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Physics researchers publish study detailing discovery of new electron emission process

Varghese Chirayath, left, and Alex Fairchild stand in the UTA Positron Lab with the advanced, variable-energy positron beam they helped to build
Varghese Chirayath, left, and Alex Fairchild stand in the UTA Positron Lab with the advanced, variable-energy positron beam they helped to build

A team of physics researchers at The University of Texas at Arlington has published a study detailing the experimental discovery of previously unexplored low-energy electron emission processes which could have a significant impact on photodynamic cancer therapies.

The paper, titled “Direct evidence for low-energy electron emission following O LVV Auger transitions at oxide surfaces”, was published in the Nature journal Scientific Reports. Lead author was Alex Fairchild, former Ph.D. student and current postdoctoral scholar in the UTA Positron Lab.

Co-authors include Positron Lab group members Varghese Chirayath, assistant professor of research; Randall Gladen, postdoctoral researcher; Ali Koymen, UTA professor of physics; and Alex Weiss, professor and chair of the UTA Department of Physics; as well as Philip Sterne of Lawrence Livermore National Laboratory in Livermore, California.

“This publication provides direct evidence for a quantum mechanical process, observed unambiguously for the first time, in which low-energy electron emission occurs following an Auger transition initiated in the 2s level of oxide materials,” Fairchild said. “The work builds on previous investigations of a similar Auger process that occurs entirely within the valence band of graphene.”

An Auger transition is a type of quantum mechanical transition in which the electrons in an excited atom rearrange themselves in such a way that an electron is emitted from the atom.

“These Auger processes are important because the low-energy electrons emitted in these processes can lead to the production of cytotoxic free radicals when they occur in cells,” Fairchild said. “They are also important in understanding the physical mechanisms behind the therapeutic effects of photodynamic therapies (PDT). PDT cancer therapies focus on using special drugs, called photosensitizing agents, along with light to kill cancer cells.

“Many of these therapies rely principally on the quantum mechanical interaction between the light and the photosensitizing agent which ultimately produces reactive oxygen species that can kill cancer cells locally.”

Electrons emitted as a result of Auger transitions cause considerable cell damage that is highly localized, thus avoiding the more widespread damage associated with chemotherapy, the researchers found. The ability to more specifically target cancer cells while leaving surrounding cells unharmed could lead to major breakthroughs in cancer treatment. Additionally, atoms that have undergone Auger transitions are left highly energized and can act as active sites for further chemical reactions with surrounding water molecules, producing reactive oxygen species.

“We employed a state-of-the-art surface spectroscopic technique developed here at UTA that utilizes positrons – the antiparticle of electrons – to discover previously unidentified low-energy electron emission processes,” Chirayath said. “Our work represents the first direct investigations of the emission of low-energy electrons as a result of O LVV Auger transitions in condensed matter systems and may be of significant importance in studies of Auger-stimulated ion desorption and photodynamic cancer therapies.”

The study’s results indicate that the low-energy electron emission following the Auger decay of O 2s hole is nearly as efficient as electron emission following the relaxation of O 1s holes in titanium dioxide (TiO2). Since TiO2 is widely used in biomedical applications, and low-energy electrons play a crucial role in the early stages of DNA radiolysis (molecular decomposition of a substance by ionizing radiation) through dissociative electron attachment, it is essential that the ways in which low-energy electrons are produced in TiO2 be identified and understood, the team wrote. In particular, TiO2 nanoparticles have recently been used in photo-assisted cancer therapies which utilize the emission of low-energy electrons from TiO2 to produce reactive oxygen species.

“There is a growing need for more targeted cancer treatments that can reduce or eliminate the dangers associated with surgery and chemotherapy,” Fairchild said. “Our group is planning more experiments to further our understanding of these difficult-to-observe Auger process and is pursing collaborations with medical-biophysics groups to apply our findings into advancing therapeutic cancer modalities.”

The paper builds on previously published research on the Auger electron emission process by members of the Positron Lab and various collaborators. That paper, titled “Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation,” was published in the July 2017 edition of Nature Communications, with Chirayath as lead author.

“Both of these papers deal with the experimental discovery of hitherto unexplored low-energy electron emission processes,” Chirayath said. “These processes will have significant impact on the understanding of the anti-cancerous properties of several photodynamic therapies as well as in the understanding of phenomena like radiation-induced DNA damage, hot electron emission and hole multiplication.”

The studies were supported by funding from the Welch Foundation.

The Positron Lab also recently completed construction of an advanced, variable-energy positron beam spectrometer. The device, which took seven years to build, is an upgraded version of a previous system constructed at UTA and can measure the energies of multiple “positron-induced” electrons in coincidence with the Doppler-shifted gamma photon resulting from the annihilation of the correlated positron. Chirayath led the design and construction of the device under the supervision of Weiss and Koymen.

The positron beam project was funded by a grant to Weiss and Koymen from the National Science Foundation’s Major Research Instrumentation (MRI) Program. It is detailed in an article titled “A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon,” published in the AIP journal Review of Scientific Instruments, with Chirayath as lead author. Co-authors included Fairchild, Koymen, Weiss, and 11 others who were involved in the project.

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