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Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K
International Journal of Mass Spectrometry 434 (2018) 136–141
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Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K P. Martini a , L. Kranabetter a , M. Goulart a , M. Kuhn a , M. Gatchell a , D.K. Bohme b , P. Scheier a,∗ a b
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria Department of Chemistry, York University, Toronto, Ontario, Canada, M3J 1P3
a r t i c l e
i n f o
Article history: Received 19 July 2018 Received in revised form 5 September 2018 Accepted 13 September 2018 Available online 20 September 2018 Dedicated to Helmut Schwarz on the occasion of his 75th birthday. He inspired generations of young physicists and chemists with his outstanding scientific achievements. Whenever we think that we have discovered a new and exciting process, it often has been already published by the group of Helmut Schwarz.
a b s t r a c t We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*− , metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n +/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state. © 2018 Elsevier B.V. All rights reserved.
Keywords: He nanodroplets Gold clusters Anions Cations Magic numbers
1. Introduction Helium nanodroplets (HNDs), formed in vacuo, allow interactions between atoms to be explored at extremely low temperatures (close to zero K) in a chemically inert environment of helium [1]. When atoms of gold are added to HNDs they cluster and, when these “golden HNDs” are exposed to ionizing electrons, both positive and negative cluster ions of gold are formed. After leaving the HNDs, the cluster ions can be monitored mass spectrometrically: the size of the cluster ions, their distribution in size, the presence of clusters of enhanced stability (so-called “magic” clusters), as well as patterns of cluster stability. Furthermore, when other atoms or molecules are added to the HNDs along with gold atoms, the reactivity of gold atom and small clusters of gold atoms toward these
∗ Corresponding author. E-mail address: [email protected] (P. Scheier). https://doi.org/10.1016/j.ijms.2018.09.020 1387-3806/© 2018 Elsevier B.V. All rights reserved.
molecules can be probed near zero K [2–4]: the high nuclear charge in a gold atom leads to relativistic effects that can be consequential in chemical bonding with atoms and molecules [5,6]. We have previously performed mass spectrometric measurements of the adsorption of He atoms on small cationic gold clusters at 0.4 K as a probe of the structures of the pure gold cluster ions [3]. Also, ionic complexes between gold and C60 were recently observed for the first time; cations and anions of the type C60 AuC60 +/− were shown to have particular stability [2]. Previous mass spectrometric investigations of pure gold cluster ions have involved a variety of different approaches. In early work by Katakuse et al mass spectra were reported for (Au)n + with n up to 250 [8] and (Au)n − with n up to 250 [9]; the gold cluster ions were produced by the bombardment of a sheet of gold with 10 keV Xe+ ions. The ion intensities decreased with cluster size and showed two types of anomalies: a variation of the ion intensities between odd and even numbers of n, with the former being greater, and discontinuities in ion intensity at particular values of n
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that could be explained by a one-electron shell model. A gas aggregate technique in which gold cluster growth occurs in an argon atmosphere before electron impact ionization revealed anomalies in the relative abundance of both singly and doubly-charged gold clusters that are explained in terms of electronic shell closing [10]. Large gold cluster cations and anions also have been generated from peptide-covered thin gold films using laser-desorption Fouriertransform ion cyclotron resonance (LD FT-ICR) mass spectrometry [11]. More recently, studies have appeared that explore the ionization and dissociation of gold cluster cations. In one study the abundance spectra of doubly and triply-charged clusters produced by the electron impact ionization of singly-charged gold cluster cations exhibited interesting features regarding the onset of these clusters, the alternation in size that inverses with charge state, and prominent signals for particular cluster sizes (magic numbers) [12]. Also, multicollision-induced dissociation (MCID) has been applied to gold clusters, Aun 2+ (n = 7–35) and Aun 3+ (n = 19–35) stored in a Penning trap and exposed to ion cyclotron resonance excitation and pulses of argon collision gas [13]. Fragmentation yields were measured as a function of the kinetic energy of the cluster ions. In general, the dissociation energy of small clusters was found to be smaller than that of the larger ones because of the influence of the Coulomb force. Methods for the determination of geometric structures of Au+ n and Au− n cluster ions with n = 3–20 have recently been reviewed by Schoss et al. [14]. Geometries were assigned by comparing experimental data, obtained primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations. Here we present an overview of our mass spectrometric detection of positive and negative cluster ions of gold atoms, including their size distribution, patterns of stability and the presence of “magic” clusters, formed by the pickup of gold by HNDs near zero K and exposed to electron impact.
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Fig. 1. Intensity profile of pure gold clusters Aun + up to 37 atoms extracted from a raw spectrum with the software IsotopeFit [21]. Error bars represent a 95% confidence interval. A log-normal fit to the data has its center at xc = 13.8.
