P A Hatherly

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.

Fragmentation of valence electronic states of CF+4 and SF+6 studied by threshold photoelectron-photoion coincidence spectroscopy

Abstract Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to measure, state selectively, the decay pathways of all the valence states of CF+4 and SF+6 in the range 15–28 eV. Radiation in the vacuum UV from a synchrotron radiation source ionises the parent molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectroscopy, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases are measured at fixed photon energies with good time resolution. The results orf the X 2T1, A 2T2 and B 2E states of CF+4 (all below the energy of He I radiation) confirm earlier measurements; results for the C 2T2 and D 2A1 states at 21.7 and 25.1 eV, respectively, are new. They confirm previous measurements that radiative decay from both states is an important process, and we have measured their state-selected fluorescence quantum yields. For SF+6, fluorescence does not occur from any of the excited valence states, and we have measured the fragmentation channels and branching ratios for all of the valence states.

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...

On the field ionisation of diatomic molecules by intense laser fields

The process of multiple ionisation using intense lasers is discussed in terms of a simple field ionisation model. The model provides predictions of laser intensities (or laser E fields) required to produce sequential ionisation in atomic Xe that are in broad agreement with experiment. In the case of diatomic molecules, mechanisms are introduced which enhance the ionisation probability. Measurement of the energies of the fragment ions resulting from the subsequent Coulomb explosion provides a basis for a detailed description of the dynamics of multielectron dissociative ionisation.

The multielectron dissociative ionization of molecular iodine in intense laser fields

The multielectron dissociative ionization of molecular iodine has been studied using a laser of wavelength 750 nm, pulse widths (FWHM) of 200 fs and 400 fs and focused intensities in the range 1013-1015 W cm-2. Covariance mapping has been employed to show that the various transient ions, ranging from I22+ to I210+, fragment in a predominantly charge-symmetrical manner. Results are compared with earlier data taken at a pulse length of 90 fs.

The double photoionisation of NH3 using the triple coincidence (PEPIPICO) technique

A simple development of the PEPIPICO technique, utilising the continuum nature of synchrotron radiation, has been used to determine thresholds for fragmentation of NH3 into the channels NH+2+H+ (35.7 and 39.6 eV), NH++H+ and NH++H+2 (43.3 eV) and N++H+ (49.2 eV). The threshold of NH2+3 metastable ion production has also been determined (33.7 eV).

Fragmentation of the C̃ 2T2 and D̃ 2A1 states of CCl+4 and SiCl+4 studied by threshold photoelectron—photoion coincidence spectroscopy

Abstract Threshold photoelectron—photoion coincidence (TPEPICO) spectroscopy has been used to study the decay pathways of the C 2 T 2 and D 2 A 1 excited electronic states of CCl + 4 and SiCl + 4 in the range 14–23 eV. Vacuum UV photons from a synchrotron radiation source ionise the parent neutral molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing ion yield curves to be obtained. The C and D states of CCl + 4 fragment exclusively to CCl + 2 and CCl + respectively, corresponding to non-statistical routes. Radiative decay from the C state of SiCl + 4 is an important process and this state fragments indirectly to SiCl + 3 by fluorescence from the C state, followed by dissociation from lower-lying repulsive states. The D state of SiCl + 4 fragments exclusively to SiCl + 2 . Kinetic energy releases into CCl + 2 + Cl 2 and SiCl + 3 + Cl have been measured at the Franck—Condon maximum of the C 2 T 2 states of the two parent ions.

Multielectron Dissociative Ionization of Co2 In Intense Laser Fields

The multiphoton ionization of CO2 has been studied using a laser of 794 nm wavelength and focused intensities in the region of 1016 W cm-2. The kinetic energy distributions and ion yields obtained using time-of-flight spectroscopy coupled with covariance mapping can be interpreted most simply in terms of charge-symmetric fragmentation, the strongest channel being O2++C2++O2+. These results am compared with two previous experiments on CO2, one using an intense laser art 248 nm, the other using Si8+ ion projectiles of 3 MeV u-1 energy.

The double photoionization/fragmentation of CF4 in the threshold region (35-80 eV)

The PEPIPICO technique has been used in conjunction with a source of continuum (synchrotron radiation) in order to determine the thresholds for fragmentation of CF4 into CF3++F+ (37.6 eV), CF2++F+ (42.4 eV), CF++F+ (47.5 eV) and C++F+ (62.0 eV). These thresholds are tentatively correlated with specific double-hole states of CF4.