Static SIMS Studies of Reactions on Mimics of Polar Stratospheric Clouds II: Low-Temperature, Low-Pressure Interactions of Cl2 and Cl2O with Solid Ice Films

Effect of Hcl Addition and Temperature on the Heterogeneous Chemistryand Photochemistry of Clono2 Adsorbed on ice Crystals

The heterogeneous chemistry and photochemistry of chlorine nitrate adsorbed on HCl-doped crystals was studied mainly at 181 K and at 190 K. Under our experimental conditions the main gaseous products found at 181 K were Cl2O and Cl2, while at 190 K, Cl2O was mainly observed. At both temperatures a net enhancement in the rate of gaseous product formation was observed when light of wavelength longer than 350 nm entered the reactor. The obtained results in conditions where the ClONO2 concentration is higher than HCl concentration, and both are high enough to maintain saturation of the surface, showed that the rate of gaseous products formation at 181 K depends on the reactive species adsorbed while at 190 K, it is independence of them. The implications of these findings to the polar stratospheric chemistry are briefly discussed.

Dynamic Reactive Ionization with Cluster Secondary Ion Mass Spectrometry

Gas cluster ion beams (GCIB) have been tuned to enhance secondary ion yields by doping small gas molecules such as CH4, CO2, and O2 into an Ar cluster projectile, Arn  + (n = 1000–10,000) to form a mixed cluster. The ‘tailored beam’ has the potential to expand the application of secondary ion mass spectrometry for two- and three-dimensional molecular specific imaging. Here, we examine the possibility of further enhancing the ionization by doping HCl into the Ar cluster. Water deposited on the target surface facilitates the dissociation of HCl. This concerted effect, occurring only at the impact site of the cluster, arises since the HCl is chemically induced to ionize to H+ and Cl– , allowing improved protonation of neutral molecular species. This hypothesis is confirmed by depth profiling through a trehalose thin film exposed to D2O vapor, resulting in ~20-fold increase in protonated molecules. The results show that it is possible to dynamically maintain optimum ionization conditions during depth profiling by proper adjustment of the water vapor pressure. H–D exchange in the trehalose molecule M was monitored upon deposition of D2O on the target surface, leading to the observation of [Mn* + H]+ or [Mn* + D]+ ions, where n = 1–8 hydrogen atoms in the trehalose molecule M have been replaced by deuterium. In general, we discuss the role of surface chemistry and dynamic reactive ionization of organic molecules in increasing the secondary ion yield.

Probing molecular solids with low-energy ions

Ion/surface collisions in the ultralow- to low-energy (1-100-eV) window represent an excellent technique for investigation of the properties of condensed molecular solids at low temperatures. For example, this technique has revealed the unique physical and chemical processes that occur on the surface of ice, versus the liquid and vapor phases of water. Such instrument-dependent research, which is usually performed with spectroscopy and mass spectrometry, has led to new directions in studies of molecular materials. In this review, we discuss some interesting results and highlight recent developments in the area. We hope that access to the study of molecular solids with extreme surface specificity, as described here, will encourage investigators to explore new areas of research, some of which are outlined in this review.

Label free biochemical 2D and 3D imaging using secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides a method for the detection of native and exogenous compounds in biological samples on a cellular scale. Through the development of novel ion beams the amount of molecular signal available from the sample surface has been increased. Through the introduction of polyatomic ion beams, particularly C(60), ToF-SIMS can now be used to monitor molecular signals as a function of depth as the sample is eroded thus proving the ability to generate 3D molecular images. Here we describe how this new capability has led to the development of novel instrumentation for 3D molecular imaging while also highlighting the importance of sample preparation and discuss the challenges that still need to be overcome to maximise the impact of the technique.

Chlorine Interactions with Water Ice Studied by Molecular Beam Techniques

The kinetics of chlorine interactions with ice at temperatures between 103 and 165 K have been studied using molecular beam techniques. The Cl(2) trapping probability is found to be unity at thermal incident energies, and trapping is followed by rapid desorption. The residence time on the surface is less than 25 microg at temperatures above 135 K and approaches 1 s around 100 K. Rate constants for desorption are determined for temperatures below 135 K. The desorption kinetics follow the Arrhenius equation, and activation energies of 0.24 +/- 0.03 and 0.31 +/- 0.01 eV, with corresponding preexponential factors of 10(12.08+/-1.19) and 10(16.52+/-0.38) s(-1), are determined. At least two different Cl(2) binding sites are concluded to exist on the ice surface. The observed activation energies are likely to be the Cl(2)-ice binding energies for these states, and the Cl(2)-surface interactions are concluded to be stronger than earlier theoretical estimates. The surface coverage of Cl(2) on ice under stratospheric conditions is estimated to be negligible, in agreement with earlier work.

Identification of surface molecular hydrates on solid sulfuric acid films

Infrared spectroscopic and secondary ion mass spectrometric studies reveal the presence of a stable molecular hydrate on the surface of condensed thin films of ionic sulfuric acid hydrates. This surface species is observed to play a role in the interaction of ammonia, reacting rapidly until the material is depleted. A slower, continuous process is also observed, attributed to a diffusion-limited reaction between incoming NH3 and H3O+ located at or near the surface.