Lesson 8: Spectroscopy

Key Learning Objectives:


In this lesson you'll find...

Electromagnetic Radiation


We perceive waves in our lives every day at every moment. All noises that we hear come into our ears in the form of waves; when we throw a stone into a lake we can see how the waves expand through the water; the use of Facebook, Instagram, TikTok and all internet platforms wouldn't be possible without the implementation of waves. Those are just some examples from thousands where waves are present in our lives. But, what is a wave and what makes it so special?

A wave is a vibrational perturbation whereby energy and information is transmitted. There are different kinds of waves and they can be classified into two main groups: mechanical and electromagnetic waves. Mechanical waves are those that can only travel through matter, e.g., acoustic waves or those formed in the water. Electromagnetic waves, on the other hand, don’t need a physical medium to travel through as they can also travel through a vacuum such as outer space. 

Electromagnetic radiation encompasses a broad spectrum of electromagnetic waves, ranging from low-frequency radio waves to high-frequency gamma rays. The spectrum is often divided into different regions, including radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

Key characteristics of electromagnetic waves include their wavelength and frequency. Wavelength is the distance between successive peaks (or troughs) of the wave, while frequency represents the number of oscillations per unit of time. The relationship between wavelength (λ) and frequency (f) is given by the equation speed of light (c) = λf.

X-rays are used in medicine to take radiographies of us; solar panels work using UV light; the heat lamps that we use in winter emit infrared radiation to make the place warmer; microwaves are used to heat our meals; and radio waves have been used in radio telescopes (such as The Dish in Parkes, NSW) and are responsible for the transmission of music and programs in radio stations.

As it was said before, all waves carry with them energy and so electromagnetic waves do. The amount of energy that they can have is inverse to their wavelength, meaning that the greater their wavelength the lower the energy they have.

Activity

Arrange the various forms of electromagnetic radiation in order of increasing energy, considering that the energy carried by a wave is inversely proportional to its wavelength.

Watch the video below for a more comprehensive introduction to electromagnetic radaition.

Energy Levels in Atoms and Molecules


In the 20th century, it was well known that atoms were made of electrons and protons. Atoms were pictured as tiny solar system where the electrons are surrounding the nucleus (made of protons) in circular orbits. However, according to the laws of classical physics, this model presented a dilemma, as it implied that electrons would continually spiral into the nucleus, colliding with and being destroyed by the protons.  

Subsequent advancements in our understanding have revealed that electrons can only occupy specific orbits with quantised energies, preventing them from spiralling down to other orbits. In essence, the energy levels (orbits) for electrons in atoms are quantised.

When two or more atoms are close enough, their electrons start to interact forming bonds between the atoms. These entities where atoms are bonded by electrons are named as molecules. Just like atoms, molecules have different energy levels that happen to be quantised as well. That means that molecules can either be in one or another energy level, but not between them.

Spectroscopy


Molecular spectroscopy is the study of the interaction between electromagnetic radiation and matter, which can manifest as either the absorption or emission of radiation by the chemical system (e.g., atom or molecule) considered. 


The Atomic Case

Although it seems like electrons are forced to be in an orbit endlesly, they can hop between the orbits when they interact with electromagnetic radiation.  Two different, but correlated, processes can happen:

Because energy levels (orbits) in atoms are quantised, the movement of electrons between these levels is restricted to transitions from one level to another. Electrons cannot remain in an intermediate state between levels; just like a ball descending a staircase that can only come to rest on one step rather than between them.


Each element  in the periodic table has its unique emission spectrum, as each one of them has its own arrangement of energy levels and interact differently with electromagnetic radiation. Therefore, the emission spectrum works as a fingerprint if one wants to identify unknown element compositions.

Upon obtaining the emission spectrum of an unidentified element, a systematic process involves comparing it with the spectral patterns of known elements. This comparison is vital for identifying specific spectral lines within the emission spectrum.

When a match is found between the observed lines and those present in the known spectrum of a particular element, the identification of the previously unknown element is successfully achieved! This method serves as a powerful tool in analytical chemistry, enabling scientists to discern and characterise elements based on their distinctive emission signatures.

The image above presents the spectral lines for some common elements. You can find more about the absorptiona and emission of electromagnaetic radiation by atoms in the video below.

Activity

As a member of a research laboratory, you are tasked with identifying two unknown samples by analysing their emission spectra provided below. Can you determine the corresponding elements for each spectrum?

The Molecular Case

Although the basis for molecular spectroscopy are pretty similar to the atomic case, as the interaction with electromagnetic waves places molecules in diverse energy levels, the way molecules and electromagnetic waves interact is different. Electromagnetic waves can either make molecules to rotate, vibrate or to be promoted to another electronic state. The figure below presents a description of all these molecular interactions based on the different types of electromagnetic radiation.


High-energy radiation (i.e., high frequency and low wavelength) can deeply penetrate the molecule, leading to interactions with its constituent electrons (UV/Vis and X-rays) and nuclei (γ-rays). Conversely, lower-energy radiation interacts with the molecule differently, inducing rotations (microwave) and vibrations (infrared).

Each one of these interactions has its own type of spectroscopy (microwave, infrared, and UV/Visible spectroscopy) and just like with atoms, each spectrum obtained works as a fingerprint for the identification of unknown molecules. But, which one works best for the identification of molecules in exoplanets atmospheres? Before answering that question, we need to have a look to some concepts in organic chemistry. 

Almost all organic molecules can be categorised using functional groups, which are a set of atoms that deliver specific features to the molecule. Functional groups play an important role in infrared spectroscopy due to their characteristic vibrational frequencies and absorption patterns. When interacting with infrared radiation, each functional group exhibits distinct vibrational frequencies that facilitate their identification in the infrared spectrum. For example, the C–H functional group absorbs at 3000 cm−1, whereas the N–H and O–H functional groups absorb at 3300 (3.03 μm) and 3600 cm−1 (2.77 μm), respectively 1. These characteristic vibrations of functional groups serve as a valuable starting point for identifying molecules in unknown infrared spectra. The figure below presents some common functional groups in chemistry along with their corresponding approximate absorption frequencies.


Scientists rely on infrared spectroscopy to characterise the molecular inventory of exoplanet atmospheres. Telescopes like the James Webb Space Telescope leverage the use of this technique to provide unprecedented insights into identifying the molecules present in the atmosphere of these distant worlds. 

The figure presents the infrared spectrum of the atmosphere of WASP39-b; a hot gas-giant exoplanet orbiting relly close to its host star. This spectrum shows, for the first time, the detection of CO2, SO2, and H2O in the atmosphere of an exoplanet.

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