The production of intermediate mass fragments ͑IMF's͒ from the four reactions 55A MeV 124,136 Xe ϩ 112,124 Sn is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF's are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF's depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source ͑or sources͒ is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced.

Multifragment disintegrations, measured for central Au + Au collisions at E/A = 35M eV , are analyzed with the Statistical Multifragmentation Model. Charge distributions, mean fragment energies, and two-fragment correlation functions are well reproduced by the statistical breakup of a large, diluted and thermalized system slightly above the multifragmentation threshold.

Adsorption involves molecules colliding at the surface of a solid and losing their incidence energy by traversing a dynamical pathway to equilibrium. The interactions responsible for energy loss generally include both chemical bond formation (chemisorption) and nonbonding interactions (physisorption). In this work, we present experiments that revealed a quantitative energy landscape and the microscopic pathways underlying a molecule’s equilibration with a surface in a prototypical system: CO adsorption on Au(111). Although the minimum energy state was physisorbed, initial capture of the gas-phase molecule, dosed with an energetic molecular beam, was into a metastable chemisorption state. Subsequent thermal decay of the chemisorbed state led molecules to the physisorption minimum. We found, through detailed balance, that thermal adsorption into both binding states was important at all temperatures.

Measuring inelastic rates with partial-wave resolution requires temperatures close to a Kelvin or below, even for the lightest molecule. In a recent experiment, Perreault, Mukherjee, and Zare [Nat. Chem. 10, 561 (2018).NCAHBB1755-433010.1038/s41557-018-0028-5] studied collisional relaxation of excited HD molecules in the v=1, j=2 state by para- and ortho-H_{2} at a temperature of about 1 K, extracting the angular distribution of scattered HD in the v=1, j=0 state. By state preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. Here, we report a first-principles simulation of that experiment which enables us to attribute the main features of the observed angular distribution to a single L=2 partial-wave shape resonance. Our results demonstrate important stereodynamical insights that can be gained when numerically exact quantum scattering calculations are combined with experimental results in the few-partial-wave regime.

The relationship between observed intermediate mass fragment and total charged particle multiplicities has been measured for Kr + Au collisions at energies between E/A = 35 and 400 MeV. Pragment multiplicities are greatest for central or near-central collisions. For these collisions, fragment production increases up to E/A-100 MeV, and then decreases at higher energies.

Multiplicities of intermediate-mass fragments (IMFs), neutrons, and charged particles were measured for 112 Sn 1 112 Sn and 124 Sn 1 124 Sn at E͞A 40 MeV. Significantly different scalings of the mean IMF multiplicities with neutron and charged-particle multiplicities are observed for the two reactions. These differences can be qualitatively understood in terms of fragment emission from an expanding evaporating source for which the initial rates of cooling by neutron and light-charged-particle emission depend on the neutron and proton numbers of the source according to statistical expectations. [S0031-9007(96)01350-6] PACS numbers: 25.70.Pq, 25.70.Gh

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A high-dimensional potential energy surface (PES) for CO interaction with the Au(111) surface is developed using a machine-learning algorithm. Including both molecular and surface coordinates, this PES enables the simulation of the recent experiment on scattering of vibrationally excited CO from Au (111). Trapping in a physisorption well is observed to increase with decreasing incidence energy. While energy dissipation of physisorbed CO is slow, due to weak coupling with both the phonons and electron-hole pairs, its access to the chemisorption well facilitates fast vibrational relaxation of CO through nonadiabatic coupling with surface electron-hole pairs. Energy transfer between molecules and metal surfaces represents a key aspect of surface processes, with important implications in a wide array of interfacial phenomena. There are two major energy exchange channels, namely the adiabatic coupling with surface phonons and the nonadiabatic interaction with electron-hole pairs (EHPs).[1-3] The lifetime of CO(ν=1) adsorbate has been measured to be 1-2 ps on Cu(001), using several experimental techniques.[4-7] Such a short lifetime for a high frequency mode (ω=2129 cm -1 ) can only be explained by its nonadiabatic coupling with surface EHPs, because its direct coupling with the low-frequency phonons is unlikely. This nonadiabatic energy dissipation mechanism has been characterized by various theoretical models, [8][9][10][11][12][13][14][15][16][17][18] cumulating with the latest first-principles calculations that quantitatively reproduced the observed lifetime. [19,20] It was thus a surprise when Shirhatti et al. reported a long lifetime (~10 2 ps) for trapped CO(ν=1) in the scattering of vibrationally excited CO(ν=2) from Au (111).[21] It was postulated that physisorption might be involved, given the relatively low desorption temperature of CO from Au (111).[22] Indeed, a recent density functional theory (DFT) study by Lončarić et al. did find such a physisorption well for CO on Au(111),[23] using the Bayesian Error Estimation Functional method with van der Waals corrections (BEEF-vdW).[24] The lifetime of physisorbed CO(ν=1) was calculated within first-principles many-body perturbation theory and found to be consistent with the experimental value.[21] The long vibrational lifetime was attributed to the weaker couplings with EHPs because of the large distance between the adsorbate and surface. The same argument has also been used to explain the vibrationally hot precursor CH4 on the Ir(111) surface. [25] However, the aforementioned theoretical work was only intended to calculate the vibrational relaxation rate for CO adsorbed on the surface, and it provides information on neither the mechanism and dynamics on how the impinging CO molecules are trapped and then desorbed, nor the accompanying energy dissipation into surface phonons. In principle, Ab Initio Molecular Dynamics (AIMD) can shed light on such issues, but the trapping and diffusion are too rare and too long to be computationally feasible for the onthe-fly ...