2. Experimental procedures The experimental apparatus is described in detail elsewhere [1,15]. He nanodroplets were produced via supersonic expansion of pre-cooled gaseous He (Messer, 99.9999% purity) under a pressure of 2.25 MPa through a 5 m diameter nozzle cooled to 9.55 K. The mean size of the produced droplets is estimated to be 3 × 105 He atoms [16,17]. The helium beam passed through an 0.8 mm diameter skimmer and entered a pickup region filled with gold vapor. The vapor was produced by solid gold heated with 118 W in an oven similar to the reported by Feng et al [18]. The doped droplets underwent ionization in a Nier-type ion source with electron kinetic energies of 80 and 20.5 eV for positive and negative ion formation, respectively. The dopants were ionized through different processes depending on the polarity [19,20]. The ionized complexes were then driven through a set of Einzel lenses into the extraction region of a commercial, reflectron time-of-flight mass spectrometer (Tofwerk AG, model HTOF) where spectra of the signal intensity versus mass per charge were obtained. The spectra were evaluated using the custom software IsotopeFit that subtracts background signals and extracts contributions from superimposed mass peaks due to isobaric ions [21]. 3. Results and discussion 3.1. Positive gold cluster ions We present first, in Fig. 1, the intensity distribution obtained for positive gold cluster ions Aun + with n up to 37. The distribution peaks early, at n = 7. A clear oscillation in intensity is seen, espe-
Fig. 2. Cluster size distributions of positively charged gold cluster ions formed upon electron ionization of HNDs doped with gold. Conditions: helium temperature before expansion: 9.55 K, pressure of He before the expansion: 2.25 MPa, electron energy 80 eV, electron current 94 A, power of gold oven: 118 W. The solid blue circles designate singly charged cations and the purple solid squares dications. The open circles represent Aun + formed as secondary ions upon 10 keV Xe+ bombardment of a gold metal sheet by Katakuse et al. [8].
cially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The oscillations seen in Fig. 1 match the alteration reported for the dissociation energies and the favored fragmentation channels of Aun + with n<14: “as a general rule we observe that even numbered cluster cations fragment via loss of neutral gold atoms while odd numbered clusters with generally higher dissociation energies than their even numbered neighbors lose neutral gold dimers” [22]. We also note that the more intense odd-numbered clusters are even-electron system with Au+ carrying the positive charge. As regards ion cluster structures, there is no obvious feature or discontinuity that speaks to a transition in ion structure from 2D to 3D at n = 7 that has been identified by computations [14]. The oscillation apparent in Fig. 1 is damped beyond n = 21 and an almost purely exponential decrease of the ion yield can be observed in Fig. 2 up to n = 70. There is a suggestion of shell closures at n = 35 and 58. Remarkably, there are strong similarities between our observed ion yield distribution and that reported previously by Katakuse et al. [8] in a study of the formation of Aun + cluster ions upon 10 keV Xe+ bombardment of a gold metal sheet and also shown in Fig. 2. Both spectra also exhibit at n = 30 a relative minimum in the ion yield. Differences are seen for the low mass ions
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approaching n = 1 which continue to rise in the metal sheet experiment and the decay in ion yield is not purely exponential and shows more structural features, but oscillations in low cluster sizes and “magic number” features are also present. The presence of the latter was explained qualitatively by a one-electron model in which free electrons are bound in a spherically symmetric potential well [8,10]. The pickup process into HNDs that is statistically governed leads to a cluster size distribution that can be fitted reasonably well by a log-normal distribution (see red line in Fig. 1) with its center at a cluster size of 13.8. In contrast, sputtering of a gold sheet results in a cluster size distribution that does not exhibit a peak-like structure. Fig. 2 also includes the ion yield distribution observed in our HND experiments for the formation of doubly-charged gold cluster cations. The yield for dications is about two orders of magnitude lower than for monocations. Even numbered dications were not resolved as they overlap with monocations (but do not influence their magnitude). The appearance size for doubly-charged gold clusters at nc = 9 agrees very well with values given in the literature [23,10]. However, in a an earlier study Saunders reported the mass spectrometric identification of dications of gold clusters down to n = 2 [24] and also experiments in Penning traps by Ziegler et al [13] indicate dications as small as Au5 2+ . Singly-charged cations of gold clusters are predominantly formed via electron transfer to an initially formed He+ . The large excess energy, due to the difference in ionization energies of He and Au (24.59 eV and 9.23 eV, respectively), will heat up the charged gold cluster and promote its fragmentation. Enhanced stability of specific cluster sizes will lead to longer lifetimes and so more efficient quenching by the ultra-cold He matrix. This explains the pronounced odd-even oscillation of the ion yield: odd numbered clusters that have an even number of valence electrons are more stable and so are recorded with higher intensity. The energy of one He+ is even high enough to ionize two gold atoms and to provide the Coulomb energy for a distance between the two cations as short as 2.35 Å. Taking the average bond lengths of gold atoms in small clusters from Gilb et al [22], a gold cluster consisting of 9 atoms having a diameter of 5.5 Å can be ionized twice by the ionization energy of He+ . However, the low relative abundance of dications in the present experiment indicates that this process has a much lower probability. Alternatively, dications from doped HNDs may also be formed via double collisions in which one He+ and one He* [25] or He*− [20,26,27] or even two metastable species interact with the dopant cluster. Fig. 3 shows ion efficiency curves for selected cations formed upon inelastic electron interactions with gold-doped HNDs. The parameters for the HND formation were 9.4 K and 2.25 MPa. The power of the gold oven was 114 W. He+ (blue line) and He2 + (black line) exhibit a resonance like structure below the ionization energy of He that has been explained by Renzler et al via a reaction of two electronically excited species, i.e., He* and He*− [28]. In contrast, all gold containing cluster cations are only weakly formed via these reaction channels and predominantly result from electron transfer reaction to an initially formed He+ . At about 43 eV, a distinct feature can be seen in the ion efficiency curves of He+ , He2 + and all Hem Au+ ions (represented by He12 Au+ in Fig. 3). In Fig. 4 the effect of the electron energy on the relative ion yield for differently sized gold clusters is shown for Aun + with n = 3,9,12, and 15. With increasing cluster size, we observe an increase of the aforementioned processes below the ionization energy of He. After subtraction of this contribution and proper normalization, all four curves exhibit the same ion efficiency curves up to 29 eV. The maximum of the ion efficiency curves shifts to lower electron energies with increasing cluster size. This is very remarkable, since the energy provided by the He+ will be the same, irrespective of the electron energy. Thus, we conclude, that one of the secondary
Fig. 3. Ion efficiency curves for selected cations formed upon electron impact on gold-doped HNDs. Helium temperature before expansion was 9.4 K and the pressure 2.2 MPa. The power of the gold oven was set to 114 W.
