7.2 History
169
determined values (Bartell 1955). To study compounds with low volatility, special
constructions of the high-temperature evaporator and effusion cell were created for
the first time by Frost et al. (1953).
Since the mid-1960s, i.e., after the advent of manufactured diffractometers, the
gas-phase electron diffraction method has been widely used in structural chemistry
(see Fig. 7.1). The application of fast computers had also a strong impact on the
development of the method.
To facilitate the control and optimization of vapor composition as well as to
study unstable species, Hargittai et al. pioneered the use of electron diffraction with
mass spectrometry (Tremmel et al. 1978). Since 1986, the synchronous electron
diffraction/mass-spectrometric experiments have been performed by Girichev et al.
(1986, 1988).
An alternative method of recording the electron scattering intensity was introduced
by Iijima et al. (1998). The image plates with radiation-sensitive layers characterized
by a very large sensitivity region can be applied instead of photographic plates without
setup of the diffractometer (Gundersen et al. 2007; Berger et al. 2009; Vogt et al.
2011a).
To study transient chemical species, the ultrafast electron diffraction (timeresolved electron diffraction is used as synonym) has been developed after pioneering
work of Ischenko et al. (1983). This new method has been explored by Wilson and
coworker theoretically (Ben-Nun et al. 1996) and by Zewail and coworker experimentally (Srinivasan et al. 2003). The developed technique is used for generation
of timed sequences of ultrafast pulses, namely a femtosecond laser pulse for the
excitation of molecules and ultrashort electron pulses to probe the ensuing structural
changes. Direct imaging is carried out by means of a charge coupling device (CCD)
camera (Ihee et al. 2001). Because of the formidable experimental challenges, there
are still only a few molecular structures studied by the ultrafast electron diffraction.
This method is reviewed in detail by Carley et al. (2005) and Ischenko et al. (2017).
7.3 A Short Description of a Conventional Gas-Phase
Electron Diffraction Experiment
When a monochromatic beam of electrons interacts with randomly oriented
molecules, a diffraction pattern arises that can be recorded by a suitable detector.
The electrons are thermally emitted from a cathode and accelerated by the potential
U between the cathode and the anode. The electron beam is focused by means of
diaphragms as well as electromagnetic lenses, and its diameter is ≈0.1 mm. The
wavelength of the electrons λ is a function of U: λ = h/(2meU)
1/2 , where m and e
are the mass and the charge of the electrons, respectively, and h is Planck’s constant.
However, at the accelerated voltages employed in the experiments (between 40 and
100 kV), the λ value must be corrected for relativistic effects (by (1 + E/mc
2 )
−1/2 ,
169
determined values (Bartell 1955). To study compounds with low volatility, special
constructions of the high-temperature evaporator and effusion cell were created for
the first time by Frost et al. (1953).
Since the mid-1960s, i.e., after the advent of manufactured diffractometers, the
gas-phase electron diffraction method has been widely used in structural chemistry
(see Fig. 7.1). The application of fast computers had also a strong impact on the
development of the method.
To facilitate the control and optimization of vapor composition as well as to
study unstable species, Hargittai et al. pioneered the use of electron diffraction with
mass spectrometry (Tremmel et al. 1978). Since 1986, the synchronous electron
diffraction/mass-spectrometric experiments have been performed by Girichev et al.
(1986, 1988).
An alternative method of recording the electron scattering intensity was introduced
by Iijima et al. (1998). The image plates with radiation-sensitive layers characterized
by a very large sensitivity region can be applied instead of photographic plates without
setup of the diffractometer (Gundersen et al. 2007; Berger et al. 2009; Vogt et al.
2011a).
To study transient chemical species, the ultrafast electron diffraction (timeresolved electron diffraction is used as synonym) has been developed after pioneering
work of Ischenko et al. (1983). This new method has been explored by Wilson and
coworker theoretically (Ben-Nun et al. 1996) and by Zewail and coworker experimentally (Srinivasan et al. 2003). The developed technique is used for generation
of timed sequences of ultrafast pulses, namely a femtosecond laser pulse for the
excitation of molecules and ultrashort electron pulses to probe the ensuing structural
changes. Direct imaging is carried out by means of a charge coupling device (CCD)
camera (Ihee et al. 2001). Because of the formidable experimental challenges, there
are still only a few molecular structures studied by the ultrafast electron diffraction.
This method is reviewed in detail by Carley et al. (2005) and Ischenko et al. (2017).
7.3 A Short Description of a Conventional Gas-Phase
Electron Diffraction Experiment
When a monochromatic beam of electrons interacts with randomly oriented
molecules, a diffraction pattern arises that can be recorded by a suitable detector.
The electrons are thermally emitted from a cathode and accelerated by the potential
U between the cathode and the anode. The electron beam is focused by means of
diaphragms as well as electromagnetic lenses, and its diameter is ≈0.1 mm. The
wavelength of the electrons λ is a function of U: λ = h/(2meU)
1/2 , where m and e
are the mass and the charge of the electrons, respectively, and h is Planck’s constant.
However, at the accelerated voltages employed in the experiments (between 40 and
100 kV), the λ value must be corrected for relativistic effects (by (1 + E/mc
2 )
−1/2 ,