Solvated electron

As a solvated electron, an electron is called, which is located in a solvent in the solution, in particular that is not bound to an atom or molecule. The name is derived from solvation. The solvated electron -containing solution is referred to as Elektridlösung.

In the solution of alkali metals in liquid ammonia, a typical blue color, on the W. Weyl in 1864 in his work came in About Metal ammonium compounds. It was already in this work as the cause of the blue color of an unknown species proposed. Not until 1962 were Edwin J. Hart ( 1910-1995 ) and Jack W. Boag identify this species as an electron, which is free by the ionization of an atom of the respective alkali metal and goes into solution.

Generation

In polar solvents such as water or alcohols, but also in nonpolar solvents such as alkanes, the solvated electron are produced artificially by radiolysis or photolysis. Different production mechanisms such as charge transfer to solvent ( CTTS ), proton transfer or ionization are possible here. The lifetime of the solvated electron can be a few hundred nanoseconds in these solvents. Electron scavenger ( scavengers) decrease in the solvent, however, the lifetime significantly.

Spectroscopic properties

Probably the most studied physical property of the solvated electron is its absorption spectrum. It is characterized by a broad structureless band extending over large regions of the visible and infrared spectral range. Maximum absorption occurs depending on the solvent in the range of 700-1100 nm, which explains the observed blue coloration. Shape, width and position of the absorption band will depend on the type of solvent, pressure and temperature.

Empirically, the absorption as a function of photon energy with maximum absorption, and are described below as a Gaussian shape for Lorentz energies above that.

The exact cause for the emergence of this waveform is so far not yet well understood as its simulation based on theoretical models indeed gives good qualitative results, but quantitatively, the measured values ​​can not describe satisfactorily.

Theoretical models

To explain the spectroscopic properties of the solvated electron various models are discussed in the literature:

• Cavity model
• Dielectric - continuum model

The cavity model is based on the assumption that the solvated electron is surrounded by a number of solvent molecules that form a solvation shell around the electron. By interaction with the sheath ( cavity ) the electron sees a potential exists in the linked equivalent to a quantum mechanical system states. Optical transitions between these states lead to the observed absorption band. Due to the limited number of molecules in the first solvation potential can be viewed as roughly spherically symmetric, and therefore the bound states of the electron in the literature often are referred to as s- and p -like. Theory works also show evidence of a splitting of the excited state into three sub-states ( lifting of degeneracy ).

Latest " ab initio " calculations show strong evidence that support this model.

Methods of investigation

Most of the early experimental work on the solvated electron deal with the spectral properties of the equilibrated ground state under various conditions:

• Different solvents
• Additives such as salts (ions) at different concentrations
• Pressure
• Temperature

With increasing knowledge about the static properties of the solvated electron, the need grew to also examine the development process in detail, which takes place on the picosecond time scale. A common method of investigation of this is the ultrafast spectroscopy: About photolysis by an ultrashort laser pulse, the generation process is set in motion. Then the time evolution of the absorption band is studied until the final formation of the solvated electron. Of interest in this connection is the question as to what steps are intermediate the formation of the solvated electron.

In addition, it is possible to excite the ground state to a higher equilibrated state and to watch the subsequent relaxation dynamics.

Special

The ubiquitous solvent, water is due to its high relevance for chemistry and biology has always been of particular interest. Probably for this reason has become the norm for the solvated electron in water, the independent term of the hydrated electron.