Many people in the chemistry and physics departments were of great help in the construction of the mass spectrometer. I would like to Dr. Hussey thanks for purchasing the necessary tools and also for my access to the chemistry department's machine shop. I would like to thank the past and present members of the Hammer group for assistance, and especially to my advisor Dr. Nathan Hammer for allowing me to work countless hours in his lab and believing in me as I struggled through construction.
MATTHEW DODD MCDOWELL: Design and Construction of a Reflector Time-of-Flight Mass Spectrometer for Multiphoton Ionization and Vibrational Spectroscopic. Its ability to select and isolate specific molecular clusters makes it an ideal tool for the fundamental spectroscopic study of the photophysical and geometric properties of molecules and molecular clusters. The design and construction of a reflectron time of flight mass spectrometer (reflectron TOF-MS) as well as its implementation for electronic and vibrational spectroscopic studies of various molecules and molecular clusters are discussed.
The second chapter covers the theoretical aspects of mass spectrometry required to create the molecular ion beams involved in TOF-MS. The third chapter contains the technical details of the construction and implementation of the many components required in a reflectron TOF-MS.
Background
- Electromagnetic Radiation - Matter Interactions
- Theory of Multiphoton Ionization
- Theory of Vibrational Spectroscopy
- References
This approach allows the Schrödinger equation to be solved more easily and is one of the fundamental approaches implemented in quantum chemistry. During this experiment he concluded that the energy of the ejected electrons was proportional to the wavelength of the light, and not to the intensity as described in classical physics. In this equation, “Tmax” is the kinetic energy of the ejected electron, “hν” is the energy of a photon, and “w” is the binding energy of the electron.
Also, it is not necessary that the sum of the photon energies used to eject the electron match perfectly. The reason is that the excess energy of the final photon, which shifts the molecule to the ionization potential, is transferred to the ejected electron in the form of kinetic energy. In fact, this is one of the basic principles driving the field of photoelectron spectroscopy.
The largest vibrations that occur can typically be described by one of the following movements: symmetrical stretching, antisymmetric stretching, wagging, rocking, twisting and shearing. The stretching motions involve changing bond lengths, and all the others involve moving the atoms relative to each other.
Mass Spectrometry
- Reflectron Time of Flight Mass Spectrometry
- Molecular Ion Clusters for Study
- Laser Interaction with Mass Spectrometry
- References
When this happens, some ions of the same mass will be exposed to the electrostatic field for different periods of time. So two particles with the same mass can have different kinetic energies. It is created by an electrostatic gradient field with the same polarity as the particle being detected. When particles of the same mass, but different kinetic energies, enter the reflectron, they are slowed down.
Many flight plans in use today were derived from a simplified plan described by Wiley and McLauren in 1955.2 Figure 2-1 is a basic diagram of a molecule. By performing this experiment in a vacuum, many of the external interactions that contribute to the spectra of the bulk sample are removed. This is the main reason that mass spectrometry is used as a platform to study molecular clusters due to its mass selective capability.
In this type of spectroscopy, a laser uses photons of specific energies to electronically excite molecules to the point of electron ejection. This makes it possible to determine the amount of energy required to ionize a molecule for different energy photons. This is done by accelerating the particle towards the detector and then calculating the location of the interaction point.
The timing of the laser is then adjusted to allow the photon to interact with the molecule or molecular cluster in flight. When interaction with a laser occurs, argon, depending on the binding energy of the charged species, may preferentially be lost in the interaction.3 The argon then disappears with some of the kinetic energy of the molecular clusters. The area under the peak is then integrated and the vibrational spectra of the molecular cluster are obtained.
A different approach can be used if the electron is bound to the molecule.4-8 A photon of given energy can then be used to remove the excess electron and the spectra can be determined by integrating either the neutral, if the detector is in the initial line of flight, or vice versa by the loss of intensity of the peak of the molecular clusters. Conversely, if cationic clusters are studied, this can be accomplished in a similar manner to the methods previously described, except for the loss of an electron method and the detection of the neutral. This type, as well as binding an excess electron, can be difficult because the binding energy is greater than the energy of the photons used to study these clusters.
