arXiv subjects

William T. P. Denman

Publications and source records attributed to William T. P. Denman.

5 recordsLinked to original sources

arXiv2026

Electron-Stimulated Desorption of D Atoms from Gibbsite (Al(OD)3) and D2O Ice: Energy and Temperature Dependence of Translational Energy Distributions

The electron-stimulated desorption (ESD) of neutral D atoms from gibbsite (\ce{Al(OD)3}) nanoplatelets and amorphous \ce{D2O} ice has been investigated using $2+1$ resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectroscopy in a high vacuum chamber at temperatures 15 and 300\,K. Electron irradiation at 540, 250, and 150\,eV produces similar translational energy distributions at $\sim$300\,K, with a dominant intermediate-temperature component ($T \sim 1500$--$2100$\,K). Cooling to 15\,K suppresses the D atom yield by approximately 50\% and removes the lowest-temperature (slowest) component. This decrease in yield is consistent with diminished hole mobility and restricted diffusion at cryogenic temperatures. Under identical conditions, \ce{D2O} amorphous solid water ice films produce approximately 20 times greater D atom signal than bare gibbsite, with significantly hotter translational distributions, reflecting the higher deuterium surface density and distinct bonding environments of bulk ice relative to the terminal hydroxyl groups on gibbsite. These results identify hole transport to terminal hydroxyl sites as the rate-limiting step for nonthermal D atom production and provide a mechanistic framework for understanding atomic hydrogen release from aluminum hydroxide phases relevant to radioactive waste storage at the Hanford Site.

physics.chem-ph↗

arXiv2026

The Fate of Frozen Carbonated Water at Europa-like Conditions

We present the results of experiments probing the retention of CO2 in crystalline water ice, frozen sodium chloride (NaCl) brines, and flash-frozen carbonated water using diffuse reflectance infrared spectroscopy. Characteristic absorptions alluding to the formation of clathrate hydrates in crystalline ices and frozen brines are observed. NaCl in frozen brines does not affect qualitatively affect the formation of clathrate hydrates. Generation and stability of clathrates in crystalline ice transiently subjected to pressure-temperature (P-T) conditions in the stability region is observed, despite conditions being unviable at the onset of freezing. Retention of CO2 in flash-frozen carbonated water is observed to be dependent on the temperature of the substrate during freezing. The state of CO2 retained in the resulting ices differs from clathrate hydrates, as inferred from the respective infrared spectra. Both mechanisms of CO2 retention are stable up to 140 K and under evacuated conditions. In the context of Europa, the P-T states traversed by the samples plausibly represent the typical conditions around endogenous CO2 if it is indeed transported from the subsurface ocean to the surface while being retained in ice/frozen brines and/or liquid emerging on the surface. However, the absorptions of CO2 in the laboratory infrared spectra do not match those detected on the leading side of Europa by the NIRSpec instrument on board JWST. Therefore, it is unlikely that the endogenous CO2 observed at the surface of Europa is sourced directly from the ocean, unless additional processes affect the observed bands of CO2 on Europa.

astro-ph.EP↗

arXiv2022

The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions

Europa's leading hemisphere chaos regions have a spectral feature at 450 nm that has been attributed to absorption by crystal defects in irradiated sodium chloride, known as F-centers. Some discrepancies exist between the laboratory data of irradiated sodium chloride and the observations, including a $\sim$10 nm shift in central wavelength of the F-center band and the lack of the prominent 720 nm absorption on Europa from M-centers, which result from the coalescence of pairs of F-centers. Here, we perform irradiation experiments on sodium chloride in an attempt to understand these discrepancies. We show that careful control of the temperature of the sample at a temperature of 120 K yields F-centers with an absorption wavelength comparable to that of Europa. In addition, we measure the effect of photobleaching -- the destruction of F-centers by photons -- and show that at the energetic particle and photon flux on Europa, an equilibrium will be reached where only a modest F-center absorption develops. The density of F-centers never reaches high enough values for the creation of secondary M-centers. Our experiments predict that F-centers grow during the night on Europa in the absence of photobleaching and then partially decay during the daytime. We show observations from the Hubble Space Telescope consistent with this prediction. All observations of the 450 nm F-center on Europa are now consistent with laboratory measurements of sodium chloride, confirming the presence of this salt on Europa.

astro-ph.EP↗

arXiv2022

A New UV Spectral Feature on Europa: Confirmation of NaCl in Leading-hemisphere Chaos Terrain

Recent visible-wavelength observations of Europa's surface obtained with the Hubble Space Telescope revealed the presence of an absorption feature near 450 nm that appears spatially correlated with leading-hemisphere chaos terrain. This feature was interpreted to reflect the presence of irradiated sodium chloride ultimately sourced from the interior. Here, we use ultraviolet spectra also collected with the Hubble Space Telescope to detect an additional previously unseen absorption near 230 nm, which spatially correlates with the 450 nm feature and with the same leading-hemisphere chaos terrain. We find that the new ultraviolet feature is also well-matched by irradiated sodium chloride at Europa-like conditions. Such confirmation of sodium chloride within geologically young regions has important implications for Europa's subsurface composition.

astro-ph.EP↗

arXiv2022

The mid-UV spectrum of irradiated NaCl at Europa-like conditions

Recent observations from the Hubble Space Telescope show a mid-UV absorption feature localized to leading hemisphere chaos regions on Europa. The same regions were previously found to have a visible absorption at 450 nm that was attributed to the presence of irradiated NaCl. The lack of any additional diagnostic absorptions for NaCl in the visible spectrum of these terrains made confirmation of this identification difficult. Here we use laboratory experiments to show that NaCl irradiated at Europa's surface temperatures develops an absorption at $\sim$220 nm consistent with the new detection in Europa's mid-UV spectrum, strongly supporting the NaCl identification. Irradiated NaCl in leading-hemisphere chaos terrain would suggest that sodium and chlorine are important components of Europa's subsurface ocean.

astro-ph.EP↗