Article pubs.acs.org/JPCA
Formation of Core−Shell Silver−Ethane Clusters in He Droplets Evgeny Loginov,† Luis F. Gomez,‡ and Andrey F. Vilesov* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States ABSTRACT: Here, we have studied the utility of large He droplets of 105−107 atoms for the growth of composite clusters consisting of an Ag core and a shell of ethane molecules. The clusters have been assembled by doping He droplets with up to 103 Ag atoms and ethane molecules in two sequential pickup cells and studied via infrared spectroscopy in the C−H stretch region of the ethane molecules. We found that the ν7 band of ethane molecules at the interface with the Ag atoms has a low frequency shift of approximately 15 cm−1 with respect to that of more distant ethane molecules away from the interface. The intensity ratio of the two bands was used for evaluation of the Ag core and ethane shell cluster structure. We found that the number of surface ethane molecules is in good agreement with a model that assumes a dense, core−shell structure for clusters containing less than about 100 atoms. However, large Ag clusters consisting of about 3000 atoms have a factor of about 5 larger surface area than that predicted by the model, indicating a ramified structure for such larger Ag clusters obtained in liquid He. Moreover, we demonstrate that He droplets behave as calorimeters for measurements of the number of captured atoms and molecules as well as the amount of absorbed laser energy.
1. INTRODUCTION
In this work, we continue our study of the growth of large clusters in He droplets and extend it to the formation of clusters consisting of Ag and ethane (Et). Helium droplets are ideal hosts for the assembly of such metal−molecule clusters by doping with two sequential pickup cells. In this work, the formation of core−shell clusters consisting of Ag atoms and Et molecules has been studied via infrared spectroscopy of the C− H stretching bands of Et in the 3 μm range. We have found that the perpendicular ν7 band of Et has two distinct features due to molecules on the interface with the Ag cluster and more distant molecules that are not in direct contact with the Ag atoms. The intensity ratio of the two bands is used to corroborate the structure of the composite clusters. Helium droplets can be viewed as calorimeters with a temperature that is fixed by the fast evaporative cooling13 of the He atoms. The absorbed energy associated with the pickup of a particle by an He droplet is proportionally related to the decrease in the droplet’s size via the known evaporation enthalpy of liquid He. This evaporative cooling property is a particularly appealing aspect of beams of large droplets for which the scattering following the capture of a large number of particles is negligible. Therefore, throughout this article we use calorimetry in He droplets in order to obtain the average number of captured Ag and Et particles as well as the absorbed IR laser energy per Et molecule. We have found that at sufficiently high laser fluency, Et absorbs a sizable amount of energy, up to about 1500 cm−1 per molecule in neat Et and core−shell Ag−Et clusters. The observed laser power saturation indicates the excitation of nearly two-thirds of all molecules in the clusters, which shows that the vibrational
Superfluid He droplets provide a unique medium for the growth of atomic and molecular clusters.1−8 Atoms embedded into the He droplets rapidly thermalize to the low temperature of T = 0.38 K5 and eventually coagulate. The kinetics and formation mechanisms of large clusters consisting of hundreds to thousands of atoms and molecules, which we refer to as particles, in superfluid solvents remain poorly understood. It is known, however, that the maximum size of the clusters formed scales with the average size of the He droplets. Therefore, in order to obtain large, nanometer-sized clusters,9,10 droplets consisting of at least 105−107 He atoms are required and are obtained through a so-called “supercritical”, low-temperature expansion.11−13 Large metal clusters produced in He droplets can be deposited onto a surface upon impact9 and imaged by electron microscopy.10 Recently, we studied the formation of Ag clusters of up to a few thousand atoms in He droplets via optical laser spectroscopy.14 We found that small Ag clusters (NAg ≈ 100) have a plasmon resonance at about 3.7 eV, similar to that obtained previously for dense, spherical clusters at higher temperatures. On the other hand, larger Ag clusters (NAg > 1000) formed in He droplets of NHe ≈ 107 atoms have an unusually broad spectrum extending into the infrared spectral range.14 The dramatic change in the spectra is in agreement with the expected change from single-cluster growth to a multiple-centered growth regime with increasing droplet size.14 The infrared absorption may also originate from elongated Ag clusters, which have transversal and longitudinal modes, with the latter being shifted toward low frequency.15 Indeed, very recent experiments show that deposited Ag clusters obtained in droplets of NHe > 108 atoms can be elongated in shape, which was ascribed to their aggregation along quantum vortices in the superfluid He droplets.16 © XXXX American Chemical Society
Special Issue: Curt Wittig Festschrift Received: March 15, 2013 Revised: June 14, 2013
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dx.doi.org/10.1021/jp402614s | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Figure 1. (a) Schematic of the He droplet beam vacuum apparatus: NZ, 5 μm diameter nozzle; SK, 1 mm diameter skimmer; PC1 and PC2, upstream and downstream pickup cells, respectively; SH, beam shutter; A1 and A2, 6 mm diameter apertures; GV1 and GV2, gate valves; EI, electron impact ionizer; IB, ion bender; QMS, quadrupole mass spectrometer. (b) Cross section of the oven inside of the pickup cell: CW, cold water jacket; CR, alumina ceramic crucible; RS, radiation shield; TF, tungsten filament; Ag, metallic silver; DB, droplet beam. (c) Typical depletion dip upon laser excitation at t = 0, as measured with mass spectrometer set at mass M = 8.