Construction of a Reflectron Time of Flight Mass Spectrometer
- Basic Design of a Time of Flight Mass Spectrometer
- Interaction with Tunable Dye Laser
- Construction of Movable Detector
- Construction of a Reflectron
- Construction of Ion Optics
- Construction of Ion Optics Controls
- Construction of Pulse Valve Controller
- Construction of Electron Gun
- Construction of the Control for an Electron Gun
- References
The output of the dye laser was then directed to the entrance window in the mass spectrometer using rotating prisms. A four-inch focusing lens was then used to focus the laser beam in the center of the source chamber directly beneath the pulse valve. Different types of dyes are used in the oscillator and amplifier stages of the dye laser, which fluoresce at different wavelengths.
When operating with a tunable dye laser, the delay between the pulse valve and the laser is varied to optimize peak intensity. Figures 3-4 and 3-5 show the moving microchannel plate detector and its circuit used in a reflector TOF-MS.5. Then one side is connected to the variable high voltage power supply and the other side is connected to ground.6 Figure 3-6 is a basic reflector diagram showing the required arrangement and the required electrical circuit.
This is best described by a Cartesian coordinate system where the flight path is defined as along the z direction and the deflector creates electrostatic fields in the x and y directions. An Einzel lens works through three concentric hollow cylinders with a voltage applied to the center cylinder and the remaining two grounded. Since the weakest field lines are in the center of the cylinder, an ion passing through it will be affected by the field lines.
The exit paths of all the ions will then intersect at a given point along the flight path (focal point), depending on the strength of the electrostatic field. Adjusting the voltage applied to the center ring adjusts the electrostatic field and the focal point of the ions. This device works similar to a convex lens in geometric optics.6 Figure 3-7 is a basic diagram of an Einzel lens showing the electrostatic field lines and their interaction with passing ions.
Since each optic requires only a fixed voltage for a given optic, a simple DC voltage divider network was designed and implemented for this purpose. A field switching device was also used to change the direction of the deflector field lines from +X to -X and from +Y to -Y to allow manipulation of the ion's in-flight path in the (+) and (-) X and Y directions. Initially an Iota One pulse valve controller from Parker Hannefin was used to control pulse valve duration.
Due to the cost of the Iota One controller and the desire for greater control over experimental parameters, a newly designed pulse valve controller was created. 9,10 are the sides of the filament and are connected to a high voltage power supply connected to -1000Vdc X deflectors.
Mass Selected Ion Spectroscopy
- Multiphoton Ionization of Methyl Iodide
- Water Cluster Precursor for Vibrational Spectroscopy
- Conclusion
- References
The sample was then excited with multiple photons to the point of ionization and then accelerated along the flight path to be detected. An alternative pathway would be for the methyl iodide ion to fragment, leaving different charged species. Since photons of different energies produce different fragment stabilities, scans of fragment intensity versus wavelength were obtained in this region.
The spectra show that as photon energy is lowered, fewer ions are produced and that the fragments are likely created from the ionized parent ion instead of fragmentation occurring before ionization. After the completion of the methyl iodide project, the logical direction to proceed was to the creation of water clusters followed by the interaction of water clusters with the study system. The first step in progressing to the final goal was creating water clusters that bind a proton.
The spectra also show an extremely stable cluster of protonated water H⋅(H2O)n+where n=21.47 The spectra of water clusters H⋅(H2O)n+where n=7-32 are shown in Figure 4-3. This was more difficult due to the lower relative stability compared to cationic clusters. Cluster identification has also been plagued by some instrumental problems, such as vacuum leakage, which are still trying to be fully resolved.
However, some of the heavy water clusters48-49 have been achieved while reaching the final goal. These heavy water clusters were first studied in Many of the other fragments such as (H2O)n- where n is obtained,50-52 but due to lack of stability in the mass spectrometer these clusters have not been studied spectroscopically. There is still much more work to be done before one can perform vibrational spectroscopy of the relevant systems.
Although much progress has been made toward the creation of a reflectron flight mass spectrometer for conducting spectroscopic studies of mass-selected clusters, it will never be completed. The biggest aspect in this area concerns the modification of current instruments, or the creation of new instruments to perform specific experiments.
