U2M1: Analyzing Light-Matter Interactions

fireworks — Different atoms emit light of characteristic colors.

One of the most important applications of chemical thinking in various fields, from forensic analysis to drug design, involves the determination of the structure of the particles that make up a substance. We may want to determine the geometry of the molecules of cocaine to better understand how this drug interferes with brain activity. We may be interested in characterizing the three dimensional structure of a protein to design a chemical substance that can inhibit its action inside the cells of a pathogen we want to eliminate. For this purpose, chemical scientists have developed a set of powerful intellectual and practical tools that help them explore or predict how atoms or ions are bonded to each other in the particles of a given substance (connectivity) and how they are arranged in space (geometry). Many of these tools rely on experimental information derived from the analysis of the interaction of the substance of interest with some form of electromagnetic radiation. It is through the analysis of the electromagnetic radiation absorbed or emitted by a substance that we can reveal its internal structure. This information has also been used by chemical and physical scientists to build theoretical models to explain and predict the properties of matter.

In our Blood

The presence of some metals, such as lead, arsenic, and mercury in human blood can lead to serious diseases. However, some other metals such as iron and copper are essential to health in very small amounts. Metal compounds are known to emit light of different colors when heated.

This module will help you develop the type of chemical thinking that is used to answer questions similar to that posed in the challenge. In particular, the central goal of Module 1 of Unit 2 is to help you understand how to use experimental data on the light emitted or absorbed by chemical substances to derive information and build models about atomic and molecular structure.

A. Spectroscopy

One of the central questions that we want to investigate in this unit is how we can use visible light, or any other form of electromagnetic (EM) radiation, to derive information about the structural properties of chemical substances. In particular, we would like to use results from the analysis of light-matter interactions to determine atom connectivity and geometry in molecules and ionic networks, as well as to detect, identify, and quantify the amount of different substances present in a system. We would also be interested in using such experimental evidence to generate models about atomic and molecular structure that could help us explain and predict the properties of materials. We will be able to face this challenge through information derived using a set of experimental techniques generically known as spectroscopy. These analytical methods are based on the analysis of different types of EM radiation absorbed or emitted by chemical substances. However, to better understand how this works, we need to discuss some basic properties of EM radiation.

The image depicts a schematic representation of an electromagnetic wave propagating through space.

Wave Components:
E (Electric Field): Shown as a blue sine wave oscillating vertically.
B (Magnetic Field): Shown as a red sine wave oscillating horizontally, perpendicular to the electric field.
Propagation Direction (k): Indicated by a rightward-pointing arrow along the horizontal axis.
Wavelength (λ): The distance between two consecutive peaks (or troughs) of the wave, marked on the diagram.

This illustration demonstrates the perpendicular relationship between the electric field (E), magnetic field (B), and the direction of wave propagation (k) in an electromagnetic wave. — **Figure 2.1** Representation of an EM wave showing its basic components: Electric field (E); magnetic field (B), and propagation direction (k). This wave is characterized by its wavelength λ.

EM radiation is a form of energy that can be generated by accelerating particles that have an electric charge. This radiation takes the form of self-propagating waves that travel through matter or through a vacuum at a characteristic speed () that depends on the nature of the medium. EM waves are comprised of electric and magnetic fields that oscillate in phases perpendicular to each other and perpendicular to the direction in which the wave propagates (see Figure 2.1). Different types of EM radiation can be characterized by either wavelength (λ) or frequency ( $LaTeX: \nu$ ). The wavelength λ is a measure of the distance over which the shape of the wave is repeated; it is measured in units of length (such as meters or centimeters). The frequency ( $LaTeX: \nu$ ) indicates the number of repeating waves that cross a certain point of space per unit of time; it is commonly measured in 1/seconds ( $LaTeX: s^{-1}$ ) also called Hertz, Hz. These two quantities are not independent of each other and their relationship depends on the speed at which the waves travel through a certain medium. You can use the following simulation to explore how the wavelength and frequency of light waves relate to each other.

The wavelength λ, frequency $LaTeX: \nu$ , and speed of propagation of a wave are related to each other through the following mathematical relationship:

(2.1) $LaTeX: {\color{#ff0000}λ}\;\times{\color{#ff0000}ν}={\color{#ff0000}v}.$

In the particular case of EM waves traveling through vacuum, the speed of propagation $v$ has a constant value $LaTeX: \nu={\color{#0000ff}c}{\color{#0000ff}=}{\color{#0000ff}\;}{\color{#0000ff}3}{\color{#0000ff}.}{\color{#0000ff}00}{\color{#0000ff}\times}{\color{#0000ff}{\color{#0000ff}10}^8}$ . This value is the same no matter what the wavelength or frequency of the radiation is. This is not necessarily the case for EM radiation traveling through a material medium, such as glass or water, where wave speed depends somewhat on wavelength. Eq. (2.1) implies that the value of the wavelength and the frequency are inversely related to each other: the shortest the wavelength $\lambda$ of the radiation, the highest its frequency $\nu$ (more whole waves cross a given point in space per unit time).

