C. L. Cocke

Differential cross sections for single ionisation of He and D2 by fast protons

Differential cross sections for single ionisation of He and D2 by protons have been measured at impact energies of 3 and 6 MeV, and proton scattering angles between 0.1 and 1.0 mrad. A clear shoulder has been observed at an angle of 0.55 mrad. This feature in the differential cross sections is attributed to binary encounters between the protons and the quasi-free target electrons. The measured data are in good agreement in shape and magnitude with both a first Born approximation calculation of the ionisation and with a simple calculation of Rutherford scattering from free electrons.

Soft X-ray-Driven Femtosecond Molecular Dynamics

The direct observation of molecular dynamics initiated by x-rays has been hindered to date by the lack of bright femtosecond sources of short-wavelength light. We used soft x-ray beams generated by high-harmonic upconversion of a femtosecond laser to photoionize a nitrogen molecule, creating highly excited molecular cations. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule exploded. We found that substantial fragmentation occurs through an electron-shakeup process, in which a second electron is simultaneously excited during the soft x-ray photoionization process. During fragmentation, the molecular potential seen by the electron changes rapidly from nearly spherically symmetric to a two-center molecular potential. Our approach can capture in real time and with angstrom resolution the influence of ionizing radiation on a range of molecular systems, probing dynamics that are inaccessible with the use of other techniques.

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Ultrafast Probing of Core Hole Localization in N2

Although valence electrons are clearly delocalized in molecular bonding frameworks, chemists and physicists have long debated the question of whether the core vacancy created in a homonuclear diatomic molecule by absorption of a single x-ray photon is localized on one atom or delocalized over both. We have been able to clarify this question with an experiment that uses Auger electron angular emission patterns from molecular nitrogen after inner-shell ionization as an ultrafast probe of hole localization. The experiment, along with the accompanying theory, shows that observation of symmetry breaking (localization) or preservation (delocalization) depends on how the quantum entangled Bell state created by Auger decay is detected by the measurement.

The ratio of cross sections for double to single ionization of helium by high energy photons and charged particles

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

Multi-hit detector system for complete momentum balance in spectroscopy in molecular fragmentation processes

A multi-hit detector system has been developed capable of measuring the complete momentum vectors of all ionic fragments after the dissociation of complex molecules induced by photon, electron or ion impact. The fragments are collected in an electrostatic field and detected with a position-sensitive micro-channel plate detector using a fast timing delay-line readout. The detector has a position resolution better than 0.2 mm and can resolve fragments with arrival times separated by at least 5 ns in time. We illustrate the features of this new detector with first measurements for the collision

Complete photo-fragmentation of the deuterium molecule

All properties of molecules—from binding and excitation energies to their geometry—are determined by the highly correlated initial-state wavefunction of the electrons and nuclei. Details of these correlations can be revealed by studying the break-up of these systems into their constituents. The fragmentation might be initiated by the absorption of a single photon, by collision with a charged particle or by exposure to a strong laser pulse: if the interaction causing the excitation is sufficiently understood, the fragmentation process can then be used as a tool to investigate the bound initial state. The interaction and resulting fragment motions therefore pose formidable challenges to quantum theory. Here we report the coincident measurement of the momenta of both nuclei and both electrons from the single-photon-induced fragmentation of the deuterium molecule. The results reveal that the correlated motion of the electrons is strongly dependent on the inter-nuclear separation in the molecular ground state at the instant of photon absorption.

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experi- ment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolu- tion in both domains simultaneously. We show that resonant absorption is impor- tant in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).

K-shell photoionization of CO and N2: is there a link between the photoelectron angular distribution and the molecular decay dynamics?

We have used COLTRIMS to measure the angular distribution of electrons released from the K-shell of N2 and the carbon K-shell of CO by absorption of one linear polarized photon. For each ionization event which leads to two charged fragments we determine the angle of the photoelectron with respect to the fragment ion momenta. In addition we determine the charge state and energy of the molecular fragments. We find a breakdown of the axial recoil approximation for CO for kinetic energy releases below 10.2 eV, whereas for N2 that approximation is found to be valid for all fragment energies. Furthermore, the photoelectron emission spectrum for N2 is found to be the same for the molecular breakup channels producing N + N + and N + N ++ . (Some figures in this article are in colour only in the electronic version)

Momentum-imaging investigations of the dissociation of D2+ and the isomerization of acetylene to vinylidene by intense short laser pulses

We present momentum images of the ionic products from the ionization of D2 and C2H2 by short laser pulses. For D2, we use a pump–probe approach to investigate the dependence of the enhanced ionization on the internuclear distance. Evidence for two (not well separated) regions of enhancement is found near internuclear distances of 6 and 10 au. In the case of acetylene, we report clear evidence for the production of both acetylene and vinylidene dications with kinetic energy releases similar to those reported earlier by core electron removal. We also find very different angular distributions for the fragments in the two channels, consistent with a finite time for the isomerization.