Chapter 12X-Ray SpectrometryX-Rays are short wavelength electromagnetic radiation produced by the deceleration ofhigh energy electrons or by electronic transitions of electrons in the inner orbital of atoms.The wavelength range of X-rays is from about 10-5Å to 100 Å; conventional X-rayspectroscopy is largely confined to the region of about 0.1 Å to 25 Å.X-ray spectroscopy is a form of optical spectroscopy that utilizes emission, absorption,scattering, fluorescence, and diffraction of X-ray radiationThe basics... X-rays are short-wavelength (hence, high frequency, and hence, relatively highenergy) electromagnetic radiation. Two ways to produce X-rays: 1) Deceleration of high-energy electrons 2) Electronic transitions involving inner-orbital (e.g. - d or f) electrons Approximate wavelength range: 10-6 nm - 10 nm Wavelength range used in conventional applications: 0.01 nm - 2.5 nmX-rays are the shortest wavelength, i.e., highest energy, electromagnetic radiationassociated with electronic transitions in atoms. Calculation of the energy states of an atomis in general, very difficult, except of course in the particular case of the hydrogen atom,where the problem is readily soluble and the results, shown schematically below, are verywell known.
The orbital picture of the H-atom associated with this energy level diagram is readilyextended to multi-electron atoms and the again familiar result is qualitatively representedbelow. Energy levels of many electron atoms in the periodic table. The insert shows a magnified view of the order near Z = 20.
An important feature of both the above diagrams is that the differences in orbital energiesdecrease as they themselves increase. This means that the energy required for excitation,or given out on relaxation of an electron from a higher orbital to a lower orbital is greaterwhen "inner" orbitals are involved and least when "outer" orbitals are involved. Except forlight elements (say, those preceding Na) the innermost orbitals are not significantlyinfluenced by bonding interactions involving the atom and, hence, their energies may beregarded as characteristic of that atom regardless of its state of combination. Inner orbitaltransitions involve X-rays, and it is for this reason that X-ray spectrometry can be a formof atom detection and, hence, of non-destructive chemical analysis.Of course, for a multi-electron atom, a conventional orbital diagram as given above cannotsimply be converted into an energy level diagram. The energy level diagram for any atomis considerably more complex and depends in detail upon the particular atom. However,for X-ray emissions of importance in elemental analysis, a simplified treatment issufficient and the diagram below is useful. Partial energy level diagram showing common transitions leading to X–radiation. The most intense lines are indicated by the widest arrows.The orbital shells for which the principal quantum number n = 1,2,3, etc. are labelled theK, L, M, etc. and, hence, emissions due to a higher energy electron entering these shells
are said to form the K, L, M, etc. series of lines. Generally, only the K and L series of X-rays are of analytical utility and the wavelengths of these lines for a selection of elementsspanning the Periodic Table are shown below. Wavelengths/Å for Intense X–ray Emission Lines Element K Series L Series a1 b1 a1 b1 Na 11.909 11.617 – – K 3.742 3.454 – – Cr 2.290 2.085 21.714 21.323 Rb 0.926 0.829 7.318 7.075 Cs 0.401 0.355 2.892 1.282 W 0.209 0.184 1.476 1.282 U 0.126 0.111 0.911 0.720 (Note that all possible electronic transitions are not of equal probability, i.e., the nature of a spectrum depends on specific selection rules, so that the complexity of a spectrum is not as great as might be expected from first consideration of an energy level diagram.)The fact that the wavelength of a line of given type decreases as the atomic number of theelement increases is rather important in that it means that an X-ray from a given elementmust be able to cause inner shell ionization and, hence, emission of radiation of lowerenergy from any lighter element.For analytical purposes, X-rays are generated in three ways: 1) bombardment of metal target with high-energy electron beam
2) exposure of target material to primary X-ray beam to create a secondary beam of X-ray fluorescence 3) use of radioactive materials whose decay patterns include X-ray emission Synchrotron radiation is an indispensible tool for x - ray absorption studies of biological materials. The XAFS technique yield information on the number, type and radial distribution of ligands coordinated to a metal are revealed at subatomic resolution. The oxidation state of a metal and the effects of substrate binding or catalysis on the metal centre can be probed. Three dimensional structural information is obtained when x - ray absorption data and protein crystal data are combined in the 3D - XAFS method.Identification and measurement of concentration of elements based on the fact thatprimary-emission x-rays emitted by an element excited by an electron beam have awavelength characteristic of that element and an intensity related to its concentration. Itmay be performed by an electron probe microanalyzer, an electron microscopemicroanalyzer, or by an electron microscope, or scanning electron microscope, fitted withan x-ray spectrometer.Below is a schematic of an X-ray tube.
