An investigation of morphology and transport in amorphous solid water via guest-host interactions

The effects of inserting energy in a buried stratum in amorphous solid water (ASW) films were investigated using pulsed 266 nm radiation. Material ejected from irradiated films was detected with time‐of‐flight mass spectrometry (TOFMS). A technique was developed using N₂O₄ in conjunction with focused UV radiation (∼1 J/cm²) that enabled facile introduction of energy in a spatially selective way via an electronic transition of N₂O₄. A variety of experiments were carried out explore the structural changes induced by the sudden addition of energy to ASW films, and an attempt was made to characterize the nature of transport within and above the surface of the film. ❧ Layered ASW/N₂O₄ films, up to 2800 monolayers (ML) thick, were grown on a MgO(100) substrate. All samples were grown at ∼100 K under ultra‐high vacuum conditions to produce porous, high quality films. Once deposited, the films were were irradiated with 266 nm radiation that was generated as the fourth harmonic of a Nd:YAG laser (10 ns pulses, reduced to 1 Hz) focused to a 0.3 mm beam waist. After a single laser pulse incident on the film, the N₂O₄ layer was converted to a hot fluid that heated the surrounding material. Heating of the film competes with cooling by the MgO(100) substrate, which acts as an efficient heat sink due to its high thermal conductivity (250 W/mK). Therefore, extreme pressure and heat gradients exist within the film upon radiation and the film cools quickly upon cessation of the laser pulse. ❧ Despite fast cooling of the film, laser‐heated N₂O₄ fluid, along with water monomer, was detected at times greater than 1 ms. This was due to a catastrophic structural change triggered by the temperature and pressure gradients, which resulted in the formation of fissures. The hot fluid of N₂O₄ and its photoproducts escaped to ultra‐high vacuum through these fissures, scraping the walls and removing H₂O molecules. Long flight times were attributed to collisions occurring above the surface of the film due to the high density of the escaping material and prevalence of fissures in the irradiated area. Fissures proved to be robust, with spectra from subsequent pulses on the same spot resembling spectra of exposed N₂O₄. ❧ This model will be explored further by implementing gold nanoparticles as fissure‐creators. Being able to form fissures of a known dimension and density within an ASW film would allow for more careful analysis of this system and would be the first demonstration of induced morphological changes in ASW that are relatively well‐defined. An outline of this technique is presented, along with preliminary results.