We report on mobility measurements of electron bubbles in superfluid helium-4. Electrons are introduced into the liquid from a plasma discharge in the vapor. For electrons with energy only a small amount above the minimum energy needed to enter the liquid, the wave function is partially transmitted into the liquid. We investigate the possibility that this partial transmission results in the formation of stable electron bubbles each of which contains only a fraction of the complete electron wave function. By measuring the mobility of these bubbles we can estimate their size. Our simulation of the mobility of these bubbles is consistent with the experimental data, and supports the idea that the interaction of the electron with the liquid helium does not result in a measurement that immediately determines that an electron is, or is not, in the bubble. Other possible explanations and their difficulties are also discussed in the paper.

Bubbles in liquid helium each containing a single electron ("normal electron bubbles" or NEB) have been studied in many experiments and are well understood. However, several experiments in which the electron source that is used is a plasma discharge in the helium vapor above the liquid reveal the existence of a number of negative ions which have a higher mobility than the NEB. In a new experiment, we have used a 5.5-MeV alpha source to inject electrons directly into the liquid and have found that with this source no high mobility negative ions are detected. This result is consistent with the proposal that the ions of high mobility are produced by electrons from the plasma that have been partially transmitted into the liquid.

We describe a new detector capable of directly measuring dark matter particles with masses as low as 1 MeV/c(2). The detector is based on the quantum evaporation of helium atoms from the surface of liquid helium and their detection using field ionization. When a dark matter particle collides with an atom in liquid helium, the deposited energy results in the evaporation of helium atoms from the liquid surface. A dense array of sharp, positively charged metal tips, known as the field ionization detector array, creates a strong electric field that ionizes the helium atoms and accelerates them into a calorimeter, which detects the impact. We studied field ionization from single tips and investigated the dependence of the ionization rate on the applied voltage. We discuss the results of these single tip field ionization experiments, as well as upcoming experiments, which will focus on studying the temperature dependence of field ionization of gaseous helium at low temperatures.

We present a review of the properties of negative ions that are produced when electrons are inserted into superfluid helium-4. It is convenient to divide the ions into two groups. The first is the ion usually referred to as the "normal electron bubble," or NEB. The structure of this ion is well understood; it is a small bubble containing a single electron and essentially no helium atoms. The second group is made up of a large number of different ions each with a mobility larger than that of the normal electron bubble. These have been named the "exotic ions," and their structure is unknown.

We present calculations of the properties of bubbles in liquid helium-4 containing 3 or 8 electrons. The pressure range in which these bubbles are stable is determined. For Z =3, we find that the bubbles are unstable except in a pressure range between − 0.78 and − 0.90 bars. For Z = 8, the bubbles are stable provided the pressure is in the range from zero to – 0.42 bars. At the upper end of the pressure range, the bubbles break into smaller bubbles each containing a fraction of the electrons; at the lower end the bubbles become unstable against unlimited expansion.

A freshman mechanics course often includes a theoretical discussion or demonstration of the motion of solid particles through fluids. The effect of viscosity is typically considered to provide a drag force on the particle which at each instant is proportional to the particle velocity. We present a brief discussion of the limitations of this approach and the importance of the so-called history effect first considered by Boussinesq and Basset in the late 19th century.

We report on studies of positive ions in superfluid helium-4. In addition to the positive ion that has been previously studied, we have been able to detect two more positive ions with slightly different mobility, and the results show some evidence for a third additional ion.

Studying the kinetics of phase transformation and phase boundary propagation during initial lithiation of silicon electrodes in lithium ion batteries is relevant to understanding their performance. Such studies are usually challenging due to the difficulties in measuring the phase boundary velocity in the interior of the sample. Here we introduce a non-invasive, in situ method to measure the progression of the phase boundary in a planar specimen geometry while maintaining well-controlled lithium flux and potential. We developed an apparatus integrating an electrochemical cell with picosecond ultrasonics to probe the propagating phase boundary in real time. Phase propagation during initial lithiation of crystalline silicon, which is an example of a high capacity anode, is investigated. The primary objective of this manuscript is to report on the experimental technique development and some preliminary results. For lithiation normal to the (100) plane, we observe the phase boundary velocity to be approximately 12 pm/s and x to be 3.73 in LixSi under galvanostatic lithiation with a current density of 40 μA/cm2. The growth rate of the lithiated phase and the reaction rate coefficient are examined using a Deal-Grove type model.