X-ray sources can emit two forms of X-rays: 1) continuous (white radiation or Bremsstrahlung - “Bremsstrah- lung” refers to radiation arising from the deceleration of particles) 2) discontinuous (line)Below is the partial energy-level diagram showing common transitions producing X-rays.The most intense lines are indicated by wider arrows.
Electron beam sources... In electron beam sources, X-rays are produced by heating a cathode to producehigh-energy electrons; these electrons are energetic enough to ionize off the cathode andrace towards a metal anode (the target) where, upon collision, X-rays are given off fromthe target material in response to the colliding electrons. By varying the conditions, onecan obtain either a continuous spectrum or a discontinuous spectrum. The reaction between the electron beam and the target material involvesdeceleration of the electron and ejection of a target photon and emission of X-rays. Theenergy lost by the electron as it smashes into the target material is equal to the energy ofthe ejected photon. Since any given electron can be retarded differently by the same targetmaterial, a range of photon energies are possible. The maximum photon energycorresponds to total stopping of the electron and is given by: hvo = (hc)/λo = Ve, where, vo is the maximum frequency, V = accelerating voltage, e = electron chargeThis is the Duane-Hunt law.Electron beam source line spectra characteristic... 1. Elements with Z > 23 exhibit two spectral series: a K line (corresponding to shorter wavelengths) and an L line (corresponding to relatively longer wavelengths). Elements with Z < 23 exhibit only the K series 2) As Z increases, so too does the minimum amount of energy required for excitation For all but the lightest elements, the X-ray line spectra are independent of
either physical or chemical states. This is because the electrons involved in the transition are not participating in any chemical bonds. Continuum Spectra from Electron Beam Sources… In an X-ray tube, electrons produced at a heated cathode are accelerated toward a metal anode by a potential as great as 100kV; upon collision, part of the energy of the electron beam is converted into X-Rays. Under some conditions only a continuum spectrum is results. The continuum X-Ray spectrum is characterized by a well-defined, short wavelength limit, which is dependent upon the accelerating voltage but independent of the target material. The continuum radiation from an electron beam source results from collisions between the electrons of the beam and the atoms of the target material.Figure 1.1: Schematic representation of an AGN continuum spectrum including a possible source for each emission component.
Line Spectra from Electron Beam Sources…Bombardment of a molybdenum target produces intense emission lines. The emissionbehavior of molybdenum is typical of all elements having atomic numbers greater than 23,that is, the X-Ray line spectra are similar when compared with ultraviolet emission andconsist of two series of lines.Line spectra are composed of distinct lines of color, or in the case of our graphs, sharppeaks of large intensity at a particular wavelength. Line spectra are characteristic ofelements and compounds when excited (energized) under certain conditions. These spectrahelped develop the current atomic theories. Line spectra thus provide a “fingerprint”unique to each element, and as with continuous spectra, the combination of the prominentlines in the spectrum produce the observe light color.The fluorescent lamp’s spectrum is a mixture of line and continuous spectra. Because ofthe exact correlation with the principal mercury vapor lines and the fluorescent lamp’slines, we can conclude that a major component of the fluorescent lamp is mercury vapor.But what produced the continuous portion of the fluorescent spectrum?A phosphor is a substance that can accept energy in one form and emit the energy in theform of visible light. Fluorescent lights are produced by coating the inside surface of the
glass tube with phosphor particles, which accepts the energy of ultraviolet photons andemits visible photons. In the case of my lamp, the phosphor coating emitted relatively highintensities of light ranging from blue to yellow in color, demonstrated by the continuouspeaks between about 480 nm and 600 nm. Because it is not an atomic source, we shouldnot expect line spectra from the phosphor particles, and so attribute the continuous portionof the plot to the activity of the phosphor. The presence of the continuous spectra also tellsus that the mercury vapor is emitting light in the ultraviolet range, which is beyond thescope of our spectrophotometer to detect directly.These three elemental vapor spectra clearly illustrate line spectra. Examining theprominent lines of neon, I would expect the light to be a deep red-orange color, which iswhat we observed. The spectral lines of krypton indicate another red light, however, weobserved a cool blue color. Argon’s prominent lines also imply a red color, which does notmatch the observed lavender-purple color. I hypothesize that the difference is because ourspectrophotometer doesn’t detect or plot the very short blue visible wavelengths (nearultra-violet), which would combine with the red lines in the spectrum to produce the blueand lavender-blue light seen from krypton and argon vapor lamps.
