Additional units

In atomic spectroscopy the photon wavelength $\lambda$ of a transition is often measured in Ångström, 1 Å=10^-10 m. In addition the photon wavenumber $\sigma= 1/\lambda$ is given in units of cm^-1 instead of in units of m^-1. Sometimes the unit Kayser is used for the wavenumber where 1 K = cm^-1. The wavenumber $2R_{\infty}=2\times109~737.315~34(13)$ cm^-1 corresponds to an energy separation of 1 E_h. Ionization potentials and electron affinities are often express in eV, where 1 E_h =27.211 396 1(81) eV.

For hyperfine strutures and isotope shifts the unit MHz is frequently used. This unit can be related to an energy and wavenumber through the relations

$$E=h\nu$$ (209)

and

$$c=\lambda \nu$$ (210)

1 MHz corresponds to a wavenumber of 3.335 641 0 $\times 10^{-5}$ cm^-1 and an energy separation of 1.519 829 $\times 10^{-10}$ E_h.

A more detailed discussion of units and their values may be found in the Atomic, Molecular, & Optical Physics Handbook (Baylis and Drake 1996) p. 1.