J S Briggs

Triply-differential cross sections for ionisation of hydrogen atoms by electrons and positrons

A derivation is given of the exact form of the three-body Coulomb wavefunction in the asymptotic region where the separation of all particles tends to infinity. Using a modification of the method of Pluvinage (1951), an approximate three-body scattering wavefunction is derived that satisfies this boundary condition. Triply-differential cross sections (TDCS) for electron impact ionisation of atomic hydrogen calculated with this scattering wavefunction, which contains no free parameters, show excellent agreement with measurements at impact energies greater than 150 eV. The corresponding TDCS for positron impact ionisation are also presented.

Differential cross sections for photo-double-ionization of the helium atom

A rather complete understanding is emerging of the process of full fragmentation of the helium atom following absorption of a single photon. In this review the major experimental and theoretical advances that have led to our understanding of this simplest photo-double-ionization process, particularly with respect to multiply differential cross sections, are described.

Fundamental processes in energetic atomic collisions

This book starts with a brief review of the formal scattering theory by Massey. The chapters that follow survey a coherent selection of interesting topics in energetic ion-atom collisions. The last third of the book contains papers on coherence and correlation in atomic collisions and on new aspects in the study of atomic collisions. The papers on coherence and correlation report progress toward complete experiments where scattering amplitudes and phases are determined in coincidence experiments. The papers on new aspects cover the use of polarized particles in atomic experiments.

Bandshapes in polymer spectra

The shape of the vibrational envelope of an absorption band in the solution spectrum of a helical polymer is related to the shape of the corresponding band in the monomer. The approximation used is expected to be valid in both the strong and weak coupling limits. In the strong coupling limit it is shown that under certain conditions the polymer band consists of an extremely narrow peak shifted from the monomer line, as has been observed in the spectra of some cyanine dyes. The identification of this peak with absorption by a propagating exciton mode is supported by a calculation of the effective distance of excitation transfer as a function of coupling strength.

The excitation of inner shells in slow atomic collisions

The development of the theory of inner-shell excitation in slow ion-atom collisions over the past decade is described. This theory recognizes that electrons in inner shells behave largely independently of other electrons and generalizes the coupled-state impact-parameter method used successfully for ion-atom collisions involving a single electron. The method utilizes one-electron molecular orbitals obtained from solutions of the stationary molecular Shcrodinger equation and the generation of such orbitals for inner-shell electrons is described. The solution of the coupled equations for the excitation of K-shell electrons in collisions of first-row atoms is considered in detail, particularly the scaling properties. A discussion is given of the limitations of the independent-electron method and the role played by the outer-shell electrons.

The J-band of organic dyes: lineshape and coherence length

Self-organised J-aggregates of dye molecules, known for over 60 years, are emerging as remarkably versatile quantum systems with applications in photography, opto-electronics, solar cells, photobiology and as supra-molecular fibres. Recently there has been much effort to achieve quantum entanglement and coherence on the nanoscale in atom traps and quantum dot aggregates (for use in quantum computing). We point out that the excitonic state of the J-aggregate is a text-book case of mesoscopic quantum coherence and entanglement. The establishment of coherence can literally be seen since the dye changes colour dramatically on aggregation due to strong shifts in the absorption spectrum. Here we reproduce in a simple theory the shifts and shapes of optical absorption spectra upon aggregation to a polymer and calculate the coherence length of quantum entanglement of monomer wavefunctions.

Vibronic energies and spectra of molecular dimers

We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipole-dipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the Jaynes-Cummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the time-dependent Schrodinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra.

Dissociation of polyatomic molecules by intense infrared laser radiation

Abstract A theory of the dissociation of polyatomic molecules by intense infrared laser radiation is presented. It is suggested that vibrational heating of the whole molecule occurs by intramolecular vibrational coupling to a given infrared active mode which is in near-resonance with the applied field. A high level of excitation of the active mode is not necessary for the dissociation to occur.

Exchange narrowing of the J band of molecular dye aggregates

The exchange narrowing of the J band of certain dye monomers upon aggregation in solution has been known since the 1930s. Here, we analyze the theoretical explanations put forward to account for these narrow absorption bands. Although the theories range from models of identical monomers interacting with vibrations to the opposite of rigid monomers with statistically distributed electronic site energies, all approaches exhibit exchange narrowing. However, we show that the origins of the narrowing are different. A unified theory incorporating the two approaches is presented in which features of both narrowing mechanisms are evident.

Sum rules for the vibronic spectra of helical polymers

The paper treats the absorption of light by a helical polymer consisting of monomer molecules whose nuclei can vibrate. The exchange of electrons between monomers is neglected. With monomers having only a single excited electronic state, sum rules are derived for the first three moments of the energy of photons absorbed into the corresponding band in the polymer. There are no further sum rules like the three obtained. The sum rules remain valid in the time-dependent Hartree approximation.