State-to-state differential and relative integral cross sections for rotationally inelastic scattering of H2O by hydrogen

State-to-state differential cross sections (DCSs) for rotationally inelastic scattering of H(2)O by H(2) have been measured at 71.2 meV (574 cm(-1)) and 44.8 meV (361 cm(-1)) collision energy using crossed molecular beams combined with velocity map imaging. A molecular beam containing variable compositions of the (J = 0, 1, 2) rotational states of hydrogen collides with a molecular beam of argon seeded with water vapor that is cooled by supersonic expansion to its lowest para or ortho rotational levels (J(KaKc) = 0(00) and 1(01), respectively). Angular speed distributions of fully specified rotationally excited final states are obtained using velocity map imaging. Relative integral cross sections are obtained by integrating the DCSs taken with the same experimental conditions. Experimental state-specific DCSs are compared with predictions from fully quantum scattering calculations on the most complete H(2)O-H(2) potential energy surface. Comparison of relative total cross sections and state-specific DCSs show excellent agreement with theory in almost all details.     

Degeneracy-averaged and polarized cross sections for rotational scattering in (Ar,LiH)

Close-coupling calculations at 500 cm^?1 for the system (Ar,LiH) are used to obtain scattering amplitudes and cross sections for the rotational transitionsj = 1 to j'= 0, 2, 3, 4, 5,j = 2 to j' = 3 and j = 3 to j'= 4. The degeneracy-averaged cross sections are in reasonable agreement with experiment for two different simple model potentials. Values for m-dependent cross sections with respect to two frames of reference are compared with various rules for Δm and with the previous results for (Ar,LiH).     

Fully quantum state-resolved inelastic scattering between He and NO(X (2)Pi)

Quantum mechanical close-coupling calculations have been used to obtain fully quantum state-resolved differential cross sections and opacity functions for the rotationally inelastic collisions of NO(X (2)Pi) with He at collision energies of 63 and 147 meV using the most recent ab initio potential energy surfaces of K?os et al. [J. Chem. Phys. 112, 2195 (2000)]. Double peaks observed in the Lambda-doublet resolved differential cross sections are shown to be related to the presence of analogous peaks in the corresponding opacity functions. These structures can be linked directly to a specific expansion term in the potential, and reflect the fact that NO is not quite homonuclear.     

Proposed standard inelastic differential electron scattering cross sections; Excitation of the 21 P level in He

Differential and integral cross section data for electron-impact excitation of the 2¹ P level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.     

Experimental determination of the potential for Na(32P)+ Ar from differential scattering cross sections

We have measured the total differential cross section for the interaction of laser excited Na(3²P) interacting with Ar at thermal energies. From these data, which show a well resolved oscillatory pattern, the respective interaction potentials have been determined. The best fit to the experimental data is found with ε = 2.55 × 10^?3 au, r_m = 5.75 au for the well depth parameter of the ²II_{$\text{1}\text{2}$} potentials.     

Total (elastic + inelastic) cross sections for electron scattering with bound and free germanium and lead atoms

Spherical complex optical potential (SCOP) approach has been used to compute the differential, total (elastic + inelastic) and momentum transfer cross sections for electrons scattering from the bound and free germanium and lead atoms in the energy range from 100–5000 eV. We find that the present calculated differential scattering cross sections (DCS) exhibit all important features (such as forward peaking, dip at middle angles and enhanced backward scattering) observed in other theoretical calculations and experimental measurements. The effect of absorption potential is generally to reduce the elastic cross section.     

Close-coupling calculations of polarization cross sections for (Ar,LiF)

To assist the interpretation of polarized rotational scattering experiments from states (j, m) to (j, m'), differential and total cross sections obtained from close-coupling calculations are given for m values corresponding to j? 3 in collisions between LiF and Ar at 20 cm^?1. The hypothesis ¦m¦-¦m'¦= 0 is proposed for the most likely transition, as an extension of the Δm = 0 rule.     

Regge pole analysis of differential cross sections for elastic molecular scattering

The angle and energy dependence of differential cross sections σ(E, θ) that exhibit diffraction oscillations is analyzed by a Regge representation. It is shown that: (1) the Regge poles can be estimated directly from the measured σ(E, θ); (2) a fixed-energy Regge pole calculation can reproduce the E-dependence of σ(E, θ).     

