Metastable de-excitation

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. 

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)... 

The method of exchange-luminescence [46, 47] is based on the phenomenon of energy transfer from the metastable levels of EEPs to the resonance levels of atoms and molecules of de-exciter. The EEP concentration in this case is evaluated by the intensity of de-exciter luminescence. This technique features sensitivity up to-10 particle/cm, but its application is limited by flow system having a high flow velocity, with which the counterdiffusion phenomenon may be neglected. Moreover, this technique permits EEP concentration to be estimated only at a fixed point of the setup, a factor that interferes much with the survey of heterogeneous processes associated with taking measurements of EEP spatial distribution. 

If the work function is smaller than the ionization potential of metastable state (see. Fig. 5.18b), then the process of resonance ionization becomes impossible and the major way of de-excitation is a direct Auger-deactivation process similar to the Penning Effect ionization a valence electron of metal moves to an unoccupied orbital of the atom ground state, and the excited electron from a higher orbital of the atom is ejected into the gaseous phase. The energy spectrum of secondary electrons is characterized by a marked maximum corresponding to the... 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

From the above-made review of literature, one may infer that the interaction of metastable atoms of rare gases with a surface of semiconductors and dielectrics is studied, but little. The study of the mechanism of transferring energy of electron-excited particles to a solid body during the processes under discussion is urgent. The method of sensor detection of rare gas metastable atoms makes it possible to obtain new information about the heterogeneous de-excitation of metastable atoms inasmuch as it combines high sensitivity with the possibility to conduct measurements under different conditions. 

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

De-excitation of Excited Rare Gas Atoms in Metastable States... 

De-excitation of the excited rare gas atoms in the resonant states has been studied less extensively than that of the metastable atoms. This is due to experimental difficulties caused by the short lifetimes of the resonant atoms. There have been reported, however, several theoretical formulations [139,140] based on a long-range dipole-dipole interaction... 

Figure 13 Plots of (

Figure 16 Relation of de-excitation cross sections (ctm) for metastable helium atoms with the Hotop-Niehaus formula for (a) He(2 S) and (b) He(2 S). (From Ref. 154.)...

Thus, it is concluded that the de-excitation of the metastable Ar( P2 and Pq) atoms is ascribed to the nonadiabatic excitation transfer at large intermolecular distance by the crossing of the intermolecular potential curves between the initial Ar -M channel and the final Ar-M channel, and the de-excitation of the resonant Ar( Pi and Pi) to the resonant excitation transfer by the dipole-dipole interaction [153]. This conclusion is compatible with the result of the above-mentioned conclusions for the de-excitation of He(2 P) by Ne [135]. 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

In this section we deal with particles which are excited in the gas phase and then approach a surface. Excitation to optically allowed state are probably not important in this context since they will remain in the excited state for a period less than 10 sec. During this period they only travel lmm. However, metastable molecules and ions have long lifetimes and therefore may be de-excited primarily at surfaces. The question, Do these excited molecules react differently from ground state molecules is the topic of this section. Since little information is available, the discussion is highly speculative. 

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