Fig. 4. Ion efficiency curves for selected cations formed upon electron impact on gold-doped HNDs. Helium temperature before expansion was 9.4 K and the pressure 2.2 MPa. The power of the gold oven was set to 114 W.
electrons from the initial ionization event of one He atom will inelastically collide with the gold dopant cluster and deposit some of its energy. Alternatively, energy can also be provided by a second projectile electron in the form of electron energy or an excited species, i,e, He* or He*− . With increasing droplet size this reaction channel becomes more likely. Large droplet size is also favorable for the formation of large gold clusters. So larger dopant clusters may be prone to fragmentation and contribute to enhanced formation of smaller gold clusters at higher electron energies, thereby leading to the observed downshifts of the maxima of the ion efficiency curves with increasing cluster size. 3.2. Negative gold cluster ions Fig. 5 shows two mass spectra of negatively charged gold cluster ions from heavily doped HNDs that were extracted from an electron energy scan. This is a common procedure to include all anions that may be formed via different, often narrow resonances in a single mass spectrum [29,30]. The anion efficiency curves extracted from these energy scans and shown in Fig. 7 exhibit a gradual blue-
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Fig. 5. Mass spectra for gold anions obtained from electron energy scans from 17.5 eV to 23 eV (lower mass spectrum) and from 23 to 28.5 eV (upper mass spectrum). The dashed lines are log-normal fits to the mass spectra and the center xc of the fits are changing from 8.2 to 13.1, respectively.
shift with increasing cluster size and no narrow resonances. Below 22 eV only free electron attachment is operative and above 22 eV also electron transfer from He*− becomes feasible. The lower mass spectrum was integrated for 3.5 h during which the electron energy was slowly scanned from 17.5 eV to 22.1 eV. The dashed line represents a log-normal fit to the mass spectrum and the center xc of this fit is at 8.2. The upper mass spectrum was integrated for 10 h where the electron energy was slowly scanned from 21 eV to 34.3 eV. The center of the log-normal fit to this cluster size distribution is at 13.1. Negatively charged ions from doped HNDs are formed either upon direct attachment of an electron (or electron bubble), or upon electron transfer from He*− [19,20]. The adiabatic electron affinities of gold clusters are quite high, generally above 2 eV, so that electron transfer from He*− is easily exothermic [31]. He*− is formed via four resonances that overlap between 21 eV and 29 eV [32]. Within the detection limit of the experiment, we did not see any gold cluster dianions in the present study. In order to unravel the anion formation process, we performed a detailed electron energy scan of the complete mass range. The cluster size distribution of the negatively charged gold cluster ions formed upon electron ionization of HNDs doped with gold is shown in Fig. 6. Again, an oscillation in intensity is seen, especially for n <21 (although the oscillations are less pronounced than for the positive cluster ions), with odd-numbered clusters being more intense than even-numbered clusters. We note again that the more intense odd-numbered clusters are even electron system with Au− carrying the negative charge. Also shown in Fig. 6 are the results obtained previously by Katakuse et al. [9] in a study of the formation of Aun − cluster ions upon 10 keV Xe+ bombardment of a gold metal sheet. Oscillations are also present in their data and no maximum is present in their ion yield distribution. Their ion yield drops sharply with cluster size, much more so than we observed for clusters with up to 35 gold atoms. Clusters with up to 67 gold atoms were observed by Katakuse et al. [9] and, as was the case with their positive cluster data, anomalies are seen at cluster sizes of 35 and 58 gold atoms. We also monitored the efficiency of the formation of the gold cluster anions as a function of the energy of the electrons that were used to cause ionization of the HNDs. Ion efficiency curves were extracted from the electron energy scan of the complete mass spectrum for all gold cluster anions and the results for seven selected anions are shown in Fig. 7. For Au12 − the raw data is indicated in light orange and a peak function, the thick smooth orange curve, is
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Fig. 6. Cluster size distributions of negatively charged gold cluster ions formed upon electron ionization of HNDs doped with gold (solid blue circles). Conditions: helium temperature before expansion: 9.6 K, pressure of He before the expansion: 2.25 MPa, electron energy was scanned between 17.5 eV and 42.5 eV, electron current 161 A, power of gold oven: 118 W. The open circles represent Aun − formed as secondary ions upon 10 keV Xe+ bombardment of a gold metal sheet by Katakuse et al [9].