Application of fixed-nuclei scattering theory to electron methane elastic and inelastic differential cross sections at 10 eV impact energy

Quantum mechanical calculations are reported for electron-methane elastic scattering and rotational excitation cross sections at 10 eV impact energy. The calculations employ a fixed-nuclei close coupling formalism with full incorporation of symmetry and are used to test previous laboratory-frame calculations employing a direct coupling approximation. Good agreement is obtained. Additional comparisons to previous theoretical and experimental work are also presented, and the contributions of the various symmetries to the cross sections are analyzed in terms of representatve matrix elements of the interaction potential.     

Anisotropic intermolecular potentials for NOAr and NOKr from total differential cross section measurements

Angular distributions of NO scattered off Ar and Kr in the thermal energy range are measured in crossed molecular beam experiments. Information on the anisotropy of the interaction potential is obtained from the damping of the rainbow structure within the framework of the infinite-order-sudden approximation. The derived potential energy surfaces are compared with the results of a previous experimental investigation and with the PES of the related O₂-and N₂-rare gas systems.     

Parity conservation and polarization of differential cross sections in complex-forming chemical reactions

For complex-forming chemical reactions, such as atom-diatom insertion reactions, quantum scattering and quantum statistical calculations usually predict sharp forward/backward peaks in the Differential Cross Sections (DCS). Conversely, the corresponding classical calculations are unable to reproduce these peaks. We show here that the basic reason for such an intriguing failure is that parity conservation is ignored in classical mechanics. A by-product of the analysis is a simple parity-restoring approximation that might significantly increase the ability of classical mechanics to describe DCSs over the whole angular range for the title processes.     

Steric effects and quantum interference in the inelastic scattering of NO(X) + Ar

Rotationally inelastic collisions of NO(X) with Ar are investigated in unprecedented detail using state-to-state, crossed molecular beam experiments. The NO(X) molecules are selected in the Ω = 0.5, j = 0.5, f state and then oriented such that either the ‘N’ or ‘O’ end of the molecule is directed towards the incoming Ar atom. Velocity map ion imaging is then used to probe the scattered NO molecules in well-defined quantum states. We show that the fully quantum state-resolved differential steric asymmetry, which quantifies how the relative efficiency for scattering off the ‘O’ and the ‘N’ ends of the molecule varies with scattering angle, is strongly affected by quantum interference. Significant changes in both integral and differential cross sections are found depending on whether collisions occur with the N or O ends of the molecule. The results are well accounted for by rigorous quantum mechanical calculations, in contrast to both classical trajectory calculations and more simplistic models that provide, at best, an incomplete picture of the dynamics.     

Degeneracy-averaged and polarized cross sections for rotational scattering in (He,LiH)

Integral and differential cross sections for rotational scattering (jmj→j'mj') in (He,LiH) were obtained by the close-coupling method with 121 channels using the ab initio potential due to Silver at an energy of 950 cm⁻¹. The results are compared with previous work using empirical potentials, with experiment, and with approximate calculations.     

Differential cross sections for fine-structure inelastic collisions of K(42P) with Ar,Kr and N2

We report measurements of differential cross sections for fine-structure inelastic collisions of potassium (4²P_3/2-4²P_1/2 with Ar, Kr and N₂. The experiment uses crossed molecular beams and a method to detect scattering angles by the analysis of Doppler shifts in laser induced fluorescence. The experimental results for KAr are compared with calculations.     

Elastic differential cross sections and intermolecular potentials for Ar + CH4 and Ar + NH3

Differential elastic cross sections are reported for CH₄ + Ar (E = μg²/2 = 8.43 kJ/mole) and NH₃ + Ar (E = 8.31 kJ/mole) in the region of the rainbow angles. Quantum interference undulations are apparently observed as well for CH₄ + Ar and, possibly, NH₃ + Ar. The measurements are fit to spherically symmetric intermolecular potentials yielding well depths and equilibrium intermolecular separations of 1.32 kJ/mole and 3.82 Å for CH₄ + Ar and 1.32 kJ/mole and 3.92 Å for NH₃ + Ar.     

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.     

Multiple scattering calculation of double and single differential cross sections for ionization of hydrogen atoms by electrons at intermediate energies

The multiple scattering approach of Das and Seal, which was applied earlier to calculate the triple differential cross section for the ionization of atomic hydrogen by electrons is now used to calculate the double and the single differential cross sections for the same system. The range of the incident electron energy is taken to be 100–250 eV. The present results are compared with the measured results of Shyn and with the available distorted wave Born approximation results.