A neon-helium laser produces a red laser beam, which is correlated on the spectral graphwith a single, sharp peak in the red portion of the spectrum. Because of this single peak,we can refer to the laser as an extremely monochromatic light source. Careful examinationof the graph reveals a minor peak on the neon vapor spectra at the same wavelength of thelaser beam, which indicates the neon component of the neon-helium laser.Absorption... Absorption of X-ray radiation follows Beer’s law like the absorption of other formsof electromagnetic radiation. For X-ray work, Beer’s law looks like: ln(Po/P) = µx where Po = incident beam power, P = transmitted beam power, µ = linear absorption coefficient (similar to molar absorbtivity), x = path length in cmWe can rewrite this to take into account the density of the sample:
ln(Po/P) = µMρx where µM is the mass absorption coefficient.Using the mass absorption coefficient, you don’t need to worry about the physical orchemical state of the sample. Nifty, huh?And, mass absorption coefficients have the additional convenience of being additivefunctions of their weight fractions of sample components: So, µM, tot = WAµA + WBµB + ... + WnµnLike many important scientific discoveries, Fraunhofers observation of spectral lines wasa complete accident. Fraunhofer wasnt looking for anything of the sort; he was simplytesting some new state-of-the-art prisms he had made. When sunlight was sent through athin slit and then through one of the prisms, it formed a rainbow-colored spectrum, just asFraunhofer had expected--but, much to his surprise, the spectrum contained a series ofdark lines.Dark lines? Thats the opposite of what weve been talking about. Youve been telling methat different elements create a series of bright lines at certain wavelengths.Thats what happens when an element is heated. In terms of the Bohr model, heating theatoms gives them some extra energy, so some of their electrons can jump up to higherenergy levels. Then, when one of these electrons drops back down to a lower level, it
emits a photon --at one of that elements special frequencies, of course.And those photons create the bright lines in the spectra you showed me.Exactly--thats called an emission spectrum. But there is another way in which elementscan produce spectra. Suppose that instead of a heated sample of some element, you havethe element in the form of a relatively cool gas. Now lets say that a source of white light--containing all visible wavelengths--is shining behind the gas. When photons from the lightsource make their way through this gas, some of them can interact with the atoms--provided that they have just the right frequency to bump an electron of that element up to ahigher energy level. Photons at those particular frequencies are thus absorbed by the gas.However, as you noted before, the atoms are "transparent" to photons of otherfrequencies...So all those other frequencies would come through okay. Then the spectrum of light thathad been through the gas would just have some gaps in it, at the frequencies that wereabsorbed.
Thats right. The spectrum with these missing frequencies is called an absorptionspectrum. (Note that the dark lines in an absorption spectrum appear at exactly the samefrequencies as the bright lines in the corresponding emission spectrum.)And thats what Fraunhofer saw?Yes. Under very careful examination, the "continuous" spectrum of sunlight turns out to bean absorption spectrum. In order to reach earth, sunlight needs to pass through the sunsatmosphere, which is a lot cooler than the part of the sun where light is emitted. Gases inthe atmosphere thus absorb certain frequencies, creating the 600 or so dark lines thatFraunhofer observed. (They are now called Fraunhofer lines in his honor.)Fraunhofer was unaware of all this, however. No one offered an explanation of spectrallines until decades later...X-ray Fluorescence... Since X-rays are rather energetic, excitation of sample electrons will give rise tofluorescence as the sample electrons are excited and return to their ground states in a seriesof electronic transitions.When a primary x-ray excitation source from an x-ray tube or a radioactive source strikesa sample, the x-ray can either be absorbed by the atom or scattered through the material.The process in which an x-ray is absorbed by the atom by transferring all of its energy toan innermost electron is called the "photoelectric effect." During this process, if theprimary x-ray had sufficient energy, electrons are ejected from the inner shells, creatingvacancies. These vacancies present an unstable condition for the atom. As the atom returnsto its stable condition, electrons from the outer shells are transferred to the inner shells andin the process give off a characteristic x-ray whose energy is the difference between thetwo binding energies of the corresponding shells. Because each element has a unique set ofenergy levels, each element produces x-rays at a unique set of energies, allowing one tonon-destructively measure the elemental composition of a sample. The process of