Fig. 7. Ion efficiency curves for seven selected gold cluster anions of the energy scans mentioned above. The raw data is included for Au12 − . Please note a shift of the maximum to higher electron energies with increasing cluster size. Also, the width of these resonance curves for anion formation increases with the number of gold atoms in the cluster.
fitted to this data. For the other six cluster anions, only the smooth fit functions are shown. With increasing cluster size, the position of the maximum of the anion efficiency curves, as well as the width of the resonance, increases. Fig. 8 shows the variation in the position of the maximum of the anion efficiency curves as a function of the gold cluster size. Except for the cluster sizes n = 1,2,3 and 6, the data follow a smooth trend that can be fitted reasonably well by a power function y(n)=a·nb , with a = 17.68 and b = 0.114 as the fitting parameters (solid line in Fig. 8). It is interesting to note that the anion efficiency curves for Aun − (n = 1,2 and 3) are closely related to the anion efficiency curve of He*− [20,32] and thus may be predominantly formed via an intermediate He*− , similar to the formation of fullerene dianions [33]. However, we could not detect dianionic gold clusters above the noise level in the present study. Nevertheless, Coulomb explosion of unstable dianions could explain this anomaly in the anion efficiency curves for these small gold cluster anions. The blue-shift of Au6 − compared to n = 1,2 and 3 fits nicely to the blue-shift of the other gold cluster anions and thus this anion may also be predominantly formed via He*− , however, at a higher electron energy that is required to liberate the anion form surrounding He (see below). The width increases more linearly with an average gain of 92 meV per gold atom (see Fig. 9). These data also exhibit an
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Fig. 8. Maxima in the anion efficiency curves as a function of the cluster size. Aun − , n = 1,2,3 and 6, are off the smooth trend of all other cluster sizes. A weak odd-even trend can be observed with even cluster sizes slightly upshifted. The red squares (taken from the energy scans also shown in Figs. 5 and 7) and blue circles represent two independent measurements (He temperature: 9.55 K, He pressure: 2.25 MPa, oven power: 118 W). The red line is the fit of a power function y(n)=a·nb to the red symbols (except n = 1,2,3 and 6) with a = 17.68 and b = 0.114.
vating the dopant cluster will be very different for dopants that lead to the evaporation of a large number of He atoms (either due to high binding energy or a large amount of internal energy). The cross section for pickup of a single gold atom into a HND, consisting of N He atoms, is proportional to its geometric cross section, i.e. N2/3 . In the present study, the average droplet size is about 300,000 He atoms [16]. The kinetic energy of an impinging gold atom at the temperature of the oven (about 1300 K) into the droplet, having a uniform velocity of 236 m/s [16], is below 0.5 eV. Thus, the sum of the kinetic energy of a gold atom colliding with the HND and the binding to an embedded gold cluster is between 3.1 and 5.2 eV. Taking the binding energy of a He atom to a HND as typically 0.6 meV, every addition of a gold atom is expected to result in the loss of 5000–8300 He atoms. If the pickup conditions and the helium droplet size is chosen such that the neutral gold clusters are still embedded in some He, we can expect that the remaining He amount increases with the number of gold atoms embedded. Larger dopant clusters emerge, on average, from larger HNDs (pickup cross section scales with N2/3 and the volume of the HND with N) and the ion yield from such clusters will, to a larger degree than for small gold clusters, be contributing to systems that still contain He in the mass spectrum. In order to evaporate this extra He after the electron attachment, additional energy is required. This energy is provided in the form of kinetic energy of the electron in the ionization process. So a higher electron energy is required to observe larger gold cluster anions. This qualitatively explains very well the observed blue shift of the anion efficiency curves with increasing size of the gold cluster anions. We expect a similar behaviour for other metal cluster anions and dopants with high internal energies, such as fullerenes, PAHs or other large molecules that are vaporized in heated ovens. With additional data from other dopants and at different pickup conditions we expect to be able to develop a model that can explain also quantitatively this unexpected blue shift of the anion efficiency curves as a function of the dopant cluster size. 4. Conclusions
Fig. 9. Widths of the resonance of the anion efficiency curves for the formation gold cluster anions as a function of the cluster size n.
odd-even oscillation, with the width of the resonance of the even numbered cluster being larger by an average of 170 meV. This pronounced blue shift of the anion efficiency curves with increasing cluster size has not been observed in our laboratory for previously investigated dopants. However, in most of these previous studies, the average HND size was substantially smaller (104
Our mass spectrometric observations of positive and negative cluster ions of gold atoms formed by the pickup of gold by HNDs near zero K and subsequent exposure to electron impact are remarkably similar to those reported by others using other methodologies that involve surface sputtering, gas aggregation and laser desorption FT-ICR. This applies to the odd/even stability pattern of smaller cluster ions as well as the presence of larger “magic” clusters. Our experiments are unique in that clusters are formed in an extremely cold helium droplet and their ionization is initiated by helium ions (both positive and negative) and metastables, as well as the attachment of electrons. In the measurements of efficiencies of formation of the gold cluster cations as a function of the energy of the electrons, the maximum of the ion efficiency curves shifts to lower electron energies with increasing cluster size. This remarkable observation is attributed to inelastic collisions of secondary electrons with gold dopant clusters. For negatively charged gold clusters, the shift of the maximum of the anion formation to higher energies as well as the increase of the widths of these resonance curves with increasing cluster size can be attributed to the need to evaporate helium from the clusters for their observation in the nascent state. Acknowledgements This work was supported by the Austrian Science Fund (FWF) Wien (project numbers P26635, P31149 and W1259), the European
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International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Full Length Article
Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K P. Martini a , L. Kranabetter a , M. Goulart a , M. Kuhn a , M. Gatchell a , D.K. Bohme b , P. Scheier a,∗ a b
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria Department of Chemistry, York University, Toronto, Ontario, Canada, M3J 1P3
a r t i c l e
i n f o
Article history: Received 19 July 2018 Received in revised form 5 September 2018 Accepted 13 September 2018 Available online 20 September 2018 Dedicated to Helmut Schwarz on the occasion of his 75th birthday. He inspired generations of young physicists and chemists with his outstanding scientific achievements. Whenever we think that we have discovered a new and exciting process, it often has been already published by the group of Helmut Schwarz.