Depending on its wavelength or frequency, EM radiation is classified as belonging into different types as illustrated in Figure 2.2. The different EM radiation types span the so-called electromagnetic spectrum and they include X-rays, ultraviolet (UV) radiation, infrared (IR) radiation, and radio waves, as well as EM radiation that is visible to the human eye (visible light). The visible part of the spectrum includes radiation with λ values that range from approximately $LaTeX: 400\;nm$ (violet region of the spectrum) to $LaTeX: 700\;nm$ (red region of the spectrum).

The image shows a detailed representation of the electromagnetic spectrum with a focus on the visible spectrum.

Top Section:

A horizontal bar represents the entire electromagnetic spectrum, labeled with different types of radiation: γ rays, X rays, UV, Visible, IR, Microwave, FM, AM, and Long radio waves.
The spectrum is annotated with increasing frequency (ν, in Hertz) from right to left and increasing wavelength (λ, in meters) from left to right. Specific values of ν and λ are marked for reference.

Bottom Section:

A magnified view of the visible spectrum is shown, spanning wavelengths from 380 nm to 750 nm.
The visible spectrum is color-coded with the corresponding wavelengths:
V (Violet): 380-450 nm
B (Blue): 450-495 nm
G (Green): 495-570 nm
Y (Yellow): 570-590 nm
O (Orange): 590-620 nm
R (Red): 620-750 nm

This illustration is used to explain the range and characteristics of the electromagnetic spectrum, highlighting the portion visible to the human eye. — **Figure 2.2** Electromagnetic spectrum. The visible region of the spectrum has been rescaled to show the range of wavelengths characteristic of different light colors.

The distinction between different types of EM radiation is relevant in chemistry because chemical substances interact in distinctive manners with different types of radiation. The type of EM radiation absorbed or emitted by a chemical substance is, in fact, a differentiating characteristic of the material.

The image illustrates a schematic diagram of a spectroscopic experiment setup.

EM Radiation Source: Located at the bottom right, emits electromagnetic radiation.
Prism: Positioned in the path of the radiation to disperse it into its component colors, creating a spectrum.
Sample: The dispersed light passes through a sample placed above the prism.
Detector: Located above the sample, captures the transmitted or emitted light.
Spectrum: A graph displayed on the top right shows the resulting spectrum, plotting intensity (or another property) versus wavelength (λ), with peaks and troughs indicating different wavelengths absorbed or emitted by the sample.

This setup demonstrates how light interacts with a sample to produce a spectrum, which can be analyzed to understand the sample's properties. — **Figure 2.3** A typical absorption spectrometer includes a EM radiation source, a device to split the radiation into specific wavelengths, and a detector.

Additionally, we can obtain different types of structural information by analyzing the EM radiation emitted or absorbed by a substance in the IR region of the EM spectrum or, for example, the UV region. That is why there are a variety of spectroscopic techniques, such as IR spectroscopy, UV-visible spectroscopy, and microwave spectroscopy. In all of these different techniques, the actual instruments include detectors that measure the intensity of the EM radiation that passes through a sample or that is emitted by a substance when heated or stimulated in some other way (such as using other types of EM radiation). The detection is done at different wavelengths which allows us to build a spectrum that shows the intensity of the radiation absorbed or transmitted as a function of λ or $LaTeX: \nu$ (see Figure 2.3).

When a chemical substance is analyzed using absorption spectroscopy, the spectrometer generates a spectrum that provides information about either the fraction of EM radiation of a specific wavelength that was absorbed by the substance (Absorbance) or the fraction of that EM radiation that went through the sample without being absorbed (Transmittance). Consider, for example, the following spectra for LaTeX: O_2(g) and LaTeX: CO_2(g) , two vital substances in our planet:

Left Graph:

Title: Not provided, but indicates properties of CO₂ (g).
Y-Axis: Transmittance (ranging from 0.6 to 1.0).
X-Axis: Wavelength in micrometers (µm), ranging from 4.10 to 4.40 µm.
Description: The transmittance of CO₂ gas shows a series of sharp dips between 4.20 µm and 4.40 µm, with the lowest point reaching around 0.6.

Right Graph:

Title: Not provided, but indicates properties of O₂ (g).
Y-Axis: Absorbance (ranging from 0 to 1.5).
X-Axis: Frequency in Gigahertz (GHz), ranging from 50 to 70 GHz.
Description: The absorbance of O₂ gas shows a series of peaks between 55 GHz and 65 GHz, with the highest peak reaching just above 1.5.

These graphs are likely used to illustrate the spectral properties of carbon dioxide and oxygen gases, focusing on their transmittance and absorbance characteristics at specific wavelengths and frequencies.

Lets Think

Now that your done reviewing this section, let's think. Test your understanding by completing the reading review below.

U2M1 A - Reading Review