Chapter 1
Introduction to Infrared Spectroscopy

Mitsuo Tasumi

Professor Emeritus, The University of Tokyo, Japan

1.1 Introduction

Infrared spectroscopy is a useful tool for molecular structural studies, identification, and quantitative analyses of materials. The advantage of this technique lies in its wide applicability to various problems in both the condensed phase and gaseous state. As described in the later chapters of this book, infrared spectroscopy is used in chemical, environmental, life, materials, pharmaceutical, and surface sciences, as well as in many technological applications. The purpose of this book is to provide readers with a practical guide to the experimental aspects of this versatile method.

In this chapter, introductory explanations are given on an infrared absorption spectrum and related basic subjects, which readers should understand before reading the later chapters, on the assumption that the readers have no preliminary knowledge of infrared spectroscopy.

As is well known, visible light is absorbed by various materials and the absorption of visible light is associated with the colors of materials. Blue materials absorb radiation with a red color, and red materials absorb radiation with a blue color. The wavelengths of radiation with a red color are longer than those with a blue color. A diagram showing quantitatively the absorption of visible light at different wavelengths from violet to red is called a visible absorption spectrum. The visible absorption spectrum closely reflects the color of the material from which the spectrum is measured.

The wavelengths of infrared radiation are longer than those of radiation with red color. Radiation with red color has the longest wavelengths among visible light, the wavelength of which increases from violet to red. Infrared radiation, though not detectable by human eyes, is absorbed by almost all materials. An infrared spectrum is a plot quantitatively showing the absorption of infrared radiation against the wavelength of infrared radiation. It is usually possible to observe an infrared absorption spectrum from any material except metals, regardless of whether the sample is in the gaseous, liquid, or solid state. This advantage makes infrared spectroscopy a most useful tool, utilized for many purposes in various fields.

Measurements of infrared spectra are mostly done for liquid and solid samples. In the visible absorption spectra of liquids and solids, only one or two broad bands are typically observed but infrared absorption spectra show at least several, often many relatively sharp absorption bands. Most organic compounds have a significant number of infrared absorption bands. This difference between the visible and infrared absorption spectra is due to the different origins for the two kinds of spectra. Visible absorption is associated with the states of electrons in a molecule. By contrast, infrared absorptions arise from the vibrational states of atoms in a molecule. In other words, the visible absorption spectrum is an electronic spectrum and the infrared spectrum is a vibrational spectrum. Vibrational motions of atoms in a molecule are called molecular vibrations.

At present, measurements of infrared spectra are widely performed in materials science, life science, and surface science. In these fields, the states of targets of research are usually liquids or solids. This book primarily aims at describing the fundamentals of infrared spectroscopy and practical methods of measuring infrared spectra from various samples in the liquid and solid states.

1.2 Fundamentals of Infrared Spectroscopy

A basic knowledge of infrared spectroscopy that readers should have before performing infrared measurements is briefly described in this section.

1.2.1 The Ordinate and Abscissa Axes of an Infrared Spectrum

It has been known for a long time that vapors, liquids, crystals, powder, glass, and many other substances absorb infrared radiation. The wavelength region of infrared radiation is not strictly defined but the wavelength regions generally accepted for near-infrared, mid-infrared, and far-infrared radiation are as follows: 700 nm to 2.5 µm for near-infrared, 2.5–25 µm for mid-infrared, and 25 µm to 1 mm for far-infrared.

The absorption intensity is taken as the ordinate axis of an infrared spectrum. The wavelength can be used as the abscissa axis of an infrared spectrum. At present, however, it is customary, in the mid-infrared region in particular, to use the wavenumber as the abscissa axis instead of the wavelength. The wavenumber is the number of light waves per unit length (usually 1 cm) and corresponds to the reciprocal of the wavelength. The wavenumber used as the abscissa axis of an infrared spectrum is always expressed in units of cm−1. In this book, the abscissa axis of an infrared spectrum is always designated as “Wavenumber/cm−1.” It should be mentioned, however, that the wavelength is often used as the abscissa axis in the near-infrared region, if a near-infrared spectrum is measured as an extension of a visible absorption spectrum.

There are publications in which the higher wavenumber (corresponding to the shorter wavelength) is placed on the left side of a spectrum, whereas it is placed on the right side in other cases. This inconsistency has occurred because infrared spectra published before the 1950s used the wavelength as the abscissa axis and placed the longer wavelength (corresponding to the lower wavenumber) on the right side. Following this tradition, many infrared spectra published since the 1960s also have placed the higher wavenumber on the left side and the lower wavenumber on the right side. In recent years, however, infrared spectra in publications which feature the direction of the abscissa axis oppositely have been increasing in number.

The wavenumber, which is the number of light waves per centimeter as mentioned above, corresponds to the frequency divided by the speed of light. Therefore, the wavenumber is proportional to the energy E of a photon as expressed in the following equation:

where is the Planck constant, the speed of light, and the wavenumber of infrared radiation. This proportionality between and is the reason why the wavenumber is now used as the abscissa axis of an infrared spectrum. In Appendix A relations closely associated with Equation (1.1) are explained in detail.

The above-mentioned wavelength regions of infrared radiation correspond to the wavenumber regions of about 14 000–4000 cm−1 for near-infrared, 4000–400 cm−1 for mid-infrared, and 400–10 cm−1 for far-infrared.

The wavenumber region of 400–10 cm−1 for far-infrared corresponds to the frequency region of or . This means that the far-infrared region approximately coincides with the terahertz frequency region. For this reason, the term terahertz spectroscopy is recently being increasingly used in place of far-infrared spectroscopy. However, the term far-infrared spectroscopy is considered a better designation because of its consistency with other optical spectroscopies.

1.2.2 The Intensity of Infrared Radiation

As infrared radiation is an electromagnetic wave, electromagnetic theory is applicable to it. In this theory, the intensity of an electromagnetic wave irradiating an area is defined as the average energy of radiation per unit area per unit time. In this book, according to the tradition of spectroscopy, the term intensity is used for this quantity. It is worth pointing out, however, that the term irradiance is increasingly used in other fields instead of “intensity.” This quantity is given in units of W m−2 (= J s−1 m−2), although its absolute value is rarely discussed in infrared spectroscopy except when lasers are involved. The intensity I is proportional to the time average of the square of the amplitude of the electric field E. In vacuum, I is expressed as

where and denote, respectively, the electric constant and the speed of light in vacuum, and the symbol means time average. This relationship will be mentioned later in Section 1.2.4.

1.2.3 Lambert–Beer's Law

Let us consider the absorption of infrared radiation which occurs when an infrared beam passes through a sample layer. As shown in Figure 1.1, a collimated infrared beam with intensity at wavenumber irradiates a sample with thickness at right angles to its surface. If the sample is transparent to the infrared beam, the infrared beam passes through the sample without losing its intensity. Here, reflection of the infrared beam at the surface of the sample is not considered. If the sample absorbs the infrared radiation of wavenumber , the infrared intensity decreases as the beam passes through the sample. If the amount of the absorption by the thin layer in Figure 1.1 is expressed by (the minus sign reflects the fact that is a negative quantity corresponding to an intensity decrease), the following equation holds.

where is a proportionality constant representing the magnitude of absorption (called the absorption coefficient) and is the intensity of the beam entering the thin layer . Integration of Equation (1.2) gives the following equation.

where the integration constant a...