Page 219 - Essentials of physical chemistry

9 Basic Spectroscopy

INTRODUCTION

An essential concept for physical chemistry in the twenty-first century is spectroscopy that inter-
twines developments in astronomy, physics, and technical innovations in instrumentation as now
applied to chemistry. While we focus on chemical applications, we can briefly mention historical
highlights in physics because the key concept is the quantization of energy. Energy quantization is a
revolutionary concept that took most of the early twentieth century to discover, prove, and describe
but it is now the backbone of spectroscopy. Spectroscopy measures various forms of light energy
that are absorbed or emitted only at specific wavelengths. That is due to the fundamental concept
that at the level of atoms and molecules, energy occurs in ‘‘quantum chunks’’ that are so small that
in everyday life, we think energy is continuous but it is not. The analogy we offer to students is the
difference between smooth peanut butter and chunky style peanut butter, because if you examine
smooth peanut butter with a simple lens you can see tiny chunks. Thus, smooth or chunky is a
matter of size in peanut butter and also in energy.
In this chapter, we will attempt to provide an overview of several forms of spectroscopy to set the
scene for more detailed descriptions in Chapter 10. Thus, we will have to compress the historical
development to focus on the important case of the hydrogen spectrum and the Bohr model of the
quantized levels within the H atom. We will revisit the details of the early twentieth century
discoveries in chemistry and physics in Chapter 10. Since this may be the end of a one-semester
course, we will stretch the Bohr model to treat x-rays but then show the need for more modern
methods. We do this to introduce several spectroscopic techniques within a simple mathematical
model and to create interest for a second semester of physical chemistry with the question ‘‘If energy
is quantized, what does this mean?’’

PLANCK’S DISCOVERY

The father of quantization was really Max Planck who received the Nobel Prize in 1918 for his work
on blackbody radiation in 1901. However, there were earlier signs of a strange new concept
regarding energy in the work of others. For instance, science historians might go all the way back
to 1814 when German optician Josef von Fraunhofer observed individual lines in the spectra of stars
and the sun. Later Gustav Kirchhoff, a German physicist, made known his work with Robert Bunsen
on observation of dark (and light) lines in the spectrum of the sun, and this work was developed
further by several amateur astronomers, most notably by William Huggins (1824–1910) who sold
his silk exchange business and set up a private observatory just outside of London. Thus, even in the
late 1800s, scientific research was still carried out by individuals at their own expense as in our
discussion of Sir Robert Boyle in the 1600s. In the late 1800s, a breakthrough in astronomy was to
attach a spectroscope (a prism) to a telescope and separate the image of stars and the sun into
separate lines=images of different color. As the science progressed, it became possible to measure
the wavelengths of the various colors of light. Measurements by Huggins and later by H. C. Vogel
(1841–1907) were then studied by Johann Balmer (1825–1898) who published an analysis of a few
visible lines from the spectrum of hydrogen, which is abundant in space and in the spectra of stars.
An excellent modern history of these developments is given by Becker [1], which gives a glimpse of
the importance of astronomy to the development of physics and chemistry.

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