Metastable De-excitation Spectroscopy

In recent years numerous investigations of clean and adsorbate covered substrates have been carried out by different methods. As most investigations use methods which give information about the behavior in at least a few layers below the surface, there is, in comparison, not so much knowledge about the electronic properties at the surface. A distinct surface sensitivity can be achieved by electron emission caused by impact of metastable noble gas atoms, a method called metastable de-excitation spectroscopy (MDS) (see, e.g., [7-9]). This technique probes predominantly the outermost atomic layer which will be demonstrated in Sect. 5.1.2 in Chap. 5. 

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For the spin resolving photoemission and spin polarized metastable de-excitation spectroscopy measurements the films were magnetized by a current pulse through a coil close to the sample along the [110] direction of the tungsten substrate. 

In the following the magnetic properties of the topmost layer will exclusively be discussed by using spin polarized metastable de-excitation spectroscopy [45]. An overview of this experimental technique was given in Chap. 2.2. 

Apparently in contradiction the photoemission spectra (see Fig. 5.3) exhibit a dominance of majority electrons near the Fermi level. However, keeping in mind the calculation of Wu et al. [50] one can now easily realize the distinct surface sensitivity of metastable de-excitation spectroscopy (MDS) which gives predominantly information from the topmost surface layer whereas in photoemission experiments the information depth is a few layers. 

In the following discussion one will see that the use of SPUPS and spin polarized metastable de-excitation spectroscopy (SPMDS) allows to give the answers due to the capability of a direct access to the adsorbate induced states and... 

MDAD Magnetic dichroism in the angular distribution of photoelectrons MDS Metastable de-excitation spectroscopy... 

SPMDS Spin polarized metastable de-excitation spectroscopy... 

Metastable De-excitation Spectroscopy (MDS), also known as Penning Ionization Electron Spectroscopy (PIES) and Metastable Quenching Spectroscopy (MQS)... 

Fig. 3.24 Electron energy scheme explaining the principle behind metastable atom electron spectroscopy. An excited atom M collides gently with an adsorbed molecule A the metastable atom is de-excited by electron transfer from the adsorbate to the core hole...

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. 

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