A.E.R. Malins

The Mobile Luminescence End-Station, MoLES: a new public facility at Daresbury Synchrotron

A new mobile end-station is described for use on multiple beamlines at the Daresbury synchrotron radiation source (overall excitation range 5 eV to 70 keV) that allows for the detection and dispersion of photoluminescence from solid-state samples in the emission range 190-1000 nm (1.2-6.5 eV). The system is fully self-contained and includes sample-cooling facilities for the temperature range 8-330 K using a closed-cycle refrigerator, thus eliminating the need for liquid cryogens. The system also includes solid-state laser sources for use with a variety of pump-probe-type experiments, and an Ar(+) surface-cleaning facility. In order to demonstrate the various capabilities of the system, the results of a variety of experiments are summarized, carried out over the excitation range 5-5000 eV on beamlines 3.2, MPW6.1 and 4.2. These include the optical detection of XAS of L-edge structure in natural minerals and archaeological ceramics, band-gap determinations of wide-band-gap silicates, and pump-probe studies of quartz.

Luminescence excitation characteristics of Ca, Na and K-aluminosilicates (feldspars) in the stimulation range 5–40 eV: determination of the band-gap energies

The first comprehensive survey of band-edge features in the ternary group of naturally occurring aluminosilicates (the feldspars) is presented. Synchrotron-based luminescence excitation of KAlSi3O8, NaAlSi3O8 and CaAl2Si2O8 allows the measurement of the evolution of the band-gap across the system, which, at 8 K, is found to vary from 7.86 eV in NaAlSi3O8 to 7.7 eV and 7.62 eV in KAlSi3O8 and CaAl2Si2O8, respectively: the band-gap energies are typically 0.1 eV smaller at 300 K. In comparison with measurements made on natural and synthetic hydrothermal α-quartz, where both the direct and indirect band-gap structures are distinctly observable, no significant post-edge band structure is discernable in the feldspars. In Ca-rich material, the luminescence is attenuated by more than two orders of magnitude for excitation energies up to 3 eV above the band-gap, partly supporting the proposition (derived from previous cyclotron resonance experiments) that the bands are simply parabolic and that the materials may be direct band-gap insulators. The luminescence excitation experiments also allow an initial survey to be made of sub-band-gap features in the materials, including low mobility, temperature sensitive band-tail states and mid-gap defects.

Threshold photoelectron–photoion coincidence spectroscopy study of CHCl2F+, CHClF+2 and CH2ClF+: Steric influence of the chlorine, fluorine and hydrogen atoms

The threshold photoelectron spectrum and threshold photoelectron-photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3-24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C-F or C-Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C-Cl vs. C-F bond cleavage in CHCl2F+ and CHClF2+.

Threshold photoelectron spectroscopy of iodine monobromide

Abstract A high-resolution (1–11 meV) threshold photoelectron spectroscopic study of ICl has been performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Accurate vibrational constants have been obtained for both spin–orbit components of the ICl + (X 2 Π i ) state through the observation of extended vibrational bands in the Franck–Condon gap regions of the molecular ion. The appearance of these vibrational bands is attributed to resonant autoionization of Rydberg states residing in this energy region of the ionic state. The adiabatic ( v + =0) ionization potentials for formation of the ground state of the ICl + ion are found to be 10.076±0.002 eV for (X 2 Π 3/2 ) and 10.655±0.002 eV for (X 2 Π 1/2 ) yielding a spin–orbit splitting in this state of 0.579±0.002 eV. The adiabatic ( v + =0) ionization potential for formation of the (A 2 Π i ) state of ICl + is estimated to be at ∼12.5 eV. Additional autoionization effects are noted in the formation of the (A 2 Π i ) and (B 2 Σ + ) band systems in ICl + .

