1 Introduction

Vibrational spectroscopies can detect and analyse vibrations in molecules. Mainly two different forms are used today: infrared and Raman spectroscopy.

In the most common form of infrared spectroscopy a substance is transmitted by infrared radiation of changing wavelength and the absorption plotted against wavelength. In the most common form of Raman spectroscopy a substance is irradiated by a laser. The intensity of the resulting radiation of lower wavelength is then plotted against its wavelength. In this case wavelength means the difference of its wavelength compared to the laser. Instead of wavelength most often its reciprocal wavenumber is used.

Vibrational spectroscopy is used by chemists to characterize their substances. If the spectra of substances are known, analytical chemists can use them to analyze a mixture of chemicals. Finally, samples may be analysed with spatial resolution, needing sophisticated (and expensive) instruments.

An exceptional (and extremely expensive) application of infrared spectroscopy is used on the James Webb Space Telescope (JWST). This instrument collects electromagnetic radiation with its 6.5 m diameter mirror (Figure 1.1). As expected, the instrument will not only take spectra of single points in space but also images of astronomical objects using focal plane arrays. The telescope carries instruments for the near infrared working in the 0.6–5 µm range and for the mid infrared working in the 5–28.5 µm region of the electromagnetic spectrum. The latter will be able to detect light from the first (oldest) galaxies. If a galaxy is very far from the observer, its visible and ultraviolet light is shifted by the cosmological redshift to this region. Early galaxies formed about 13.5 billion years from now, that is only a few hundred million years from the “Big Bang”. The Hubble Space Telescope already has a limited NIR capability, enabling it to look at galaxies 500 million years from the “Big Bang”, whereas Webb will be able to image galaxies formed only 200 million years from the beginning of the universe. Infrared detectors can see newly forming stars and faintly visible comets as well as objects in the Kuiper Belt. As an example, Figure 1.2 shows the carina nebula imaged by Hubble. With visible light, only a few stars can be detected in dust and gas. Near infrared light penetrates the dust and shows a plethora of stars.

Figure 1.1: The James Webb Space Telescope (artist’s concept above) will be one of the primary instruments scientists use to continue the search for planets outside our solar system. Credits: NASA Goddard Space Flight Center from Greenbelt, MD, USA – James Webb Space Telescope: https://www.jwst.nasa.gov/content/webbLaunch/assets/images/mirrorAlignment/instrumentsCommOverallCompositeImage-1200px.jpg.

Figure 1.2: Carina nebula in visible light (left) and near-infrared light (right) as seen by the Hubble space telescope. Credits: NASA, ESA.

The detectors of the JWST are described in detail by Rauscher et al. (2014).

Infrared radiation is also emitted from cooler objects and it is able to penetrate gas and dust clouds better than visible light. This is clearly visualized in the spectacular Figure 1.1.

The near-infrared imager and slitless spectrograph part of the FGS/NIRISS will be used to investigate the following science objectives: first light detection, exoplanet detection and characterization, and exoplanet transit spectroscopy.

If the planet moves along the surface of its star, the starlight will be absorbed in the region of the chemicals in the planets atmosphere. Signals of vegetation (if present) should be visible too.

The focal plane of the infrared detector consists of two HgCdTe sensor chip assemblies. Each chip is a 2D array of 2,048 × 2,048 pixels, 18 μm pitch, hybridized onto a dedicated Read-Out integrated circuit. It will enable researchers to identify the “fingerprints” of molecules (Figure 1.3) like water, carbon dioxide, methane, and ammonia, which can’t be identified with any other existing instruments.

Figure 1.3: Compounds in space which can be detected by Webb. Credits: NASA, ESA, the Hubble Heritage Team, and M. McClure (Universiteit van Amsterdam) and A. Boogert (University of Hawaii).

