Atomic electron tomography (AET) enables the determination of 3D atomic structures by acquiring a sequence of 2D TEM projection measurements of a particle and then computationally solving for its underlying 3D representation. AET is a challenging and labor-intensive experiment! In this talk, we offer two computational methods to alleviate these challenges and make the reconstruction procedure more robust.

First, we describe a method that solves directly for the locations and properties of individual atoms from projection measurements. This is in contrast to classical tomography algorithms that first solve for a volume and then extract the atomic structure. We parameterize a particle as a collection of atoms each represented by a Gaussian. We show that this parameterization imparts a strong prior on the reconstruction that avoids physically implausible artifacts often present in volumetric reconstructions due to noise and missing wedge effects. These reconstruction improvements further translate to higher fidelity atomic structure identification.

Second, we tackle the problem of carbon contamination: over the time it takes to capture the tomographic projection series, amorphous carbon often accumulates on the sample surface, making it difficult to reconstruct the underlying static sample and causing laboriously collected datasets to be discarded. We use implicit neural representations, which compactly represent large 3D+time data cubes and impose flexible space-time priors, to directly model and solve for the 3D temporal dynamics of the sample  This allows us to computationally remove the contamination and recover an uncorrupted reconstruction of the static sample of interest. 

NASA science missions are often complex systems of systems, involving various stakeholders, including the United States’ Congress. To ensure a clear and concise communication of expectations, requirements, and constraints, NASA has adopted the Science Traceability Matrix (STM). STM provides a logical flow from the decadal survey to science goals and objectives, mission and instrument requirements, and data products. STM serves as a summary of what science will be achieved and how it will be achieved, with a clear definition of what mission success will look like. In this seminar, I will present the STM from the Parker Solar Probe (PSP), including requirements relating to the plasma instrument for which I am a co-investigator. I will describe how our team used the STM to map the mission’s top-level requirements to mission success criteria and helped to eliminate any single point of failure that could end the mission prematurely. I will then present my own research on magnetic switchbacks in the PSP magnetic and plasma observations and their role in solar wind acceleration and heating. I will conclude the seminar by discussing how my research on the temporal evolution of switchbacks in the solar wind led to a new STM, and helped to chart a multidisciplinary path to designing a ground-breaking science mission concept, titled Space Weather Investigation Frontier (SWIFT), with the potential to improve space weather forecasting lead times by up to 40%.

Linear spectroscopy is used to learn about transitions from the ground states of systems. Nonlinear spectroscopies, such as transient absorption (TA) spectroscopy, first excite the system and then probe after some time delay, giving dynamical information about excited states and spectral information about their excitations. If the pump pulses are strong enough, then some molecules are excited multiple times, and the signal has contributions from singly excited molecules mixed with those from multiply excited molecules. Such mixed signals are hard to interpret, so TA spectra are often acquired with a sufficiently weak pump pulse that the higher-order contributions can be neglected. But the signal-to-noise ratio becomes worse when the pump is weak.

I will describe a general method to systematically separate spectroscopic orders of response by acquiring spectra with multiple pump-pulse energies, with applications in many forms of spectroscopy. This method allows acquisitions with increased pump intensities that improve signal-to-noise while systematically removing contaminations from higher-order processes. High-order responses have not previously been separable, and I will give examples of the spectral and dynamical information that they can contain, from exciton-exciton-annihilation kinetics to revealing masked signals in congested spectra. I will show experimental demonstrations from TA and two-dimensional electronic spectroscopy. I will show how to choose the pulse intensities to give the best extractions of response orders, given the noise present.

TBA

From planting crops to making trains run efficiently, clocks have been an important tool throughout most of human history. Atomic clocks, based on quantum-mechanically-defined transitions in atoms, are currently the most accurate realizations of the second and underlie important technologies such as the global positioning system (GPS) and high-speed communications. This lecture will describe how atomic clocks work and their history, with a focus on compact clocks and the applications in which they are used.

The Department of Biochemistry invites professors and scientists from other universities and institutes to present seminars at the University of Colorado Boulder throughout the academic year. These seminars provide an opportunity for faculty and students to learn about exciting current research.

TBA

Solar and stellar flares are explosive phenomena in which magnetic energy stored around starspots is suddenly released through magnetic reconnection. The radiation emitted during flares covers a broad range of wavelengths from radio to X-rays, each tracing different aspects of the flare process. In X-rays, the emission arises from hot thermal plasma heated by nonthermal electrons that travel upward from the chromosphere into the corona.

RS CVn-type binaries and protostars exhibit giant flares that are several orders of magnitude more energetic than those on the Sun, and it remains unclear whether their underlying physical processes are fundamentally the same as in solar flares. Furthermore, the impact of high-energy radiation from flares on exoplanetary environments has attracted increasing attention. In particular, X-ray emission from protostars has recently drawn significant interest from the star and planet formation community in the context of X-ray–driven chemistry, as it may strongly affect the surrounding protoplanetary disks.

In X-ray observations of stellar flares, NICER — with its combination of large effective area and high observational agility — has played a key role. Furthermore, XRISM, which has an order of magnitude higher energy resolution than NICER, was launched in 2023. In this talk, I focus on the Fe K-shell emission lines in X-ray, which are covered by both of these satellites, and introduce the physics of stellar flares and its effect on exoplanetary environments that can be inferred from their line intensities and structures.

The Department of Biochemistry invites professors and scientists from other universities and institutes to present seminars at the University of Colorado Boulder throughout the academic year. These seminars provide an opportunity for faculty and students to learn about exciting current research.

The Department of Biochemistry invites professors and scientists from other universities and institutes to present seminars at the University of Colorado Boulder throughout the academic year. These seminars provide an opportunity for faculty and students to learn about exciting current research.

The Department of Biochemistry invites professors and scientists from other universities and institutes to present seminars at the University of Colorado Boulder throughout the academic year. These seminars provide an opportunity for faculty and students to learn about exciting current research.

The Department of Biochemistry invites professors and scientists from other universities and institutes to present seminars at the University of Colorado Boulder throughout the academic year. These seminars provide an opportunity for faculty and students to learn about exciting current research.