A unit used in photochemistry, mid-20ᵗʰ century, equal to the energy of a mole of photons. Its value varies with the frequency of the light in the reaction being studied. One einstein is Avogadro's constant times the energy of a single photon at the frequency used. (A photon's energy is Planck's constant times the frequency, or Planck's constant times the speed of light, divided by the wavelength.) Dividing 1.1962 × 10⁸ by the wavelength in nanometers will give the value of the einstein in joules.

McGlashan (page 60, footnote 4) remarks: “The mole is sometimes given the special but unnecessary (see Section 3.4, page 18) name 'einstein' in the particular case that the specified elementary particle is a photon...”

M. L. McGlashan.
Physico-Chemical Quantities and Units. The Grammar and Spelling of Physical Chemistry.
London: The Royal Institute of Chemistry, 1968.

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