Cluster beam apparatus

Figure 7-1. Schematic side view of differentially pumped cluster beam apparatus and quadrupole mass spectrometer. The temperature of the nozzle in the stagnation region is regulated by a circulating chiller. Reprinted with permission from Vaidyanathan et al. 1991b. Copyright 1991 American Institute of Physics.

Figure 1. A schematic diagram of the pulsed cluster beam apparatus.

CO2 laser pyrolysis of silane in a gas flow reactor and the extraction of the resulting silicon nanoparticles into a cluster beam apparatus has been shown to offer an excellent means for the production of homogeneous films of size-separated quantum dots. Their photoluminescence varies with the size of the crystalline core. All observations are in perfect agreement with the quantum confinement model, that is, the photoluminescence is the result of the recombination of the electron-hole pair created by the absorption of a UV photon. Other mechanisms involving defects or surface states are not operative in our samples. 

In September 1985 British chemist Harold Kroto (Figure 4 65) of the University of Sussex collaborated with Americans Richard E. Smalley, Robert F. Curl, James R. Heath and Sean C. O Brien at Rice University in Houston, Texas in some experiments on graphite. Kroto had an interest in molecules found in interstellar space and had wanted to show that molecules containing long chains of carbon atoms could be formed under the conditions believed to be typical of the outer atmospheres of stars known as red giants. Smalley had developed a cluster beam apparatus which could vaporize small samples of solid graphite into carbon atoms which could be rapidly cooled and analysed. 

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

In 1985. similar experiments were conducted at Rice University. In a 1988 paper. Curl and Smalley (Rice University) outlined their experiments with carbon cluster beams, essentially using the clusler-generaung apparatus previously described by the Exxon researchers. Initially, this experimentation was motivated by an interest that had been shown by die astrophysicist, Krotu (University of Sussex), who had been modeling the formation of carbon molecules in circumstellar shells. As a consequence, the Rice University team concentrated its studies on the smaller (2- to 30-atom) carbon clusters. As pointed out in the Curl-Smalley paper, the objective was to determine if some or all of the species had the same form as the long linear carbon chains known to be abundant in interstellar space."... 

For the FEM experiments described below, the cluster beam was directed through a small collimation capillary into a separately pumped deposition chamber, which is kept at 1 x 10 8 Torr. A transfer cell equipped with a 2 1/s ion pump enabled a tungsten FEM tip to be 1. inserted into the deposition chamber and positioned with its apex in the cluster beam, 2. withdrawn and transported at 2 x 10 7 Torr to a UHV field emission microscope, and 3. inserted into the field emission apparatus and positioned properly for field emission measurements (6). 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

The CLARA apparatus is connected through a gate valve to a small UHV deposition chamber, where a silicon substrate is placed on a 3-axis translating and z-axis rotating holder. During the deposition the holder intersects the cluster beam, so that a circular spot 200 nm thick (estimated by means of a quartz microbalance rate measurement) and with a diameter of 8 mm is obtained. The deposition rate is typically 4 nm/min. 

One of the great issues in the field of silicon clusters is to understand their photoluminescence (PL) and finally to tune the PL emission by controlling the synthetic parameters. The last two chapters deal with this problem. In experiments described by F. Huisken et al. in Chapter 22, thin films of size-separated Si nanoparticles were produced by SiLL pyrolysis in a gas-flow reactor and molecular beam apparatus. The PL varies with the size of the crystalline core, in perfect agreement with the quantum confinement model. In order to observe an intense PL, the nanocrystals must be perfectly passivated. In experiments described by S. Veprek and D. Azinovic in Chapter 23, nanocrystalline silicon was prepared by CVD of SiH4 diluted by H2 and post-oxidized for surface passivation. The mechanism of the PL of such samples includes energy transfer to hole centers within the passivated surface. Impurities within the nanocrystalline material are often responsible for erroneous interpretation of PL phenomena. 

Fig. 4.19 Time-of-flight molecular beam apparatus for the measurement of cluster fragmentation...

Figure 24.1 shows a schematic layout of a crossed-beam apparatus in which the Na (FCH3) (n = 1 to 5) clusters were produced by the pick-up technique. For this, a hot effusive beam of Na atoms is crossed with a pulsed, cold supersonic beam of FCH3. 

Figure 24.1 Top schematic view of the pick-up technique a molecular beam apparatus to investigate the spectroscopy and dynamics of sodium-containing clusters is shown. The metallic cluster is produced by the pick-up technique under crossed-beam conditions. Adapted with permission from Polanyi et al, J. Phys. Chem. 99 13691. Copyright 1995 American Chemical Society. Bottom schematic view of a pick-up technique based on a beam-gas arrangement. After nozzle expansion, a skimmer extracts the beam that subsequently collides with the particles in the gas cell. The cluster beam is ionized by a pulsed laser and mass analysed in a TOE mass spectrometer. Reproduced from Nahler et al, J. Chem. Phys., 2003, 119 224, with permission of the American Institute of Physics...

Figure 24.20 Schematic view of the crossed molecular-beam apparatus to study reactions of metal complexes deposited at the surface of large-size clusters. Reproduced from Mestdagh etal, Int Rev. Phys. Chem., 1997, 16 215, with permission...

The details of the crossed-beam apparatus used in our experiment can be found in many earlier publications [17,18]. Briefly, the alkali dimer source consisted of a resistively heated molybdenum oven and nozzle assembly, with the temperatures of the nozzle and the oven being controlled independently by different heating elements. Sodium vapour carried by an inert gas, which was either He or Ne, expanded out of the 0.2 mm diameter nozzle to form a supersonic beam of Na/Na2/inert gas mixture. The Na2 concentration was about 5% molar fraction of the total sodium in the beam when He was used as carrier gas. The beam quality dropped severely when we seeded Na2 in Ne so the dimer intensity became much weaker. No substantial amount of trimers or larger clusters was detected under our experimental conditions. The Na2 beam was crossed at 90 by a neat oxygen supersonic beam in the main collision chamber under single collision conditions. The O2 source nozzle was heated to 473 K to prevent cluster formation. Both sources were doubly differentially pumped. The beams were skimmed and collimated to 2 FWHM in the collision chamber. Under these conditions, the collision energies for the reaction could be varied from 8 kcal/mol to 23 kcal/mol. 

Both methods require a crossed molecular beam apparatus with high angular and velocity resolution in order to separate the different contributions. It is noted that the scattering process with He transfers a small amount of energy to the cluster which can be measured by analyzing the scattered clusters by time—of—flight methods. ... 

Endohedral metal fullerenes can be detected in relatively small amounts in the mass spectra in the laser vaporization cluster beams (vide supra). However, macroscopic quantities of these compounds may be produced rather readily either by vaporization in a laser furnace apparatus or by arc-burning of a composite rod of graphite and the corresponding metal oxide. In Fig. 4.50, a mass spectrum which illustrates the formation of a series of fullerene endohedral yttrium complexes obtained by laser vaporization of a composite graphite/ Y2O3 rod at 1200 ""C is reproduced. Among these species there is also one, Y2 Cs2, which corresponds to the inclusion of a metal cluster in the fullerene ball. 

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