# What is the largest wavelength that can excite an atom?

What is the largest wavelength that can excite an atom? Or is there even a largest wavelength?

It depends on the atom, the state it is in, and the definition of excite.

Here is the simplest atom, the hydrogen atom.

Normally the electron will be at the n=1 energy level. To get the electron excited to the n=2 energy level one needs a photon of energy $13.6-3.4=9.2$ eV, which gives a wavelength of $5.35*10^{-7}$meters. If previously the electron has been excited close to the ionization energy level, the wavelength can be as large as the energy difference to complete ionization, a very large number as n goes to infinity in diminishing steps.

The same is true for all atoms, except the energy levels are different, unique for each atom.

• So in principle it can be as large as we want it to be. But what would be the largest wavelength for an atom in the groundstate or metastable state? – yasalami Aug 15 '18 at 12:30
• look at the diagram above. It has specific energies for transitions . When it is in the ground state the transition to the next one will be the smallest energy, and thus the largest wavelength from the ground state. Transitions to higher states would need more energy and thus smaller wavelengths. I have given a link to change energy to wavelength. – anna v Aug 15 '18 at 14:04

An atom can be excited/ionized in a sufficiently strong electromagnetic field of arbitrarily low frequency through multi-photon excitation/ionization. I believe an atom can be ionized even in a sufficiently strong static electromagnetic field.

This depends on the atom. I assume you are interested in stable atoms only. I take "excite" as meaning that the atom is in its ground state initially. I am also assuming that forbidden transitions are allowed. Then my best guess is the Cesium 6s J=$\frac{1}{2} \rightarrow$ 6p J=$\frac{1} {2}$. The wave length is 894.3 nm.

• By what mechanism would the forbidden transitions be allowed?.. – Ruslan Aug 15 '18 at 16:38
• @Ruslan They are not allowed, but forbidden. They occur with low transition matrix element via for example spin-orbit coupling. – my2cts Aug 15 '18 at 16:54