Electron diffraction of molecules in superfluid helium droplets

In this dissertation, I describe the experimental investigation in electron diffraction of molecules in superfluid helium droplets. The project is part of an overall scheme called ‘single molecule serial electron diffraction imaging’ (SS-EDI), with the ultimate goal of building an apparatus to determine atomic structures from oriented macromolecules. In SS-EDI, protein ions are doped in helium droplets and the doped droplets are then oriented by an elliptically polarized laser and exposed to a coherent electron beam for diffraction. The specific goal of my project is to explore the feasibility for electron diffraction of molecules in helium droplets. I started by installing several diagnostic components and systematically characterizing the behavior of the helium droplet beam, including its timing profile, size distribution, and doping statistics. I then moved on to electron diffraction of molecules in helium droplets. In the work on CBr4, the background issue from helium atoms was solved by increasing the doping path length or pressure of gaseous CBr4. In the experiment of ferrocene, the velocity slip of our pulsed droplet beam has been utilized to separate different sized droplets and to achieve efficient single doping in one droplet. The resulting success testifies to the capability of size control of our experiment. In the work of I2, the investigation was further expanded to different sized droplets containing different sized iodine clusters. From this study, we observed for the first time halogen bonded iodine clusters and bi-layer iodine nanocrystals. The work in this dissertation represents a major step in demonstrating the working principle of the overall idea of SS-EDI. The successes of these efforts imply that by embedding the sample in a superfluid helium droplet, the structurally relevant diffraction information of the sample can still be retrieved, as long as one can control the amount of helium surrounding the sample.Keywords: electron diffraction, helium drople

Electron impact ionization and multiphoton ionization of doped superfluid helium droplets: A comparison

We compare characteristics of electron impact ionization (EI) and multiphoton ionization (MPI) of doped super fluid helium droplets using the same droplet source. Selected dopant ion fragments from the two ionization schemes demonstrate different dependence on the doping pressure, which could be attributed to the different ionization mechanisms. While EI directly ionizes helium atoms in a droplet therefore has higher yields for bigger droplets (within a limited size range), MPI is insensitive to the helium in a droplet and is only dependent on the number of dopant molecules. The optimal timing of the ionization pulse also varies with the doping pressure, implying a velocity slip among different sized droplets. Calculations of the doping statistics and ionization probabilities qualitatively agree with the experimental data. Our results offer a word of caution in interpreting the pressure and timing dependence of super fluid helium droplets, and we also devise a scheme in achieving a high degree of doping while limiting the contribution of dopant clusters

Communication: Electron diffraction of ferrocene in superfluid helium droplets

We report electron diffraction of ferrocene doped in superfluid helium droplets. By taking advantage of the velocity slip in our pulsed droplet beam using a pulsed electron gun, and by doping with a high concentration of ferrocene delivered via a pulsed valve, we can obtain high quality diffraction images from singly doped droplets. Under the optimal doping conditions, 80% of the droplets sampled in the electron beam are doped with just one ferrocene molecule. Extension of this size selection method to dopant clusters has also been demonstrated. However, incomplete separation of dopant clusters might require deconvolution and modeling of the doping process. This method can be used for studies of nucleation processes in superfluid helium droplets

Electron Diffraction of Superfluid Helium Droplets

We present experimental results of electron diffraction of super fluid helium droplets and droplets doped with phthalocyanine gallium chloride, and discuss the possibility of performing the same experiment with a laser aligned sample. The diffraction profile of pure droplets demonstrates dependence on the nozzle temperature, i. e., on the average size of the droplets. Larger clusters demonstrate faster decay with increasing momentum transfer, while smaller clusters converge to isolated gas phase molecules at source temperatures of 18 K and higher. Electron diffraction of doped droplets shows similar modified molecular scattering intensity as that of the corresponding gas phase molecules. Based on fittings of the scattering profile, the number of remaining helium atoms of the doped droplets is estimated to be on the order of hundreds. This result offers guidance in assessing the possibility of electron diffraction from laser aligned molecules doped in superfluid helium droplets.Keywords: Coherent single molecule diffraction, Doped superfluid helium droplets, Electron diffraction, Superfluid helium droplets, Phthalocyanine gallium chloride, Gas phase electron diffractio

Facile time-of-flight methods for characterizing pulsed superfluid helium droplet beams

We present two facile time-of-flight (TOF) methods of detecting superfluid helium droplets and droplets with neutral dopants. Without an electron gun and with only a heated filament and pulsed electrodes, the electron impact ionization TOF mass spectrometer can resolve ionized helium clusters such as He₂ ⁺ and He₄ ⁺, which are signatures of superfluid helium droplets. Without ionizing any helium atoms, multiphoton non-resonant laser ionization of CCl₄ doped in superfluid helium droplets at 266 nm generates complex cluster ions of dopant fragments with helium atoms, including (He)[subscript]nC⁺, (He)[subscript]nCl⁺, and (He)[subscript]nCCl⁺. Using both methods, we have characterized our cryogenic pulsed valve—the Even-Lavie valve. We have observed a primary pulse with larger helium droplets traveling at a slower speed and a rebound pulse with smaller droplets at a faster speed. In addition, the pickup efficiency of dopant is higher for the primary pulse when the nozzle temperature is higher than 13 K, and the total time duration of the doped droplet pulse is only on the order of 20 μs. These results stress the importance of fast and easy characterization of the droplet beam for sensitive measurements such as electron diffraction of doped droplets.Article Copyright 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Review of Scientific Instruments and may be found at http://scitation.aip.org/content/aip/journal/rs

Study on Breakage Mechanism in the Swirl Generating Stage of an Oil-Water Separator for Marine Oil Extraction and Its Verification

High-efficient oil-water separator is badly-needed in marine oil extraction. Droplet breakage is common during the conversion from pipe to swirl flow in separators. Avoiding oil droplets break into small ones in the swirl generating stage is beneficial to improve the separator's separation efficiency. Information regarding the breakage mechanism and dispersed droplet distribution is critical for optimum design of the conversion structure, such as guiding vanes and prediction of the oil-water separation performance. However, little work has been related to the study of droplet sizes in a swirl flow produced by guiding vanes. The present work focuses on the oil droplet sizes generated by the passage of oil-water mixture goes through guiding vanes in a vane-type separator and the different breakage mechanism. Experiments were performed under different flow rates and maximum droplet sizes were measured in situ downstream from the guiding vanes. The maximum droplet size was found to fit a modified-T model. Besides, Modified-T model was found to fit different studies data the best in the noncoalescence system. The studies shows that reducing the energy loss also reduces the probability of droplet breakage which put forward a new method to improve the separator's design

Electron Diffraction of Superfluid Helium Droplets

We present experimental results of electron diffraction of superfluid helium droplets and droplets doped with phthalocyanine gallium chloride and discuss the possibility of performing the same experiment with a laser aligned sample. The diffraction profile of pure droplets demonstrates dependence on the nozzle temperature, that is, on the average size of the droplets. Larger clusters demonstrate faster decay with increasing momentum transfer, whereas smaller clusters converge to isolated gas phase molecules at source temperatures of 18 K and higher. Electron diffraction of doped droplets shows similar modified molecular scattering intensity as that of the corresponding gas phase molecules. On the basis of fittings of the scattering profile, the number of remaining helium atoms of the doped droplets is estimated to be on the order of hundreds. This result offers guidance in assessing the possibility of electron diffraction from laser aligned molecules doped in superfluid helium droplets