Basic physics of X-ray absorption and scattering

Abstract

X-rays This chapter provides a brief description of the basic X-ray physics needed to design XAFS experiments. We start with the basics. X-rays are short-wavelength electromagnetic (EM) radiation; except for their wavelength, they are essentially the same as radio waves, microwaves, infrared, visible, ultraviolet, and gamma radiation. The frequency f is related to the wavelength λ by f λ = c , where c is the speed of light, ≈ 3 × 10 8 m/s. In free space, EM waves are transverse: the electric and magnetic field vectors of the wave are perpendicular to each other, and also to the direction of propagation. The electric and magnetic field vectors oscillate in phase, and their magnitudes are proportional to each other. The direction of the electric field is described by the “electric polarization vector”, which is a unit vector in the direction of the wave's electric field vector. The direction of wave propagation is given by the wave vector where. From a quantum perspective, the electromagnetic waves of classical physics consist of swarms of photons, which carry energy, linear momentum, and angular momentum. Such a wave is illustrated in Figure 2.1. The wavelength λ of all particles, including photons and electrons, is related to their momentum p through the De Broglie relation λ = h/p , where h is Planck's constant. Similarly, the particle frequency f is related to the energy E by f = E/h .