ZEKE spectroscopy

The ZEKE spectroscopy (ZEKE short for English zero-electron kinetic energy or even zero kinetic energy ) is a special form of photoelectron spectroscopy is, in which only electrons are detected, the (almost) no kinetic energy have ( zero kinetic energy ).

functionality

Spherically symmetric Coulomb potential (left) and the Coulomb potential in the electric field (right).

The gas to be examined is irradiated with a short laser pulse. After this laser pulse has subsided, the time is waited. During this time, all electrons move out of the examination area. With the help of an electric field all remaining electrons are sucked out and measured after the expiration of . With this method, electrons in particular can be detected at the ionization limit. These zero-energy electrons are a signature of the different Rydberg states and can be filtered through a pipe channel and detected with the CW method or by pulse delay after all hot electrons have been extracted. Since no charged surfaces are used here, the resolution is increased by many orders of magnitude compared to normal photoelectron spectroscopy. ${\displaystyle \tau }$ ${\displaystyle E>E_{c}}$ ${\displaystyle \tau }$

Estimate of ${\displaystyle E_{c}}$

Typical dimensions of the examination area are in the millimeter range, can be set in the millisecond range. Furthermore, a radial symmetry of the examination area ( r  = 1 mm) and an ionization in its center are assumed. In order to get out of the examination area and consequently no longer to be measured, an electron with the energy has to travel a distance of 1 mm. In the case of non-relativistic electrons applies to the kinetic energy ${\displaystyle \tau }$ ${\displaystyle E}$

${\displaystyle E={\frac {m}{2}}v^{2}}$

and thus

${\displaystyle v={\sqrt {\frac {2E}{m}}}}$

An electron leaves the examination area if applies

${\displaystyle r'=v\tau ={\sqrt {\frac {2E}{m}}}\tau >r}$

It is therefore true

${\displaystyle E_{c}=\left({\frac {r}{\tau }}\right)^{2}\cdot {\frac {m}{2}}}$

With , and results ${\displaystyle m=\mathrm {9{,}11\cdot 10^{-31}\,kg} }$ ${\displaystyle r=\mathrm {1\cdot 10^{-3}\,m} }$ ${\displaystyle \tau =\mathrm {1\cdot 10^{-2}\,s} }$

${\displaystyle E_{c}=\mathrm {9{,}11\cdot 10^{-33}\,{\frac {kg\cdot m^{2}}{s^{2}}}} =\mathrm {5{,}6\cdot 10^{-14}\,eV} }$

For the specified values, only electrons with a kinetic energy below are detected. ${\displaystyle \mathrm {5{,}6\cdot 10^{-14}\,eV} }$

Advantages of ZEKE Spectroscopy

In contrast to classic photoemission spectroscopy, with ZEKE spectroscopy it is not necessary to separate the electrons according to their energy. The devices become simpler. Furthermore, the yield of electrons is significantly greater, as these are normally distributed in all spatial directions. Only a small part of the electrons of a certain energy is measured. In ZEKE spectroscopy, however, all electrons are detected that have the corresponding energy. Compared to PES and many other methods, distortions caused by the surface potential of the spectrometer are avoided here.

See also

• Rydberg state

literature

• Eckhard Waterstradt: ZEKE spectroscopy on molecules and molecule clusters with XUV, VUV and multiphoton excitation. Herbert Utz Verlag, 1999, ISBN 3896751905 .
• Hans-Jörg Dietrich: Long-lived, molecular Rydberg states in ZEKE spectroscopy: ionization dynamics and new experimental techniques. Herbert Utz Verlag, 1996, ISBN 978-3896751591 .
• EW Schlag : ZEKE Spectroscopy. Cambridge University Press 1998, ISBN 978-0521675642

Individual evidence

1. ↑ Gerhard Drechsler: Photoelectron spectroscopy with zero-energy electrons. In: News from chemistry, technology and the laboratory. 40, No. 1, 1992, pp. 20-22, doi : 10.1002 / nadc.19920400107 .