Effusive Sources Research Articles

Molecular beam/infrared reflection-absorption spectroscopy apparatus for probing heterogeneously catalyzed reactions on functionalized and nanostructured model surfaces.

A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams-two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflection-absorption spectroscopy for the detection of surface-adsorbed species, and quadrupole mass spectrometry for gas phase analysis. The supersonic beam is generated in a pulsed supersonic expansion and can be modulated via a variable duty-cycle chopper. The effusive beams are produced by newly developed compact differentially pumped sources based on multichannel glass capillary arrays. Both effusive sources can be modulated by a vacuum-motor driven chopper and are capable of providing high flux and high purity beams. The apparatus contains an ambient pressure cell, which is connected to the preparation chamber via an in situ sample transfer system and provides an experimental possibility to study the reactivity of well-defined nanostructured model catalysts in a broad range of pressure conditions-up to ambient pressure-with the gas phase analysis based on gas chromatography. Additionally, a dedicated deposition chamber is connected to the preparation chamber, which is employed for the in situ functionalization of model surfaces with large organic molecules serving as promoters or modifiers of chemical reactions. We present a general overview of the apparatus as well as a description of the individual components and their interplay. The results of the test measurements involving the most important components are presented and discussed.

Conformational Landscape of Tri-n-butyl Phosphate: Matrix Isolation Infrared Spectroscopy and Systematic Computational Analysis.

The conformations of tri-n-butyl phosphate (TBP) were studied using matrix isolation infrared spectroscopy and density functional theory (DFT) calculations. TBP was trapped in a N2 matrix using both effusive and supersonic sources, and its infrared spectra were recorded. The computational exploration of TBP is a very demanding problem to confront, due to the presence of a large multitude of conformations in TBP. To simplify the problem, computations were done on model compounds, dimethyl butyl phosphate (DMBP) and dibutyl methyl phosphate (DBMP), to systematically arrive at the conformations of TBP that are expected to contribute to its chemistry at room temperature. Some predictive rules seem to simplify this complex conformational landscape problem. The predictive rules that were formulated enabled us to search the relevant portion of the conformational topography of this molecule. The computations were performed at the B3LYP level of theory using the 6-31++G(d,p) basis set. Vibrational wavenumber calculations were also performed for the various conformers to assign the infrared features of TBP, trapped in solid N2 matrix.

An adaptable dual species effusive source and Zeeman slower design demonstrated with Rb and Li.

We present a dual-species effusive source and Zeeman slower designed to produce slow atomic beams of two elements with a large mass difference and with very different oven temperature requirements. We demonstrate this design for the case of (6)Li and (85)Rb and achieve magneto-optical trap (MOT) loading rates equivalent to that reported in prior work on dual species (Rb+Li) Zeeman slowers operating at the same oven temperatures. Key design choices, including thermally separating the effusive sources and using a segmented coil design to enable computer control of the magnetic field profile, ensure that the apparatus can be easily modified to slow other atomic species. By performing the final slowing using the quadrupole magnetic field of the MOT, we are able to shorten our Zeeman slower length making for a more compact system without compromising performance. We outline the construction and analyze the emission properties of our effusive sources. We also verify the performance of the source and slower, and we observe sequential loading rates of 12 × 10(8) atoms/s for a Rb oven temperature of 140 °C and 1.1 × 10(8) atoms/s for a Li reservoir at 460 °C, corresponding to reservoir lifetimes for continuous operation of 10 and 4 years, respectively.

Cold and Controlled Molecular Beams: Production and Applications

The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies.

Soft Ionization of Thermally Evaporated Hypergolic Ionic Liquid Aerosols

Isolated ion pairs of a conventional ionic liquid, 1-Ethyl-3-Methyl-Imidazolium Bis(trifluoromethylsulfonyl)imide ([Emim(+)][Tf(2)N(-)]), and a reactive hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Dicyanamide ([Bmim(+)][Dca(-)]), are generated by vaporizing ionic liquid submicrometer aerosol particles for the first time; the vaporized species are investigated by dissociative ionization with tunable vacuum ultraviolet (VUV) light, exhibiting clear intact cations, Emim(+) and Bmim(+), presumably originating from intact ion pairs. Mass spectra of ion pair vapor from an effusive source of the hypergolic ionic liquid show substantial reactive decomposition due to the internal energy of the molecules emanating from the source. Photoionization efficiency curves in the near threshold ionization region of isolated ion pairs of [Emim(+)][Tf(2)N(-)] ionic liquid vapor are compared for an aerosol source and an effusive source, revealing changes in the appearance energy due to the amount of internal energy in the ion pairs. The aerosol source has a shift to higher threshold energy (∼0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicrometer aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally "cooler" source of isolated intact ion pairs in the gas phase compared to effusive sources.

