Excess Translational Energy Research Articles

Understanding the Effect of an Amorphous Surface on the Ultrafast Dynamics of a Heterogeneous Photoinduced Reaction: CD3I Photoinduced Reaction on Amorphous Cerium Oxide Films.

In this work, to understand how an amorphous surface influences the dynamics of surface photoinduced reactions, pump-probe spectroscopy in conjunction with mass spectrometry is employed to track the ultrafast evolution of intermediates and final products with time, mass, and energy resolution. As a model system, the photoinduced reaction of CD3I adsorbed on amorphous cerium oxide films is investigated. A fraction of the first intermediates produced on a freshly prepared surface is trapped to passivate the surface. After the A-band excitation, the minimum dissociation time of CD3I indicates that CD3I adsorption geometries with either CD3 or I facing the gas phase exist; however, the transient data suggest that most molecules are adsorbed with the I atom facing the surface. CD3 and I are consumed to form I2 and reform CD3I, which are produced at a steady rate only after the intermediates have lost the excess translational energy released from photodissociation.

Direct Molecular Simulation of Nitrogen Dissociation Under Adiabatic Postshock Conditions

Direct molecular simulations (DMS) of nitrogen dissociation in adiabatic reservoirs covering a wide enthalpy range are presented. These conditions more realistically reproduce the gas state in a hypersonic shock layer than prior isothermal DMS studies. The Minnesota ab initio potential energy surface is used for nitrogen molecule-molecule () and nitrogen atom-molecule () classical trajectory calculations. Profiles of gas parameters (mixture composition, gas temperature, etc.) are reported, and the molecules’ internal energy population distributions during dissociation examined. It is observed that the evolution of the gas mixture under adiabatic conditions can be divided into two phases. Early on, excess translational energy is available for molecules to dissociate, regardless of their internal energy. This causes a sudden drop in the reservoir kinetic temperature and slows down subsequent dissociation. At later stages, the gas settles into a quasi-steady-state (QSS) regime, where dissociation proceeds primarily from higher-lying rovibrational levels near and above the dissociation threshold energy. The dissociation rate becomes limited by depletion of high-lying internal energy levels. Subsequently, adiabatic simulations are compared with prior isothermal DMS calculations performed at similar reservoir temperatures. The same shape is effectively observed for population distributions in both cases. This retroactively justifies the use of isothermal conditions to obtain QSS dissociation rate coefficients.

Inert Gas Scattering from Liquid Hydrocarbon Microjets

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 O2 from liquid dodecane (C12H26, Pvap = 0.1 Torr at 295 K) and from a reference liquid, squalane (C30H62, Pvap = 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 O2 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.

Quantum-state resolved reactive scattering at the gas-liquid interface: F+squalane (C30H62) dynamics via high-resolution infrared absorption of nascent HF(v,J)

Exothermic chemical reaction dynamics at the gas-liquid interface have been investigated by colliding a supersonic beam of F atoms [E(com)=0.7(3) kcalmol] with a continuously refreshed liquid hydrocarbon (squalane) surface under high vacuum conditions. Absolute HF(v,J) product densities are determined by infrared laser absorption spectroscopy, with velocity distributions along the probe axis derived from high resolution Dopplerimetry. Nascent HF(v<or=3) products are formed in a highly nonequilibrium (inverted) vibrational distribution [E(vib)=13.2(2) kcalmol], reflecting insufficient time for complete thermal accommodation with the surface prior to desorption. Colder, but still non-Boltzmann, rotational state populations [E(rot)=1.0(1) kcalmol] indicate that some fraction of molecules directly scatter into the gas phase without rotationally equilibrating with the surface. Nascent HF also recoils from the liquid surface with excess translational energy, resulting in Doppler broadened linewidths that increase systematically with internal HF excitation. The data are consistent with microscopic branching in HF-surface dynamics following the reactive event, with (i) a direct reactive scattering fraction of newly formed product molecules leaving the surface promptly and (ii) a trapping desorption fraction that accommodates rotationally (though still not vibrationally) with the bulk liquid. Comparison with analogous gas phase F+hydrocarbon processes reveals that the liquid acts as a partial "heat sink" for vibrational energy flow on the time scale of the chemical reaction event.

