Free-electron laser, FEL

Freedom of Information Electronic Reading Room (ODER), 15 701-702 Free electron lasers (FELs), 1 720 Free energy, of antigen-antibody binding events, 74 138... 

We have considered in particular the case of multiphoton transitions, to be observed with the help of intense high frequency fields as produced by X-ray Lasers or Free-Electron Lasers (FEL). As a result of our analysis, we have shown that two-photon bound-bound transition amplitudes in high-Z hydrogenic systems are significantly affected by relativistic corrections, even for low values of the charge of the nucleus. For instance at Z = 20, the corrections amount to about 10%, a value much higher than what is observed for standard one-photon transitions in X-ray spectroscopy measurements for which the non-relativistic dipole (NRD) approximation agrees with the exact result to within 99% at comparable frequencies. 

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

In conventional lasers, the emitted radiation occurs between energy states involving bound electrons in atoms or molecules. Energy state population is modified by pumping the initial state to higher levels by means of an external source. The electron modes of motion are discrete so that absorption and emission of photons involves two (or more) quantized levels (except where the final state in the laser emission process is above the dissociation limit e.g.in some excimer lasers). On the other hand, free electron lasers (FEL) operate on free-free transitions i.e. by energy changes of unbound electrons, so that any state of electron motion... 

FIGURE 11 The average brilliance predicted for the Linac Coherent Light Source (LCLS) and TESLA Test Facility (TTF) free-electron laser (FEL) facilities in comparison with present-day synchrotron light sources. APS, ESRF, ALS, SBLC. 

Free-electron lasers (FELs) are essentially an outgrowth and extension of modem synchrotron light sources. The amplification and radiation are achieved by a beam of free electrons forced into transverse oscillations by a spatially periodic wiggler magnetostatic field, thereby emitting magnetic bremsstrahlung radiation in the for-... 

Microfluidics represents a good sample ground for the forthcoming free electron laser (FEL) experiments on soft biological matter. Investigating the structure and dynamics... 

While the practical application of IRMPD spectroscopy to mass-selected molecular ions had thus been shown, its widespread use as a structural tool in irai chemistry was impeded by the limited tunability of the CO2 laser and the absence of other useful laser sources featuring a high power and wide tunability across the IR spectrum. The interest in IRMPD spectroscopy of gaseous ions revived around the mm of the millennium, when IR free electron lasers (FELs) as well as novel high pulse-energy OPOs were coupled with ion storage tandem mass spectrometers. 

Unfortunately, powerful IR lasers tunable over a significant frequency range are difficult to obtain. Currently, the most effective but highly demanding approach to this end is represented by the free-electron laser (FEL). FEL facilities are limited and offer access for researchers to perform their experiments at dedicated ports on a tight schedule (Fig. 9.35). Alternatively, optical parametric oscil-lator/amplifiers (OPO/As) can serve as tunable IR light sources [139,156]. 

In recent years a completely novel concept of a tunable laser has been developed that does not use atoms or molecules as an active medium, but rather free electrons in a specially designed magnetic field. The first free-electron laser (FEL) was realized by Madey and coworkers [5.213]. A schematic diagram of the FEL is shown in Fig. 5.105. The high-energy relativistic electrons... 

A few interesting sources for future FT-IR spectrometers have been reported in the past 10 years, including the synchrotron and free electron laser (FEL) [4]. Using the radiation from a synchrotron beam line, spectra of samples as small as 10 pm in diameter (the diffraction Unfit) may be measured with veiy high SNR in times as short as 1 second. Obviously the use of these sources requires the spectroscopist to travel to a synchrotron or FEL facility with a mid-infrared beam line equipped with a FT-IR nficrospectrometer. Such facilities are available in several countries and can be used at minimal cost provided that the potential user can make a good case for the measurement... 

During the last fifteen years, Meijer and coworkers have pioneered the application of infrared (IR) Free Electron Lasers (FELs) to obtain vibrational spectra of gas-phase species. The IR-FELs have provided access to weak modes in the far-infrared part of the spectrum corresponding to, e.g. metal-metal vibrations. Additionally, the spectroscopy in the gas phase has made it possible to extract the low-frequency modes which, in the case of deposited or embedded clusters, are often obscured by absorption in the substrate. 

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