Pavia Spectroscopy 4th Pdf
Yair Nixon
Introduction to SpectroscopySpectroscopy is a complex art - but it can be very useful in helping scientistsunderstand how an object like a black hole, neutron star, or active galaxyis producing light, how fast it is moving, and even whatelements it is made of. A spectrum is simply a chart or a graph thatshows the intensity of light being emitted over a range of energies. Spectracan be produced for any energy of light - from low-energy radio waves tovery high-energy gamma-rays.Spectra are complex because each spectrum holds a wide variety ofinformation. For instance, there are many different mechanisms by which anobject, like a star, can produce light - or using the technical term forlight, electromagnetic radiation. Each of these mechanisms has acharacteristic spectrum.Let's look at a spectrum and examine each part of it.To the right is an X-ray spectrum made using data from the ASCA satellite.It is of a supernova remnant (SNR) - a SNR is a huge cloud of gaseousmatter swept up from the explosion of a massive star. The X-axis showsthe range of energy of light that is being emitted. The Y-axis of the graphshows the intensity of the light recorded by the instrument from the SNR -- that is, the number of photons of light the SNR is giving off at each energy,multiplied by the sensitivity of the instrument at that energy.We can tell that the light, or radiation, from this SNR is very high energy -if we look at the units of the X-axis - we can see that the photons of lighthave energys measured in keV, or kilo-electron Volts.A kilo-electron Voltis 1000 electron Volts (eV). This puts is the X-ray range of theelectromagnetic spectrum.The graph shows a decreasing curve, with lots of bumps in it. Thecurve itself is called a continuum - it represents X-ray photonsemitted at all energies continuously. The X-rays that are producing this continuum can be caused by several mechanism that are completely different than those producing the X-raysat thevarious peaks and bumps on the curve. The peaks and bumps are called lineemission. Not only are these two different kind of X-ray emission(continuum and line) produced differently, but they each tell usdifferent things about the source that is emitting them.The Electromagnetic SpectrumWhite light (what we call visible or optical light) can be split upinto its colors easily and with a familiar result - therainbow. All we have to do is use a slit to focus a narrow beam ofthe light at a prism. This set-up is actually a basic spectrometer.The resultant rainbow is really a continous spectrum that shows us thedifferent energies light (from red to blue) present invisible light. But the electromagnetic spectrum encompasses morethan justoptical light - it covers all energies of light extending fromlow-energy radio waves, to microwaves, to infrared, to optical light,to ultraviolet, to very high-energy X- and gamma-rays.Line EmissionInstead of using our spectrometer on a light bulb, what if we were to use it tolook a tube of gas - for example, hydrogen? We would first need toheat the hydrogen to very high temperatures, or give the atoms of hydrogenenergy by running an electric current through the tube. This would causethe gas to glow - to emit radiation. If we looked at the spectrum of lightgiven off by the hydrogen gas with our spectroscope, instead of seeing acontinuum of colors, we would just see a few bright lines. Below we seethe spectrum, the unique fingerprint of hydrogen.These bright lines are called emission lines. Remember how we heatedthe hydrogen to give the atoms energy? By doing that, we excited the electronsin the atom - when the electrons fell back to their ground state, they gaveoff photons of light at hydrogen'scharacteristic energies. If we altered theamount or abundance of hydrogen gas we have, we could change theintensity of the lines, that is, their brightness, because more photons wouldbe produced. But we couldn'tchange their color - no matter how much or how little hydrogen gas waspresent, the pattern of lines would be the same. Hydrogen's patternof emission lines is unique to it. The brightness of the emissionlines can give us a great deal of information about the abundance of hydrogenpresent. This is particularly useful in a star, wherethere are many elements mixed together.Each element in the periodic table can appear in gaseous form and will eachproduce a series of bright emission lines unique to that element. Thespectrum of hydrogen will not look like the spectrum of helium, or thespectrum of carbon, or of any other element.Hydrogen:
Helium:
Carbon:
We know that the continuum of the electromagnetic spectrum extends fromlow-energy radio waves, to microwaves, to infrared, to optical light,to ultraviolet, to X and gamma-rays. In the same way, hydrogen's uniquespectrum extends over a range, as do the spectra of the other elements.The above spectra are in the optical range of light.Line emission can actually occur at any energy of light (i.e. visible, UV, etc.) and with any type of atom, however, not all atoms have lineemission at all wavelengths. The difference in energy between levelsin the atom is not great enough for the emission to be X-rays inatoms of lighter elements, for example.Different Graphical Representations of SpectraThe sample spectra above represent energy emission as lines, the amount ofphotons of light represented by the brightness and width of the line.But we can alsomake a graphical representation of a spectrum. Instead of the emission ofa characteristic energy being shown as a line, it can be shown as a peakon a graph. In this case, the height and width of the peak show itsintensity. One example of this is the very first spectrum we looked at - theone of the supernova remnant. The peaks and bumps on the graph are simplya graphical representation of the emission lines of different elements.Below, you will see the spectrum of the Sunat ultraviolet wavelengths. There are distinct lines (in the topgraph) and peaks (in the bottom one) and if you look at the X-axis,you can see what energies they correspond