emissions of characteristic x-rays is called "X-ray Fluorescence," or XRF. Analysis usingx-ray fluorescence is called "X-ray Fluorescence Spectroscopy." In most cases theinnermost K and L shells are involved in XRF detection. A typical x-ray spectrum from anirradiated sample will display multiple peaks of different intensities.Spectrum taken using Amptek XR-100CR 25mm2X500µm X-Ray Detector (20µs shapingtime - optional feature) and Amptek MCA8000A Multichannel Analyzer.The characteristic x-rays are labeled as K, L, M or N to denote the shells they originatedfrom. Another designation alpha (a), beta (b) or gamma (g) is made to mark the x-rays thatoriginated from the transitions of electrons from higher shells. Hence, a Ka x-ray isproduced from a transition of an electron from the L to the K shell, and a Kb x-ray isproduced from a transition of an electron from the M to a K shell, etc. Since within theshells there are multiple orbits of higher and lower binding energy electrons, a furtherdesignation is made as a1, a2 or b1, b2, etc. to denote transitions of electrons from theseorbits into the same lower shell.The XRF method is widely used to measure the elemental composition of materials. Sincethis method is fast and non-destructive to the sample, it is the method of choice for fieldapplications and industrial production for control of materials. Depending on theapplication, XRF can be produced by using not only x-rays but also other primaryexcitation sources like alpha particles, protons or high energy electron beams.
Sometimes, as the atom returns to its stable condition, instead of emitting a characteristicx-ray it transfers the excitation energy directly to one of the outer electrons, causing it tobe ejected from the atom. The ejected electron is called an "Auger" electron. This processis a competing process to XRF. Auger electrons are more probable in the low Z elementsthan in the high Z elements.X-ray Diffraction... Like other forms of electromagnetic radiation, X-rays are also susceptible todiffraction. This property is most useful in determining the structure of single crystalsamples. Here’s the basic idea: In crystals, we have an essentially infinitely repeating sequence of atoms that areusually layered one on top of another. In X-ray diffraction, parallel beams of X-rays aresent into a sample; some will immediately be reflected by the surface layer; some willpenetrate into inner layers. As the X-rays travel into the inner layers, some will bereflected and some will continue through the sample and so. We now have a series ofreflected X-rays that can form an interference pattern with each other; we will obtain aninterference pattern where dark regions correspond to destructive interference and lightregions correspond to constructive interference. The Bragg equation gives the angle ofincidence where constructive interference occurs in terms of the interatomic distancebetween crystal planes: sin θ = (nλ)/2d where, θ= angle of incidence λ = wavelength d = interplane distance of crystalBelow is a figure of the diffraction of X-rays by a crystal.
Diffraction and Braggs LawDiffraction occurs as waves interact with a regular structure whose repeat distance is aboutthe same as the wavelength. The phenomenon is common in the natural world, and occursacross a broad range of scales. For example, light can be diffracted by a grating havingscribed lines spaced on the order of a few thousand angstroms, about the wavelength oflight.It happens that X-rays have wavelengths on the order of a few angstroms, the same astypical interatomic distances in crystalline solids. That means X-rays can be diffractedfrom minerals which, by definition, are crystalline and have regularly repeating atomicstructures.When certain geometric requirements are met, X-rays scattered from a crystalline solid canconstructively interfere, producing a diffracted beam. In 1912, W. L. Bragg recognized apredicatable relationship among several factors.1. The distance between similar atomic planes in a mineral (the interatomic spacing) whichwe call the d-spacing and measure in angstroms.2. The angle of diffraction which we call the theta angle and measure in degrees. Forpractical reasons the diffractometer measures an angle twice that of the theta angle. Notsurprisingly, we call the measured angle 2-theta.3. The wavelength of the incident X-radiation, symbolized by the Greek letter lambda and,in our case, equal to 1.54 angstroms.
The DiffractometerA diffractometer can be used to make a diffraction pattern of any crystalline solid. With adiffraction pattern an investigator can identify an unknown mineral, or characterize theatomic-scale structure of an already identified mineral.There exists systematic X-ray diffraction data for thousands of mineral species. Much ofthese data are gathered together and published by the JCPDS-International Centre forDiffraction Data.
