How to Build a Laser Death Ray
Ionization
|
|
How to Build a Laser Death RayIonization |
|
|
Under certain conditions, light can strip electrons from atoms. This process is called ionization. The electrically charged atoms left behind are called ions. The ions and free electrons together are called a plasma.
Ionization requires energy. This energy comes from the beam. When you have ionization, you are therefore losing energy from your death ray beam. As a result, the range at which the beam can cause damage will decrease, and ionization should be avoided whenever possible. If the plasma becomes too dense, the beam will be entirely absorbed. This is conventionally described using a parameter called the plasma frequency ωp
ωp = 54.1 n1/2
where n is the number of electrons per cubic meter, and the result is in hertz. All light with a wavelength longer than c/ωp will be absorbed within a few wavelengths, where c is the speed of light = 3×108 meters per second. All light with a shorter wavelength will pass through the plasma. Air at sea level contains 2.4×1025 molecules per cubic meter (and twice that number of atoms per cubic meter). If each
atom has one electron ionized, the plasma frequency will be 5.41 sqrt(4.8×1025) = 3.7×1013 hertz, and all light with a wavelength longer than 8×10-6 meters will be blocked. If all the valence electrons (loosely bound electrons capable of participating in chemical bonding) are removed from each atom, the electron density is 25.0×1025 particles per cubic meter, and the plasma frequency is 8.5×1014 hertz and all light with a wavelength longer than 0.35×10-6 meters will be stopped.
Faint ionization of the atmosphere will cause a green glow due to the recombination of electrons with the air molecules. As the plasma becomes denser, it will start to blaze a brilliant while. There are three main sources of ionization ... Single photon ionizationLight is composed of individual packets, or quanta, of energy. A single quanta of light is called a photon. The energy of one photon is 1.24×10-6 divided by the light's wavelength in meters. The energy is in units of electron volts, or eV, a common unit used for describing energies on the atomic or molecular scale. One eV = 1.602×10-19 Joules.When the energy of one photon is greater than the energy which holds an electron to an atom, the atom can absorb the photon and be ionized. This is a linear process, so all the effects of single photon ionization can be handled using the methods described in the section on Linear Absorption and Scattering. The threshold for single photon ionization generally occurs near 2×10-7 meters. This is the upper wavelength for the vacuum ultraviolet (VUV) part of the spectrum, so called because single photon ionization is so effective that VUV light simply cannot travel through the air farther than a few millimeters. Even the soft x-rays cannot go more than a few centimeters through air. It is only when you reach the very short hard x-ray and gamma ray wavelengths that ionizing light can travel an appreciable distance in air, but even these have attenuation lengths of less than a kilometer. As a result, VUV, x-ray, and gamma ray lasers are primarily useful in the vacuum of space where their short wavelength allows them to be focused at great distances due to limited diffraction effects. Multi-photon ionizationWhen light becomes sufficiently intense, two or more photons can be absorbed simultaneously. This process is non-linear. For example, the rate of two photon ionization depends on the square of the intensity, the rate of three photon ionization depends on the cube of the intensity, and so on.Multi-photon ionization is more involved to quantitatively describe than single photon ionization. Here, I will give a few examples. Two photon ionization can be significant for ultraviolet beams between about 0.4 microns and 0.2 microns. Light with a wavelength of 0.193 microns and an intensity of 5×1010 watts/square meter will cause a steady state electron density of 8×1013 particles per cubic meter (http://www.patentstorm.us/patents/5675103-description.html). Multi-photon ionization is important in the process of filamentation, when light intensities reach 1017 to 1018 watts per square meter. The ionization is easily visible, but not so severe as to immedeately stop the beam. Cascade ionizationConsider an ion floating in the air when a ray of light comes by. Ions are electrically charged atoms. Light is a wave in the electromagnetic field. This means the light causes an oscillating electrical field as it passes. The electrical field will exert a force on the ion, accelerating it first in one direction, then the other. If the ion can accelerate in one direction for long enough, before the field changes direction or before it bumps into another molecule, it can gain enough energy to knock an electron off another atom or molecule. Thus, when the ion does hit another air molecule, it creates another ion. This new ion can then create more ions through the same process. Pretty soon the air breaks down and becomes a plasma.How can you avoid it? The more intense your light, the faster the ions are accelerated so the more energy they gain. Lowering the intensity will therefore stop cascade ionization. But we want a very intense beam to make a death ray! So, if the electric field of the light changes direction faster, the ions will not have enough time to build up much energy before they start slowing down again. This means we want to use light of a higher frequency, which means we must use shorter wavelengths. This can be quantified by defining a quiver energy
Eosc=9.3×10-6 I λ2
where I is the laser intensity (in watts per square meter), λ is the wavelength of the laser light (in meters), and the quiver energy is given in eV. If the quiver energy is greater than the energy needed to remove an electron, cascade ionization occurs. For air, the ionization energy is around 15 eV (15.6 eV for nitrogen molecules, 13.6 eV for oxygen molecules).
|