In 1970, Doake and Gribbon reported the discovery of a negative ion in superfluid helium that has a mobility about six times higher than the mobility of the normal electron bubble. We report on measurements of the variation of the mobility of this “fast ion” with applied pressure.

We report on experiments in which we study cavitation resulting from electrons in liquid helium. Electrons are introduced into the liquid by a radioactive source. After an electron comes to rest in the liquid, it forces open a small cavity referred to as an electron bubble. To study cavitation, a sound pulse is generated by means of a hemispherical piezoelectric transducer producing a large-amplitude pressure oscillation at the acoustic focus. If an electron is in the vicinity of the focus and the negative-going pressure swing exceeds a critical value, a cavitation bubble is produced which can be detected by light scattering. Two distinct critical pressures \( P_{\text{el}} \) and \( P_{\text{rare}} \) have been measured. The first corresponds to cavitation resulting from the application of a reduced pressure to liquid containing an electron which has already formed an electron bubble. The second is the critical pressure needed to lead to cavitation when an electron enters the liquid at a time and place where there is already a reduced pressure. We have measured these two pressures as a function of temperature and consider possible explanations for the difference between them. In addition to these clearly seen cavitation thresholds, there are some cavitation events that have been detected with a threshold that is at an even smaller negative pressure than \( P_{\text{el}} \) and \( P_{\text{rare}} \).

An electron bubble in liquid helium-4 under the saturated vapor pressure becomes unstable and explodes if the pressure becomes more negative than −1.9 bars. In this paper, we use focused ultrasound to explode electron bubbles. We then image at 30,000 frames per second the growth and subsequent collapse of the bubbles. We find that bubbles can grow to as large as 1 mm in diameter within 2 ms after the cavitation event. We examine the relation between the maximum size of the bubble and the lifetime and find good agreement with the experimental results.

We report on experiments in which focused laser light is used to induce optical breakdown in liquid helium-4. The threshold intensity has been measured over the temperature range from 1.1 to 2.8 K with light of wavelength 1064 nm. In addition to the measurement of the threshold, we have performed experiments to study how the breakdown from one pulse modifies the probability that a subsequent pulse will result in breakdown.

We present the results of computer simulations of the motion of an electron bubble through superfluid helium-4 when acted upon by an electric field. The simulations are based on an extended version of the Gross–Pitaevskii equation. The temperature is assumed to be sufficiently low for the drag exerted on the bubble by thermal excitations to be negligible, and the calculations are made for velocities below the critical velocitie for nucleation of vortices and roton production. We calculate the effective mass \(m*\) of the bubble and obtain results in excellent agreement with the measurements of Poitrenaud and Williams, and Ellis, McClintock, and Bowley.

Multielectron bubbles (MEBs) are charged cavities in liquid helium which provide an interesting platform for the study of electrons on curved surfaces. Very recently, we have reported an experiment to trap these objects in a two-dimensional Paul trap, where they could be observed from ten to hundreds of milliseconds. During this time, the vapor inside the bubble condensed which resulted in a steady reduction in their size such that beyond a certain time the MEBs could no longer be detected. In this paper, we present experimental data on the lifetime of the bubbles as a function of their initial radius and compare the results with a theoretical model.

We describe a method for dark matter detection based on the evaporation of helium atoms from a cold surface and their subsequent detection using field ionization. When a dark matter particle scatters off a nucleus of the target material, elementary excitations (phonons or rotons) are produced. Excitations which have an energy greater than the binding energy of helium to the surface can result in the evaporation of helium atoms. We propose to detect these atoms by ionizing them in a strong electric field. Because the binding energy of helium to surfaces can be below 1 meV, this detection scheme opens up new possibilities for the detection of dark matter particles in a mass range down to 1  MeV/c^{2}.

We report on a method for determining the stress near the surface of a crystal that is partly transparent. A pump light pulse is applied to the sample to set up a stress near to the sample surface. This stress relaxes and launches a strain pulse into the sample. The propagation of this strain pulse can be monitored by means of a time-delayed optical probe pulse. The reflectivity of this probe light pulse contains a component which varies periodically with the time delay. We show how the period of this component can be precisely extracted from the data, how the period can be used to determine the stress in the sample, and the sensitivity of the technique for stress measurements.

