n ranging between 1 and 10. These ions have been fragmented in two different ways. One is CID with helium and the other photofragmentation at 532, 355 and 266 nm. The mechanism transferring energy into the cluster is very different in each case, hence the fragmentation mechanism is different, and the information provided on the cluster is complementary.
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Present address: Chemical Sciences Division, Lawrence Berkeley Laboratory, 6-2101 ALS LBL, Berkeley, CA 94720, USA.
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Present address: Département de chimie, Université de Nice Sophia-Antipolis, C.M.O.M., F-06108 Nice Cedex 2, France.