# Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations
## 2 Methods  
### Photofragmentation Experiments  
Gas phase clusters were produced by laser ablation and inert gas condensation using an experimental setup detailed elsewhere [36]. For the production of \(M\) (\(M=\) Ag, Pt, Pd) doped gold clusters, two independent nanosecond pulsed Nd:YAG lasers (2 nd harmonic, 532 nm; Spectra-Physics, Santa Clara, CA, USA) were focused on a gold and an \(M\) target, right after a short pulse of He carrier gas (backing pressure of 7 bar) was introduced in the source. By collisions with the carrier gas and subsequent expansion into a vacuum, the ablated plasma condensated and formed a distribution of clusters of various sizes and compositions. This distribution can be tuned by production conditions, including the relative energy of the ablation lasers, their relative firing time, and the pressure of the He gas [37]. In order to obtain information about the relative stability of the clusters, irrespective of the production conditions of the source, the initially charged clusters were electrostatically deflected from the molecular beam and neutral species were excited by a focused excimer F\({}_{2}\) laser (157 nm). This excitation induces a fast ionization process followed by extensive fragmentation [25]. The abundances of the photofragmented species were then analyzed by time-of-flight mass spectrometry, allowing for the identification of relative stability patterns. This approach has been used in the past to investigate the relative stability of several clusters of different sizes and compositions [38; 39; 27; 35]. For Ag and Pd doping, there was no mass overlap between different species and the different clusters can be easily distinguished in the mass spectra. This is shown in Figure 1, where fractions of typical mass spectra of photofragmented AgAu\({}_{N-1}\)\({}^{+}\) and PdAu\({}_{N-1}\)\({}^{+}\) clusters are presented in panels (a) and (b), respectively. Continuous lines connect the \(M\)Au\({}_{N-1}\)\({}^{+}\) species to visualize the size-dependent abundances. This situation is different for the case of Pt doping. Platinum has a distinct isotopic pattern (192 u (0.008%), 194 u (0.329%), 195 u (0.338%), 196 u (0.253%), and 198 u (0.072%)) and the Au atom mass resides in between (197 u). In addition, the strong binding energy of the Pt\({}_{2}\) dimer (4.63 eV compared to 3.15 eV of Au\({}_{2}\) in our calculations) makes it difficult to find production conditions under which only pure Au\({}_{N}\)\({}^{+}\) and singly doped PtAu\({}_{N-1}\)\({}^{+}\) clusters are formed. Thus, pure Pt\({}_{N}\)\({}^{+}\) clusters were also produced, as well as mixed Pt\({}_{x}\)Au\({}_{N-x}\)\({}^{+}\) (\(x\) = 0-\(N\)) species. A part of a typical mass spectrum of the Pt\({}_{x}\)Au\({}_{N-x}\)\({}^{+}\) clusters is presented in Figure 1c. For this reason, a deconvolution process needed to be performed to quantify the intensity of each cluster composition. An example is presented in Figure 1c. Each Pt\({}_{x}\)Au\({}_{N-x}\)\({}^{+}\) (\(x\) = 0-\(N\)) intensity profile was assumed to be composed of a set of Gaussian functions matching the natural isotopic distribution of the cluster. The width of a single Gaussian function was determined by recording a mass spectrum of photofragmented pure Au\({}_{N}\)\({}^{+}\) clusters (with a single isotope for each size) under the same experimental conditions, in order to account for the mass-dependent resolution of the mass spectrometer. Therefore, the center and the width of each Gaussian function was fixed, while the total intensities of each Pt\({}_{x}\)Au\({}_{N-x}\)\({}^{+}\) composition were used as fitting parameters. The analysis was restricted to the size range for which this deconvolution could be applied satisfactorily (\(N\leq 6\)). As a corroboration of the procedure, the extracted abundances of photofragmented pure Au\({}_{N}\)\({}^{+}\) and Pt\({}_{N}\)\({}^{+}\) clusters were compared with results from previous measurements [35; 40], showing an almost identical result.