Metastable Impact Electron Spectroscopy

MIES is a very surface sensitive examination method, where metastable rare gas atoms of thermal energy interact with the surface of solids under the emission of electrons. The kinetic energy of the metastable rare gas atoms, in our case, helium is about 58 meV. This energy is neither sufficient to penetrate the sample surface, nor to cause mechanical sputtering effects. All processes take place at a distance of about 3-10 Å in front of the surface. Upon impact of  the metastable helium atoms on the surface these are partially deexcitated and undergo a transition to the ground state. The probability for this is ranging from 0.1 to 0.6 depending on the work function of the solid surface. Upon transition to the ground state electrons from the sample surface can be emitted. Their energy distribution allows to obtain insights into the electronic structure of the surface. For proper interpretation of the measurement results the mechanisms causing the emission of electrons and their probabilities must be known. The mechanisms are the following.

Auger Deexcitation (AD)

The Auger deexcitation process (schematic diagram in Fig. 3.2) is also referred to as surface Penning ionization. It takes place largely independent of the work function upon impact of a metastable helium atom on a solid surface of an insulator, semiconductor or metal with a transition rate in the range of 3*1014 s-1 .

AD-Prozess
Fig. 3.2: The AD process

An electron from an occupied state of the sample surface with the binding energy EB transfers to the unoccupied 1s state of the He* atom. Subsequently, in an interatomic Auger process the energy EB - E1s is transferred to the 2s electron, which is emitted. The following energy balance results from Fig. 3.2 .:

Gleichung 3.9 (3.9)
Gleichung 3.10 (3.10)
(3.11)

Substituting the excitation energy E* of the metastable helium atoms by hw, the equation (3.10) assumes the shape of the equation for UPS. Evening out the differences in excitation energies by superimposing the spectra with matching Fermi levels the MIES-spectra resulting from an auger deexcitation process can be compared with UPS-spectra. However it has to be noted that the Auger deexcitation spectrum contains only information of the electronic structure of the surface, while in the UPS spectrum this information is superimposed by bulk properties.

Resonance transfer and Auger neutralization (RT + AN)

The resonance transfer with subsequent Auger neutralization is only possible if the 2s electron of the He* atoms can tunnel into an unoccupied state of the solid surface. This process takes place on metal and semiconductor surfaces but not on insulators, since the valence band of an insulator is below the 2s level of the  He*. Fig. 3.3 shows a schematic representation of these two processes.

 

RT + AN - Prozess
Fig. 3.3: Resonance transfer and Auger neutralization

The work function of the solid surface must be greater than the asymptotic ionization potential (IP*) of the metastable helium. This is understandable due to the greater overlap of the excited orbital of He* with free states above the Fermi level compared to the overlap of the ground state orbital with occupied states of the metal conduction band. Therefore the resonance transfer process takes place at a greater distance to the surface than the Auger deexcitation process. For a sufficiently large work function (approximately 3.8 eV) the resonance transfer with subsequent Auger neutralization thus predominates in general. On resonance transfer the 2s electron of He* tunnels into an unoccupied surface state. The thereby resulting positively charged helium ion (He+) is neutralized by an electron of the solid. Within this neutralization process, the transition energy is transferred without electromagnetic radiation to another electron of the solid which is emitted. The emitted Auger electrons originate from the outermost atomic layer, since the transition rate (1*1016 s-1)  of the Auger neutralization process essentially depends on the overlap of the solid surfaces occupied states and the unoccupied 1s state of the helium ion. The energy distribution N(E) of the emitted electrons is described by a self convolution of the spectroscopically studied states.

Gleichung 3.12 (3.12)

D(E) denotes the spectroscopically studied surface density of states (SDOS), M1 and M2 denote the transition matrix elements of electron transitions (in Fig. 3.3 referred to as RT(1) and AN(2)). The spectra consist of a convolution, since there are always two electrons of the solid with independent transition matrix elements (M1, M2) involved in the RT + AN process. Because of this convolution one obtains relatively featureless spectra compared to AD spectra. The convolution the spectra consist of can be approximately deconvoluted by differentiation.

Energy analysis

For energy analysis, we consider Fig. 3.3.

(3.13)

In the following equations E denotes the average energy of the two electrons involved in the Auger neutralization process.

(3.14)
(3.15)
(3.16)
(3.17)

Electrons originating from the Fermi level, have the maximum kinetic energy. From (3.16) follows with E = EF:

(3.18)

With Ekin,Pr = 0 follows from (3.17) the low-energy use of the spectrum:

(3.19)

The width of the spectrum results from (3.18) and (3.19):

(3.20)

Resonant transfer (RT) with autodetachment (AU) or Auger deexcitation (AD)

In case of a very small work function (< 2.2 eV), an electron from an occupied state of a solid, a metal or a semiconductor can tunnel resonant in the singly occupied (2s) state of He* (RT). With insulators this cannot happen, since the upper edge of the valence band is much lower than the He*(2s) state. The resultant excited helium ion He-*(1s2s2) may undergo a transition in the ground state by autodetachment (AU) or by Auger deexcitation (AD) (Fig. 3.4).

RT+AU und RT+AD
Fig. 3.4: Resonant transfer + autodetachment, resonant transfer + Auger deexcitation


RT + AU

One 2s electron of He-* transitions in an intraatomic Auger process into the unoccupied (1s) state. The transition energy is transmitted to the second 2s electron which is emitted. In the spectrum this process leads to a sharp structure in the vicinity of the Fermi level, since this process can take place only when the 2s level of the He-* is below the Fermi level of the solid.

RT + AD

The unoccupied (1s) state is occupied in an interatomic Auger process by an electron originating from an occupied state in the solid. The transition energy is transferred to the two 2s electrons of the He-*, which are emitted.

Singlet-triplet conversion

The processes described so far are independent of the spin state of helium. They take place in both the singlet (21S0), as well as in the triplet state (23S1). The singlet to triplet ratio was determined by collisions of He-* and argon. For  our source this is 1:7. The singlet state has an excitation energy that is by 0.8 eV higher than the one of the triplet state. In front of metallic solids the singlet state is converted to the triplet state before the above-mentioned processes take place. The following mechanisms are leading to this conversion (Fig. 3.5).

Singulett-Triplett-Konversion
Fig. 3.5: singlet-triplet conversion, REC model, ADC model, AEC model

 

REC model (Resonant Excitation Conversion - model)

If the Fermi level of the solid is located between the singlet level and the triplet level, the 21S-electron of He-* (in the singlet state) can tunnel into an unoccupied state in the solid, after which the triplet state of the resultant He+ is occupied resonant by an electron from the solid.

ADC model (Auger Deexcitation Conversion - model)

This model applies to the case that the Fermi level of the solid is above the (21S) state of He*. An electron from the solid occupies the (23S) state in He* forming a He-*(1s2s2S) ion. The transition energy is transmitted to the electron in the (21S) state which is emitted. The He* (1s2s2S) ion transitions in the He* (23S) state.

AEC model (Auger excitation Conversion - model)

As in (f) the Fermi level of the solid body is here above the (21S)-state of He*. The (23S) state is occupied by an electron from the solid. The transition energy is transferred in an intramolecular Auger process to the electron in the (21S) state, which then occupies an unoccupied state in the solid. For insulators there is no conversion, since the upper edge of the valence band is much lower than the He*(2s) level, so that the conversion processes cannot take place.

For more information see here.

 

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