Inverse Volcano: A New Molecule-Surface Interaction Phenomenon.

Explosive desorption of guest molecules embedded in amorphous solid water upon its crystallization is known as the "molecular volcano." Here, we describe an abrupt ejection of NH_{3} guest molecules from various molecular host films toward a Ru(0001) substrate upon heating, utilizing both temperature programmed contact potential difference and temperature programmed desorption measurements. NH_{3} molecules abruptly migrate toward the substrate due to either crystallization or desorption of the host molecules, following an "inverse volcano" process considered a highly probable phenomenon for dipolar guest molecules that strongly interact with the substrate.

Chemical Reactivity of Strongly Interacting, Hydrogen-Bond-Forming Molecules Following 193 nm Photon Irradiation: Methanol in Amorphous Solid Water at Low Temperatures.

Mixtures of methanol and amorphous solid water (ASW) ices are observed in the interstellar medium (ISM), where they are subject to irradiation by UV photons and bombardment by charged particles. The charged particles, if at high enough density, induce a local electric field in the ice film that potentially affects the photochemistry of these ices. When CD3OD@ASW ices grown at 38 K on a Ru(0001) substrate are irradiated by 193 nm (6.4 eV) photons, products such as HD, D2, CO, and CO2 are formed in large abundances relative to the initial amount of CD3OD. Other molecules such as D2O, CD4, acetaldehyde, and ethanol and/or dimethyl ether are also observed, but in smaller relative abundances. The reactivity cross sections range from (2.6 ± 0.3) × 10-21 to (3.8 ± 0.3) × 10-25 cm2/photon. The main products are formed through two competing mechanisms: direct photodissociation of methanol and water and dissociative electron attachment (DEA) by photoelectrons ejected from the Ru(0001) substrate. An electric field of 2 × 108 V/m generated within the ASW film during Ne+ ions bombardment is apparently not strong enough to affect the relative abundances (selectivity) of the photochemical products observed in this study.

Distribution of Weakly Interacting Atoms and Molecules in Low-Temperature Amorphous Solid Water.

Understanding the distribution and mixing of atoms and molecules in amorphous solid water (ASW) at low temperatures is relevant to the exploration of the astrochemical environment in the interstellar medium (ISM) that leads to the formation of new complex molecules. In this study, a combination of temperature programmed desorption (ΔP-TPD) experiments and Ne+ ion sputtering is used to determine the extent of mixing and distribution of guest atoms and molecules within thin ASW films deposited at 35 K on a Ru(0001) substrate, prior to sputtering. The mixing of krypton atoms and methyl chloride molecules within thin ASW films is directed by the physical properties of the respective species and the nature of their interaction with the host water molecules. While the Kr-H2O interaction may be described as a weak van der Waals attraction, the CD3Cl-H2O interaction can be characterized as weakly hydrophobic in nature. This leads to differences in the level of homogeneity in mixing and distribution of the guest species in the ASW film. Both krypton atoms and methyl chloride molecules reveal a propensity to migrate toward the ASW-vacuum interface. The krypton atoms migrate through both diffusion and displacement by incoming H2O molecules, while the methyl chloride molecules tend to move toward the vacuum interface primarily via displacement. This behavior results in more homogeneous mixing of Kr in ASW at 35 K compared to the dipole moment containing molecule CD3Cl. As a general outcome of our study, it is observed that mixing in ASW at low temperatures is more homogeneous when the guest atom/molecule is inert and does not possess a constant dipole moment.

Formamidinium Halide Perovskite and Carbon Nitride Thin Films Enhance Photoreactivity under Visible Light Excitation.

Photochemical and photocatalytic activity of adsorbates on surfaces is strongly dependent on the nature of a given substrate and its resonant absorption of the (visible) light excitation. An observation is reported here of the visible light photochemical response of formamidinium lead bromide (FAPbBr3) halide perovskite and carbon nitride (CN) thin-film materials (deposited on a SiO2/Si(100) substrate), both of which are known for their photovoltaic and photocatalytic properties. The goal of this study was to investigate the role of the substrate in the photochemical reactivity of an identical probe molecule, ethyl chloride (EC), when excited by pulsed 532 nm laser under ultrahigh vacuum (UHV) conditions. Postirradiation temperature-programmed desorption (TPD) measurements have indicated that the C-Cl bond dissociates following the visible light excitation to form surface-bound fragments that react upon surface heating to form primarily ethane and butane. Temperature-dependent photoluminescence (PL) spectra of the FAPbBr3 films were recorded and decay lifetimes were measured, revealing a correlation between length of PL decay and the photoreactivity yield. We conclude that the FAPbBr3 material with its absorption spectrum in resonance with visible light excitation (532 nm) and longer PL lifetime leads to three times faster (larger cross-section) photoproduct formation compared with that on the CN substrate. These results contrast the behavior under ambient conditions where the CN materials are photochemically superior due, primarily, to their stability within humid environments.

