1 Introduction to Spectroscopic methods Spectroscopy: Study of interaction between radiation (or other forms of energy) and matter (a branch of science). Spectrometry: Analytical methods based on atomic and molecular spectroscopy

2 Types of Analytical Spectroscopy Absorption Absorption Fluoresence and Phosphoresence Fluoresence and Phosphoresence Emission (atomic with flames, arcs, sparks, and palsmas) Emission (atomic with flames, arcs, sparks, and palsmas) Chemilumenesence and Biolumenesence Chemilumenesence and Biolumenesence Reflection Reflection

The Electromagnetic Spectrum = c / E = h

4 What about E? = c / E = h

5 Kinds of Spectroscopy Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

6 LIGHT Electro-magnetic radiation

7 Light as a Wave Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

8 Light as a Wave Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

9 Light as a Wave Frequency = Frequency = Velocity of propagation = v = Velocity of propagation = v = Speed of light in a vacuum = c = 3.00 x 10 8 m/s Wavenumber (reciprocal of ) = = k = /v Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

10 Effect of the Medium on a Light Wave Frequency remains the same. Frequency remains the same. Velocity and Wavelength change. Velocity and Wavelength change. Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

11 Mathematic Description of a Wave Y = A sin(  t +  ) A = Amplitude  = angular frequency = 2  =  = phase angle Y = A sin(2  t +  ) Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

12 Mathematic Description of a Wave Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007 Sine waves with different amplitudes and with a phase different of 90 degree

13 If two plane-polarized waves overlap in space, the resulting electromagnetic disturbance is the algebraic sum of the two waves. Coherence: When two waves have an initial phase difference of zero or it is constant for a long time they are considered coherent. Superposition of Waves Y = A 1 sin(2  1 t +   ) + A 2 sin(2  2 t +   ) +……. A 2 sin(2  2 t +   ) +……. Optical Interference: The interaction of two or more light waves yielding an irradiance that is not equal to the sum of the irradiances.

14 Optical Interference Constructive Interference 1) Have identical frequency 2)  2 –  1 =  =  m2  Destructive Interference 1) Have identical frequency 2)  2 –  1 =  = (2m+1)  Figure 3-4 – Ingle and Crouch, Spectrochemical Analysis  2 –  1 = 180 deg or integer of multiple of 360 deg.  2 –  1 = 0, or 360 deg or integer of multiple of 360 deg.

15 Superposition of sinusoidal wave: (a) A1 < A2, (  1 -  2) = 20º, 1 = 2; (b) A1 < A2, (  1 -  2) = 200º, 1 = 2 Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

16 Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007 Superposition of tw sinusoidal wave of different frequencies but identical amplitudes. Should be

17 Diffraction: The Bending of Light as It Passes Through an Aperture or Around a Small Object FraunhoferDiffraction Narrow Slit Diffraction Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007 Diffraction is a consequence of interference

18 Eugene Hecht, Optics, Addison-Wesley, Reading, MA, Diffraction increases as aperture size  Diffraction increases as aperture size  Diffraction of Waves in a Liquid

19 Diffraction Pattern From Multiple Slits Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

20 Diffraction Pattern From Multiple Slits Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

21 Diffraction Pattern From Multiple Slits Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007 CF = BC sin  = n n is an integer called order of interference

22 Coherent Radiation  Conditions for coherent of two sources of radiation are: 1.Identical frequencies and wavelength 2.Phase relationship remains constant with time

23 Eugene Hecht, Optics, Addison-Wesley, Reading, MA, Conservation Law  T  = 1  = Fraction Absorbed  = Fraction Reflected T  = Fraction Transmitted What happens when light hits a boundary between two media? Refraction: change in direction of radiation as it passes from one medium to another with different density Physics of Refraction

24 Refractive index (n) the velocity (v) of EM radiation depends on the medium through which it travels the velocity (v) of EM radiation depends on the medium through which it travels n i = c/v i (>1). n i = c/v i (>1). the ratio of the velocity in vacuum over the velocity in the medium the ratio of the velocity in vacuum over the velocity in the medium n depends on the frequency of the light n depends on the frequency of the light

25Refraction n 1 sin  1 = n 2 sin  2 Snell’s Law v 2 sin  1 = v 1 sin  2 Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

26 Refraction

27 Transmission: The Refractive Index Eugene Hecht, Optics, Addison-Wesley, Reading, MA, n is wavelength (frequency) dependent. In glass n increases as decreases.