Measuring modulated luminescence using non-modulated stimulation: ramping the sample period

Conventional methods of recording linearly modulated (LM) optically stimulated luminescence (OSL) require control over either the exciting light intensity, or the ability to pulse the source. For many light sources (e.g. constant-power CW lasers, arc lamps and synchrotrons) this can be problematic. Directly analogous results to LM-OSL can, however, be achieved with non-modulated excitation sources, by ramping the sample period (RSP) of luminescence detection. RSP-OSL has the distinct advantage over LM-OSL in that, since the excitation remains at full power, data accumulation times (that can be considerable) can be reduced by typically 50%. RSP methods are universally applicable and can be employed, for example, where the excitation source is constant heat, rather than light: here, iso-thermal decay of phosphorescence becomes recorded as a sequence of peaks, corresponding to de-trapping of charge from different defect levels, and is particularly useful for analysing shallow-trap effects. RSP methods are also useful in providing significant compaction of data sets, where signal analysis is required of overlapping systems having a wide range of decay kinetics.

Luminescence excitation characteristics of Ca-, Na- and K-aluminosilicates (feldspars), in the stimulation range 20–500 eV: optical detection of XAS

We demonstrate that the visible/UV luminescence from common feldspar crystals (NaAlSi3O8, KAlSi3O8 and CaAl2Si2O8) can be used to detect detailed L-edge and associated near-edge absorption structure of the main constituent atoms (Ca, K, Na, Al, Si), when exciting in the energy range 20–500 eV. Comparisons of the spectral features are drawn with similar measurements made on the associated materials SiO2, Al2O3 and CaCO3. The potential for using optically detected x-ray absorption spectroscopy as a method for identifying the luminescent components of mixed mineral samples is considered.

The photoionization dynamics of the three structural isomers of dichloroethene

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron spectrum, threshold photoelectron photoion coincidence spectrum and ion breakdown diagram of the 1,1, cis-1,2 and trans-1,2 isomers of C2H2Cl2 have been recorded in the range 9–23u2009eV. The energies of the peaks in the threshold photoelectron spectrum are in good agreement with outer-valence Greens function calculations. The major difference between the isomers, both predicted and observed experimentally is that the and states of are approximately degenerate for 1,1 and trans-1,2, but well separated for the cis-1,2 isomer. The ground and low-lying valence states of are bound, with higher-lying states dissociating to C2H2Cl+ or . The translational kinetic energy release into C2H2Cl+u2009+u2009Cl is determined as a function of energy. Isolated-state behaviour for the low-lying electronic states of becomes more statistical as the energy increases.

Threshold photoelectron photoion coincidence spectroscopy of trichloroethene and tetrachloroethene

The threshold photoelectron, the threshold photoelectron photoion coincidence and ion breakdown spectra of trichloroethene and tetrachloroethene have been recorded from 9–22 eV. Comparisons with the equivalent data for the three dichloroethene molecules and theoretical calculations highlight the nature of the orbitals involved during photoionisation in this energy range. The ground electronic state of ( ) is bound, with excited valence states dissociating to ( ) and C2HCl+ ( ). Appearance energies suggest that C2HCl+ forms from by loss of two chlorine atoms, whereas forms from by loss of a Cl2 molecule. The translational kinetic energy release into ( ) + Cl is determined as a function of energy. In both cases, the fraction of the available energy released into translational energy of the two products decreases as the photon energy increases.

Threshold photoelectron spectroscopy and photoionization total ion yield spectroscopy of simple organic acids, aldehydes, ketones and amines

We have initiated a research program to investigate the ionization behavior of some simple organic molecules containing the carboxyl group (R2C=O), where R could be H, OH, NH2, or CH3 or other aliphatic or aromatic carbon groups, using threshold photoelectron spectroscopy and photoionization total ion yield spectroscopy. We report here on the simplest organic acid, formic acid, and two simple aldehydes: acetaldehyde and the simplest unsaturated aldehyde, 2-propenal (acrolein). The objective of this study was to characterize the valence cationic states of these molecules with vibrational structural resolution.