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water.

UV photons and low-energy electrons play an important role in the evolution of various molecules in the interstellar medium (ISM). Here, we examined the product molecule formation as a result of irradiation of 193 nm photons and 6.4 eV electrons (same energy under identical laboratory conditions) on D2O|CH4 + ND3|D2O sandwiched films deposited on Ru(0001) substrate at 25 K in ultrahigh vacuum as a model for processes in the ISM. Temperature-programmed desorption spectra performed following the irradiation revealed the signature of hydrazine and formamide product molecules. These molecules were, however, formed exclusively following the photons' irradiation. These results were compared with the products obtained from a D2O|CH4|D2O sample without ammonia, where deuterated formaldehyde was the dominant product, formed also by photons only. Our results indicate that the photon-induced activation of the cofrozen molecules within D2O occurs via direct (partial) dissociation of the host and embedded molecules, followed by sample annealing. The electron-induced activation occurs through a direct dissociative electron attachment mechanism. The results presented here suggest possible pathways to generate various C-N, C-O, C-C, N-O, and N-H bonds containing molecules in the ISM.

Spontaneous polarization of thick solid ammonia films.

Ammonia molecules have an important role in the radiation-induced chemistry that occurs on grains in the cold interstellar medium and leads to the formation of nitrogen containing molecules. Such grains and surfaces are primarily covered by water ices; however, these conditions allow the growth of solid ammonia films as well. Yet, solid ammonia know-how lags the vast volume of research that has been invested in the case of films of its "sibling" molecule water, which, in the porous amorphous phase, spontaneously form polar films and can cage coadsorbed molecules within their hydrogen-bonded matrix. Here, we report on the effect of growth temperature on the spontaneous polarization of solid ammonia films (leading to internal electric fields of ∼105 V/m) within the range of 30 K-85 K on top of a Ru(0001) substrate under ultra-high vacuum conditions. The effect of growth temperature on the films' depolarization upon annealing was recorded as well. By demonstrating the ability of ammonia to cage coadsorbed molecules, as water does, we show that temperature-programmed contact potential difference measurements performed by a Kelvin probe and especially their temperature derivative can track film reorganization/reconstruction and crystallization at temperatures significantly lower than the film desorption.

The role of thermal history on spontaneous polarization and phase transitions of amorphous solid water films studied by contact potential difference measurements.

Monitoring thermal processes occurring in molecular films on surfaces can provide insights into physical events such as morphology changes and phase transitions. Here, we demonstrate that temperature-programmed contact potential difference (TP-∆CPD) measurements employed by a Kelvin probe under ultrahigh vacuum conditions and their temperature derivative can track films' restructure and crystallization occurring in amorphous solid water (ASW) at temperatures well below the onset of film desorption. The effects of growth temperature and films' thickness on the spontaneous polarization that develops within ASW films grown at 33 K-120 K on top of a Ru(0001) substrate are reported. Electric fields of ∼106 V/m are developed within the ASW films despite low average levels of molecular dipole alignment (<0.01%) normal to the substrate plane. Upon annealing, an irreversible morphology-dependent depolarization has been recorded, indicating that the ASW films keep a "memory" of their thermal history. We demonstrate that TP-∆CPD measurements can track the collapse of the porous structure at temperatures above the growth and the ASW-ice Ic and ASW-ice Ih transitions at 131 K and 157 K, respectively. These observations have interesting implications for physical and chemical processes that take place at the interstellar medium such as planetary formation and photon- and electron-induced synthesis of new molecules.

Highly efficient photoinduced desorption of N2O and CO from porous silicon.

Photoinduced desorption (PID) of N(2)O and CO from porous silicon (PSi) samples is reported. Both adsorbates exhibit unusually large cross sections for PID at 193 nm, up to 10(-15) cm(2), 2-3 orders of magnitude larger than the literature values for similar processes on flat Si. Under this UV irradiation, N(2)O molecules undergo photodissociation (a competing process leading to surface oxidation) with a cross section that is 2 orders of magnitude smaller than photodesorption. In the case of CO desorption is the exclusive photodepletion mechanism. PID efficiency decreases with increasing CO coverage suggesting PID hindrance by interactions among the desorbing CO molecules leading to re-adsorption at higher coverage. The wavelength and fluence dependence measurements exclude the possibility of laser induced thermal desorption for both adsorbates. The proposed mechanism for this phenomenon is desorption induced by hot electron transfer from the substrate to the adsorbate. Enhanced lifetime of transient negative adsorbate due to stabilization by localized holes on PSi nanotips can explain the observed abnormally large PID efficiency on top of porous silicon.