Molecular Dynamics Simulations of Atmospheric Oxidants at the Air−Water Interface: Solvation and Accommodation of OH and O3

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.

Packing density and structure effects on energy-transfer dynamics in argon collisions with organic monolayers

A combined experimental and molecular-dynamics simulation study has been used to investigate energy-transfer dynamics of argon atoms when they collide with n-alkanethiols adsorbed to gold and silver substrates. These surfaces provide the opportunity to explore how surface structure and packing density of alkane chains affect energy transfer in gas-surface collisions while maintaining the chemical nature of the surface. The chains pack standing up with 12 degrees and 30 degrees tilt angles relative to the surface normal and number densities of 18.9 and 21.5 A(2)molecule on the silver and gold substrates, respectively. For 7-kJmol argon scattering, the two surfaces behave equivalently, fully thermalizing all impinging argon atoms. In contrast, these self-assembled monolayers (SAMs) are not equally efficient at absorbing the excess translational energy from high-energy, 35 and 80 kJmol, argon collisions. When high-energy argon atoms are scattered from a SAM on silver, the fraction of atoms that reach thermal equilibrium with the surface and the average energy transferred to the surface are lower than for analogous SAMs on gold. In the case of argon atoms with 80 kJmol of translational energy scattering from long-chain SAMs, 60% and 45% of the atoms detected have reached thermal equilibrium with the monolayers on gold and silver surfaces, respectively. The differences in the scattering characteristics are attributed to excitation efficiencies of different types of surface modes. The high packing density of alkyl chains on silver restricts certain low-energy degrees of freedom from absorbing energy as efficiently as the lower-density monolayers. In addition, molecular-dynamics simulations reveal that the extent to which argon penetrates into the monolayer is related to packing density. For argon atoms with 80-kJmol incident energy, we find 16% and 7% of the atoms penetrate below the terminal methyl groups of C(10) SAMs on gold and silver, respectively.

Dynamics of the gas-phase reactions of chloride ion with fluoromethane: high excess translational activation energy for an endothermic S(N)2 reaction.

Guided ion beam tandem mass spectrometry techniques are used to examine the competing product channels in the reaction of Cl(-) with CH(3)F in the center-of-mass collision energy range 0.05-27 eV. Four anionic reaction products are detected: F(-), CH(2)Cl(-), FCl(-), and CHCl(-). The endothermic S(N)2 reaction Cl(-) + CH(3)F --> CH(3)Cl + F(-) has an energy threshold of E(0) = 181 +/- 14 kJ/mol, exhibiting a 52 +/- 16 kJ/mol effective barrier in excess of the reaction endothermicity. The potential energy of the S(N)2 transition state is well below the energy of the products. Dynamical impedances to the activation of the S(N)2 reaction are discussed, including angular momentum constraints, orientational effects, and the inefficiency of translational energy in promoting the reaction. The fluorine abstraction reaction to form CH(3) + FCl(-) exhibits a 146 +/- 33 kJ/mol effective barrier above the reaction endothermicity. Direct proton transfer to form HCl is highly inefficient, but HF elimination is observed above 268 +/- 95 kJ/mol. Potential energy surfaces for the reactions are calculated using the CCSD(T)/aug-cc-pVDZ and HF/6-31+G(d) methods and used to interpret the dynamics.

Desorption dynamics in N 2O decomposition on Pd(110)

The decomposition of N 2O was studied on Pd(110) through analysis of the angular and velocity distributions of desorbing products by means of angle-resolved thermal desorption combined with time-of-flight techniques. Both desorption and decomposition of N 2O(a) were completed below 170 K, simultaneously emitting N 2. N 2 showed two desorption peaks, β 1-N 2 at 152 K and β 2-N 2 at 134–140 K. The former revealed an inclined emission at ±43° off the normal in a plane along the (001) direction and a hyper-thermal translational energy, whereas the latter showed a cosine angular distribution without an excess translational energy. Different desorption channels were proposed to yield these desorption.

