Time-resolved studies of ultrafast wavepacket dynamics in hydrogen molecules

Recent advances in the study of quantum vibrations and rotations in the fundamental hydrogen molecules are reported. Using the deuterium molecules (D₂⁺ and D₂) as exemplars, the application of ultrafast femtosecond pump-probe experiments to study the creation and time-resolved imaging of coherent nuclear wavepackets is discussed. The ability to study the motion of these fundamental molecules in the time-domain is a notable milestone, made possible through the advent of ultrashort intense laser pulses with durations on sub-vibrational (and sub-rotational) timescales. Quantum wavepacket revivals are characterised for both vibrational and rotational degrees of freedom and quantum models are used to provide a detailed discussion of the underlying ultrafast physical dynamics for the specialist and non-specialist alike.     

Norbornane: an investigation into its valence electronic structure using electron momentum spectroscopy, and density functional and Green's function theories

We report on the results of an exhaustive study of the valence electronic structure of norbornane (C(7)H(12)), up to binding energies of 29 eV. Experimental electron momentum spectroscopy and theoretical Green's function and density functional theory approaches were all utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among all the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all of the 20 valence orbitals of norbornane. This experimentally validated quantum chemistry model was then used to extract some chemically important properties of norbornane. When these calculated properties are compared to corresponding results from other independent measurements, generally good agreement is found. Green's function calculations with the aid of the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than 22.5 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet photoemission and newly presented (e,2e) ionization spectra, except for the band associated with the 1a(2) (-1) one-hole state, which is probably subject to rather significant vibronic coupling effects, and a band at approximately 25 eV characterized by a momentum distribution of "s-type" symmetry, which Green's function calculations fail to reproduce. We note the vicinity of the vertical double ionization threshold at approximately 26 eV.     

Inter-cycle variation in whole-body vibration exposures of operators driving track-type loader machines

Whole-body vibration (WBV) measurements are an important aspect of performing risk assessments for those exposed to vibration. A large array of variables affect the outcome of a vibration measurement and its extrapolation to a daily dose measure: e.g. variability in driving style, road surface roughness, loading. The variability in vibration emission is an inherent property for most vibrating environments and there is a risk that a vibration measurement might not be representative of the long-term exposures. It is important to acknowledge the variation inherent to WBV exposure to help understand how this variation will affect health risk assessments. A field investigation was conducted in order to characterise the variation of WBV magnitudes between work cycles of track-type loaders. Six different track-type loaders were measured at four different work sites. The vibrations were measured at the operators seat in three translational axes (x-, y-, and z-axis) in accordance with ISO 2631-1 (1997). The findings indicate the worst axis of vibration for the track-type loaders was predominantly the fore-and-aft (x-axis), for most operations. The most severe emission values were measured for machine C at site 2 (1.12 ms⁻² rms) and machine D at site 2 (1.03 ms⁻² rms). These machines would exceed the action value of the Physical Agents (Vibration) Directive within 2 h of exposure. All of the machines measured would exceed the exposure action value of the Directive within an 8 h working period. The lateral (y-axis) produced the greatest amount of variability between work cycles (coefficient of variation up to 20%). It is concluded that the inherent variability between work cycles and tasks reinforces the requirement to perform a full task analysis prior to measuring WBV exposures to ensure that all tasks are measured and that adequate cycles are measured to obtain a reliable indication of the vibration emission.     

Atomic and molecular ions in intense ultra-fast laser fields

The interaction of a 60 fs 790 nm laser pulse with beams of Ar⁺, C⁺, H₂ ⁺, HD⁺ and D₂ ⁺ are discussed. Intensities up to 10¹⁶ Wcm^-2 are employed. An experimental z-scanning technique is used to resolve the intensity dependent processes in the confocal volume.Received: 6 January 2003, Published online: 15 July 2003PACS: 32.80.Fb Photoionization of atoms and ions - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

Theory of oscillation type viscometers: The oscillating cup

This is the first of a series of papers dealing with the theory of oscillation type viscometers with particular emphasis on those in which a finite right circular cylinder (cup or disk) oscillates in contact with a fluid. The purpose is to obtain formulae which relate the density and viscosity of the fluid to the frequency and logarithmic decrement of the oscillation and the various physical properties of the suspension system. The study is to include an accurate analysis of the fluid motion in the vicinity of the edges since this is the main source of error in most existing theories. The first paper gives a general formulation of the problem and an exact solution (including transients) for the oscillating cup viscometer with the fluid inside the cup.

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Zusammenfassung Diese Abhandlung ist der erste Bericht aus einer Veröffentlichungsreihe, die sich mit der Theorie schwingender Viskosimeter befasst. Dabei sind besonders die behandelt, bei denen ein endlicher Kreiszylinder (Becher oder Scheibe) von einem flüssigen Medium gefüllt oder umgeben schwingt. Die Aufgabe der Untersuchung ist es, die Abhängigkeit der Frequenz und des logarithmischen Dekrementes der Schwingung von der Dichte und der Viskosität der Flüssigkeit und von den verschiedenen physikalischen Grössen des Aufhängungssystems zu finden. Weiterhin ist eine genaue Analyse der Flüssigkeitsbewegung in den Randzonen eingeschlossen, da diese die Hauptursache der Fehler in den meisten der vorhandenen Theorien war. Die erste Abhandlung gibt die grundlegende Formulierung des Problemes und eine exakte Lösung (einschliesslich der Einschwingzeit) für das Viskosimeter, bei dem ein mit der zu messenden Flüssigkeit gefüllter schwingender Becher verwendet wird.

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List of Symbols The following is a partial list of symbols including only those that are used repeatedly: A surface of contact between fluid and cylinder - D(s) defined in equation (16) - f() transient, equation (42) - h half height of cylinder or height of liquid in an open cup - I moment of inertia of suspension system - I moment of inertia of fluid inside cup - I _k modified Bessel function of orderk - J _k Bessel function of orderk - M() frictional moment exerted by the fluid - n normal toA outward from the fluid - r radial coordinate in the fluid This work was supported by the United States Air Force through the Air Force Office of Scientific Research and Development Command under contract No. AF 18(600)1548. Part of the work was done while one of the authors (G. F. N.) was being supported by a grant from the Alfred P. Sloan Foundation.     

Semi-Resonant Interactions and Frequency Dividers

A theory describing the behavior of a system as it evolves slowly through internal nonlinear resonances is presented. The energy sharing process is seen to be quite complex as it depends crucially on both nonlinear and frequency detuning effects. Two phenomena are discussed in detail although the general ideas are applicable to many situations. Firstly we examine the interaction between the quadratically coupled oscillators whose natural frequencies are in the ratio 2:1 for a limited period of time. Such a system is shown to be an extremely useful switching device. Secondly we examine the time dependent Duffing equation and find that smooth forward and reverse transitions occur without the presence of dissipation.     

Suppression of multiple ionization of atomic ions in intense ultrafast laser pulses

The interaction of an intense laser field with a beam of atomic ions has been investigated experimentally for the first time. The ionization dynamics of Ar+ ions and Ar neutrals in a 60 fs, 790 nm laser pulse have been compared and contrasted at intensities up to 10(16) W cm-2. Our results show that nonsequential ionization from an Ar+ target is strongly suppressed compared with that from the corresponding neutral target. We have also observed for the first time the strong field ionization of high lying target metastable levels in the Ar+ beam.