M. Lundqvist

Resolution and signal‐to‐background enhancement in gas‐phase electron spectroscopy

The most important factors contributing to line broadening and background intensity in electron spectra are identified and new design principles for high quality measurements are presented. It is shown that time‐dependent potential gradients in the gas cell are responsible for a large part of the ‘‘normal’’ line broadening, while scattering processes inside the gas cell are responsible for a large part of the background intensity. By designing the experiment according to these results, gas‐phase electron spectra can be recorded routinely at an instrument resolution level of better than 5 meV even at comparatively high gas cell pressures necessary to give high intensity for weak lines, and in principle unlimited counting times, at a much improved signal‐to‐background level. The resulting improvements in the spectral quality are demonstrated by spectra of Ar and HBr.

Doppler-free kinetic energy release spectrum of

A Doppler-free kinetic energy release spectrum of has been recorded using a coincident time-of-flight technique, resolving vibrational structure in the optically active A , D and D electronic states. A comparison is made with earlier photofragment and optical measurements. The v = 7 - 10 levels of the A state, dissociating into the lowest atomic level , are observed in the spectrum. The lifetime of the v = 7 level was determined to ns while the absence of lower levels indicates lifetimes above . The D state was found to support only two vibrational levels, dissociating into . The observed levels of the A and D states may dissociate either via tunnelling through the potential barriers separating the levels from the dissociation continuum or via electronic predissociation. Experimental data of the D state indicate that optical transitions into the state dominate the decay of the v = 0 level, while electronic predissociation dominates the decay of higher levels. For the v = 0 and 1 levels of the D state predissociation is observed into the and the ionic states, while predissociation into the states is observed for the v = 2 and 3 levels.

Observation of predissociation and tunnelling processes in O-2(2+): A study using Doppler free kinetic energy release spectroscopy and ab initio CI calculations

A Doppler free kinetic energy release spectrum of O-2(2+) has been recorded at a line width (FWHM) of 40-60 meV using a coincidence time-of-flight technique. Vibrational structure has been resolved ...

A study of the vibronic structure in the HeI excited photoelectron spectrum of CO2 involving the X 2Πg and A 2Πu ionic states

A study of the vibronic structure in the HeI excited photoelectron spectrum of CO2 involving the X 2Pig and A 2Piu ionic states

AN EXPERIMENTAL STUDY OF THE VALENCE SHELL PHOTOELECTRON SPECTRUM OF HYDROGEN SULPHIDE

Abstract The complete valence shell photoelectron spectrum of hydrogen sulphide has been studied in the 10 to 25 eV binding energy region using He I, He II and synchroton radiation. Using monochromated synchroton radiation, photoelectron angular distributions and branching ratios have been determined in the 14 to 120 eV photon energy range. The results suggest that most of the prominent inner valence features should be associated with ionisation from the 4a 1 molecular orbital. High resolution spectra have been obtained over a similar binding energy range using He I and He II excitation. Several narrow photoelectron bands have been observed in the region between 22.5 and 24.5 eV, and are associated with vibrational progressions belonging to two different electronic states. The He I excited spectrum has allowed a detailed analysis to be made of the vibrational and rotational fine structure exhibited in the photoelectron bands associated with ionisation from the 2b 1 and the 5a 1 orbitals. For the A 2 A 1 state, comparisons are made with the rovibronic structure predicted by Duxbury et al.

A study of turn-off limitations and failure mechanisms of GTO thyristors by means of 2-D time-resolved optical measurements

The failure mechanisms of Gate-Turn-Off (GTO) thyristors are investigated. Measurements based on a time-resolved free-carrier absorption (FCA) technique are used to support the presented models. The measurements serve to map the local carrier densities two-dimensionally, at any time of the switching cycle. Inductively loaded GTOs under snubberless operation are studied close to the safe-operating area (SOA) limit. Several important features of the destructive process are established. First, the existence of a quasi space-charge region, QSC, in the n base and charge focusing at the anode side of the device is noticed during the fall-time period. Secondly, a piling up of holes in the p base is followed by current filamentation during the tail period. The sequential behaviour of different phenomena and their possible causal explanations are also established. The expansion of the QSC towards the anode emitter causes the enhancement and focusing of the charge distribution in the vicinity of this junction. Two possible failure mechanisms, viz. local dynamic punch-through in the n base and local dynamic avalanche injection in the blocking junction are discussed in the light of the experimental results. It is also suggested that both the above mentioned failure mechanisms can be part of the chain of events leading to device failure. Regardless of which mechanism dominates, a high peak of excess holes appears in the p base in the beginning of the tail period. This charge debiases the cathode junction locally, i.e. compensates the negative bias of the cathode junction, and current filaments connecting the cathode and anode sides of the device are formed. The excessive power dissipation in the dominant filament causes failure and permanent damage.