28 Dispersion and Prisms Dispersion The variation in refractive index of a substance with wavelength or frequency

29 Dispersion and Prisms Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

30 A ray of single-wavelength incident on a prism 11 22 33   : angle of deviation Cai ® 2007

31 A ray of white-wavelength incident on a prism RR BB White light Cai ® 2007

32

33 Reflection of Radiation I 0 : intensity of incident light I r : reflected intensity For monochromatic light hitting a flat surface at 90 0

34 Reflection of Radiation

35 Specular reflection: Reflection of light from a smooth surface Diffuse reflection: Reflection of light from a rough surface Smooth or rough surface ???????

36 Reflection Refraction M1M1 M2M2

37 Ingle and Crouch, Spectrochemical Analysis  at different interfaces Reflectance is the fraction of the incident radiant energy refelcted.

38 Scattering of Radiation Rayleigh scattering Rayleigh scattering Molecules or aggregates of molecules smaller than Molecules or aggregates of molecules smaller than Scattering by big molecules Scattering by big molecules Used for measuring particle size Used for measuring particle size Raman Scattering Raman Scattering Involves quntized frequency changes Involves quntized frequency changes The fraction of radiation transmitted at all angles from its original path

39 Serway, Physics, 4 th edition, 1996

40 Light as Particles Eugene Hecht, Optics, Addison-Wesley, Reading, MA, h = Planck Constant = 6.63  Js

41 The Photoelectric Effect Vo: Stopping voltage (the negative voltage at which the photocurrent is zero) eV 0 = h -   : work needed to remove e - Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

42 Cut-off Current is proportional to the intensity of the radiation Current is proportional to the intensity of the radiation V 0 depends on the frequency of the radiation and the chemical composition of the coating V 0 depends on the frequency of the radiation and the chemical composition of the coating V 0 depends on the chemical composition of the coating on the photocathode V 0 depends on the chemical composition of the coating on the photocathode V 0 independent of the intensity of the radiation V 0 independent of the intensity of the radiation Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

43 Energy States of Chemical Quantum theory by Planck (1900) Quantum theory by Planck (1900) Black body radiation Black body radiation Atoms, ions, and molecules exist in discrete states Atoms, ions, and molecules exist in discrete states Characterized by definite amounts of energy Characterized by definite amounts of energy Changes of state involve absorption or emission of energy Changes of state involve absorption or emission of energy E 1 -E 0 = h = hc/

44 Interaction of Radiation and Matter Emission and Chemiluminescence Process

45 Interaction of Radiation and Matter Absorption Process

46 Interaction of Radiation and Matter Photoluminescence method (Fluorescence and phosphorescence)

47 Interaction of Radiation and Matter Inelastic Scattering in Raman Spectroscopy

48 Emission of Radiation Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, Emission Emission X *  X + h X *  X + h Excitation needs energy! Particle bombardment (e-) Electrical currents (V) Fluorescence Heat

49 Emission: Saltwater in a flame

50 Line Spectra Individual atoms, well separated, in a gas phase

51 Band Spectra Small molecules and radicals Vibrational levels

52 Continuum Spectra Produced when solid are heated to incandescence. Produced when solid are heated to incandescence. Blackbody Radiation (Thermal Radiation) Blackbody Radiation (Thermal Radiation)

53 Blackbody Radiation A blackbody is a theoretical object, (i.e. emissivity = 1.0), which is both a perfect absorber and emitter of radiation. Common usage refers to a source of infrared energy as a "blackbody" when it's emissivity approaches 1.0 (usually e = 0.99 or better) and as a "graybody" if it has lower emissivity. A blackbody is a theoretical object, (i.e. emissivity = 1.0), which is both a perfect absorber and emitter of radiation. Common usage refers to a source of infrared energy as a "blackbody" when it's emissivity approaches 1.0 (usually e = 0.99 or better) and as a "graybody" if it has lower emissivity.blackbody emissivityblackbody emissivity Important sources of infrared, visible, and long wavelength UV for analytical instruments Important sources of infrared, visible, and long wavelength UV for analytical instruments

54 Wien’s Displacement Law Eugene Hecht, Optics, Addison-Wesley, Reading, MA, Stefan-Boltzman Law P =  T 4  =  Wcm -2 K -4 Blackbody Radiation Both max and radiation power (P) are related to TEMPERATURE and current!

55 Continuum Source Line Source Continuum + Line Source Ingle and Crouch, Spectrochemical Analysis Al + Mg

56 Ranges of Common Sources Ranges of Common Sources Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

57 Optical Source Characteristics Ingle and Crouch, Spectrochemical Analysis

58 Nernst Glower Rare earth oxides formed into a cylinder (1-2 mm diameter, ~20mm long). Pass current to give: T = 1200 – 2200 K. Ingle and Crouch, Spectrochemical Analysis Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

59 Globar Silicon Carbide Rod (5mm diameter, 50 mm long). Heated electrically to 1300 – 1500 K. Positive temperature coefficient of resistance Electrical contact must be water cooled to prevent arcing. Ingle and Crouch, Spectrochemical Analysis

60 Tungsten Filament Ingle and Crouch, Spectrochemical Analysis Heated to 2870 K. Useful Range: 350 – 2500nm

61 Tungsten / Halogen Iodine added. Reacts with gaseous W near the quartz wall to form WI 2. W is redeposited on the filament. Gives longer lifetimes Allows higher temperatures (~3500 K).

