Atomic and molecular physics involve the study of atoms, molecules, and their properties. Over the years, increasingly advanced techniques have become available to us. These techniques aid us in the characterization of atoms and molecules. While the responsibility of studying chemical properties lies with the chemical scientist, physicists study the quantum mechanics of atoms and molecules and try to describe their energy levels, spin-orbit coupling, and various other properties.
In this tutorial, we will study a beautiful method of studying elements that involve focusing on their emission spectra. The wavelengths visible in emission spectra can be equated via formulae to arrive at the values we seek.
Emission simply means discharge. That is, something from inside the body is being expelled or emitted out. In the physicist’s world, emission usually refers to the production and subsequent expulsion of either gases or radiation.
For example, vehicles that run on fossil fuels emit gases that are dangerous to the atmosphere. On the other hand, when electrons and holes recombine in a p-n junction diode, they emit radiation. Similarly, when electrons jump from one energy level to another, they also emit radiation.
When it comes to characterizing elements, the emission of radiation is what concerns us more. Electrons in different energy levels can fall to lower levels and emit radiation. These energy levels might refer to orbits, vibrational levels, or even rotational levels. As long as there is a change in energy, there can be an emission of radiation.
As you might be aware, a spectrum is a range of continuous values. For instance, the rainbow is a spectrum of colors. The temperature of the water inside a water heater could be referred to as a spectrum since it varies from room temperature to whatever you set your heater at.
Thus, emission spectrum refers to the range of frequencies that electromagnetic radiation emitted from a certain chemical compound can have. Each chemical element has a different emission spectrum, which is unique. For instance, the spectra of $\mathrm{H_2O_2}$ and $\mathrm{H_2O}$ will be completely different. Note that “spectra” is the plural form of the word “spectrum”.
Fig:1 Emission spectrum of Iron
Nilda, Emission spectrum-Fe, marked as public domain, more details on Wikimedia Commons
The image above is the emission spectrum of iron. The emission spectrum is plotted as a series of lines. The horizontal axis represents the wave number $\mathrm{(\nu=\frac{1}{f})}$, while the vertical axis represents intensity.
Emission spectra, as stated previously, are created via transitions between different energy states. These can be vibrational, rotational, or electronic. The energy of the radiation emitted is equal to the energy difference between the two energy states.
For instance, you must have studied how to find the wavelength of the light emitted when an electron falls from an orbit $\mathrm{n_2}$ to $\mathrm{n_1}$.
$$\mathrm{\lambda=R(\frac{1}{n_1^{2}}-\frac{1}{n_2^{2}})}$$
This is an example of an emission caused by a transition between orbits. Similarly, we can have transitions between different vibrational and rotational levels, the calculations for which, are given via quantum mechanics.
We can induce these transitions via applying heat or electric discharge.
Emission Spectra can be divided into the following three Categories:
The line spectrum becomes visible as a series of discrete lines. Line spectra are observed in atoms and can be analyzed by passing electrical discharge via gas tubes. The pattern obtained in line spectra is characteristic of the element being studied. Thus, by studying the line spectrum, we can easily infer the element we are studying.
For example, the line spectrum of sodium has two lines at 589.6nm and 589 nm
A continuous spectrum is the opposite of a line spectrum, and it contains a continuous range of frequencies between two certain values. The colors obtained in this spectrum depend on the temperature of the sample.
For example, the light bulbs we use in our homes emit a continuous spectrum.
Band spectrum consists of several continuous lines, which are closely spaced in some areas, while distant in others. This gives the appearance of bands being formed in the spectrum. In reality, with high enough resolution, we can see the lines.
These bands are sharply defined on one end and fade off on the other. Further, a band spectrum is characteristic of a particular molecule and thus, can be used to infer the identity of unknown molecules.
A halide lamp emits a continuous spectrum with peaks at certain wavelengths. The intensity is represented on the vertical axis, with the wavelength on the x-axis.
Fig:2 Spectrum of halide lamp
Varistor60, Metal Halide Rainbow, CC BY-SA 4.0
Hydrogen molecules $\mathrm{(H_2)}$ emit a relatively simple line spectrum, though, in high resolution, even this simple spectrum becomes much more detailed.
Fig:3 Spectrum of hydrogen
OrangeDog, Hydrogen spectrum, CC BY-SA 3.0
Astronomers study the emission spectra of distant celestial objects. This allows them to list what elements are present in the star or planet in question. This can potentially help us find planets that can sustain life.
Emission spectra hold information about the electronic structure of the element emitting them. Thus, studying emission spectra can help us identify unknown elements in a sample.
Certain elements, when applied on a platinum wire and held up to a flame, give highly characteristic and specific colors. This concept is used in performing salt analysis of compounds.
Emission in physics is the production and subsequent discharge of gases or radiation. For instance, light is emitted by electron-hole recombination in junction diodes. Similarly, electronic transitions also cause emissions.
Emission spectrum refers to the range of frequencies emitted by a certain element upon electronic transition between different energy levels corresponding to rotational, vibrational, or orbital states. The study of emission spectra of elements can be used to identify them or study their properties.
Emission spectra can be line spectra, continuous spectra, or band spectra. Line spectrum is mostly emitted by atoms, while band spectra are characteristic of molecules. For instance, the line spectrum of Sodium contains two highly distinct lines at 589.6 and 589.0 nm.
Q1. If emission spectra calculations involve quantum mechanics, how can we have a continuous spectrum when energies in quantum mechanics are quantized?
Ans. In reality, the spectrum is not continuous. It just happens to be so closely spaced that it looks like a continuum.
Q2. Are there many ways to induce transitions in elements?
Ans. Yes. Application of heat is a simple method. However, less intrusive methods of transition induction include the application of electric field, magnetic field, X-ray photon-based induction, etc.
Q3. Is the study of emission spectra a fully quantum-based topic?
Ans. Not exactly. In most practical situations, a semi-classical approach to emission spectra suffices. A modern method is to study emission spectra using quantum electrodynamics. However, the semi-classical approach is still used.
Q4. Do all materials have an emission spectrum?
Ans. Yes. However, a lot of elements have spectral lines that are beyond the visible range. In either case, all elements emit radiation in some form or another and thus, must have an emission spectrum in some form or another.
Q5. Are emission spectra visible to the naked eye?
Ans. Some colors from some elements might be visible to the naked eye. However, even what we see can be resolved further into more well-defined lines via spectroscopy. For instance, sodium emits an amber-yellow color. But under a spectroscope, we see two well-defined lines situated 6 Angstrom apart.
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