Showing papers by "Nicholas Winograd published in 2001"

Explosive Boiling of Water Films Adjacent to Heated Surfaces: A Microscopic Description †

Abstract: Molecular dynamics simulations are used to investigate the separation of water films adjacent to a hot metal surface. The simulations clearly show that the water layers nearest the surface overheat and undergo explosive boiling. For thick films, the expansion of the vaporized molecules near the surface forces the outer water layers to move away from the surface. These results are of interest for mass spectrometry of biological molecules, steam cleaning of surfaces, and medical procedures.

Understanding collision cascades in molecular solids

Abstract: This paper describes simulations of the sputtering of a molecular solid that uses a reactive potential with both covalent bonding and van der Waals interactions. Recently, the adaptive intermolecular REBO (AIREBO) potential has been developed, which incorporates intermolecular interactions in a manner that maintains the reactivity of the original reactive empirical bond-order (REBO) potential. Preliminary simulations of the keV bombardment of a molecular solid have been performed using the AIREBO potential. Molecules that are initially struck by the bombarding particle break into fragments. The fragments initiate molecular collision cascades leading to the ejection of intact molecules and molecular fragments from the surface.

Thickness effects of water overlayer on its explosive evaporation at heated metal surfaces

Abstract: Molecular dynamics (MD) simulations have been employed to investigate the effect of the thickness of a water overlayer on the character of its ejection from a heated Au surface. The simulations are performed for five systems differing in the thickness of the water overlayer which was adsorbed on a metal substrate heated to 1000 K. For each system, an explosive evaporation occurs in the part of the water film adjacent to the metal surface and the upper part of the film is pushed off by the generated force. The average maximum temperature of the water film decreases as the film thickness increases. In contrast, the temperature achieved by the fast cooling due to the explosive evaporation shows an inverse trend. The significance of these model calculations to matrix-assisted laser desorption and ionization (MALDI) mass spectrometry is discussed.

Emission of neutral molecules from ion-bombarded thiol self-assembled monolayers

Abstract: We have investigated ion-stimulated desorption of neutral molecules emitted from 8 keV Ar + ion-bombarded self-assembled monolayers (SAMs) of phenethyl mercaptan (PEM) C 6 H 5 CH 2 CH 2 –SH and 2-(4 ′ -methyl-biphenyl-4yl)-ethanethiol (BP2) CH 3 C 6 H 4 C 6 H 4 CH 2 CH 2 –SH deposited on Au(1 1 1) substrate. Neutral molecules were detected by laser postionization mass spectrometry. Only molecular fragments were detected from ion-bombarded systems. The mass spectra obtained for sputtered and gas phase fragments indicate that molecules recorded during ion bombardment were indeed emitted from the surface and were not the result of photofragmentation induced by the ionizing laser beam. From experimentally obtained time-of-flight (TOF) distributions, it was determined that the majority of desorbed neutral molecules leave the surface with very low translational energies. As the sample temperature is reduced, the distributions become broader and shift to longer flight times. The shift is more pronounced for molecules from BP2 and increases with the mass of the recorded molecular fragment. We postulate that the emission of molecules is initiated by processes which gently break molecular bonds (e.g., chemical reactions, secondary electrons). The formed fragments are loosely bound to the surface and can be removed by evaporation. At the investigated temperature range (170–350 K), the observed emission delay is attributed to the time required for the molecule to evaporate from the surface and is not influenced by the bond breaking rate.

Desorption of silver atoms from benzene-covered Ag(1 1 1) by energetic Ar+ bombardment

Abstract: Experiments have been conducted to gain insight into the processes of desorption of neutral species from surfaces covered with organic molecules due to bombardment with keV particles. The system is comprised of benzene molecules adsorbed onto Ag(1 1 1) and bombarded with 8 keV Ar+ ions. Molecular dynamics (MD) simulations of the same system have been performed. Results show that the presence of the benzene alters the yield, the kinetic energy distributions, and the angular distributions of the silver atoms. These changes of the desorption characteristics are the result of collisions between the Ag atoms and the benzene molecules adsorbed to the surface. As more benzene is adsorbed to the surface, the changes to the Ag atom desorption characteristics become more pronounced. The simulations reproduce the modifications to the Ag atom energy and angle distributions.