a b s t r a c t We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*− , metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n +/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state. © 2018 Elsevier B.V. All rights reserved.
Keywords: He nanodroplets Gold clusters Anions Cations Magic numbers
1. Introduction Helium nanodroplets (HNDs), formed in vacuo, allow interactions between atoms to be explored at extremely low temperatures (close to zero K) in a chemically inert environment of helium [1]. When atoms of gold are added to HNDs they cluster and, when these “golden HNDs” are exposed to ionizing electrons, both positive and negative cluster ions of gold are formed. After leaving the HNDs, the cluster ions can be monitored mass spectrometrically: the size of the cluster ions, their distribution in size, the presence of clusters of enhanced stability (so-called “magic” clusters), as well as patterns of cluster stability. Furthermore, when other atoms or molecules are added to the HNDs along with gold atoms, the reactivity of gold atom and small clusters of gold atoms toward these
∗ Corresponding author. E-mail address: [email protected] (P. Scheier). https://doi.org/10.1016/j.ijms.2018.09.020 1387-3806/© 2018 Elsevier B.V. All rights reserved.
molecules can be probed near zero K [2–4]: the high nuclear charge in a gold atom leads to relativistic effects that can be consequential in chemical bonding with atoms and molecules [5,6]. We have previously performed mass spectrometric measurements of the adsorption of He atoms on small cationic gold clusters at 0.4 K as a probe of the structures of the pure gold cluster ions [3]. Also, ionic complexes between gold and C60 were recently observed for the first time; cations and anions of the type C60 AuC60 +/− were shown to have particular stability [2]. Previous mass spectrometric investigations of pure gold cluster ions have involved a variety of different approaches. In early work by Katakuse et al mass spectra were reported for (Au)n + with n up to 250 [8] and (Au)n − with n up to 250 [9]; the gold cluster ions were produced by the bombardment of a sheet of gold with 10 keV Xe+ ions. The ion intensities decreased with cluster size and showed two types of anomalies: a variation of the ion intensities between odd and even numbers of n, with the former being greater, and discontinuities in ion intensity at particular values of n
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that could be explained by a one-electron shell model. A gas aggregate technique in which gold cluster growth occurs in an argon atmosphere before electron impact ionization revealed anomalies in the relative abundance of both singly and doubly-charged gold clusters that are explained in terms of electronic shell closing [10]. Large gold cluster cations and anions also have been generated from peptide-covered thin gold films using laser-desorption Fouriertransform ion cyclotron resonance (LD FT-ICR) mass spectrometry [11]. More recently, studies have appeared that explore the ionization and dissociation of gold cluster cations. In one study the abundance spectra of doubly and triply-charged clusters produced by the electron impact ionization of singly-charged gold cluster cations exhibited interesting features regarding the onset of these clusters, the alternation in size that inverses with charge state, and prominent signals for particular cluster sizes (magic numbers) [12]. Also, multicollision-induced dissociation (MCID) has been applied to gold clusters, Aun 2+ (n = 7–35) and Aun 3+ (n = 19–35) stored in a Penning trap and exposed to ion cyclotron resonance excitation and pulses of argon collision gas [13]. Fragmentation yields were measured as a function of the kinetic energy of the cluster ions. In general, the dissociation energy of small clusters was found to be smaller than that of the larger ones because of the influence of the Coulomb force. Methods for the determination of geometric structures of Au+ n and Au− n cluster ions with n = 3–20 have recently been reviewed by Schoss et al. [14]. Geometries were assigned by comparing experimental data, obtained primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations. Here we present an overview of our mass spectrometric detection of positive and negative cluster ions of gold atoms, including their size distribution, patterns of stability and the presence of “magic” clusters, formed by the pickup of gold by HNDs near zero K and exposed to electron impact.
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Fig. 1. Intensity profile of pure gold clusters Aun + up to 37 atoms extracted from a raw spectrum with the software IsotopeFit [21]. Error bars represent a 95% confidence interval. A log-normal fit to the data has its center at xc = 13.8.
