Collisions between gases and high vapor pressure liquids can be investigated by coupling narrow diameter liquid jets with gas-surface scattering experiments. In these initial studies, we monitor the scattering of Ar, Ne, and O 2 from liquid dodecane (C 12 H 26 , P vap = 0.1 Torr at 295 K) and from a reference liquid, squalane (C 30 H 62 , P vap = 10 −7 Torr). Collisions of Ar with a cylindrical squalane jet and a flat squalane film reveal similar scattering patterns despite differences in geometry. Further studies indicate that 50 kJ mol −1 Ne atoms scatter impulsively upon collision and transfer ∼55% of their energy to each liquid. Higher values are found for O 2 collisions, where the overall energy transfer is 70−75% of the 30 kJ mol −1 incident energy. These studies imply that hot gases readily transfer their excess translational energy to liquid alkanes in processes such as the heating and evaporation of fuel droplets. SECTION: Kinetics and Dynamics

We have monitored the speeds of evaporating helium atoms dissolved in liquid octane, isooctane, 1-methylnaphthalene, dodecane, squalane, ethylene glycol, and two jet fuels. In all cases, the average kinetic energies of the evaporating He atoms exceed the Maxwellian value of 2RT. The energies roughly track solvent surface tensions; this correlation may reflect the tighter packing and attractions of interfacial solvent molecules that restrict the gaps through which He atoms escape. Mixtures of dodecane, squalane, and 1-methylnaphthalene generate He evaporation energies that lie between the pure liquid values. We find, however, that He atoms evaporate from pure 1-methylnaphthalene with kinetic energies lower than expected based on its high surface tension, perhaps because the sideways packing of the aromatic rings provides more direct channels for the escaping He atoms. Additionally, He evaporates from two complex fuel mixtures, Jet A and JP-8, with nearly identical energies, implying that the extra additives in JP-8 do not segregate to the surface in ways that alter the dynamics of evaporation. ■ INTRODUCTIONHelium atom scattering is an enormously powerful tool to elucidate the surface structure and surface vibrations of crystalline solids, organic monolayers, and even polymer films. 1−5 The diffraction features that make this technique so useful are unfortunately lost for room-temperature liquids because of long-range disorder and extensive thermal motions that promote energy transfer between the He and surface atoms. However, it may still be possible to glean insights into interfacial motions and packing by replacing He atom scattering with He atom evaporation from liquids. 6 We continue our studies here by measuring the energy distributions of He atoms evaporating from a wide range of linear and branched hydrocarbon liquids, including mixtures, jet fuels, and ethylene glycol (as a model for a deicing additive). The experiments are performed by dissolving He atoms into each liquid, injecting the liquids into a vacuum as microjets, 7,8 and monitoring the velocities of the evaporating He atoms.The evaporation of He atoms dissolved in hydrocarbons, alcohols, and salty water appears to be special: these small and weakly attractive atoms evaporate at speeds that are faster than predicted by a Maxwell−Boltzmann (MB) distribution. This behavior stands in contrast to larger and more polarizable species such as Ar, N 2 , O 2 , H 2 O, CO 2 , HCl, and HNO 3 , which evaporate with flux-weighted average energies of 2RT liq consistent with a MB energy distribution for a liquid at temperature T liq . 6,9 We recently found that the He speed distributions depend on the nature of the solvent molecules, with average kinetic energies ranging from 1.14 × 2RT liq for He evaporation from dodecane to 1.7 × 2RT liq from a 7.5 M LiBr/ H 2 O solution. 6 Detailed balancing of the incoming and outgoing fluxes implies that He atoms must preferentially dissolve at higher kinetic energies as well. 10 In this timereversed picture, the attractive...