The steric effect in a full dimensional quantum dynamics simulation for the dissociative adsorption of H2 on Cu(111)

The rotational alignment of the dissociative adsorption of H2 on the Cu(111) surface has been studied by a six-dimensional quantum dynamics simulation. The theoretical rotational alignment is in excellent agreement with the experimental measurement of Hou et al. [Science 277, 80 (1997)]. The translational energy threshold of the dissociation is found to increase with increase of rotational quantum number j then to decrease after j=4 or 5. No substantial difference in the dependence of rotational alignment on the excess translational energy has been found between the dissociation of H2 and D2 on the Cu(111) surface. The variation of rotational alignment as a function of excess translational energy is almost independent of the rovibrational level (v,j) of the initial state. The theoretical study further predicts that the rotational alignment curve (a function of translational energy) would first shift toward high translational energy with increasing j, then shift back toward low translational energy after j=5.

Dynamics of molecular surface diffusion: Energy distributions and rotation–translation coupling

Surface diffusion rates have been simulated using classical molecular dynamics in a model of CO adsorbed on Ni(111). This paper describes the energy distribution among adsorbate modes at the transition state, energy relaxation after crossing the transition state, and correlations among adsorbate modes near the transition state. The adsorbate bending (frustrated rotation) mode is strongly coupled to lateral translational motion. This molecular mode provides an important source of energy for reaching the transition state to diffusion, and an important frictional force that dissipates excess lateral translational energy. In this model, the molecular bending mode is a more important source (and sink) of lateral translational energy than the surface at short times. This result is interpreted as a consequence of directional bonding to the surface, and it should be generally important in surface diffusion of chemisorbed molecules.

Molecular dynamics simulation of nanoscale thermal conduction and vibrational cooling in a crystalline naphthalene cluster

A molecular dynamics simulation of crystalline naphthalene is used to study nanometer scale thermal transport in solids. One molecule in a cluster of 75 is heated to a large initial temperature and then allowed to cool. Stochastic boundary conditions which preserve the time averaged volume of the cluster are used. The excess translational and librational energy of the hot molecule is lost within 1 ps. The excess vibrational energy is lost on the 100 ps time scale. Translational and librational energy propagates rapidly throughout the cluster at velocities which are comparable to the speed of sound. Despite the far slower rate of vibrational energy loss from the hot molecule, the growth of vibrational energy occurs uniformly on the other molecules in the cluster. Therefore intermolecular vibrational energy transfer occurs primarily via an indirect mechanism. Vibrational excitations are first converted into translational and librational excitations, which propagate throughout the cluster and then excite vibrations on distant molecules via multiphonon up pumping. Examination of the molecular neighbors shows that intermolecular transfer of mechanical energy can be anisotropic, since the hot molecule can only transfer energy where it contacts atoms on adjacent molecules. Energy transfer along the b- and c-crystallographic axes is more efficient than along the a axis. The most efficient energy transfer is in the direction of two of the four nearest neighbors. Transient hot spots are produced on these neighboring molecules. The implications of this anisotropic conduction for the propagation of thermal reactions, e.g., the decomposition of high explosives, are discussed briefly.

ChemInform Abstract: Low Energy (0‐10 eV) Electron Attachment to CF3Cl Clusters: Formation of Product Ions and Analysis of Excess Translational Energy.

