In this thesis I describe the second generation of a rotating supersonic beam source. The purpose of this device is to produce velocity augmented molecular beams for use with scattering experiments or subsequent slowing methods. The beam emerges from a nozzle inserted at the tip of a hollow aluminum rotor which can be spun at high speeds in either the forward or backwards direction. The forward direction mode increases the laboratory frame velocity distribution of the emitted beam and the backward direction mode decreases this velocity distribution.\n\nBoth rotor modes are analyzed theoretically and experimentally within the text. I introduce a pulsed gas inlet system for the rotating source as well as cryocooling of the vacuum chamber. This new version provides moderately intense beams of slow molecules, containing ∼1012 molecules at lab speeds as low as 35 m/s, and very intense beams of fast molecules, containing ∼1015 molecules at 400 m/s. Beams of any molecule available in gas phase can be produced utilizing this system. For collision experiments, the ability to scan the velocity utilizing the rotating source is very advantageous when using two merged beams. If the two velocities can be closely matched, very low relative collision energies can be produced without making either beam slow.
In this thesis I describe the second generation of a rotating supersonic beam source. The purpose of this device is to produce velocity augmented molecular beams for use with scattering experiments or subsequent slowing methods. The beam emerges from a nozzle inserted at the tip of a hollow aluminum rotor which can be spun at high speeds in either the forward or backwards direction. The forward direction mode increases the laboratory frame velocity distribution of the emitted beam and the backward direction mode decreases this velocity distribution.\n\nBoth rotor modes are analyzed theoretically and experimentally within the text. I introduce a pulsed gas inlet system for the rotating source as well as cryocooling of the vacuum chamber. This new version provides moderately intense beams of slow molecules, containing ∼1012 molecules at lab speeds as low as 35 m/s, and very intense beams of fast molecules, containing ∼1015 molecules at 400 m/s. Beams of any molecule available in gas phase can be produced utilizing this system. For collision experiments, the ability to scan the velocity utilizing the rotating source is very advantageous when using two merged beams. If the two velocities can be closely matched, very low relative collision energies can be produced without making either beam slow.
We describe a pulsed rotating supersonic beam source, evolved from an ancestral device [M. Gupta and D. Herschbach, J. Phys. Chem. A 105, 1626 (2001)]. The beam emerges from a nozzle near the tip of a hollow rotor which can be spun at high-speed to shift the molecular velocity distribution downward or upward over a wide range. Here we consider mostly the slowing mode. Introducing a pulsed gas inlet system, cryocooling, and a shutter gate eliminated the main handicap of the original device in which continuous gas flow imposed high background pressure. The new version provides intense pulses, of duration 0.1-0.6 ms (depending on rotor speed) and containing ∼10(12) molecules at lab speeds as low as 35 m/s and ∼10(15) molecules at 400 m∕s. Beams of any molecule available as a gas can be slowed (or speeded); e.g., we have produced slow and fast beams of rare gases, O(2), Cl(2), NO(2), NH(3), and SF(6). For collision experiments, the ability to scan the beam speed by merely adjusting the rotor is especially advantageous when using two merged beams. By closely matching the beam speeds, very low relative collision energies can be attained without making either beam very slow.
We describe modifications of a pulsed rotating supersonic beam source that improve performance, particularly increasing the beam density and sharpening the pulse profiles. As well as providing the familiar virtues of a supersonic molecular beam (high intensity, narrowed velocity distribution, and drastic cooling of rotation and vibration), the rotating source enables scanning the translational velocity over a wide range. Thereby, beams of any atom or molecule available as a gas can be slowed or speeded. Using Xe beams in the slowing mode, we have obtained lab speeds down to about 40 ± 5 m/s with density near 10(11) cm(-3) and in the speeding mode lab speeds up to about 660 m/s and density near 10(14) cm(-3). We discuss some congenial applications. Providing low lab speeds can markedly enhance experiments using electric or magnetic fields to deflect, steer, or further slow polar or paramagnetic molecules. The capability to scan molecular speeds facilitates merging velocities with a codirectional partner beam, enabling study of collisions at very low relative kinetic energies, without requiring either beam to be slow.
Mounting a molecular beam source near the tip of a hollow high-speed rotor provides a means to shift the velocity distribution of the beam downward or upward over a wide range. We describe the construction of such a device and experiments and model calculations characterizing its operation, for both supersonic and effusive beams of rare gases, O 2, CH 3 F, and SF 6 . For example, the flow velocity of a rotating supersonic beam of O 2 was accelerated to above 1000 m/s (corresponding to a kinetic energy of 2200 K and deBroglie wavelength of 0.1 Å) and decelerated (when seeded in Xe) to below 70 m/s (corresponding to a kinetic energy below 10 K and deBroglie wavelength of nearly 2 Å). With improvements in prospect, the rotating beam source offers a versatile and relatively simple way to enhance techniques for manipulating molecular trajectories.
Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.
The flow velocity of gas emerging from a supersonic nozzle mounted on a high-speed rotor can be largely canceled by the rotor velocity, thereby producing an intense beam of molecules traveling in a vacuum with translational speeds slowed to a few tens of meters per second. Centrifugal action significantly enhances the supersonic character of the gas flow from the rotating nozzle, further narrowing the spread of velocities in the emerging beam. These features are demonstrated by model calculations and experimental results for beams of Xe and for O 2 and CH 3 F seeded in Xe.
Beams of atoms and molecules are stalwart tools for spectroscopy and studies of collisional processes. The supersonic expansion technique can create cold beams of many species of atoms and molecules. However, the resulting beam is typically moving at a speed of 300-600 m/s in the lab frame, and for a large class of species has insufficient flux (i.e. brightness) for important applications. In contrast, buffer gas beams can be a superior method in many cases, producing cold and relatively slow molecules in the lab frame with high brightness and great versatility. There are basic differences between supersonic and buffer gas cooled beams regarding particular technological advantages and constraints. At present, it is clear that not all of the possible variations on the buffer gas method have been studied. In this review, we will present a survey of the current state of the art in buffer gas beams, and explore some of the possible future directions that these new methods might take.