## Equation of the Day #17: The Rydberg Formula

**Akano**, in Math/Physics Aug 04 2016 · 213 views

**Hydrogen is the simplest and most common neutral atom in the universe. It consists of two particles – a positively charged proton and a negatively charged electron. The equation that describes the hydrogen atom (or any one-electron atom) in the nonrelativistic regime is the Schrödinger equation, specifically**

**where**

*ħ*

**is the reduced Planck constant,**

*μ*

**is the reduced mass of the electron-nucleus system,**

*Z*

**is the number of positive charges in the nucleus that the electron is orbiting,**

*e*

**is the charge of a proton,**

*τ*

**is the circle constant,**

*ε*

_{0}

**is the vacuum permittivity, and**

*ψ*

**is the wavefunction. Solving this equation (which is nontrivial and is usually done after a semester of Advanced Quantum Mechanics) yields a surprisingly simple formula for the energies of the atom,**

** ,**

**where**

*h*

**is Planck's constant,**

*c*

**is the speed of light,**

*m*

_{e}**is the rest mass of the electron, and**

*n*

**is any integer larger than or equal to 1. The constant**

*R*

_{∞}

**is known as the Rydberg constant, named after Swedish physicist Johannes Rydberg, the scientist who discovered a formula to predict the specific colors of light hydrogen (or any hydrogen-like atom) would absorb or emit. Indeed, the formula I gave,**

*E*/

_{n}*hc*

**, is equivalent to the inverse wavelength, or spatial frequency, of light that it takes for the atom in its**

*n*

^{th}**e**nergy state to free the electron of its atomic bond. Indeed, this was a puzzle in the early 20^{th}century. Why was it that hydrogen (and other atoms) only absorbed and emitted specific colors of light? White light, as Isaac Newton showed, is comprised of all visible colors of light, and when you split up that light using a prism or similar device, you get a continuous rainbow. This was not the case for light emitted or absorbed by atoms.**The equation above was first derived by Niels Bohr, who approached solving this problem not from using the Schrödinger equation, but from looking at the electron's angular momentum. If electrons could be considered wavelike, as quantum mechanics treats them, then he figured that the orbits of the electron must be such that an integer number of electron wavelengths fit along the orbit.**

**Left: Allowed orbit. Right: Disallowed orbit. Image: Wikimedia commons**

**This condition requires that**

**The wavelength of the electron is inversely related to its momentum,**

*p*=

*mv*

**, via Planck's constant,**

*λ*=

*h/p*

**. The other relation we need is from the physics of circular motion, which says that the centripetal force on an object moving in a circular path of radius**

*r*

**is**

*mv*

^{2}/

*r*

**. Equating this to the Coulomb force holding the proton and electron together, we get**

**Plugging this into the quantization condition, along with some algebra, yields the energy equation.**

What's incredible is that hydrogen's energy spectrum has a closed-form solution, since most problems in physics can't be solved to produce such solutions, and while this equation only works exactly for one-electron atoms, it can be modified to work for so-called Rydberg atoms and molecules, where a single electron is highly excited (large

What's incredible is that hydrogen's energy spectrum has a closed-form solution, since most problems in physics can't be solved to produce such solutions, and while this equation only works exactly for one-electron atoms, it can be modified to work for so-called Rydberg atoms and molecules, where a single electron is highly excited (large

*n*

**) and orbits a positive core, which need not be a nucleus, but a non-pointlike structure. In my lab, we consider two types of Rydberg molecules.**

**The example on the left is an electronic Rydberg molecule, while the one on the right is called an ion-pair Rydberg state, where a negative ion acts as a "heavy electron" co-orbiting a positive ion. To model the energies of these kinds of states, we use a modified energy equation.**

**where I.P. represents the ionization energy of the electron, and the new quantity**

*δ*

**is known as the quantum defect. It's a number that, for electronic Rydberg states, has a magnitude that's usually less than 1, while for ion-pair states can be quite large (around –60 or so in some cases); it in some sense contains information of how the core ion, e.g. H**

_{2}^{+}, is oriented, how the electron is spread over space, how its polarized, and so on. It's a vessel into which we funnel our ignorance in using the approximation that the molecule is behaving in a hydrogen-like manner, and it is surprisingly useful in predicting experiments. Currently my research involves studying electronic Rydberg states of molecular nitrogen, N_{2}, and looking at heavy Rydberg states of the hydrogen molecule, H_{2}to gain a better understanding of the physics of certain states that have been experimentally observed in both systems.