2. Experimental procedures The experimental apparatus is described in detail elsewhere [1,15]. He nanodroplets were produced via supersonic expansion of pre-cooled gaseous He (Messer, 99.9999% purity) under a pressure of 2.25 MPa through a 5 m diameter nozzle cooled to 9.55 K. The mean size of the produced droplets is estimated to be 3 × 105 He atoms [16,17]. The helium beam passed through an 0.8 mm diameter skimmer and entered a pickup region filled with gold vapor. The vapor was produced by solid gold heated with 118 W in an oven similar to the reported by Feng et al [18]. The doped droplets underwent ionization in a Nier-type ion source with electron kinetic energies of 80 and 20.5 eV for positive and negative ion formation, respectively. The dopants were ionized through different processes depending on the polarity [19,20]. The ionized complexes were then driven through a set of Einzel lenses into the extraction region of a commercial, reflectron time-of-flight mass spectrometer (Tofwerk AG, model HTOF) where spectra of the signal intensity versus mass per charge were obtained. The spectra were evaluated using the custom software IsotopeFit that subtracts background signals and extracts contributions from superimposed mass peaks due to isobaric ions [21]. 3. Results and discussion 3.1. Positive gold cluster ions We present first, in Fig. 1, the intensity distribution obtained for positive gold cluster ions Aun + with n up to 37. The distribution peaks early, at n = 7. A clear oscillation in intensity is seen, espe-
Fig. 2. Cluster size distributions of positively charged gold cluster ions formed upon electron ionization of HNDs doped with gold. Conditions: helium temperature before expansion: 9.55 K, pressure of He before the expansion: 2.25 MPa, electron energy 80 eV, electron current 94 A, power of gold oven: 118 W. The solid blue circles designate singly charged cations and the purple solid squares dications. The open circles represent Aun + formed as secondary ions upon 10 keV Xe+ bombardment of a gold metal sheet by Katakuse et al. [8].
cially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The oscillations seen in Fig. 1 match the alteration reported for the dissociation energies and the favored fragmentation channels of Aun + with n<14: “as a general rule we observe that even numbered cluster cations fragment via loss of neutral gold atoms while odd numbered clusters with generally higher dissociation energies than their even numbered neighbors lose neutral gold dimers” [22]. We also note that the more intense odd-numbered clusters are even-electron system with Au+ carrying the positive charge. As regards ion cluster structures, there is no obvious feature or discontinuity that speaks to a transition in ion structure from 2D to 3D at n = 7 that has been identified by computations [14]. The oscillation apparent in Fig. 1 is damped beyond n = 21 and an almost purely exponential decrease of the ion yield can be observed in Fig. 2 up to n = 70. There is a suggestion of shell closures at n = 35 and 58. Remarkably, there are strong similarities between our observed ion yield distribution and that reported previously by Katakuse et al. [8] in a study of the formation of Aun + cluster ions upon 10 keV Xe+ bombardment of a gold metal sheet and also shown in Fig. 2. Both spectra also exhibit at n = 30 a relative minimum in the ion yield. Differences are seen for the low mass ions
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approaching n = 1 which continue to rise in the metal sheet experiment and the decay in ion yield is not purely exponential and shows more structural features, but oscillations in low cluster sizes and “magic number” features are also present. The presence of the latter was explained qualitatively by a one-electron model in which free electrons are bound in a spherically symmetric potential well [8,10]. The pickup process into HNDs that is statistically governed leads to a cluster size distribution that can be fitted reasonably well by a log-normal distribution (see red line in Fig. 1) with its center at a cluster size of 13.8. In contrast, sputtering of a gold sheet results in a cluster size distribution that does not exhibit a peak-like structure. Fig. 2 also includes the ion yield distribution observed in our HND experiments for the formation of doubly-charged gold cluster cations. The yield for dications is about two orders of magnitude lower than for monocations. Even numbered dications were not resolved as they overlap with monocations (but do not influence their magnitude). The appearance size for doubly-charged gold clusters at nc = 9 agrees very well with values given in the literature [23,10]. However, in a an earlier study Saunders reported the mass spectrometric identification of dications of gold clusters down to n = 2 [24] and also experiments in Penning traps by Ziegler et al [13] indicate dications as small as Au5 2+ . Singly-charged cations of gold clusters are predominantly formed via electron transfer to an initially formed He+ . The large excess energy, due to the difference in ionization energies of He and Au (24.59 eV and 9.23 eV, respectively), will heat up the charged gold cluster and promote its fragmentation. Enhanced stability of specific cluster sizes will lead to longer lifetimes and so more efficient quenching by the ultra-cold He matrix. This explains the pronounced odd-even oscillation of the ion yield: odd numbered clusters that have an even number of valence electrons are more stable and so are recorded with higher intensity. The energy of one He+ is even high enough to ionize two gold atoms and to provide the Coulomb energy for a distance between the two cations as short as 2.35 Å. Taking the average bond lengths of gold atoms in small clusters from Gilb et al [22], a gold cluster consisting of 9 atoms having a diameter of 5.5 Å can be ionized twice by the ionization energy of He+ . However, the low relative abundance of dications in the present experiment indicates that this process has a much lower probability. Alternatively, dications from doped HNDs may also be formed via double collisions in which one He+ and one He* [25] or He*− [20,26,27] or even two metastable species interact with the dopant cluster. Fig. 3 shows ion efficiency curves for selected cations formed upon inelastic electron interactions with gold-doped HNDs. The parameters for the HND formation were 9.4 K and 2.25 MPa. The power of the gold oven was 114 W. He+ (blue line) and He2 + (black line) exhibit a resonance like structure below the ionization energy of He that has been explained by Renzler et al via a reaction of two electronically excited species, i.e., He* and He*− [28]. In contrast, all gold containing cluster cations are only weakly formed via these reaction channels and predominantly result from electron transfer reaction to an initially formed He+ . At about 43 eV, a distinct feature can be seen in the ion efficiency curves of He+ , He2 + and all Hem Au+ ions (represented by He12 Au+ in Fig. 3). In Fig. 4 the effect of the electron energy on the relative ion yield for differently sized gold clusters is shown for Aun + with n = 3,9,12, and 15. With increasing cluster size, we observe an increase of the aforementioned processes below the ionization energy of He. After subtraction of this contribution and proper normalization, all four curves exhibit the same ion efficiency curves up to 29 eV. The maximum of the ion efficiency curves shifts to lower electron energies with increasing cluster size. This is very remarkable, since the energy provided by the He+ will be the same, irrespective of the electron energy. Thus, we conclude, that one of the secondary
Fig. 3. Ion efficiency curves for selected cations formed upon electron impact on gold-doped HNDs. Helium temperature before expansion was 9.4 K and the pressure 2.2 MPa. The power of the gold oven was set to 114 W.
