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###### Atomic spectra

Physics (Year 12) - Wave Particle Duality and Quantum Theory

Dev Lohar

The understanding of absorption and emission spectrums were important to convince scientists around the world that light can have particle-like behaviour. These spectrums are now widely used in the field of astronomy. There are two common forms of spectrums for both absorption and emission; line, and band. The basics and the foundational physics are similar for all of them, except a few differences. It is also important to note that absorption and emission spectrums are the opposites of one another; this will be further explained below.

# Neil Bohrâ€™s model of the atom

In 1913, Neil Bohr provided an explanation, based on quantum ideas, for the observed absorption and emission spectrum of hydrogen. Based on these explanations, he was able to predict the wavelengths of all lines in the emission spectrum of hydrogen. This led to a set of ideas which are relevant to all emission and absorptions spectrums of any atom:

• Inside an atom, electrons move in circular orbits around the nucleus. There are multiple orbits for each atom (the orbits are labelled as n = 1, 2, 3, â€¦) and atoms can only occupy these orbits. Each orbit corresponds to a specific amount of energy. An electron is typically in the lowest energy orbit; called the ground state.

• Photons can only be absorbed by atoms if the energy of the photon (given by E = hf) is exactly equal to the energy differences between the occupied orbit and a higher orbit. The same goes for when an excited electron returns from a high energy orbit to a low energy orbit. The energy difference between the 2 orbits corresponds to the energy of the photon emitted.

Below is an energy level diagram for hydrogen. It shows the different energy levels electrons can move to and from. The arrows represent the transition pathways an electron could take when moving up or down energy levels. It is observant that there are more transition pathways when moving down an energy level, compared to moving up. This is because electrons can only move up from the ground state, to a higher energy state. Whereas electrons can move down from any state to a lower energy state.

*diagram*

# Line spectrums

Line emission spectrums work as explained below:

• When a low pressurised gas is excited (for example, by increasing the temperature or decreasing the pressure), the electrons in the atoms get excited and so move to high energy levels

• These excited electrons are unable to stay at the high levels for too long, hence they return to the ground state through a series of energy transitions

• These energy transitions release photons with specific wavelengths; corresponding to the energy level difference of the transition

• The emitted photons are observed and they appear as coloured lines on a dark background as seen below

*diagram*

The line absorption spectrum would simply be the inverse of the line emission spectrum; the coloured lines in the emission spectrum would be the dark lines in the absorption spectrum. Not all the lines in the emission spectrum would be in the absorption spectrum, but all the dark lines in the absorption spectrum would be in the emission spectrum. This is because there are more transition pathways for an electron coming back to the ground state, compared to an electron being excited and transitioning to a higher energy state. This corresponds to more different wavelengths of light that could be emitted, compared to the wavelengths of light that could be absorbed.

*diagram*

# Band spectrums

Band emission spectrums work as explained below:

• When a polyatomic compound is excited (for example, by increasing the temperature or decreasing the pressure), the electrons in the atoms get excited and so move to high energy levels

• These excited electrons are unable to stay at the high levels for too long, hence they return to the ground state through a series of energy transitions

• These energy transitions release photons with multiple wavelengths; corresponding to the energy level difference of the transition

• These wavelengths will appear to be closer together in bands and appear as coloured bands on a dark background

The band absorption spectrum would simply be the inverse of the band emission spectrum; the coloured band in the emission spectrum would be the dark bands in the absorption spectrum

*diagram*

# Fluorescence

Fluorescent chemicals are chemicals that re-emit light after they have been excited by light. They work because of the following:

• Electrons inside fluorescent chemical atoms are excited by shining UV light, which causes them to move up energy levels

• These electrons return to the ground state through step-by-step transitions and hence release photons of visible light

*diagram*

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