Spectroscopy Theory PDF

Section 1 Spectroscopy is the science that studies the interaction of various types of radiation (light energy) with a substance (atoms and molecules) to be analysed for its identification (i.e qualitative analysis) as well as determination of its amount (i.e quantitative analysis). Electromagneti...

Section 1 Spectroscopy is the science that studies the interaction of various types of radiation (light energy) with a substance (atoms and molecules) to be analysed for its identification (i.e qualitative analysis) as well as determination of its amount (i.e quantitative analysis). Electromagnetic radiation is a type of energy that is transmitted through space at enormous velocities in form of waves. (as light). Light is described as electromagnetic waves of energetic particles ; photons and is usually described in terms of wave properties and photon properties I- Wave properties (Light as a wave) 1- Wave properties : Electromagnetic radiation consists of perpendicularly oscillating electric and magnetic fields that propagate through space along a linear path and with a constant velocity. Figure 1 a) Wavelength , λ , lambda. the lenght of the cycle or wave. It is defined as the distance between two successive maxima or minima. Figure 2. Fig 2 : wavelength and wave amplitude are shown on the electric field oscillation Region Unit Definition X-ray Angstrom unit A0 10-10m UV/ Visible Nanometer nm 10-9m IR Micrometer µm 10-6m b) Wave number , ύ. The number of waves per cm. It is the reciprocal of wavelength expressed in cm-1. ύ = 1/ λ waves / cm c) Frequency , ν The number of waves passing a point in space in one second. ν =c/λ waves per sec. c = velocity of light in vacuum = 3 ×1010cm / sec = 3× 108 m/ sec ν = c / λ = cm.s-1/cm = s-1 or Hertz (Hz)  The wave properties of light is described by three parameters 1- λ wavelength 2- ν frequency 3- ύ wave number ύ = 1/λ = ν /c The unit of frequency , ν , is the Hertz (Hz ) which corresponds to one cycle per second. That ,is 1 Hz = s-1  The refractive index η It measures the extent of interaction between electromagnetic radiation and the medium through which it passes For example , η of water at room temp is 1.33 , which means that radiation passes through water at a rate of c/1.33 or 2.26×1010 cm s-1. (In other words light travels 1.33 times slower in water than it does in vacuum) N.B. The velocity and wavelength of radiation become proportionally smaller as the radiation passes from vacuum , or from air , to a denser medium while the frequency remains constant. II- Photons Properties (Light as energy) 2. Photons Properties : - In many radiation / matter interaction , it is useful to consider light as consisting of photons or quanta.(energy packets) - Relation between energy of photon ( E ) to its wavelength , frequency and wave number is E=hν =hc/λ =hcύ Where h is Planck's constant (6.63 ×10-34 J s ). N.B. ν & ύ are directly proportional to E λ is inversely proportional to E i.e. the shorter the wavelength , the greater the energy of the photons and the more powerful the radiation. Example : The λ of the sodium D line is 589 nm. What are the E , ν , ύ for this line ? hc (6.626 × 10-34 J. s ) ( 3.00 × 108 m/s) E = ------ = ----------------------------------------------- λ 5.89 × 10-9 m = 3.37 × 10 -19 J c 3.00 × 108 m/s ν = ------ = ------------------- = 5.09× 1014 s-1 λ 589 × 10-9 ( cycle / sec. ) 1 1 ύ = ------ = -------------------- =1.7 × 10-4 cm-1 λ 589 × 10-7 cm Electromagnetic radiation is divided into different regions based on the type of atomic or molecular transition that gives rise to the absorption or emission of photons. N.B. The boundaries describing electromagnetic spectrum are not rigid, and an overlap between spectral regions is possible. - Regions of electromagnetic spectrum Spectroscopy  Absorption spectroscopy When monochromatic beam of ,light bass through a sample cell part will be absorbed - part transmitted - This beam of light is a part of the electromagnetic waves  Colorimetry When we have a solution of a colored compound so it can absorb a beam from the visible light region and the absorbed light intensity is directly proportional to the concentration of the compound  Spectrophotometry the compounds can absorb a beam from the ultraviolet light region and the absorbed light intensity is also directly proportional to the concentration of the compound 21