L. J. Frasinski

Covariance Mapping: A Correlation Method Applied to Multiphoton Multiple Ionization

In some cases there are hidden correlations in a highly fluctuating signal, but these are lost in a conventional averaging procedure. Covariance mapping allows these correlations to be revealed unambiguously. As an example of the applicability of this technique, the dynamics of fragmentation of molecules ionized by an intense picosecond laser are analyzed.

Dissociative photoionisation of molecules probed by triple coincidence ; double time-of-flight techniques

Two novel experiments producing three-dimensional histograms of time-of-flight correlations have recently been performed at the Daresbury Synchrotron Radiation Source. In the first, the two photoions from the double ionisation of SF6 were detected; in the second, the photoelectron and the O+ ion from the predissociation of O2+. This technique allows a visual insight into the details of dissociative photoionisation which has not been available previously.

Femtosecond X-ray-induced explosion of C 60 at extreme intensity

Understanding molecular femtosecond dynamics under intense X-ray exposure is critical to progress in biomolecular imaging and matter under extreme conditions. Imaging viruses and proteins at an atomic spatial scale and on the time scale of atomic motion requires rigorous, quantitative understanding of dynamical effects of intense X-ray exposure. Here we present an experimental and theoretical study of C60 molecules interacting with intense X-ray pulses from a free-electron laser, revealing the influence of processes not previously reported. Our work illustrates the successful use of classical mechanics to describe all moving particles in C60, an approach that scales well to larger systems, for example, biomolecules. Comparisons of the model with experimental data on C60 ion fragmentation show excellent agreement under a variety of laser conditions. The results indicate that this modelling is applicable for X-ray interactions with any extended system, even at higher X-ray dose rates expected with future light sources.

Nonradiative decay pathways of electronic states of group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

The nonradiative decay channels of the valence electronic states of the gas‐phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three‐dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic‐energy releases in the fragmentation pathw...

Coulomb explosion of simple molecules in intense laser fields

Abstract When diatomic and triatomic molecules are exposed to subpicosecond laser pulses (focused intensities well in excess of 1014 W cm−2; wavelengths of the order 600 nm), they are highly ionized. The laser electric fields are so large that they compete with, and in some cases exceed, the internal fields binding the valence electrons. The result is that conventional perturbation theory is inapplicable and one has to resort to alternative approaches. A simple starting point is the field ionization, Coulomb explosion model. This model is partially successful in explaining the ionization and subsequent fragmentation of the heavy molecule, I2, exposed to short (sub-100 fs) laser pulses, but even here the fine details are not understood. The dynamics of the fragmentation of the lighter molecules such as N2 and CO are not understood at all; one complication is that the time scales of dissociation and stepwise ionization overlap. In the case of the simplest molecular ion, H+ 2, there has been some progress, l...

Using covariance mapping to investigate the dynamics of multi-photon ionization processes of Ne atoms exposed to X-FEL pulses

We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.

Covariance mapping of two-photon double core hole states in C2H2 and C2H6 produced by an x-ray free electron laser

Few-photon ionization and relaxation processes in acetylene (C2H2) and ethane (C2H6) were investigated at the linac coherent light source x-ray free electron laser (FEL) at SLAC, Stanford using a highly efficient multi-particle correlation spectroscopy technique based on a magnetic bottle. The analysis method of covariance mapping has been applied and enhanced, allowing us to identify electron pairs associated with double core hole (DCH) production and competing multiple ionization processes including Auger decay sequences. The experimental technique and the analysis procedure are discussed in the light of earlier investigations of DCH studies carried out at the same FEL and at third generation synchrotron radiation sources. In particular, we demonstrate the capability of the covariance mapping technique to disentangle the formation of molecular DCH states which is barely feasible with conventional electron spectroscopy methods.

Multiphoton multiple ionisation of N2O probed by three-dimensional covariance mapping

Abstract One can attempt to understand the complex dynamics of multiphoton multiple ionisation of triatomic molecules using conventional time-of-flight mass spectrometry, but only by applying three-dimensional covariance mapping can processes involving the simultaneous production of three charged fragments be unequivocally assigned to a specific parent molecular ion.

Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N 4+ ‐N 5+ channel for nitrogen and up to the I 8+ ‐I 9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.

Fragmentation of valence electronic states of SF+6 studied with synchrotron radiation

Fragmentation of the seven valence electronic states of SF+6 has been studied in the range 440–810 A by photoionisation mass spectrometry. The ion yield for SF+5 shows two thresholds at the adiabatic ionisation energy of the ground (X 2T1g) and first two excited states (A2T1u and B 2T2u) of SF+6. SF+4 shows a threshold at the third state (C 2Eg) of SF+6, SF+3 shows two thresholds at the fourth and fith states (D 2T2g and E 2T1u), and SF+2 turns on at the sixth state (F 2A1g). Thus in all cases the appearance energy of a fragment ion occurs at the adiabatic ionisation potential of an electronic state of SF+6, and not at the lower-lying thermochemical energy of the fragment ion. Kinetic energy releases have been measured in SF+5, SF+4 and SF+3 at photon energies just above threshold for production of each ion. In the case of SF+5 a substantial fraction of the available energy is converted into translational energy of the fragments. The relative photoionisation yields of SF+5, SF+4 and SF+3 are compared with values expected from the photoelectron branching ratios of electronic states of SF+6. The latter results overestimate the yields of SF+3 and SF+4 relative to SF+5, and possible explanations are discussed.