X-ray photoelectron spectroscopy of amorphous ice

Core-excitation of water ice releases many different molecules and ions in the gas phase. Studying these desorbed species and the underlying mechanisms can provide useful information on the effects of X-ray irradiation in ice. We report a detailed study of the X-ray induced desorption of a number of neutral, cationic and anionic species from amorphous solid water. We discuss the desorption mechanisms, and the relative contributions of Auger and secondary electrons (X-ray induced Electron Stimulated Desorption) and initial excitation (direct desorption) as well as the role of photochemistry. Anions are shown to desorb not just through processes linked with secondary electrons but also through direct dissociation of the core-excited molecule. The desorption spectra of oxygen ions (O + , OH + , H2O + , O − , OH − ) give a new perspective on their previously reported very low desorption yields for most types of irradiation of water, showing that they mostly originate from the dissociation of photoproducts such as H2O2.

Section: Desorption Of Protonated Clustersmentioning

confidence: 58%

Desorption of neutrals, cations, and anions from core-excited amorphous solid water

Dupuy

Féraud

Bertin

et al. 2020

“…The UV photon energy used in the present experiment (10-11 eV) is lower than the vertical ionization energy of water molecules in gas phase (12.6 eV), but greater than the photoionization threshold of amorphous ice (∼9 eV) observed in photoelectron emission spectroscopic studies. 52 Therefore, we consider it possible that a similar direct photoionization process occurs in the present ice samples. In this case, the H 3 O + forming process can be the photoionization of water molecules to H 2 O + in the ice (Reaction (R3)) followed by proton transfer from H The photoelectrons generated in the first step may initially be trapped in the ice film and might then participate in subsequent reactions in the ice, or they may be ejected from the sample into vacuum or a Ru substrate.…”

Section: Discussionmentioning

confidence: 86%

Metastable hydronium ions in UV-irradiated ice

Moon

Kang

2012

We show that the irradiation of UV light (10-11 eV) onto an ice film produces metastable hydronium (H(3)O(+)) ions in the ice at low temperatures (53-140 K). Evidence of the presence of metastable hydronium ions was obtained by experiments involving adsorption of methylamine onto UV-irradiated ice films and hydrogen-deuterium (H∕D) isotopic exchange reaction. The methylamine adsorption experiments showed that photogenerated H(3)O(+) species transferred a proton to the methylamine arriving at the ice surface, thus producing the methyl ammonium ion, which was detected by low energy sputtering method. The H(3)O(+) species induced the H∕D exchange of water, which was monitored through the detection of water isotopomers on the surface by using the Cs(+) reactive ion scattering method. Thermal and temporal stabilities of H(3)O(+) and its proton migration activity were examined. The lifetime of the hydronium ions in the amorphized ice was greater than 1 h at ∼53 K and decreased to ∼5 min at 140 K. Interestingly, a small portion of hydronium ions survived for an extraordinarily long time in the ice, even at 140 K. The average migration distance of protons released from H(3)O(+) in the ice was estimated to be about two water molecules at ∼54 K and about six molecules at 100 K. These results indicate that UV-generated hydronium ions can be efficiently stabilized in low-temperature ice. Such metastable hydronium ions may play a significant role in the acid-base chemistry of ice particles in interstellar clouds.

“…14,22 Low energy ions neutralize at the surface, resulting in positive holes (ions) in the solid. 20,21 Since the ionization energies for Xe and He (12.15 and 24.5 eV, respectively) 25 are greater than the ionization threshold for ice (∼11 eV), 26 the formation of hydronium is energetically favorable as 21,27 penetration depth (<20 nm). Effects of backscattered ions and sputtered ions are negligible in our experiments.…”

Section: A Dielectric Constant At 30 Kmentioning

confidence: 99%

“…Effects of backscattered ions and sputtered ions are negligible in our experiments. Secondary electrons are ejected by different mechanisms; 22 their energy is limited to a few eV due to the large gap (∼11 eV) 26 between the top of the valence band and the vacuum level of ASW. If the secondary electrons escape into vacuum, additional holes would result in the solid, near its surface (the electron escape depth is a few nm).…”

Section: A Dielectric Constant At 30 Kmentioning

confidence: 99%

Proton transport in ice at 30–140 K: Effects of porosity

Baragiola

2015

We examined the role of porosity, a crucial characteristic of amorphous solid water (ASW), on electrostatic charging and discharging of ASW films with 500 eV He(+) and Xe(+) ions, by measuring the surface potentials with a Kelvin probe. When a charged ASW film is heated, its surface potential decreases sharply, at temperatures that depend on the maximum temperature the film was once subject to. This sharp decrease of the surface potential is not due to a large thermally induced increase of the dielectric constant ε as proposed in other studies, since measurements of ε yielded a value of ∼3 below ∼100 K. Rather, the potential drop can be explained by the transport of the surface charge to the substrate, which depends on film porosity. We propose that the charge migrates along the walls of the pores within the ASW film, facilitated by the thermally induced reorientation of the incompletely coordinated molecules on the pore walls.