We propose the thermal-mode spectroscopy (TMS), a new experimental technique for thermal conductivity of minute specimen. The TMS method is based on the excitation and detection of "thermal modes" using an optical pump-probe technique. All existing methods are based on "heat transport", and they are unsuitable for minute and high-heat-diffusivity specimens because of many ambiguous parameters. Thermal conduction is recognized as "overdamping" phenomenon of thermal wave, and it is characterized by the temperature relaxation time (corresponding to eigenvalue), which is a function of dimensions and thermal diffusivity of the specimen. By solving the eigenvalue problem of heat equation, we theoretically obtain the relaxation time and then the thermal conductivity. Because the TMS method needs no ambiguous parameters owing to the eigenvalue method and the non-contact measurement, we can determine thermal conductivity more accurately than other previous methods.

We report on the development of a new type of scanning acoustic microscope. We use a femtosecond light pulse to generate a short sound pulse, and then focus this sound onto the sample by means of a specially designed and microfabricated acoustic lens of radius a few microns. The sound travels to the sample through a thin layer of water. The sound reflected from the sample is collected by the lens and then passes through a monolithically integrated optical resonant cavity. The induced change in the properties of this cavity are measured using a time-delayed probe light pulse. We describe some of the challenges involved in the construction and operation of this high-precision metrology apparatus and present some preliminary results.

An electron in liquid helium forces open a cavity referred as an electron bubble. These objects have been studied in many past experiments. It has been discovered that under certain conditions other negatively charged objects can be produced but the nature of these “exotic ions” is not understood. We have made a series of experiments to measure the mobility of these objects, and have detected at least 18 ions with different mobility. We also find strong evidence that in addition to these objects there are ions present which have a continuous distribution of mobility. We then describe experiments in which we attempt to produce exotic ions by optically exciting an electron bubble to a higher energy quantum state. To within the sensitivity of the experiment, we have not been able to detect any exotic ions produced as a result of this process. We discuss three possible explanations for the exotic ions, namely impurities, negative helium ions, and fission of the electron wave function. Each of these explanations has difficulties but as far as we can see, of the three, fission is the only plausible explanation of the results which have been obtained.

The goal of this grant was the development of a new type of scanning acoustic microscope for nanometer resolution ultrasound imaging, based on ultrafast optoacoustics (>GHz). In the microscope, subpicosecond laser pulses was used to generate and detect very high frequency ultrasound with nanometer wavelengths. We report here on the outcome of the 3-year DOE/BES grant which involved the design, multifaceted construction, and proof-of-concept demonstration of an instrument that can be used for quantitative imaging of nanoscale material features – including features that may be buried so as to be inaccessible to conventional lightwave or electron microscopies. The research program has produced a prototype scanning optoacoustic microscope which, in combination with advanced computational modeling, is a system-level new technology (two patents issues) which offer novel means for precision metrology of material nanostructures, particularly those that are of contemporary interest to the frontline micro- and optoelectronics device industry. For accomplishing the ambitious technical goals, the research roadmap was designed and implemented in two phases. In Phase I, we constructed a “non-focusing” optoacoustic microscope instrument (“POAM”), with nanometer vertical (z-) resolution, while limited to approximately 10 micrometer scale lateral recolution. The Phase I version of the instrument which was guided by extensive acoustic and optical numerical modeling of the basic underlying acoustic and optical physics, featured nanometer scale close loop positioning between the optoacoustic transducer element and a nanostructured material sample under investigation. In phase II, we implemented and demonstrated a scanning version of the instrument (“SOAM”) where incident acoustic energy is focused, and scanned on lateral (x-y) spatial scale in the 100 nm range as per the goals of the project. In so doing we developed advanced numerical simulations to provide computational models of the focusing of multi-GHz acoustic waves to the nanometer scale and innovated a series fabrication approaches for a new type of broadband high-frequency acoustic focusing microscope objective by applying methods on nanoimprinting and focused-ion beam techniques. In the following, the Phase I and Phase II instrument development is reported as Section II. The first segment of this section describes the POAM instrument and its development, while including much of the underlying ultrafast acoustic physics which is common to all of our work for this grant. Then, the science and engineering of the SOAM instrument is described, including the methods of fabricating new types of acoustic microlenses. The results section is followed by reports on publications (Section III), Participants (Section IV), and statement of full use of the allocated grant funds (Section V).