The diffractometer in the IPFW Geosciences Department is a Philips APD3520 built in1986. It consists of several parts.A. The chiller provides a source of clean water to cool the X-ray tube.B. The regulator smooths our building current to provide a steady and dependable sourceof electricity to the diffractometer and its peripherals.C. The computer sends commands to the diffractometer and records the output from ananalysis. We are currently using a 486-100 running DR-DOS7 to run the diffractometer,and provide interfacing with this web page. We process most of the information digitally,although we can make hardcopy analog patterns directly on the;D. Strip-chart recorder.
E. The tube provides an X-ray source. (An old tube, shown upside down, is on the countertop.) Inside there is a 40,000 volt difference between a tungsten filament and coppertarget. Electrons from the filament are accelerated by this voltage difference and hit thecopper target with enough energy to produce the characteristic X-rays of copper. We useone part of the copper spectrum (with a wavelength of 1.54 angstrom) to make thediffraction pattern. The radiation is monochromatized by a graphite crystal mounted justahead of the scintillation counter.F. The theta compensating slit collimates the X-rays before they reach the sample.G. The sample chamber holds the specimen. We grind our samples to a fine powder beforemounting them in the diffractometer, and then close the chamber to allow the collimatedX-rays to enter from the left. The X-rays hit and scatter from the sample. The diffractedbeams leave the chamber to the right where they can be detected by the;H. Scintillation counter which measures the X-ray intensity. It is mounted on the;I. Goniometer which literally means angle-measuring device. The goniometer is motorizedand moves through a range of 2-theta angles. Because the scintillation counter is connectedto the goniomter we can measure the X-ray intensity at any angle to the specimen. Thatshow we determine the 2-theta angles for Braggss Law.Diffraction PatternsA diffraction pattern records the X-ray intensity as a function of 2-theta angle. All thediffraction patterns youll see on this web site were prepared as step-scans. To run a step-scan we mount a specimen, set the tube voltage and current, and enter the followingparameters:--A starting 2-theta angle.--A step-size (typically 0.005 degrees).--A count time per step (typically 0.05-1 second).--An ending 2-theta angle.Once started, the goniometer moves through its range, stopping at each step for the alottedtime. The X-ray counts at each step are saved to a file on the computer. Once finished, the
data are smoothed with a weighted moving average and a diffractogram like the one belowis printed or displayed.Consider the following areas on the diffractogram.A. The diffraction pattern is labelled with the sample name and other information pertinentto the experiment. This happens to be a pattern of ground calcite from the France StoneQuarry in Fort Wayne, Indiana. The sample was randomly mounted using the backpacktechnique. The diffraction pattern was prepared on March 24, 1993. The diffractometerwas running at 40 kv and 30 ma. Steps were in increments of 0.005 degrees, and countswere collected for 0.25 seconds at each step. The data were smoothed with a 15-pt(weighted, moving average) filter.B. The vertical axis records X-ray intensity. The horizontal axis records angles in degrees2-theta. Low angles (large d-spacings) lie to the right.C. This is one of the X-ray peaks. It happens to be the one with the smallest angle which Imeasured as 23.04 degrees. Solving Braggs Law (with n=1 and wavelength=1.54 ang) wefind that 23.04 degrees 2-theta corresponds to a d-spacing of 3.86 angstrom.
D. This is another peak picked for no special reason. I measured the peak at 39.37 degrees2-theta. This corresponds to a d-spacing of 2.287 angstrom.E. This is the largest peak on the pattern. It actually extends several times the height of thisimage. Many factors affect the intensity of a given peak. Some of these factors areintrinsic to the mineral under study; some of these factors are peculiar to the way aspecimen is mounted in the diffractometer. (The random/backpack mounting methodlimits, but does not eliminate, these peculiarities).When solving scientific problems it is often useful to ask three questions: What do IKnow? What can I measure? What do I want to find out?In Braggs law:1. we know lambda (the wavelength) = 1.54 angstrom and we assume n=1.2. We can measure 2-theta from the diffraction pattern. Divided by 2, these values becometheta.3. That leaves us with an equation with one unknown -d- the d-spacing we want to findout.Instruments... Instruments for X-ray absorption, emission, fluorescence, and diffraction are allsimilar to other optical spectroscopy instruments. Thus, the basic components will be asource (X-ray tube, radioisotopes, or secondary fluorescent sources), a filter system (i.e.monochromators), sample holder, detector/transducer, signal processor, and output device.Below is the figure of an X-ray monochromator and detector.