Fig. 4. Ion efficiency curves for selected cations formed upon electron impact on gold-doped HNDs. Helium temperature before expansion was 9.4 K and the pressure 2.2 MPa. The power of the gold oven was set to 114 W.
electrons from the initial ionization event of one He atom will inelastically collide with the gold dopant cluster and deposit some of its energy. Alternatively, energy can also be provided by a second projectile electron in the form of electron energy or an excited species, i,e, He* or He*− . With increasing droplet size this reaction channel becomes more likely. Large droplet size is also favorable for the formation of large gold clusters. So larger dopant clusters may be prone to fragmentation and contribute to enhanced formation of smaller gold clusters at higher electron energies, thereby leading to the observed downshifts of the maxima of the ion efficiency curves with increasing cluster size. 3.2. Negative gold cluster ions Fig. 5 shows two mass spectra of negatively charged gold cluster ions from heavily doped HNDs that were extracted from an electron energy scan. This is a common procedure to include all anions that may be formed via different, often narrow resonances in a single mass spectrum [29,30]. The anion efficiency curves extracted from these energy scans and shown in Fig. 7 exhibit a gradual blue-
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Fig. 5. Mass spectra for gold anions obtained from electron energy scans from 17.5 eV to 23 eV (lower mass spectrum) and from 23 to 28.5 eV (upper mass spectrum). The dashed lines are log-normal fits to the mass spectra and the center xc of the fits are changing from 8.2 to 13.1, respectively.
shift with increasing cluster size and no narrow resonances. Below 22 eV only free electron attachment is operative and above 22 eV also electron transfer from He*− becomes feasible. The lower mass spectrum was integrated for 3.5 h during which the electron energy was slowly scanned from 17.5 eV to 22.1 eV. The dashed line represents a log-normal fit to the mass spectrum and the center xc of this fit is at 8.2. The upper mass spectrum was integrated for 10 h where the electron energy was slowly scanned from 21 eV to 34.3 eV. The center of the log-normal fit to this cluster size distribution is at 13.1. Negatively charged ions from doped HNDs are formed either upon direct attachment of an electron (or electron bubble), or upon electron transfer from He*− [19,20]. The adiabatic electron affinities of gold clusters are quite high, generally above 2 eV, so that electron transfer from He*− is easily exothermic [31]. He*− is formed via four resonances that overlap between 21 eV and 29 eV [32]. Within the detection limit of the experiment, we did not see any gold cluster dianions in the present study. In order to unravel the anion formation process, we performed a detailed electron energy scan of the complete mass range. The cluster size distribution of the negatively charged gold cluster ions formed upon electron ionization of HNDs doped with gold is shown in Fig. 6. Again, an oscillation in intensity is seen, especially for n <21 (although the oscillations are less pronounced than for the positive cluster ions), with odd-numbered clusters being more intense than even-numbered clusters. We note again that the more intense odd-numbered clusters are even electron system with Au− carrying the negative charge. Also shown in Fig. 6 are the results obtained previously by Katakuse et al. [9] in a study of the formation of Aun − cluster ions upon 10 keV Xe+ bombardment of a gold metal sheet. Oscillations are also present in their data and no maximum is present in their ion yield distribution. Their ion yield drops sharply with cluster size, much more so than we observed for clusters with up to 35 gold atoms. Clusters with up to 67 gold atoms were observed by Katakuse et al. [9] and, as was the case with their positive cluster data, anomalies are seen at cluster sizes of 35 and 58 gold atoms. We also monitored the efficiency of the formation of the gold cluster anions as a function of the energy of the electrons that were used to cause ionization of the HNDs. Ion efficiency curves were extracted from the electron energy scan of the complete mass spectrum for all gold cluster anions and the results for seven selected anions are shown in Fig. 7. For Au12 − the raw data is indicated in light orange and a peak function, the thick smooth orange curve, is
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Fig. 6. Cluster size distributions of negatively charged gold cluster ions formed upon electron ionization of HNDs doped with gold (solid blue circles). Conditions: helium temperature before expansion: 9.6 K, pressure of He before the expansion: 2.25 MPa, electron energy was scanned between 17.5 eV and 42.5 eV, electron current 161 A, power of gold oven: 118 W. The open circles represent Aun − formed as secondary ions upon 10 keV Xe+ bombardment of a gold metal sheet by Katakuse et al [9].