Matrix isolation infrared and ab initio studies of the conformations of dimethoxymethylsilane

Conformers of dimethoxymethylsilane were studied using matrix isolation infrared spectroscopy, by trapping it in nitrogen and argon matrixes. In addition to a room temperature effusive source, supersonic and hot nozzle effusive sources were also used to deposit the matrix. These experiments were done to aid assigning the vibrational features, by altering the population of the various conformers in the matrix. As a result of these experiments, the infrared spectral features have been assigned to the ground state conformer, G ±G ∓ and higher energy conformers, G −G − and G −T. Our experimental observations were supported by computations performed at HF and B3LYP levels using 6-31++G** basis set.

Conformations of Trimethoxymethylsilane: Matrix Isolation Infrared and Ab Initio Studies

Conformations of trimethoxymethylsilane were studied using matrix isolation infrared spectroscopy and ab initio computations. Trimethoxymethylsilane was trapped in both argon and nitrogen matrixes using heated nozzle effusive sources and a supersonic jet source, in an effort to alter the conformational population in the matrix. Ab initio calculations were carried out at the HF and B3LYP level using 6-31++G basis set to support our experimental observations. The frequencies computed at the B3LYP level was found to fit well with our experimental data. A conformer with a C1(g(+/-)g(+/-)t) structure was predicted by our computations to be the ground state conformer.

Conformational stabilities of 1-methoxy-2-(methylthio)ethane and relevant intramolecular CH⋯O interaction studied by matrix-isolation infrared spectroscopy and density functional calculations

Conformational stabilities of 1-methoxy-2-(methylthio)ethane were studied by matrix-isolation infrared spectroscopy, and the relevant intramolecular 1,5-CH⋯O interaction was examined by density functional calculations. The conformer with trans– trans– gauche ± around the CH 3O–CH 2–CH 2–SCH 3 bonds is the most stable in an argon matrix and the conformer with trans– gauche ±– gauche ∓ is the second most stable. The energy difference between the two conformers was determined to be 0.7±0.4 kJ mol −1 using thermal effusive sources. The 1,5-CH⋯O interaction is responsible for the high stability of the trans– gauche ±– gauche ∓ conformer, since the nonbonded (C)H⋯O distance associated with this interaction is shorter than the corresponding van der Waals separation by 0.15 Å. The energy of the 1,5-CH⋯O interaction was evaluated to be 4.0 kJ mol −1 .

Angular profiles of molecular beams from effusive tube sources: I.Experiment

New measurements of the angular distributions of effusive gas beamsemanating from two collimating tube sources are presented. These sources are asingle tube of aspect ratio 25 and a multi-capillary array of aspect ratio100. The data were collected for a wide variety of gases (He, Ne, Ar, Kr, Xe,H2, N2, CO2 and C2H2) over a broadrange of driving pressures. The profiles were obtained by rotating theionization gauge gas detector about the centre of the exit of the tube. Thedata were acquired as a function of the gas flow rate and pressure in thesource line. Comparisons of the present measurements with other experimentalwork and models are made.

Design and characterization of collimated effusive gas beam sources: Effect of source dimensions and backing pressure on total flow and beam profile

The total flux and the flow profile of gas-phase molecular beams generated by a number of assemblies involving different combinations of apertures and capillary arrays were measured for a wide range of backing pressures covering the molecular and viscous flow regimes. Specifically, δ=10 and 50 μm diameter, L=2 mm thick glass capillary arrays were combined with single apertures of φ=0.17, 1.0, and 11.4 mm diameters and L=0.1, 0.5, and 2 mm thickness in order to design high-flux beam dosers with a high degree of collimation. The variations in the total flux and the spatial profile of the beams were tested as a function of the backing pressure, which was varied between 10−4 and 102 Torr, by a sampling movable skimmer. The data obtained under the molecular flow (low backing pressure) regime corroborate some conclusions from previous reports. In particular, it is shown here that the conductance of the dosers (the ratio of the total flux of the beam to the backing pressure) depends only on their geometry in that regime. The beam profile, on the other hand, deteriorates with increasing backing pressure because of the increase in gas–wall and gas–gas collisions in the intermediate “opaque” flow regime where the mean free path of the gas, λ, is larger than the diameter of the capillaries, δ, but smaller than the length of the tubes, L. As λ approaches δ, a transition “slip” regime is reached, and a drop in the conductance of the doser is observed. Finally, by pressures where λ<0.05δ, a viscous laminar flow is established where the conductance of the doser increases with pressure and its directionality improves as well. The implications of our results to the design of molecular beams for specific applications are briefly discussed.