Multiple proton energy irradiation for improved GTO thyristors

Abstract Proton irradiation using three proton energies and a variety of fluence combinations has been used to improve the trade-off between steady-state and turn-off losses for Gate Turn-Off (GTO) thyristors. Using a fluence of 9 · 10 10 cm −2 near the anode, 4 · 10 10 cm −2 in the middle of the device and 1 · 10 10 cm −2 close to the blocking junction, the turn-off energy is reduced by 50% relative to electron-irradiated standard components. The electrical results are consistent with the results of infrared free carrier absorption measurements performed on single GTO thyristor cells during both steady state and turn-off. The charge dynamics in selected regions of the thyristor is discussed and charge carrier lifetime is deduced by analytical methods for various proton fluence and energy combinations.

A novel technique for the simultaneous measurement of ambipolar carrier lifetime and diffusion coefficient in silicon

Abstract Using the classical semiconductor continuity equation, a new method for the simultaneous extraction of the ambipolar carrier lifetime and the ambipolar diffusion coefficient for high injection levels is developed. The method uses carrier decay measurements based on the free-carrier absorption (FCA) technique. The measurements are performed in the base of p-i-n type diodes, where the middle region is a lightly n-doped base region. By varying the forward bias, the ambipolar lifetime and diffusion coefficients are studied for different excess-carrier concentrations ranging approx. 2–4 decades above the base doping. Both the lifetime and diffusion coefficients agree well with theoretical models. The theoretical ambipolar diffusion coefficients agree well with theoretical models. The theoretical ambipolar diffusion coefficient is calculated using the values of Dn and Dp, which, in turn, are determined from mobility models and the Einstein relation. The mobility models used include carrier-carrier scattering effects which are important in the explanation of high-injection dependence.

Doppler-free kinetic energy release spectrum of NO2+ and ab initio CI calculations

Abstract The nitric oxide dication, NO 2+ , is studied using Doppler-free kinetic energy release spectroscopy and ab initio CI calculations. The C 2 Σ + and 2 4 Π states have been observed and studied at vibrational resolution. The B 2 Σ + state has also been investigated. The ν = 0 level of the C 2 Σ + state dissociates by tunnelling through a potential barrier into N + ( 3 P) + O + ( 2 D). Spin-orbit interaction with a repulsive 6 Σ + state accounts for the dissociation of the ν = 0 level in the 2 4 Π state into the ground state ion pair N + ( 3 P) + O + ( 2 P)). Predissociation into N + ( 3 P) + O + ( 4 S) due to (N + ( 3 P) + O + ( 2 D) and N + ( 3 P) + O + ( 2 P)). Predissociation into N + ( 3 P) + O + ( 4 S) due to spin-orbit coupling with the 1 4 Π state governs the dissociation of the first three vibrational levels in the B 2 Σ + state. The apparent relative population of the vibrational levels in this state is strongly influenced by radiative transitions to the X 2 Σ + state.

ROTATIONAL FINE-STRUCTURE IN THE UV PHOTOELECTRON-SPECTRA OF HF AND HCL

Abstract He I and Ne I excited photoelectron spectra of the HF and HCl molecules have been recorded at a photoelectron linewidth (FWHM) of 2.5–3.5 meV. At this resolution much of the rotational structure has been resolved in the photoelectron bands associated with the X 2II state of HF+ and the A 2∑+ states of HF+ and HCl+. For the X 2II state of HCl+ a marked influence from the rotational structure has been observed on the line profile. From these spectra the following adiabatic binding energies have been obtained: 16.0460 eV (X, HF+), 19.1154 eV (A, HF+), 12.7447 eV (X, HCl+) and 16.2678 eV (A, HCl+). A detailed analysis of the rotational structure has been made for HF by comparison with the results of recent ab initio calculations. A good overall agreement is found between the experimental and theoretical results. The experimental results indicate the presence of a dissociation barrier in the potential curve of the A state that is 50 meV in HF+ and 60 meV in HCl+. For the A state of HCl+ the rotational structure disappears in the range of v = 8,9 and 10, suggesting a lifetime in this range of about 10−13 s. This lifetime limitation is associated with an interaction with repulsive states leading to predissociation at the location of the curve crossings.