which contains two identical, 6 cm long differentially pumped pickup cells (PC1 and PC2). In this work, the upstream (PC1) cell hosts a resistively heated oven filled with metallic Ag. The cross section of the oven is shown in Figure 1b. The oven is based on a commercial (MDC, Re-Vap 900) evaporator, which is equipped with a custom-made (R.D. Mathis) tungsten filament (TF). The filament coil holds a custom-made alumina ceramic crucible (CR) (Almath Crucibles) with a length of 40 mm, diameter of 12.5 mm, and two orifices of 5 mm diameter each for the He droplet beam. The oven is surrounded with a radiation shield (RS). The temperature inside of the oven can be increased up to about 1200 °C, as obtained by direct thermocouple measurements. The operating vapor pressure of Ag in the oven is estimated to be around 10−3 mbar and is adjusted by the regulated power supply of the filament. Ethane gas (99.96%, Matheson Gas Products) was admitted into PC2 via a leak valve (MDC, MLV-21) for pressures in the range of 10−4−10−3 mbar. The doped He droplets pass through an additional differential pumping stage and enter into the ultrahigh vacuum chamber, where the intensity of the beam is measured with a quadrupole mass spectrometer (QMS) (Extrel, MAX 300), which is equipped with an electron beam ionizer (EI) and 90° ion bender (IB). In this work the mass filter was set to M = 8 (He2+), which is known to be the dominant splitter ion upon electron impact ionization of large He droplets.17,18 The infrared spectra of the clusters were obtained using a pulsed optical parametric oscillator amplifier (LaserVision, pulse width 7 ns, pulse energy 2−5 mJ, repetition rate 20 Hz,
cooling of the Et clusters in He droplets is much slower than the laser pulse duration of 7 ns.
2. EXPERIMENT The schematic of the He droplet beam setup is shown in Figure 1a. Helium nanodroplets are formed in the source chamber by expanding high-purity (99.9999%) helium gas at a pressure of 20 bar into vacuum through a nozzle (NZ) of 5 μm nominal diameter. Droplets having an average size of ⟨NHe⟩ = 2.0 × 107, 5.3 × 106, 1.8 × 106, and 3.3 × 105 are obtained at nozzle temperatures of T0 = 7, 8, 9, and 9.5 K, respectively. The sizes of the He droplets that are given were measured recently by the droplet attenuation technique.13 In ref 13, ⟨NHe⟩ = 1 × 107 was reported at T0 = 7 K; however, subsequent measurements have given 2 × 107, which is used in this work. The last value is also in good agreement with previous beam deflection measurements.12 It should be noted that our measurements of the droplet sizes13 via the attenuation technique were performed more recently and with a different nozzle orifice than the measurements described in this work. Therefore, the average droplet sizes (and number of particles in the clusters grown within) are probably only accurate to within a factor of 2, as discussed in ref 13. The droplets sizes in the beam are welldescribed by an exponential distribution.12 In this work, we did not attempt to study the effects of the droplet size distribution; an average droplet size, NHe, was used in all equations that follow. Approximately 10 mm downstream, the He droplets pass through a skimmer (SK) into the main high-vacuum chamber, B
dx.doi.org/10.1021/jp402614s | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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spectral resolution 2 cm−1). Absolute frequencies were calibrated against the gas spectrum of methane in a photoacoustic cell and are expected to be accurate to within about 2 cm−1 over the course of the measurements. The infrared laser beam, with a diameter of about 5 mm, propagated anticollinear to the droplet beam. The laser pulse energy remained constant throughout the scans to within about 20%. The laser fluency along the molecular beam was measured to be about 10 mJ/ cm2. In most of the described experiments, the divergent OPO laser beam was collimated with a 1 m lens placed in front of the entrance window to the vacuum apparatus, which led to an increased fluency by about a factor of 5. The laser fluency remained constant during experimental runs conducted within the same day, which are presented in the same