Electron-impact-induced fluorescence-spectrometry

Electron-impact induced fluorescence is a fundamental process in atomic and molecular physics and has been the topic of intense research for more than 100 years. Experimental cross sections have been important to validate theoretical descriptions of electron–atom or electron–molecule interactions in discharges, plasmas, controlled nuclear fusion or astrophysics. Electron impact excitation of rare gas atoms is a living topic, but is usually used to investigate the collision energy dependence of excitation cross sections of distinct energetic levels.

Excitation by fast electron impact is comparable to polychromatic excitation by electromagnetic waves, resonant excitation, ionization excitation, and (for molecules) dissociative (ionization) excitation may occur together for the same electron impact energy and will cause a multitude of fluorescence lines. These are investigated with this method.

The experiments are performed by a telefocus electron source attached to a target cell combined with a 1 m normal incidence spectrometer (see overview of Fig. (a)). The electrons are emitted by thermal emission of a tungsten tip with a maximum beam current of 400 µA. The source volume is defined by a set of three insulated Wehnelt-cylinders (Fig. (c)), which may be independently set on different electrostatic potentials leading to a focusing of the electron beam (Fig. (b)). For the electron acceleration the whole source, i.e., tip and Wehnelt-cylinders is set to the required high-potential, tunable between −1 and −25 kV. Electrons transmitted through the target cell are collected by a Faraday cup behind the interaction region and their current is measured.

Measurement of photon-photon and photon-electron coincidences

The measurement of time- and angle-resolved correlated photons and electrons emitted in coincidence is primarily performed using position- and time-sensitive detectors which are also used in the PIFS setup. These detectors are equipped with a photocathode for photon-measurements and in any case microchannel plates (MCPs) that amplify the incoming signal whose position is measured using a position-sensitive anode. Time information about the event is gathered from the foremost MCP that shows a drop in its operating voltage when an event is detected.

The voltage pulses that encode position and time information are amplified and recorded by a time-to-digital converter (TDC). In such a measurement, the detector that first detects an event starts the time measurement.

The same signal is delayed and recorded together with the time signal from the other detector and the bunchmarker signal for the light pulse from the synchrotron. The difference between those signals contains the information for the coincidence of detected events. Also, the position signals from the detectors that hold the information on angular correlations are recorded during this procedure.

The TDC records these signals with a timing resolution of 50 ps and sends them to the connected PC.

Photon-induced fluorescence-spectrometry

Fluorescence spectrometry of gas phase samples excited (and/or ionized) by monochromatized and polarized synchrotron radiation (photon-induced fluorescence spectrometry (PIFS)) has been proven to be remarkably fruitful as a complement to the more usual photoelectron spectrometry. As the bandwidth of the exciting radiation is decoupled from the resolution of the fluorescence spectrometer, being advantageous for high resolution experiments and for experiments where the energy of the exciting photons is scanned over a larger range at a constant spectrometer resolution. Moreover fluorescence spectrometry can be used for investigations of molecular dissociation processes into excited neutral fragments, where photoelectron spectrometry fails, and for multiple ionization processes, as long as excited states are formed, without the necessity of difficult electron coincidence experiments. 

The figure shows an overview of the current apparatus and its components: The synchrotron radiation (SR) is focused by the last SR-beamline refocusing mirror into the interaction region. Insulatedly mounted pin holes confine the interaction region; with an applied voltage the total ion yield can be measured. A photodiode, mounted as a beam dump, collects the transmitted radiation. Through two exit slits the fluorescence light of the excited atoms, one horizontal, the other vertically down enters the spectrometers. The fluorescence light is dispersed by normal incidence gratings in two 1 m spectrometers and recorded by single photon counting and position sensitive MCP Detectors (investigable wavelength range 40 - 700 nm). For horizontally linear polarized SR, a Wollaston prism splits up the linear polarized parts of the (visible range 350 - 700 nm) fluorescence. Then both polarizations are measured simultaneously, but independently. A third outlet (under the magic angle regarding the propagation vector of the SR) can be utilized to measure the grade of circular polarization of the fluorescence. This set-up utilizes a parallelizing lens, a quarterwave plate, a polarizer, a bandpass filter for the wavelength of the transition of interest, and a photomultiplier tube on which the transmitted light is focused with a second lens.