ChemInformVolume 21, Issue 42 Preparative Organic Chemistry ChemInform Abstract: Low Energy (0-10 eV) Electron Attachment to CF3Cl Clusters: Formation of Product Ions and Analysis of Excess Translational Energy. A. + KUEHN, A. + KUEHN Inst. Phys. Theor. Chem., FU Berlin, D-1000 Berlin 33Search for more papers by this authorE. ILLENBERGER, E. ILLENBERGER Inst. Phys. Theor. Chem., FU Berlin, D-1000 Berlin 33Search for more papers by this author A. + KUEHN, A. + KUEHN Inst. Phys. Theor. Chem., FU Berlin, D-1000 Berlin 33Search for more papers by this authorE. ILLENBERGER, E. ILLENBERGER Inst. Phys. Theor. Chem., FU Berlin, D-1000 Berlin 33Search for more papers by this author First published: October 16, 1990 https://doi.org/10.1002/chin.199042096AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue42October 16, 1990 RelatedInformation

Low energy (0–10 eV) electron attachment to CF3Cl clusters: Formation of product ions and analysis of excess translational energy

Electron attachment to CF3Cl clusters has been studied mass spectrometrically in a beam experiment. In addition to the fragments known from dissociative attachment to the isolated compound (F−, Cl−, etc.), a veriety of larger complexes such as M−n, Mn⋅Cl−, Mn⋅F−, n≥1 (M=CF3Cl) could be observed. The resonance profiles of the ion yield curves suggest that the initial step of electron capture proceeds via the formation of a temporary CF3Cl− ion within the target aggregate followed by different decomposition reactions. Among the various products, the parent radical anion (CF3Cl−) is generated in its relaxed configuration not accessible in electron capture by the isolated molecule. A time-of-flight (TOF) analysis of the Cl− products reveals two decay channels, one associated with high excess translational energy and a second releasing the Cl− ion with only thermal energy.

Energy partitioning in the unimolecular decomposition of cyclic perfluororadical anions

Associative and dissociative electron attachment to the unsaturated cyclic compounds perfluorobenzene (C 6F 6), perfluorotoluene (C 6F 5CF 3) and perfluorocyclopentene ( c-C 5F 8) and to the saturated compound 1,2-dimethylperfluorocyclobutane ( c-C 4F 6(CF 3) 2) is studied in the energy range 0–20 eV in a beam experiment. All compounds capture thermal electrons to form a long-lived parent radical ion (M −). In the aromatic systems the complementary ions (with respect to the additional charge), F −/ (M-F) − are simultaneously generated within a resonance near 4.8 and 8.7 eV. A TOF analysis reveals that the resonance near 8.7 eV decomposes into (M-F) − with high excess kinetic energy (3.2±0.4 V in the case of perfluorobenzene), while the complementary reaction yielding F −+neutral fragment(s) is associated with low kinetic energy release (<0.1 ev). In contrast, the variety of negative fragment ions generated from the saturated compound at any energy are associated with low excess translational energy. The results are discussed in terms of the molecular orbitals involved in electron capture and in terms of the subsequent unimolecular decomposition mechanism.

Atomic nitrogen in aurora: Production, chemistry, and optical emissions

The production of ground configuration nitrogen atoms in aurora is investigated. The two principal mechanisms are electron impact dissociation of N2 and chemical‐ionic reactions, with the latter dominating the production of N(²D). Between 60 and 70% of the N atoms are produced in the excited ²P and ²D states over a large altitude range. Simultaneous field‐aligned measurements of the 5200‐Å and 3466‐Å lines of N I and the 4278‐Å first negative group band of N2+ were made in the northern nightside auroral oval under varying auroral conditions. These data, taken over a continuous 4‐hour period on the night of February 5, 1978, yield an average emission rate ratio of the 5200‐Å/3466‐Å lines near 0.5. Model calculations yield ratios of 0.5 and 1.4 for particle characteristic energies (alpha) of 2 keV and 0.1 keV in an assumed Maxwellian electron spectrum in good agreement with the observations reported here and consistent with those reported in the literature for very soft precipitation in the dayside cusp region. The production rate of energetic (hot) N(4S) atoms in the aurora is evaluated from considerations of the excess translational energy associated with N atom production. It may be a significant factor in the production rate of NO and the odd nitrogen density in general in the auroral atmosphere.

