Prompt in situ emission of gold adducts from single impacts of large gold clusters on organics solids

doi:10.1016/j.ijms.2007.03.011

International Journal of Mass Spectrometry

Volume 263, Issues 2–3, 1 June 2007, Pages 298-303

https://doi.org/10.1016/j.ijms.2007.03.011 Get rights and content

Abstract

We report the first observation of single impacts of 136 keV Au_n⁴⁺ (100 ≤ n ≤ 400) on organic solids, generating prolific emission of Au and Au₂ - containing fragments and molecular ions. We show that the individual impacting cluster is both the source of energy stimulating the emission, and the donor of atoms for adducts synthesis. The emission of Au and Au₂ was observed when n ≥ 100. The most abundant species is Au(CN)₂⁻. The adduct yields behave as follows when the projectile size varies from n = 100 to 400: (a) they increase with n; (b) the dependence with n for the formation of Au(CN)₂⁻ differs from those for more complex species suggesting different pathways of synthesis; (c) the combined yields of the Au or the Au₂ - adducts are the same for different targets. There is evidence that the projectiles were implanted virtually intact in the organic targets, thus, the adduct synthesis involves a small number of Au atoms ablated from the projectile, which implies extensive ionization of the detached atoms. The abundance of three-body assemblies, e.g. Au(CN)₂⁻, Au(CN) (M–H)⁻, suggests that the adduct formation occurs likely in a dense phase.

Introduction

It was shown some years ago that bombardment with polyatomic projectiles results in a non-linear enhancement of the secondary ion (SI) emission, specifically the emission of cluster and molecular ions [1], [2]. For example, the SI yields obtained with Au₃⁺–Au₅⁺ projectiles are at least 10 times higher than those obtained with Au⁺. This comparison is for projectiles at equal velocities in the range of 5–60 keV. Recent work with Bi_n⁺ (1 ≤ n ≤ 7) shows similar trends for the SI emission [3]. Experiments with more complex projectiles, in particular C₆₀⁺, demonstrate that the SI yield enhancement is correlated with the energy deposited in the very near surface region (<10 nm). Experimental work [4] and molecular dynamics (MD) simulations [5], [6] indicate that, at equal impact energy, C₆₀⁺ is more efficient in generating ionized ejecta than Au₃⁺_. Thus, the energy density is the main parameter of the SI emission. One way to further increase the energy density is to use massive clusters containing a large number of heavy atoms. An example is Au₄₀₀⁴⁺ [7], [8], [9], [10]. We show below that the conditions reached with such massive projectile–target systems lead to relaxation via new pathways as evidenced in surprising ion chemistry.

We report here the first observation of Au-adducts produced with single impacts of massive Au_n clusters, i.e. the formation and release of ejecta made from projectile atoms and target components. The experiments involved Au_n^q+ clusters (100 ≤ n ≤ 400, q = 4, 34q keV total energy) impacting on various organic substrates, i.e. glycine, histidine and guanine. Given the constraint of a single impact in the static mode bombardment, the projectile strikes an unperturbed area of the target meaning that the formation of adducts must involve atoms from the projectile. Such a prompt in situ formation has not been predicted theoretically, nor has it been observed in MD simulations for clusters within this impact energy range. A recent MD simulation shows the formation of possible chemical bonds between carbon and silicon atoms following the impact of C₆₀⁺ (250 eV/atom of impact energy) on a silicon target [11]. Ejection of the newly formed species is however not mentioned.

In the projectile–target combinations studied, the emission of Au-containing adducts is surprisingly high, with about one adduct detected per three projectile impacts. The chemistry involved must be ultra-fast (<0.1 ns) and occur under transient high pressure and temperature conditions. The newly observed production of Au-adducts is described below as a function of the parameters involved in their formation, i.e. the number of gold atoms in the projectile and the nature of the target, and compared to the yields of those for the customary secondary ions emitted from the organic solids.

Section snippets

Experimental

Targets of similar chemical properties, i.e. 1.6 < pK_a1 < 2.34, 9.2 < pKa2 < 9.8, glycine (C₂H₅NO₂, M_w = 75.07), histidine (C₆H₉N₃O₂, M_w = 155.16) and guanine (C₅H₅N₅O, M_w = 151.13) were bombarded with large gold clusters, Au_n⁴⁺ of mean size n = 100, 200, 300 and 400. The targets were identically prepared by vapor deposition of analyte (film thickness ∼1 μm) onto a stainless steel plate. This method of preparation assures the substrate uniformity over a large area (1 cm²). Projectiles were impacting the targets

Mass spectra

We present in Fig. 1(a) the mass spectrum of negative ions emitted from a guanine target from a summation of N₀ ∼ 2 × 10⁵ individual impacts of 136 keV Au₄₀₀⁴⁺ projectiles. The area exposed to bombardment was ∼1 mm². Thus, the bombardment occurred under “super-static” conditions, i.e. each Au₄₀₀⁺ impacted a fresh area of the target. The peaks observed in the low mass range of the spectrum correspond to “conventional” secondary ions e.g., CN⁻, CNO⁻, C₃N⁻, and larger fragments (m/z = 66, 90, 106 and 133)

Phenomenological approach

A closer examination of the yield dependence with the projectile size n for the various classes of adducts reveals some differences. The yields for Au(CN)₂⁻ and Au₂(CN)₃⁻ from guanine are plotted in Fig. 5(a) along with the yield for the re-emitted Au⁻ from glycine. As noted earlier, Au⁻ is not present in the mass spectra for guanine. The yield for Au⁻ follows a fit in n^α, with α ∼ 1.8. For Au(CN)₂⁻ and Au₂(CN)₃⁻, a fit with a function of the type (n − n₀)^α prevails. The values obtained for α were

Conclusions

A key finding is that the combined yields of the adducts are the same for different targets. We infer that the ejected matter contains an overall constant ionic charge. A further item of note is the threshold for adduct production which occurs at n ∼ 70 under the conditions of this study.

The prompt in situ formation of Au-adducts is prolific and involves highly efficient interactions with the Au ablated from the projectile. It must be noted that the adduct emission does not scale with the

Acknowledgements

We thank Stanislas Verkhoturov for fruitful discussions. E.A.S thanks the Robert A. Welch Foundation (A-1482) and the National Science Foundation (CHE-0449312) for funding this research. S.D.N. thanks the CNRS “direction des relations européennes et internationales” (CNRS-Etats-Unis 2006) for funding a collaborative project.

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Prompt in situ emission of gold adducts from single impacts of large gold clusters on organics solids

Abstract

Introduction

Section snippets

Experimental

Mass spectra

Phenomenological approach

Conclusions

Acknowledgements

Nucl. Instrum. Meth. B

Int. J. Mass Spectrom.

Nucl. Instrum. Meth. B

Appl. Surf. Sci.

Int. J. Mass Spectrom.

Appl. Surf. Sci.

Nucl. Instrum. Meth. B

Int. J. Mass Spectrom.