Fig. 7. Ion efficiency curves for seven selected gold cluster anions of the energy scans mentioned above. The raw data is included for Au12 − . Please note a shift of the maximum to higher electron energies with increasing cluster size. Also, the width of these resonance curves for anion formation increases with the number of gold atoms in the cluster.
fitted to this data. For the other six cluster anions, only the smooth fit functions are shown. With increasing cluster size, the position of the maximum of the anion efficiency curves, as well as the width of the resonance, increases. Fig. 8 shows the variation in the position of the maximum of the anion efficiency curves as a function of the gold cluster size. Except for the cluster sizes n = 1,2,3 and 6, the data follow a smooth trend that can be fitted reasonably well by a power function y(n)=a·nb , with a = 17.68 and b = 0.114 as the fitting parameters (solid line in Fig. 8). It is interesting to note that the anion efficiency curves for Aun − (n = 1,2 and 3) are closely related to the anion efficiency curve of He*− [20,32] and thus may be predominantly formed via an intermediate He*− , similar to the formation of fullerene dianions [33]. However, we could not detect dianionic gold clusters above the noise level in the present study. Nevertheless, Coulomb explosion of unstable dianions could explain this anomaly in the anion efficiency curves for these small gold cluster anions. The blue-shift of Au6 − compared to n = 1,2 and 3 fits nicely to the blue-shift of the other gold cluster anions and thus this anion may also be predominantly formed via He*− , however, at a higher electron energy that is required to liberate the anion form surrounding He (see below). The width increases more linearly with an average gain of 92 meV per gold atom (see Fig. 9). These data also exhibit an
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Fig. 8. Maxima in the anion efficiency curves as a function of the cluster size. Aun − , n = 1,2,3 and 6, are off the smooth trend of all other cluster sizes. A weak odd-even trend can be observed with even cluster sizes slightly upshifted. The red squares (taken from the energy scans also shown in Figs. 5 and 7) and blue circles represent two independent measurements (He temperature: 9.55 K, He pressure: 2.25 MPa, oven power: 118 W). The red line is the fit of a power function y(n)=a·nb to the red symbols (except n = 1,2,3 and 6) with a = 17.68 and b = 0.114.
vating the dopant cluster will be very different for dopants that lead to the evaporation of a large number of He atoms (either due to high binding energy or a large amount of internal energy). The cross section for pickup of a single gold atom into a HND, consisting of N He atoms, is proportional to its geometric cross section, i.e. N2/3 . In the present study, the average droplet size is about 300,000 He atoms [16]. The kinetic energy of an impinging gold atom at the temperature of the oven (about 1300 K) into the droplet, having a uniform velocity of 236 m/s [16], is below 0.5 eV. Thus, the sum of the kinetic energy of a gold atom colliding with the HND and the binding to an embedded gold cluster is between 3.1 and 5.2 eV. Taking the binding energy of a He atom to a HND as typically 0.6 meV, every addition of a gold atom is expected to result in the loss of 5000–8300 He atoms. If the pickup conditions and the helium droplet size is chosen such that the neutral gold clusters are still embedded in some He, we can expect that the remaining He amount increases with the number of gold atoms embedded. Larger dopant clusters emerge, on average, from larger HNDs (pickup cross section scales with N2/3 and the volume of the HND with N) and the ion yield from such clusters will, to a larger degree than for small gold clusters, be contributing to systems that still contain He in the mass spectrum. In order to evaporate this extra He after the electron attachment, additional energy is required. This energy is provided in the form of kinetic energy of the electron in the ionization process. So a higher electron energy is required to observe larger gold cluster anions. This qualitatively explains very well the observed blue shift of the anion efficiency curves with increasing size of the gold cluster anions. We expect a similar behaviour for other metal cluster anions and dopants with high internal energies, such as fullerenes, PAHs or other large molecules that are vaporized in heated ovens. With additional data from other dopants and at different pickup conditions we expect to be able to develop a model that can explain also quantitatively this unexpected blue shift of the anion efficiency curves as a function of the dopant cluster size. 4. Conclusions
Fig. 9. Widths of the resonance of the anion efficiency curves for the formation gold cluster anions as a function of the cluster size n.
odd-even oscillation, with the width of the resonance of the even numbered cluster being larger by an average of 170 meV. This pronounced blue shift of the anion efficiency curves with increasing cluster size has not been observed in our laboratory for previously investigated dopants. However, in most of these previous studies, the average HND size was substantially smaller (104
Our mass spectrometric observations of positive and negative cluster ions of gold atoms formed by the pickup of gold by HNDs near zero K and subsequent exposure to electron impact are remarkably similar to those reported by others using other methodologies that involve surface sputtering, gas aggregation and laser desorption FT-ICR. This applies to the odd/even stability pattern of smaller cluster ions as well as the presence of larger “magic” clusters. Our experiments are unique in that clusters are formed in an extremely cold helium droplet and their ionization is initiated by helium ions (both positive and negative) and metastables, as well as the attachment of electrons. In the measurements of efficiencies of formation of the gold cluster cations as a function of the energy of the electrons, the maximum of the ion efficiency curves shifts to lower electron energies with increasing cluster size. This remarkable observation is attributed to inelastic collisions of secondary electrons with gold dopant clusters. For negatively charged gold clusters, the shift of the maximum of the anion formation to higher energies as well as the increase of the widths of these resonance curves with increasing cluster size can be attributed to the need to evaporate helium from the clusters for their observation in the nascent state. Acknowledgements This work was supported by the Austrian Science Fund (FWF) Wien (project numbers P26635, P31149 and W1259), the European
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