Dynamics of interaction of H 2 and D 2 with Ni(110) and Ni(111) surfaces

Abstract Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(111) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

Angle-dependence of ion kinetic energy spectra obtained by using mass spectrometers I. Theoretical consequences of conservation laws for collisions

Collision-induced processes for ion beams in the kilovolt energy range have been studied in mass spectrometers since the inception of the technique (‘Aston bands’). In most such studies to date, the fast collision products bave been collected over a range of angles relative to the initial projectile trajectory, this range being defined by the ion optics of the instrument used. More recently, there has been some interest in controlling and exploiting the collection angle for the fast collision products. The motivation behind such studies may be expressed qualitatively in terms of the more violent collisions favouring higher-energy processes, and also resulting in a larger scattering angle for the projectile ion in the activating collision. Thus, it might be hoped that collection angle could serve as an experimental parameter whose variation controls the energetics of the collision-induced processes actually observed. The present work examines the probable limitations of such an approach, on the basis of classical collision theory. Even without knowledge of an appropriate potential function for the projectile ion-neutral target interaction, it is possible to obtain useful quantitative information concerning such collisions. The extensive work on monatomic projectiles and targets is reviewed, and an attempt made to extend these results to polyatomic species of more interest to the mass spectrometrist. When the collision induced process to be studied is chemical fragmentation, the observed angular distribution is a convolution of two effects of comparable importance, namely the scattering angle-energy deposition relation which is the desired result of the experiment, and the internal energy of the projectile (precursor) ion released as excess translational energy of the fragments. The most recent experimental work on angle-resolved mass spectrometry is critically discussed in the light of these considerations.

Catalytic oxidation of CO on Pt(111): The influence of surface defects and composition on the reaction dynamics

Angular distributions of CO 2, I(γ r), formed by catalytic reaction between CO and O 2 on a stepped Pt(111) surface, have been measured by means of molecular beam techniques as a function of CO and O coverages, in the presence of “subsurface oxide”, and at varying temperatures. The resulting data can well be fitted by I( γ r ) = a cos γ r + (1− a) cos 7 γ r with a being an adjustable parameter which varies with the surface conditions. The cos γ r part is ascribed to particles which were completely accommodated with their translational energy to the surface temperature before leaving the surface, while the cos 7γ r channel corresponds to molecules carrying excess translational energy from crossing the activation barrier of the surface reaction. The fraction of thermally accommodated particles increases by the presence of steps or “subsurface oxide”, and decreases with O and CO coverage as well as with increasing temperature. The effect of coverage can qualitatively be attributed to the varying participation of defect sites in product formation, while the influence of surface temperature is well reproduced in the framework of a general theoretical model. Scattering experiments with CO 2 revealed that the trapping probability for this molecule (in the temperature range interesting here) is of the order of 0.5, thus confirming the conclusion of incomplete thermal accommodation of a CO 2 molecule interacting with a Pt(111) surface.

Photophysical processes in the vapour phase measured by the optic–acoustic effect. Part 7.—Vibrational–translational relaxation in I2, X state, and collisionally induced predissociation in the B state

The spectrophone signal in gaseous I2, at 0.3 Torr and 295 K, and following modulated excitation at 546 nm, is analysed in terms of contributions from thermal energy release and from particle density increase due to predissociation. The former contribution is shown to be dominant and allows the measurement of the vibrational–translational relaxational relaxation time in I2(X state). The measurement is novel in that the excess translational energy produced by predissociation is frequency-monitored as it equilibrates with the thermally accessible vibrational levels. The extent of energy released is analysed to show that in the collisional predissociation process almost no energy (< 5%) is transferred to the vibration of the I2 collision partner. There is no evidence for a significant yield of direct (Xâ†�B, I2) intersystem crossing. “Hot” I atom excitation of I2 vibrations is also discussed.