62 Intensity Spectrum of the Tungsten-Halogen Lamp Weak intensity in UV range Good intensity in visible range Very low noise Low drift

63 Arc Lamps Ingle and Crouch, Spectrochemical Analysis Electrical discharge is sustained through a gas or metal vapor. Continuous emission due to rotational/vibrational energy levels and pressure broadening.

64 H 2 or D 2 Arc Lamps Ingle and Crouch, Spectrochemical Analysis D 2 + E e-  D 2 *  D’ + D” + h D 2 + E e-  D 2 *  D’ + D” + h Energetics: E e- = E D 2 * = E D’ + E D” + h E e- = E D 2 * = E D’ + E D” + h Useful Range: 185 – 400 nm.

65 Intensity Spectrum of the Xenon Lamp High intensity in UV range High intensity in visible range Medium noise

66 Hg Arc Lamp Continuum + Line Source High Power Source. Often used in photoluminescence. Ingle and Crouch, Spectrochemical Analysis

67 Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, Hollow Cathode Discharge Tube. Apply ~300 V across electrodes. Ar + or Ne + travel toward the cathode. If potential is high enough cations will sputter metal off the electrode. Metal emits photons at characteristic atomic lines as the metal returns to the ground state.

68 Hollow Cathode Discharge Tube. Line Widths are typically 0.01 – 0.02 Å. Ingle and Crouch, Spectrochemical Analysis

69 Absorption of Radiation Is a quantized process??? Is a quantized process??? The energy absorbed is released, although not necessarily all as light energy (e.g. heat) The energy absorbed is released, although not necessarily all as light energy (e.g. heat) Results in excitation of a molecule to a higher energy state Results in excitation of a molecule to a higher energy state E= E electronic + E vibrational + E rotational E= E electronic + E vibrational + E rotational

70 Absorption of Radiation Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

71 Atomic absorption

72 Rotational energy levels associated with each vibrational level not shown

73 Relaxation Resonance fluorescence F = A Non- Resonance fluorescence F  A Stokes shift F > A

74 Quantitative Aspects of Spectrochemical Measurements Radiation power P Radiation power P The energy of the a beam of radiation that reaches a given area per second The energy of the a beam of radiation that reaches a given area per second S =kP S =kP S is an electrical signal Dark current Dark current Response of the detector in the absence of radiation Response of the detector in the absence of radiation S =kP + k d S =kP + k d

75 Quantitative Aspects of Spectrochemical Measurements Transmittance Transmittance T = P/P o (definition) T = P/P o (definition) P o - incident light power P o - incident light power P - transmitted light power P - transmitted light power %T = P/P o x 100 % %T = P/P o x 100 % Absorbance Absorbance A = - log T (definition) A = - log T (definition) Beer’s Law (physical law applicable under certain conditions) Beer’s Law (physical law applicable under certain conditions) A =  b c (basis of quantitation) A =  b c (basis of quantitation)  - molar absorptivity (L mol -1 cm -1 )  - molar absorptivity (L mol -1 cm -1 ) b - pathlength (cm) b - pathlength (cm) c - concentration (mol L -1 ) c - concentration (mol L -1 )

76 Non-radiative relaxation Vibrational Relaxation: Vibrational Relaxation: A molecule can give off some of its energy from absorbed light (usually uv-vis) by jumping to a lower energy vibrational state. A molecule can give off some of its energy from absorbed light (usually uv-vis) by jumping to a lower energy vibrational state. The excess energy is used to make the conversion. No light is given off. The excess energy is used to make the conversion. No light is given off. Internal Conversion: Internal Conversion: The molecule transitions to a lower energy electronic state without giving off light. The molecule transitions to a lower energy electronic state without giving off light. Excess energy is used to covert the molecule from one electronic state to another. Excess energy is used to covert the molecule from one electronic state to another. Poorly understood Poorly understood External conversion: External conversion: The molecule gives off energy to an external source, such as by collision with another similar molecule or solvent molecule. This is called “quenching” The molecule gives off energy to an external source, such as by collision with another similar molecule or solvent molecule. This is called “quenching” Intersystem Crossing: Intersystem Crossing: The molecule goes from a singlet to triplet excited state and uses up energy changing the spin of an electron. The molecule goes from a singlet to triplet excited state and uses up energy changing the spin of an electron.