to. For example, we knowthat helium emits light at a wavelength of 304 angstroms, so if we seea peak at that wavelength, we know that there is helium present.Spectra and AstronomyIn a star, there are actually many elements present. The way we can tellwhich ones are there is by looking at the spectra of the star. In fact, theelement helium was first discovered in the Sun, before it was everdiscovered on Earth. The element is named after the Greek name for the Sun,Helios.The science of spectroscopy is quite sophisticated. From spectrallines astronomers can determine not only the element, but thetemperature and density of that element in the star. Emission lines canalso tell us about the magnetic field of the star. The width ofthe line can tell us how fast the material is moving, giving usinformation about stellar wind. If the lines shift back and forth, it meansthat the star may be orbiting another star - the spectrum will givethe information necessary to estimating the mass and size of the star systemand the companion star. If the lines grow and fade instrength we can learn about the physical changes in the star.Spectral information, particularly from energies of light other thanoptical, can tell us about material around stars. This material mayhave been pulled from a companion star by a black hole or a neutronstar, where it will form an orbiting disk. Around a compact object(black hole, neutron star), the material in thisaccretion disk is heated to the point that it gives off X-rays,and the material eventually falls onto the black hole or neutronstar. It is by looking at the spectrum of X-rays being emitted bythat object and its surrounding disk, that we can learn about the natureof these objects.Continuum Emission Just like visible light, with its range of energies from red to blue,X-rays have a continuum, or a range of energies associated with it.X-rays usually range in energy from around 0.5 keV up to around 1000 keV.Like line emission, continuum X-ray emission involves charged particles.Continuum emission is a result of the acceleration of a population ofcharged particles. All X-ray sources contain such particles. These particles must beat least partially ionized - their electrons need to be unbound from theirnuclei to be free to zip around when they are heated toextreme temperatures. For an electronto radiate X-rays, the gas containing the electronmust have extreme conditions, such as temperatures of millions of degrees,super-strong magnetic fields, or the electrons themselves must be movingat nearly the speed of light. Extreme conditionscan be found in disks of matter orbiting black holes or in supernova remnants.Strong magnetic fields, like those created in the wake of a supernovaexplosion, can also accelerate fast moving ions in spirals around thefield lines to the point of X-ray emission. Electrons can be acceleratedto nearly the speed of light in the shockwave created by a supernova explosion.There are three mechanisms that will produce a continuum X-ray emission.They are Synchrotron Radiation, Bremsstrahlung, and ComptonScattering. The radiation produced is continuous, and not at thediscreet energies of line emission because the populations ofelectrons have a continuous range of energies, and they can beaccelerated through a range of energies,.
courtesy of University of HertfordshireSychrotron radiation is emitted when a fast electroninteracts with a magnetic field. A magnetic field in an area anelectron is traveling in will cause the electron to change direction byexerting a force on it perpendicular to the direction the electron is moving.As a result, the electron will be accelerated, causing it to radiateelectromagnetic energy. This is called magneticbremsstrahlung or synchrotron radiation (after radiationobserved from particle accelerators by that name). If the electrons and themagnetic field are energetic enough, the emitted radiation can be in the formof X-rays.Bremsstrahlung occurs when an electron passes close to apositive ion, and the strong electric forces cause its trajectory tochange. The acceleration of the electron in this way causes it toradiate electromagnetic energy - this radiation is calledbremsstrahlung, (literally, from the German meaning 'brakingradiation'). Thermal bremsstrahlung occurs in a hot gas, where manyelectrons are stripped from their nuclei, leaving a population ofelectrons and positive ions. If the gas is hot enough (millions ofdegrees Kelvin), this kind of radiation will primarily take the formof X-rays.
courtesy of University ofHertfordshire
courtesy of University of HertfordshireComptonization is when a photon collides with an electron - the photonwill either give up energy to or gain energy from the electron,changing the electron's velocity as a result.What Are Some Examples of Continuum Emission?Gas that is hotter than 10 million degrees, such as the gasheated by a supernova explosion, produces most of its emission in X-raysfrom thermal Bremsstrahlung. Gas can be heated to these temperatures bythe outward moving shock of a supernova explosion, or in an accretiondisk around a black hole or neutron star. Synchrotronradiation can produce X-rays around supernova remnants (SNR), where themagnetic fields are strong and ions have been accelerated by the shockwave to high energies. X-rays produced by SNR require electrons withenergies of about 104 GeV (Giga electron-Volts) each (you would have to heatan electron to a temperature of about ten trillion degrees for it to havethis much energy)! Synchrotron radiation and Compton scattered radiationare major components of the diffuse X-ray background and emission fromactive galaxies. For the StudentUsing the text, define the following terms:spectroscopy, keV, continuum, continuum emission, line emission,electromagnetic spectrum, synchrotron radiation, bremmstrahlung,comptonization. Reference URLs:Spectroscopy
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#railAlign width:700px;margin-right:-22px; A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Andy Ptak (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC