Rydberg constant


Rydberg constant

The Rydberg constant, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to atomic spectra in the science of spectroscopy. Rydberg initially determind its value empirically from spectroscopy, but then later, it was found that its value could be calculated from more-fundamental constants by using quantum mechanics.

The "Rydberg constant" represents the limiting value of the highest wavenumber (the inverse wavelength) of any photon that can be emitted from the hydrogen atom, or, alternatively, the wavenumber of the lowest-energy photon capable of ionizing the hydrogen atom from its ground state. The spectrum of hydrogen can be expressed simply in terms of the Rydberg constant, using the Rydberg formula.

Value of the Rydberg constant

Making use of the simplifying assumption that the mass of the atomic nucleus is infinite compared to the mass of the electron, the Rydberg constant is (according to 2002 CODATA results):

:R_infty = frac{m_e e^4}{8 epsilon_0^2 h^3 c} = 1.0973731568525(73) imes 10^7 ,mathrm{m}^{-1}::where,::: h is the Planck's constant,::: m_e is the rest mass of the electron,::: e is the elementary charge,::: c is the speed of light in vacuum, and::: epsilon_0 is the permittivity of free space.

This constant is often used in atomic physics in the form of an energy::h c R_infty = 13.6056923(12) ,mathrm{eV} equiv 1 ,mathrm{Ry}

Two complications arise. One is that one may wish to discuss a hydrogenlike ion, i.e., an atom with atomic number Z that has only one electron. In this case, the wavenumbers and photon energies are scaled up by a factor of Z^2. The other is that the mass of the atomic nucleus is not actually infinite compared to the mass of the electron. The predicted spectrum must then be corrected by substituting the reduced mass for the mass of the electron, resulting in:

:R_M = frac{R_infty}{1+m_e/M} ::where,:::R_M is the Rydberg constant for a certain atom with one electron with the rest mass m_e :::M is the mass of the atomic nucleus.

The Rydberg constant is one of the most well-determined physical constants, with a relative experimental uncertainty of less than 7 parts per trillion. The ability to measure it directly to such a high precision constrains the proportions of the values of the other physical constants that define it.

Alternate expressions

The Rydberg constant can also be expressed as the following equations.

:R_infty = frac{alpha^2 m_e c}{4 pi hbar} = frac{alpha^2}{2 lambda_e} and:h c R_infty = frac{h c alpha^2}{2 lambda_e} = frac{h f_C alpha^2}{2} = frac{hbar omega_C}{2} alpha^2 where:: h is Planck's constant,:: c is the speed of light in a vacuum,:: alpha is the fine-structure constant,:: lambda_e is the Compton wavelength of the electron,:: f_C is the Compton frequency of the electron,:: hbar is the reduced Planck's constant, and:: omega_C is the Compton angular frequency of the electron.

The Derivation of Rydberg constant from quantum mechanics

Historically, the Rydberg equation was found "empirically" (experimentally), and it predated the development of quantum theory. To understand its significance in terms of the quantum theory, we can start from the equation

: E_mathrm{total} = frac{- m_e e^4}{8 epsilon_0^2 h^2}. frac{1}{n^2}

for the energy of the electron in the nth energy state, as can be derived either from the Bohr model or from a fully quantum-mechanical treatment of the hydrogen atom. Therefore a change in energy in an electron changing from one value of n to another is

: Delta E = frac{ m_e e^4}{8 epsilon_0^2 h^2} left( frac{1}{n_mathrm{initial}^2} - frac{1}{n_mathrm{final}^2} ight)

We simply change the units to wavelength left( frac{1}{ lambda} = frac {E}{hc} ightarrow Delta{E} = hc Delta left( frac{1}{lambda} ight) ight) and we get

: Delta left( frac{1}{ lambda} ight) = frac{ m_e e^4}{8 epsilon_0^2 h^3 c} left( frac{1}{n_mathrm{initial}^2} - frac{1}{n_mathrm{final}^2} ight)

where:: h is Planck's constant,:: m_e is the rest mass of the electron,:: e is the elementary charge,:: c is the speed of light in vacuum, and:: epsilon_0 is the permittivity of free space.:: n_mathrm{initial} and n_mathrm{final} being the electron shell number of the hydrogen atomWe have therefore found the Rydberg constant for hydrogen to be

: R_H = frac{ m_e e^4}{8 epsilon_0^2 h^3 c}

ee also

*Rydberg formula

References

* [http://physics.nist.gov/cuu/Constants/codata.pdf CODATA recommendations 2006]
* [http://scienceworld.wolfram.com/physics/Rydberg.html Mathworld]


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