Properties of Electromagnetic Radiation

Recognizing that the wave travels a distance X in time tg, we can derive a second relationship  

the wavelength and frequency of a given wave are not independent quantities rather, they are inversely propor- 

In addition to its wave properties, electromagnetic radiation also exhibits certain behavior characteristic of particles. A particle, or quantum, of radiation is called a photon. For our purposes the most important particlelike property of a photon is its energy ( ). Each photon possesses a discrete amount of energy that is directly proportional to its frequency (if we regard it as a wave). This relationship can be written 

To summarize, if two photons possess the same energy, they correspond to waves (or wavelets) of the same frequency and the same maximum amplitude. The total intensity of an electromagnetic beam is therefore the number of photons delivered per second. 

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The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

Thus, for electromagnetic radiation of frequency, V, the wavelength in vacuum is longer than in other media. Another unit used to describe the wave properties of electromagnetic radiation is the wavenumber, V, which is the reciprocal of wavelength... 

The energy of a photon provides an additional characteristic property of electromagnetic radiation. 

In the previous section we defined several characteristic properties of electromagnetic radiation, including its energy, velocity, amplitude, frequency, phase angle, polarization, and direction of propagation. Spectroscopy is possible only if the photon s interaction with the sample leads to a change in one or more of these characteristic properties. 

A significant change in the theoretical treatment of atomic structure occurred in 1924 when Louis de Broglie proposed that an electron and other atomic particles simultaneously possess both wave and particle characteristics and that an atomic particle, such as an electron, has a wavelength X = h/p = h/mv. Shortly thereafter, C.J, Davisson and L.H. Germer showed experimentally the validity of this postulate. Dc Broglie s assumption that wave characteristics are inherent in every atomic particle was quickly followed by the development of quantum mechanics, in its most simple form, quantum mechanics introduces the physical laws associated with the wave properties of electromagnetic radiation into the physical description of a system of atomic particles. By means of quantum mechanics a much more satisfactory explanation of atomic structure can be developed. 

Chemists study atoms by observing the properties of electromagnetic radiation they emit. They then build up a model of the structure of the atom that accounts for these properties. The analysis of the electromagnetic radiation emitted or absorbed by substances is a branch of chemistry called spectroscopy. We shall see that atomic spectroscopy—spectroscopy applied to atoms—can be used to determine their internal structure. 

Particlelike Properties of Electromagnetic Radiation The Planck Equation... 

The speed of operation is interlinked with the properties of electromagnetic radiation at the corresponding frequencies. The frequencies of operation in modem IT devices have reached some GHz in the region of microwave radiation. A further increase would step to infrared radiation, visible light and so on see Fig. 1. 

Infrared (IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy (Chapter 14) both use a form of electromagnetic radiation as their energy source. To understand IR and NMR, therefore, you need to understand some of the properties of electromagnetic radiation— radiant energy having dual properties of both waves and particles. 

Particle properties of electromagnetic radiation Behavior that is consistent with radiation acting as small particles or quanta of energy. 

The wave properties of electromagnetic radiation are described by two interdependent variables, as Figure 7.1 shows ... 

Many of the properties of electromagnetic radiation are conveniently described by means of a classical sinusoidal wave model, which embodies such characteristics as wavelength, frequency, velocity, and amplitude, in contrast to other wave phenomena, such as sound, electromagnetic radiation requires no supporting medium for its transmission and thus passes readily through a vacuum. 

The properties of electromagnetic radiation are discussed elsewhere [3-5]. Absorption of such radiation is essential to start the photoreaction. Ac-... 

A few years after de Broglie pubUshed his theory, the wave properties of the electron were demonstrated experimentally. When X-rays pass through a crystal, an interference pattern results that is characteristic of the wavelike properties of electromagnetic radiation. This phenomenon is called X-ray diffraction. As electrons pass through a crystal, they are similarly diffracted. Thus, a stream of moving electrons exhibits the same kinds of wave behavior as X-rays and all other types of electromagnetic radiation. 

The major part of this book will be concerned with the wave properties of matter, but it will be helpful, at the outset, to spend a little time looking at the particle properties of electromagnetic radiation because similar concepts apply in both cases. 

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