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Laser-Induced Dissociation of the Monolayer of Adsorbed Methanol Molecules
摘要： Astronomical observations indicate a high abundance of methanol molecules in the gas phase of molecular-cloud dense cores, which cannot be explained by gas-phase chemical reactions only. A significant contribution to the methanol abundance should be provided by chemical reactions on the dust particle surface with subsequent desorption of the produced molecules into the gas phase. For the development and refinement of models involving these processes, laboratory studies of photo-induced processes occurring in the adsorbed material are necessary. In this paper, the experiment results of adsorbed methanol molecules are presented. A methanol molecule monolayer, physically adsorbed on fused silica surface cooled by liquid nitrogen (Т ～ 100 K), was irradiated in high vacuum by nanosecond pulses of an excimer KrF laser with a fixed wavelength λ = 248 nm. The photodissociation products of three-photon laser excitation were recorded by a quadrupole mass spectrometer. Relative yields of photofragments H, OH, and CH3 were determined. Photolysis of partially deuterated CH3OH molecules has shown that hydrogen atoms can be ejected both from hydroxyl and methyl groups. In contrast to the isolated molecule photolysis in the gas phase and dissociation of the multilayer molecular coatings, photoexcitation of adsorbed methanol monolayer even in the energy region of 10 eV does not cause noticeable chemical transformations and does not lead to the formation of molecular components H2 and CH4. Due to existing astrochemical modeling problems, possible application methods of the obtained laboratory results are considered.
关键词： chemical reactions,astronomical observations,adsorbed monolayer,mass spectrometry,photo-induced processes,gas phase,dust particle surface,KrF laser,photodissociation,methanol
更新于2025-09-23 15:21:01
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Electrospray-Surface Enhanced Raman Spectroscopy (ES-SERS) for probing surface chemical compositions of atmospherically relevant particles
摘要： We present electrospray-surface enhanced Raman spectroscopy (ES-SERS) as a new approach to measuring the surface chemical compositions of atmospherically relevant particles. The surface-sensitive SERS is realized by electrospraying Ag nanoparticle aerosols over analyte particles. Spectral features at v(SO42-), v(C-H) and v(O-H) modes were observed from the normal Raman and SERS measurements of laboratory-generated supermicron particles of ammonium sulfate (AS), AS mixed with succinic acid (AS/SA) and AS mixed with sucrose (AS/sucrose). SERS measurements showed strong interaction (or chemisorption) between Ag nanoparticles and surface aqueous sulfate [SO42-] with [SO42-]AS/sucrose > [SO42-]AS/SA > [SO42-]AS. Enhanced spectra of the solid AS and AS/SA particles revealed the formation of surface-adsorbed water on their surfaces at 60% relative humidity. These observations of surface aqueous sulfate and adsorbed water demonstrate a possible role of surface-adsorbed water in facilitating the dissolution of sulfate from the bulk phase into its water layer(s). Submicron ambient aerosol particles collected in Hong Kong exhibited non-enhanced features of black carbon and enhanced features of sulfate and organic matter (carbonyl group), indicating an enrichment of sulfate and organic matter on the particle surface.
关键词： Electrospray-Surface Enhanced Raman Spectroscopy,succinic acid,atmospherically relevant particles,surface chemical compositions,ES-SERS,sucrose,Ag nanoparticles,surface-adsorbed water,organic matter,ammonium sulfate,black carbon
更新于2025-09-09 09:28:46
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The role of adsorbed water on TiO <sub/>2</sub> particles in the electrorheological effect
摘要： TiO2 particles prepared by the hydrolysis of titanium butoxide adsorb water molecules with different adsorption energies. The electrorheological (ER) ?uid composed of TiO2 particles behaves as the polar-molecule giant electrorheological (GER) ?uid at 25?C even after thermal treatment at 75?C. After thermal treatment at 125?C and 175?C, the water molecules with low adsorption energy are removed and the ER ?uid behaves as traditional ER ?uid at 25?C. However, at 175?C, the ER ?uid treated at 175?C behaves as a polar-molecule GER ?uid when the external electric ?eld is larger than 2 kV/mm. These results show that the water molecules with low adsorption energy act as polar molecules responsible for the GER effect and, with the help of thermal energy, water molecules with high adsorption energy can work as those with low adsorption energy, resulting in the GER effect.
关键词： TiO2 particles,electrorheological effect,polar-molecule GER ?uid,adsorbed water,thermal treatment
更新于2025-09-09 09:28:46
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The Sommerfeld ground-wave limit for a molecule adsorbed at a surface
摘要： Using a mid-infrared emission spectrometer based on a superconducting nanowire single-photon detector (SNSPD), we observe the dynamics of vibrational energy pooling of CO adsorbed at the surface of a NaCl crystal. After exciting a majority of the CO molecules to their first vibrationally excited state (v = 1), we observe infrared emission from states up to v = 27. Kinetic Monte Carlo simulations show that vibrational energy collects in a few CO molecules at the expense of those up to eight lattice sites away by selective excitation of NaCl’s transverse phonons. The vibrating CO molecules behave like classical oscillating dipoles, losing their energy to NaCl lattice-vibrations via the electromagnetic near-field. This is analogous to Sommerfeld’s description of the Earth’s influence on radio transmission by ground waves.
关键词： vibrational energy pooling,NaCl crystal,superconducting nanowire single-photon detector,electromagnetic near-field,Kinetic Monte Carlo simulations,Sommerfeld ground-wave limit,CO adsorbed
更新于2025-09-04 15:30:14