Methods... X-ray fluorescence (XFR) is one of the most widely used of all analytical methodsfor qualitative identification of elements with Z>8. (Skoog, Leary, p. 373) Below is adiagram of a XFR.
Below is a X-Ray fluorescence spectrum. X-ray absorption is not too convenient and is relatively awkward and time-consuming with respect to X-ray fluorescence methods. Also, X-ray absorption tends togive spectra with lots of peaks; to make this technique useful, absorption is often restrictedto analysis of single, heavy components in a matrix of relatively lighter components. Forexample, X-ray absorption is used to determine the amount of lead in gasoline or theamount of sulfur/halogens in hydrocarbons. X-ray diffraction is most powerful when used for crystal structure determination.Structurally complex compounds, such as steroids, vitamins, and antibiotics are beingstudied via X-ray diffraction. In addition to natural products, X-ray diffraction is also goodfor qualitative identification of crystalline compounds via the X-ray powder diffractionmethod. This method is distinct in that it is the only analytical method capable ofproviding both qualitative and quantitative information about the components within asolid sample. X-ray powder diffraction works on the basis that every compound has aunique X-ray diffraction pattern. The electron microprobe is an X-ray emission technique that is used to gatherinformation about the chemical and physical nature of surfaces. It can provide bothqualitative and quantitative information. It works by focusing a narrow beam of electronsonto a surface, giving rise to X-ray emission; the emitted X-rays are then detected viawavelength- or energy-dispersive spectrometers and then analyzed.
X-ray spectraFigure 1.2: Schematic diagram displaying the key features of a Seyfert 1 X-ray spectrum (from Fabian 1998b).(eps)
-To first order, the X-ray spectra of AGN are power laws of the form S with ~ 0.7,modified by photoelectric absorption at soft energies (Mushotzky 1980, Halpern 1982,Rothschild et al. 1983). ROSAT and Ginga, sensitive in the ranges 0.1 - 2 and 2 - 30 keVrespectively, revealed a more complicated structure with a softer underlying power law of ~ 0.9. Superimposed on this continuum are a number of features illustrated in figure 1.2.A strong emission line is seen at a rest frame energy of ~ 6.4 keV (e.g. Pounds etal. 1990). This is interpreted as fluorescent Fe K emission, but with an equivalent widthtoo great to be the result of absorbing material in the line of sight (Makishima 1986). Thespectra are seen to harden beyond this feature to form a bump peaking at ~ 30 keV. Thiscan be explained by a reprocessing (Compton scattering or `reflection) of the X-ray powerlaw within optically thick material (e.g. George & Fabian 1991). The intensity of thiscomponent is consistent with the reflector subtending a solid angle of 2 (Nandra etal. 1991), most likely the accretion disc itself. Photoelectric absorption by elements withinthe disc result in Compton reflection only becoming dominant beyond the absorption edgeof iron. Beyond the peak at ~ 30 keV the spectrum steepens due to the effects of energyloss to electron recoil and reduction in the scattering cross-section.At soft X-ray energies, the power law continuum is often absorbed by ionised material inthe line of sight. This is known as the `warm absorber and is generally characterised bythe K-shell absorption edges of ionised oxygen which fall within the ROSAT and ASCAbandpasses. Absorption by cold material is also seen in some objects although these areusually classified as obscured AGN (see section 1.2). Photoelectric absorption may oftenmask the presence of the `soft excess. Identification of this feature is also hampered by itsposition at the bottom of the bandpass for most X-ray telescopes. However, recent XMM-Newton spectra of a sample of 6 Seyfert I galaxies reveals an excess soft emissioncomponent in each case (Pounds & Reeves 2002).A final important aspect of AGN high energy spectra is an exponential cut-off at ~ 200keV (e.g. Seyfert I galaxy IC 4329a, Madejski et al. 1995). Although the exact energy ofthis cut-off is uncertain, no radio-quiet AGN has been detected beyond several hundredkeV.Models for the X-ray spectra of AGN generally attempt to explain the underlying powerlaw and high energy cut-off, considering the remaining spectral features to be due to the
