Molecular Beam Kinetics
Molecular beam scattering from surfaces is a powerful experimental tool for studying the dynamics and kinetics of the interaction of molecules with surfaces. The coupling of surface science, molecular beam, and laser technologies makes possible the measurement of total energy disposal and redistribution in gas-surface scattering. Previously, these experimental methods were used to acquire detailed surface kinetics and state-to-state scattering measurements of molecules interacting with metallic substrates. While such experiments have resulted in a fairly detailed understanding of surface chemistry on metals, researchers currently do not have a similar understanding of the elementary dynamical and kinetic processes occurring on ice and oxide surfaces. Such interactions are clearly important from an environmental viewpoint, since they form the molecular-level basis for the complex physiochemical processes that take place on the surface of atmospheric aerosols, at the aqueous-mineral geochemical interface, and at the vapor-liquid interface.
The goal of EMSL researchers is to apply and extend molecular beam surface-scattering techniques to these systems in an effort to elucidate the relevant interactions. Toward this goal, researchers have designed and constructed three new state-of-the-art molecular beam-surface scattering and kinetics instruments. This unique set of instrumentation located in EMSL's Chemistry and Physics of Complex Systems Facility allows researchers to investigate the dynamics and kinetics of surface interactions in unprecedented detail.
The following information briefly describes the instrument configurations and provides an operational overview to facilitate user planning. All work with these instruments and at EMSL must be performed in compliance with EMSL practice and permits.
Molecular Beam Surface Scattering Instruments
EMSL researchers have designed and constructed two molecular beam-scattering machines (Instruments I and II) and a low-energy ion beam line. A plan view of these instruments is depicted in Figure 1. These ultrahigh vacuum (UHV) machines have a base pressure below 2 x 10-10 Torr in the scattering chambers. Instruments I and II are both equipped with three coplanar molecular beam lines that intersect at the sample target location within the scattering chamber. The two outer beams are at an angle of ±15 degrees with respect to the central beam. Each beam can be independently controlled and operated as a continuous or pulsed effusive or supersonic beam. The three beams can be synchronously modulated via computer-controlled high-speed electromechanical shutters and rotating chopper wheels. The primary difference between Instruments I and II is the distance the beam sources reside from the scattering target. Instrument I has an increased source-to-target distance (70 cm), which allows direct backscattering geometries to be examined. The source-to-target distance on Instrument II (40 cm) is much shorter, allowing for more intense beams to be generated.
The surface scattering target is mounted on a four-axis manipulator and placed at the intersection point of the three molecular beams. The sample is cooled with a closed-cycle helium refrigerator and can attain a base temperature of 20 K. The sample stage is designed to allow rapid thermal cycling between 25 K and 1200 K. A programmable temperature controller maintains the sample temperature.
Both the incident and scattered molecular beams can be detected via a differentially pumped neutral atom/molecule detector. This detector is mounted on a large rotary flange assembly whose axis of rotation is perpendicular to the plane defined by the molecular beams and passes through the intersection point of the beams. The detector consists of an electron impact ionizer, an electrostatic quadrupole bender, and a quadrupole mass spectrometer. The detector is mounted inside a double differentially pumped manifold to significantly increase the signal-to-background ratio for the desorbed/scattered flux of atoms or molecules. This detector can be used to measure velocity and angular distributions of the scattered/desorbed flux. In addition, the detector is designed to allow the incident beam to pass through it so that it can measure the directly backscattered flux in Instrument I.
The target sample manipulator is also mounted on a large rotary flange assembly that allows the sample to be positioned either at the intersection point of the molecular beams or in front of a number of surface preparation and characterization instruments. Both instruments are equipped with a sputter ion gun for sample cleaning, and an Auger electron spectrometer (AES) and a low-energy electron diffraction (LEED) spectrometer for monitoring surface composition and order, respectively. The UHV scattering chamber can also house a number of effusive beam evaporators, which can be used to synthesize compositionally tailored nanoscale oxide and ice surfaces.
In addition to the dedicated surface analytical instrumentation described above, Instruments I and II share a suite of specialized instrumentation comprised of an X-ray photoelectron spectrometer (XPS), a Fourier transform infrared (FTIR) spectrometer, a Kelvin probe, a six-axis sample manipulator, and a low-energy ion beam line. The ion beam line can produce mass selected, monoenergetic ion beams with energies ranging from 10 eV to 3000 eV and can be operated in two different modes, depending on the final energy of the ions. For high-energy beams (Ei ≥ 400 eV), the ions are transported through the beam line at their final energy. However, due to space charge spreading of the ion beam, this approach does not work for low-energy beams. Therefore, for Ei < 400 eV, the ions are transported through the beam line at 400 eV and decelerated to their final energy just prior to striking the sample. An electrostatic quadrupole bender can be used to direct the ion beam into either Instrument I or Instrument II. The ion beam is focused onto the target using a retractable deceleration lens assembly. This assembly also can be employed to deposit very low-energy (<5 eV) ions onto the target. The scattered ion flux can be detected with a rotatable electrostatic energy analyzer and/or quadrupole mass spectrometer.
Beam Surface Kinetics Instrument
EMSL researchers designed and constructed a state-of-the-art molecular beam surface-scattering instrument for examining surface kinetics. This instrument enables the simultaneous detection of both gaseous and surface species while the target is exposed to a flux of two reagents, and the modular design facilitates interchangability and adaptability for collaborative research. A plan view of the beam surface kinetics instrument is displayed in Figure 2. In this instrument, two molecular beams intersect at 90 degrees on a target surface residing within a UHV scattering chamber. Each beam can be independently controlled and operated as a continuous or pulsed effusive or supersonic beam. Both beams can be synchronously modulated via computer-controlled high-speed electromechanical shutters and rotating chopper wheels. Each beam line is independently triply-differentially pumped while maintaining a source-to-target distance sufficiently short to enable intense beams having fluxes in excess of 10 monolayers/sec to be used. A differentially pumped stationary neutral atom/molecule detector resides between the two incident beams and can detect particles scattering or desorbing along the surface normal.
Other geometries can be examined by rotating the target surface. The detector is an electron impact quadrupole mass spectrometer whose axis lies within the principal scattering plane. This detector also can be used to detect ions issuing from the surface or generated via laser photoionization.
The surface scattering target is mounted on a four-axis manipulator and placed at the intersection point of the two molecular beams. The sample is cooled with a closed-cycle helium refrigerator and attains a base temperature of 20 K. The sample stage is designed to allow rapid thermal cycling between 25 K and 1200 K. A programmable temperature controller maintains the sample temperature. The target sample manipulator also is mounted on a large rotary flange assembly which allows the sample to be positioned either at the intersection point of the molecular beams or in front of a number of surface preparation and characterization instruments.
The instrument is equipped with a sputter ion gun for sample cleaning; and an AES and a LEED spectrometer for monitoring surface composition and order, respectively. In addition to the dedicated surface analytical instrumentation described above, the instrument has an FTIR spectrometer and a secondary ion mass spectrometer that can be used while the sample target is being exposed to reagent fluxes from the two molecular beams. This novel feature enables both the gaseous and surface species to be simultaneously monitored under actual reaction conditions.
This instrument may be used for research in collaboration with EMSL researchers. For example, researchers from the University of Washington worked with EMSL staff using this instrument to study adsorption of small alkane molecules on MgO(100) and to study particle size effects on the adsorption and dissociation of methane on model catalysts consisting of size-controlled palladium nanoclusters supported on MgO(100).
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