figures. The fluency could, however, vary by up to a factor of about 3 between runs conducted on different days due to the slow deterioration of the laser output and inaccuracies in the laser beam and optics adjustments. Following the absorption of multiple photons by the Et molecules in the clusters and the concomitant heating of the droplet, a large number of He atoms will evaporate from the droplet, which results in a transient depletion of the mass spectrometer signal, I8, as shown in Figure 1c. The depletion lasts for approximately 6 ms, which is the time of flight of the droplets over 110 cm from the pickup cell to the ionizer of the mass spectrometer. The shape of the signal reflects the changing effective laser fluency along the flight length. The tail of the dip corresponds to clusters excited close to the pickup cell at the center of the droplet beam. Such clusters interact with the central part of the laser beam, which has higher fluency and thus larger depletion at the tail of the dip. On the other hand, the initial part of the pulse originates from the even overlap of the droplet and laser beams, each of about 6 mm in diameter, close to the ionizer of the mass spectrometer giving rise to a smaller depletion signal. The magnitude of the depletion signal at short times may also be influenced by the vibrational relaxation time of the Et molecules if the last is comparable to the time of flight.
take Ecoh(Et) = 1230 cm−1, which equals the evaporation enthalpy of liquid Et at T = 184 K. This implies that Et clusters in He are glassy, with a pair distribution similar to that in the liquid. This conjecture is supported by a recent study of micrometer-sized Et clusters, which indicated the formation of liquid clusters at T = 78 K, which is well below the freezing temperature at about 90 K. The subsequent freezing and transition from phase I to phase II was on the time scale of hundreds of seconds.22,23 In the case that the clusters have the crystalline phase II structure, the binding is larger by the enthalpy of fusion and enthalpy of transition from phase I to phase II of 230 and 190 cm−1, respectively.24 The cohesive energy is smaller in clusters due to the large fraction of surface particles and approximately scales with the number of the particles in the cluster, n, as Ecoh(n) ≈ Ecoh(∞)(1 − 0.8n−1/3) as obtained for liquid rare gas clusters and Ag clusters.25 Considering that the clusters encountered in this work consist of about 100−1000 particles, we take Ecoh in clusters as 15% less than that in the bulk; in effect, we take Ecoh = 20300 and 1050 cm−1 for Ag and Et, respectively, irrespective of the size. In addition, Ecoh could be even smaller due to the possibility of noncompact aggregate structures14 formed in liquid He; this effect, however, cannot be quantified without any knowledge on the microscopic structure of the clusters. The energy quotient in eq 1 is the inverse number of evaporated He atoms per added particle of M, which is estimated at 3500 and 330 for Ag atoms and Et molecules, respectively. The same numbers have been used for the mixed clusters. The expected core−shell structure of the clusters and interaction between Ag and Et components of the clusters will introduce an additional inaccuracy in the number of evaporated atoms, which is difficult to quantify. We believe that the obtained average numbers of the Ag and Et particles are accurate to within a factor of 2 as a result of inaccuracies in the average droplet size and the number of evaporated He atoms per molecule, as discussed above. On the other hand, the ratios of the number of embedded particles in one particular run when the He droplet size was kept constant and the cluster composition varied are expected to be accurate to within about 20%. In this work, relatively large levels of attenuation, A1,Ag = 0.4−0.8, of the He droplet beam by Ag atoms captured in the first pickup cell were employed, and NAg were obtained from A1,Ag using eq 1. For Et, however, due to its smaller Ecoh(Et), a correspondingly smaller attenuation A2,Et is obtained for a given number of embedded particles than that in the case of Ag; this attenuation A2,Et could not be measured reliably if