1.1 Vibrational spectral libraries

A few libraries of vibrational spectra are available, some free, but most of them have to be bought or paid per spectrum (descriptions taken from the companies website). Some of the libraries (but not all) are described below:

1) Aldrich Raman Condensed Phase Library

This library represents a comprehensive collection of 18,454 FT-Raman spectra. It contains many common chemicals found in the Aldrich Handbook of Fine Chemicals.

A list of the Aldrich spectra can be accessed:

https://assets.thermofisher.com/TFS-Assets/CAD/Specification-Sheets/D02332~.pdf

2) Nicolet Library of FT-IR and Raman Spectra

The Nicolet library of Fourier transform infrared (FT-IR) and Raman spectra contains a broad choice of common chemicals that can be found in the Aldrich Handbook of Fine Chemicals. This spectral collection includes 3,119 FT-IR spectra and 3,119 matched Raman spectra, which are of interest to analytical laboratories.

https://www.thermofisher.com/order/catalog/product/833-009601

3) FT-Raman Forensic Library

This library is designed to assist forensic scientists and investigators in fast and easy identification of common street drugs. It contains a collection of 175 common drugs and related compounds frequently encountered in forensic analysis. It contains spectra of controlled substances, common contaminants, and “cutting” agents. The Raman technique provides a unique benefit in forensic science since sample analysis can be completed directly through plastic bags and glass containers.

http://www.thermo.com.cn/Resources/200802/productPDF_2080.pdf

4) Cayman Chemical™ Raman Library

This library, containing 187 spectra, includes synthetic cannabinoids, synthetic cathinones, synthetic piperazines, and tryptamines, which are the basis for newer drugs of abuse such as spice, bath salts, legal ecstasy, and novel psychoactive drugs.

http://www.thermofisher.com/order/catalog/product/834-105000

5) National Institute of Advanced Industrial Science and Technology (AIST) Spectral Database for Organic Compounds (SDBS)

From this database 3,573 Raman spectra of organic compounds are available and about 54,100 FT-IR spectra.

This is a free service!

http://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi

6) KnowItAll Raman Spectral Library

Bio-Rad produced high-quality Raman spectral databases from their renowned Sadtler databases. The KnowItAll Raman Spectral Library offers access to over 339,000 IR spectra and 25,000 Raman spectra

https://sciencesolutions.wiley.com/solutions/technique/ir/knowitall-ir-collection/

7) FDM

FDM publishes FT-IR, ATR/FT-IR, and Raman libraries.

Separate libraries can be obtained for polymers, retail adhesives and sealants, plastic, organics, and inorganics. Together the FT-IR libraries contain 2,728 spectra taken at a spectral range of 400 to 4,000 cm−1 with a resolution of 4 cm−1, and the Raman libraries contain 3,050 spectra measured with 780 nm excitation and a spectral range of 3,417 to 200 cm−1.

http://www.fdmspectra.com/

8) S.T. Japan-USA

This database contains more than 170,000 ATR-FT-IR, FT-IR transmission, Raman, and NIR spectra. New spectra are continously added to the spectral libraries. The NIR database – 4,200 cm−1 to 10,000 cm−1 – includes 6,049 NIR spectra.

The largest FT-IR collection, the KBr database, contains 22,995 spectra measured using KBr sample preparation with a spectral range of 4,000 to 400 cm−1.

The Raman collection contains 20,112 Raman spectra. The samples were measured with an excitation laser wavelength of 1,064, 785, 532, or 488 nm and are covering a spectral range from 4,000 cm−1 to 200 cm−1.

http://www.stjapan-usa.com

9) NIST Chemistry WebBook

The NIST Chemistry WebBook shows evaluated infrared reference spectra from the Coblentz IR spectral collection.

This is a free service!

https://webbook.nist.gov/

The websites mentioned have been last accessed in February 2023. We cannot take responsibility for the content of the websites whose links are refered to in this book.

Primpke et al. (2018) describe reference database design for the automated analysis of microplastic samples based on FT-IR spectroscopy.