effects of reprocessing. The majority ascribe the power law to the inverse Comptonscattering of soft photons in a bath of `hot electrons. Variations in this model depend onthe energy distribution of these electrons and their location in relation to the accretion disc.In the `non-thermal model (Zdziarski et al. 1990, Zdziarski & Coppi 1991), electrons areaccelerated to relativistic energies, perhaps due to magnetic reconnection events.Collisions with photons and other electrons cause a `pair cascade which then ComptoniseUV and optical photons from the disc. The pairs then thermalise to temperatures of a fewkeV resulting in further upscattering of UV photons to form the soft excess. A majorproblem with this model is that the pairs should eventually annihilate to produce a strongemission line at 511 keV. This line has never been observed. It is therefore thought likelythat the scattering electrons have a thermal (`Maxwellian) energy distribution. This wouldpredict a high energy cut-off (Ec) at Ec/1.6 kTplasma for a Comptonising plasma attemperature Tplasma (e.g. Pietrini & Krolik 1995).The geometry of this Comptonising plasma remains uncertain. A simple `sandwich modeldescribes a cool, optically thick accretion disc covered by a hot optically thin layer (e.g.Maraschi & Haardt 1996). A certain fraction (f) of the gravitational energy is dissipated inthe hot corona, with the remainder (1-f) dissipated in the disc. Isotropic Comptonisationresults in half the emission escaping directly from the corona. Of the half which impingeson the disc, 10 - 20% is reflected to produce the Compton reflection bump and the Fe Kfluorescence line. The remaining 80 - 90% is photoelectrically absorbed and eventuallyreemitted as thermal radiation in the `big blue bump.Problems with this model geometry arise on attempting to fit the slope of the X-rayspectra. It is found the fraction, f, needs to be 1 or too many soft `seed photons will beproduced. This would require the entire optical-UV bump to be reprocessed emission. Inreality UV luminosities appear to be larger than bolometrically corrected X-rayluminosities for Seyfert galaxies (Walter & Fink 1993). An alternative geometry consistsof the `patchy corona model. Here the magnetic field releases energy into the corona indiscrete active, or flaring regions (Galeev, Rosner & Vaiana 1979). In these regions f 1while elsewhere on the disc f 0 and the primary UV emission is relatively unimpeded.This model is supported by observations of continuum variability. The reprocessedcontribution to the UV bump would be expected to vary simultaneously with the X-ray,
while the primary thermal disc emission would vary much more slowly. Variations in theoptical-UV and medium X-ray on timescales of weeks to months, have been observed tobe well correlated in a number of AGN (NGC 4151: Warwick et al. 1996, NGC 5548:Tagliaferri et al. 1996). The UV and X-ray variations are, however, not perfectlycorrelated as required by the `sandwich model geometry.These models still do not explain the source of the soft X-ray excess. The emission processis poorly constrained due to the large gap in the data between the UV and soft X-ray. Anearly candidate was the tail end of a multi-temperature thermal accretion disc spectrum,the cool end of which is the `big blue bump. However this requires an extremely hightemperature which appears unlikely, even for the inner edge of the disc. In the case ofNGC 5548 at least, the soft excess requires an entirely separate spectral component inorder to fit the data (Magdziarz et al. 1998). Other models include blended soft X-ray lineemission (e.g. Turner et al. 1991), or reprocessed Comptonised emission from the surfacelayers of the disc. However, none of these models is completely satisfactory. In particular,recent evidence that variations in the soft excess may lead variations in the medium X-rayband (Chiang et al. 2000) causes major problems for the reprocessing scenario.A feature of AGN X-ray spectra that appears to manifest well beyond the accretion disc isthe signature of the `warm absorber. Absorption edges of O VII at 739 eV and O VIII at870 eV are found in at least half of all Seyfert Is and a few quasars (Mathur et al. 1994).In the same objects, UV and optical absorption features are also typically found althoughthe warm absorber has a higher ionisation state (Crenshaw et al. 1999). In all cases,absorption features are blue shifted indicating an outflow of material. For the X-rayabsorption lines typical velocities are hundreds to several thousand km s-1 (e.g. Kaastra etal. 2000, Kaspi et al. 2001, Collinge et al. 2001), considerably slower than the broad-lineregion clouds. The warm absorber is thought to be an ionised wind, (perhaps evaporatedoff the inner edge of a dusty torus, Krolik & Kriss 2001) which may also be responsiblefor scattering the nuclear light in obscured AGN.References:http://www.anachem.umu.se/jumpstation.htm