Laser photo-induced dissociation using tandem time-of-flight mass spectrometry

A novel tandem time-of-flight (TOF) mass spectrometer has been developed for studying the photo-induced dissociation of large molecules and elemental clusters. It consists of a linear first stage TOF analyser for primary mass separation and precursor ion selection, and a second orthogonal reflecting field TOF analyser for product ion analysis. The instrument is equipped with a large volume throughput molecular beam source chamber allowing the production of jet-cooled molecules and molecular clusters, as well as elemental clusters, using either a pulsed laser vaporisation source (LVS) or a pulsed are cluster ion source (PACIS). A second differentially pumped chamber can be used with effusive sources, or for infrared laser desorption of large molecules, followed by laser ionisation. These primary ions can then be irradiated with a second, high energy laser to induce photodissociation. Detailed information about the fragmentation mechanisms can be deduced from the product ion mass spectra. Preliminary results on the photo-induced dissociation (PID) of the molecule ion of aniline at 266 nm are presented. In this case the molecule ions were generated via two-photon laser ionisation at 266 nm using an effusive source. Results for the collision-induced dissociation (CID) of the aniline molecule ion, using a commercial mass spectrometer equipped with an atmospheric pressure electrospray ionisation interface, are also presented. Copyright 2000 John Wiley & Sons, Ltd.

On the design of capillary and effusive gas dosers for surface science

We present a theoretical analysis of flux distributions and enhancement factors for dosers of two types: capillary arrays and arrays of effusive sources. Our analysis introduces an important distinction between the flux per solid angle and the flux per unit area of the sample, and encompasses array diameters both smaller and larger than the sample. We find, contrary to earlier assertions, that a single effusive source gives unacceptably large flux gradients unless it is located so far from the sample that its enhancement factor is negligible. Arrays of a small number of effusive sources located near the sample edge, however, are competitive with capillary arrays for doser-to-sample distances comparable to the sample radius, and are readily adapted to large or unusually shaped samples. Capillary arrays outperform effusive arrays at all doser-to-sample distances, and are particularly effective when placed very close to the sample. If flux uniformity is a concern, a capillary doser should generally be at least 20% larger than the sample. Additional effects due to trapping by cold surfaces in the chamber and multiple collisions of the emitted molecules are discussed qualitatively.

Thermochemical studies of the gaseous uranium chlorides

The gaseous uranium halides UCl, UCl2, UCl3, UCl4, and UCl5 were generated under equilibrium conditions using several types of effusive beam sources, and were identified and characterized thermochemically by mass spectrometry. Five independent gaseous equilibria involving these species were studied in sufficient depth so that both reaction enthalpy and entropy changes could be evaluated accurately from second law analysis. The enthalpy data yield the sequence of bond dissociation energies at 298 K in kcal/mol: D° (U–Cl)=108; D° (ClU–Cl)=117; D° (Cl2U–Cl)=117; D° (Cl3U–Cl)=101; and D° (Cl4U–Cl)=50, all ±2 kcal/mol. As expected, the absolute entropies of the gaseous chlorides show that the electronic partition functions of the lighter species are substantial at the experimental temperatures, leading to electronic entropies of 5–7 cal/deg mol at about 2100 K.

Determination of molecular beam characteristics for condensable molecules by optical interferences

A method is presented which allows the determination of the characteristics of effusive molecular beams formed by heavy molecules. It is based on condensing the beam molecules on a cooled metal surface and measuring the thickness of the condensed layer by means of interference when the layer is lighted with monochromatic light. The information about the beam characteristics is provided by the growth of the layer thickness. The results in the case of the investigated benzene beam are in agreement with the theoretical predictions of a model of molecular beam formation by effusive sources, as far as this model can be assumed to be correct.

Molecular Beams of Short Lived Radioactive Nuclides

The advantages of the use of molecular beams labeled with short lived (T½≈10−5−10−1 sec) nuclides in crossed beam studies of molecular interactions are discussed. Theoretical expressions useful for the design of effusive sources of such beams are presented. Experiments to verify the theory and develop apparatus capable of generating usefully intense beams are described, with particular reference to beams of HAt made with 0.032 sec 217At. Beams of 4×105 molecules/min incident on a spot 0.4 cm in diameter, 2.5 cm from the collimator, and with angular divergence of 4° have been achieved.

High Intensity Nozzle Beam Source for Use in Molecular Total Cross Section Measurements

A nozzle source which produces beams of neutral molecules, a factor of 102 to 103 more intense than that from effusive sources, is described. Further, the velocity distribution from this source is much narrower than that from effusive sources. The properties of the source and a description of its use in total cross section measurements for the case of a He beam on a H2 scatterer are presented.