To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The NaCl(H2O)n complexes were formed and probed in liquid helium nanodroplets, and IR spectra were recorded for n = 1 → 4. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na+Cl− ion-pair. Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers.

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A model for debris clouds produced by impact of hypervelocity projectiles on multiplate structures Appl. Phys. Lett. 93, 211905 (2008) Examining the temporal evolution of hypervelocity impact phenomena via high-speed imaging and ultraviolet-visible emission spectroscopy The temporal evolution of a previously observed hypervelocity impact-induced vapor cloud [Mihaly et al., Int. J. Impact Eng. 62, 13 (2013)] was measured by simultaneously recording several full-field, near-IR images of the resulting emission using an OMA-V high-speed camera. A two-stage light-gas gun was used to accelerate 5 mg Nylon 6/6 right-cylinders to speeds between 5 km/s and 7 km/s to impact 1.5 mm thick 6061-T6 aluminum target plates. Complementary laserside-lighting [Mihaly et al., Int. J. Impact Eng. 62, 13 (2013); Proc. Eng. 58, 363 (2013)] and frontof-target (without laser illumination) images were also captured using a Cordin ultra-high-speed camera. The rapid expansion of the vapor cloud was observed to contain a bright, emitting exterior, and a darker, optically thick interior. The shape of this phenomenon was also observed to vary considerably between experiments due to extremely high-rate (>250 000 rpm) of tumbling of the cylindrical projectiles. Additionally, UV-vis emission spectra were simultaneously recorded to investigate the temporal evolution of the atomic and molecular composition of the up-range, impact-induced vapor plume. A PI-MAX3 high-speed camera coupled to an Acton spectrograph was utilized to capture the UV-vis spectra, which shows an overall peak in emission intensity between approximately 6-10 ls after impact trigger, corresponding to an increased quantity of emitting vapor/plasma passing through the spectrometer slit during this time period. The relative intensity of the numerous spectral bands was also observed to vary according to the exposure delay of the camera, indicating that the different atomic/molecular species exhibit a varied temporal evolution during the vapor cloud expansion. Higher resolution spectra yielded additional emission lines/bands that provide further evidence of interaction between fragmented projectile material and the 1 mmHg atmosphere inside the target chamber. A comparison of the data to down-range emission spectra also revealed differences in the relative intensities of the atomic/molecular composition of the observed vapor clouds. V C 2014 AIP Publishing LLC.

Infrared (IR) spectra of complexes between NaCl and methanol have been recorded for the first time. These complexes were formed in liquid helium nanodroplets by consecutive pick-up of NaCl and CH 3 OH molecules. For the smallest NaCl(CH 3 OH) n , complexes where n = 1−3, the IR data suggest that the lowest-energy isomer is the primary product in each case. The predominant contribution to the binding comes from ionic hydrogen bonds between the OH in each methanol molecule and the chloride ion in the NaCl, as established by the large red shift of the OH stretching bands compared with the isolated CH 3 OH molecule. For n ≥ 4, there is a dramatic shift from discrete vibrational bands to very broad absorption envelopes, suggesting a profound change in the structural landscape and, in particular, access to multiple low-energy isomers. ■ INTRODUCTIONAlkali halides (MX) are archetypal univalent salts. The crystalline structure of the solid is disrupted by water, and these salts readily dissolve to form separated ions in dilute aqueous solutions. If the solid represents one extreme, then the counterpart is an isolated MX molecule. How might this smallest entity of salt, a single MX molecule, behave in the presence of a small water cluster, (H 2 O) n ? This problem can be addressed in experiments by forming isolated MX(H 2 O) n complexes and then using techniques, such as spectroscopy, to determine their properties. Such studies offer insight into the interaction between the constituent ionic particles and the solvent and can be reinforced through quantum chemical calculations. Understanding these interactions and the balance between them is critical in being able to carry out accurate simulations of bulk solutions. An early infrared (IR) spectroscopic study of a NaCl/H 2 O mixture in an argon matrix provided a tentative assignment of vibrational bands of the NaCl(H 2 O) complex.2 The first detailed spectroscopic study of several different NaCl(H 2 O) n complexes (n = 1−3) was obtained by microwave spectroscopy, and this revealed important structural information. 3,4 We have recently shown that complexes between NaCl and water molecules can be formed inside liquid helium nanodroplets, and this made it possible to record the first IR spectra of several small NaCl(H 2 O) n complexes. 5 The spectra for n = 1−3 are consistent with structures in which an ionic hydrogen bond (IHB) is formed between a single OH group in each water molecule and the Cl − in the salt. Evidence for interwater hydrogen bonding only begins to appear for n ≥ 4.An alternative solvent to water is methanol. Most solid alkali halides will dissolve in methanol, but their solubility is far lower than that in water. 6,7 The reduced solubility of salts in methanol when compared with that in water is a consequence of the hydrophobic (CH 3 ) group in the former. It would therefore be interesting to investigate how this amphiphilic character affects the interactions between a single salt molecule and one or more methanol molecules. It is well-known that...

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to