Zn­(O,S) thin films have a tunable band gap and are useful as conduction and valence band buffers in various types of solar cells. Previous growth of Zn­(O,S) thin films by atomic layer deposition (ALD) has utilized alternating cycles of ZnO ALD and ZnS ALD. Controlling the composition of the Zn­(O,S) alloys using alternating cycles is complicated because of an efficient exchange reaction between ZnO and gaseous H2S given by ZnOH* + H2S → ZnSH* + H2O. This facile exchange reaction leads to a higher than expected sulfur content in the Zn­(O,S) films. In this study, the effect of this exchange reaction on the composition of Zn­(O,S) films was examined by varying the reaction conditions. For growth using alternating cycles, the Zn­(O,S) film composition was strongly affected by the temperature and the size of the H2S exposure. An alternative method that avoids alternating cycles of ZnO ALD and ZnS ALD was also employed to grow Zn­(O,S) thin films. This alternative method uses codosing of H2O and H2S at 100 °C. Codosing allows the composition of the Zn­(O,S) film to be controlled by the mole fraction of the dosing mixture. The relative magnitudes of the exchange reaction rate (k 1) for ZnOH* + H2S → ZnSH* + H2O and the competing exchange reaction rate (k 2) for ZnSH* + H2O → ZnOH* + H2S could be estimated using this method. Through the study of the composition of the Zn­(O,S) films for different H2O and H2S partial pressures during codosing, the ratio of the exchange reaction rates, k 1/k 2, was determined to be 71–231. Band gaps were measured for the Zn­(O,S) thin films grown using the alternating cycle method and the codosing method. The band gaps could be produced with the most control by varying the mole fraction of H2S in the H2S/H2O codosing mixture.

Atomic and molecular solutes evaporate and dissolve by traversing an atomically thin boundary separating liquid and gas. Most solutes spend only short times in this interfacial region, making them difficult to observe. Experiments that monitor the velocities of evaporating species, however, can capture their final interactions with surface solvent molecules. We find that polarizable gases such as N2 and Ar evaporate from protic and hydrocarbon liquids with Maxwell-Boltzmann speed distributions. Surprisingly, the weakly interacting helium atom emerges from these liquids at high kinetic energies, exceeding the expected energy of evaporation from salty water by 70%. This super-Maxwellian evaporation implies in reverse that He atoms preferentially dissolve when they strike the surface at high energies, as if ballistically penetrating into the solvent. The evaporation energies increase with solvent surface tension, suggesting that He atoms require extra kinetic energy to navigate increasingly tortuous paths between surface molecules.

Atomic layer processing such as atomic layer deposition (ALD) and thermal atomic layer etching (ALE) is usually described in terms of sequential, self-limiting surface reactions. This picture for ALD and thermal ALE leaves out the possibility that the metal precursor in ALD and thermal ALE can also convert the surface material to another new material. This perspective introduces the previous evidence for conversion reactions in atomic layer processing based on a variety of studies, including Al2O3 ALD on ZnO, growth of Zn(O,S) alloys, “self-cleaning” of III-V semiconductor surfaces, and thermal ALE of ZnO and SiO2. The paper then focuses on the reaction of Al(CH3)3 [trimethylaluminum (TMA)] on ZnO as a model conversion system. A variety of techniques are utilized to monitor ZnO conversion to Al2O3 using TMA at 150 °C. These techniques include FTIR spectroscopy, quadrupole mass spectrometry (QMS), x-ray reflectivity (XRR), gravimetric analysis, x-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. The various studies focus on ZnO conversion to Al2O3 for both hydroxyl-terminated and ethyl-terminated ZnO substrates. FTIR studies observed the conversion of ZnO to Al2O3 and provided evidence that the conversion is self-limiting at higher TMA exposures. QMS studies identified the volatile reaction products during the TMA reaction with ZnO as CH4, C2H4, C2H6, and Zn(CH3)2. The CH4 reaction product preceded the appearance of the Zn(CH3)2 reaction product. XRR investigations determined that the thickness of the Al2O3 conversion layer on ZnO limits at ∼1.0 nm at 150 °C after larger TMA exposures. A gravimetric analysis of the conversion reaction on ZnO nanoparticles with a diameter of 10 nm displayed a percent mass loss of ∼49%. This mass loss is consistent with an Al2O3 shell of ∼1 nm on a ZnO core with a diameter of ∼6 nm. XPS studies revealed that ZnO ALD films with a thickness of 2 nm were almost completely converted to Al2O3 by large TMA exposures at 150 °C. QCM investigations then measured the mass changes for lower TMA exposures on hydroxyl-terminated and ethyl-terminated ZnO films. More mass loss was observed on ethyl-terminated ZnO films compared with hydroxyl-terminated films, because TMA does not have the possibility of reacting with hydroxyl groups on ethyl-terminated ZnO films. The mass losses also increased progressively with temperatures ranging from 100 to 225 °C on both hydroxyl-terminated and ethyl-terminated ZnO films. The perspective concludes with a discussion of the generality of conversion reactions in atomic layer processing.