1. Lecture Date: January 22 nd, 2013 Introduction to Spectroscopy

2. What is Spectroscopy?  The study of the interaction between radiation and matter  “Analytical spectroscopy”, as defined in this class, covers applications of spectroscopy to chemical analysis

3. History of Analytical Spectroscopy  1666: Isaac Newton (England) shows that white light can be dispersed into constituent colors, and coins the term “spectrum” –Newton also produced the first “spectroscope” based on lenses, a prism, and a screen  1800: W. Herschel and J. W. Ritter show that infrared (IR) and ultraviolet (UV) light are part of the spectrum  1814: Joseph Fraunhofer noticed that the sun’s spectrum contains a number of dark lines, developed the diffraction grating  1859: G. Kirchoff obtains spectra of the elements, explains the sun’s spectrum

4. The Visible Spectrum of the Sun (Black lines are absorption by elements in the cooler outer region of the star) Figure from National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation, http://www.noao.edu/image_gallery/html/im0600.html

5. History of Analytical Spectroscopy  1870: J. C. Maxwell formalizes and combines the laws of electricity and magnetism  1900 to present: More than 25 Nobel prizes awarded to spectroscopists, including: –1902: H. A. Lorentz and P. Zeeman –1919: J. Stark –1933: P. A. M. Dirac and E. Schrodinger –1945: W. Pauli …. –1999: A. Zewail

6. Introduction to Spectroscopy Figures from NASA (www.nasa.gov)  The electromagnetic spectrum  Each color you see is a specific (narrow) range of frequencies in this spectrum

7. The Electromagnetic Spectrum  Modern life (not just analytical spectroscopy) revolves around the EM spectrum!

8. Properties of Electromagnetic Radiation  Wave/particle duality  Perpendicular E and B components –E = electric field –B = magnetic field  Wave properties: –Wavelength (frequency) –Amplitude –Phase Long wavelength (low frequency) Short wavelength (high frequency) c = the speed of light (~3.00 x 10 8 m/s) = the frequency in cycles/second (Hz) = the wavelength in meters/cycle Note – this figure shows polarized radiation!

9. Interference of Radiation  Monochromatic: radiation containing a single frequency  Polychromatic: radiation containing multiple frequencies  Constructive interference: when two waves reinforce each other  Destructive interference: when two waves cancel each other

10. The Interaction of Radiation and Matter  Electromagnetic radiation travels fastest in a vacuum  When not travelling in a vacuum, radiation and matter can interact in a number of ways  Some key processes (for spectroscopy): –Diffraction –Refraction –Scattering –Polarization –Absorption

11. Transmission of Radiation  The velocity at which radiation travels (or propagates) through a medium is dependent on the medium itself  When radiation travels through a medium and does not undergo a frequency change, it cannot be undergoing a permanent energy transfer  However, radiation can still interact with the medium –Radiation, an EM field, polarizes the electron clouds of atoms in the medium –Polarization is a temporary deformation of the electron clouds

12. Transmission and Refraction  The refractive index (n i ) of a medium is given by: c = the speed of light (~3.00 x 10 8 m/s) i = the velocity of the radiation in the medium in m/s n i = the refractive index at the frequency i  Refractive index measures the degree of interaction between the radiation and the medium –Liquids: n i ~ 1.3 to 1.8 –Solids: n i ~ 1.3 to 2.5  Refractive index can be used to identify pure liquid substances

13. Refraction  When radiation passes through an interface between two media with different refractive indices, it can abruptly change direction  Snell’s law: 1 = the velocity of the radiation in medium 1 in m/s n 1 = the refractive index in medium 1  Snell’s law is a consequence of the change in velocity in the media  Reflection always occurs at an interface. Its extent depends on the refractive indices of the media 11 22 Medium 1 Medium 2

14. Diffraction  Fraunhofer diffraction: –Also known as far-field diffraction, parallel beam diffraction –Important in optical microscopy  Fresnel diffraction –Also known as near-field diffraction

15. Diffraction  Diffraction gratings: –Widely used in spectroscopic instruments to separate frequencies (can be made precisely) http://www.astro.virginia.edu/research/observatories/40inch/fobos/images/grating.jpg  Bragg diffraction – multiple slit Fraunhofer diffraction: –Important for instrument design, crystallography

16. Scattering  Rayleigh scattering (an elastic process): –Scattering of small amounts of radiation by molecules and atoms (whose size is near to the wavelength of the radiation)  Mie scattering: applies to large particles, involves scattering in different directions. –Practical use in particle size analysis

17. Polarization  Polarization of EM radiation – a simple classical picture: Figure from Sears, et al., “University Physics”, 7 th Ed., 1988

18. Coherent Radiation  Coherent radiation fulfils two conditions: (1) it has the same frequency or set of frequencies, and (2) it has a well-defined and constant phase relationship –Coherent radiation is “cross- corelated” in that the properties of one beam can be used to predict those of the other beam  Examples of coherent radiation: –Lasers –Microwave sources (masers) Coherent radiation: different frequencies (colors) with a defined phase relationship interfere to produce a pulse Diagram from wikipedia.org (public domain)

19. Incoherent Radiation  Produced by “random” emission, e.g. individual atoms in a large sample emitting photons  Actually is coherent, but just to a tiny (undetectable) extent  Also known as “continuous” radiation  Examples of incoherent radiation: –Incandescent light bulbs –Filament sources –Deuterium lamps Incoherent radiation: different frequencies (colors) combined to produce continuous radiation with varying phase, frequency and amplitude Diagram from wikipedia.org (public domain)

20. More Properties of Electromagnetic Radiation  Wave and particle behavior: photons behave as both waves and particles –Quantum mechanics developed around the concept of the photon, the elementary unit of radiation  Planck’s law:  E is the energy of the photon in joules  h is Planck's constant (6.624 x 10 -34 joule seconds)  is the frequency of the radiation

21. Absorption and Emission  Absorption is a process accompanied by an energy change –involves energy transfer of EM radiation to a substance, usually at specific frequencies corresponding to natural atomic or molecular energies  Emission occurs when matter releases energy in the form of radiation (photons  E = h Higher energy Lower energy AbsorptionEmission

22. Energy Levels  Several types of quantum-mechanical energy levels occur in nature: –Electronic –Rotational –Vibrational (including phonons and heat) –Nuclear spin (other nuclear energy levels usually need high energies to access)  For each of these, a discrete quantum state and energy- driven transitions between these states can be studied (as opposed to a continuous range of energies)

23. Selection Rules  Selection rules:  Simple rules that are derived from transition moment integrals (usually via symmetry arguments) that express which energy level transitions are allowed  Example (for rotational energy levels of a rigid linear rotor such as a diatomic molecule):  A forbidden transition is usually still possible, but often is weaker than allowed transitions

24. The Uncertainty of Measurements  Because the lifetimes of quantum states can persist for short periods, it can be difficult to measure their energies accurately  This is usually stated in the form of an “energy-time uncertainty”:

25. The Uncertainty Principle  The uncertainty principle: it is not possible to know both the location and the momentum of a particle exactly – a fundamental limit on all measurements  In Heisenberg’s terms, the act of measuring a particle’s position affects its momentum, and vice versa  In equation form: –In other words, if you know the position of a particle to within  x, then you can specify its momentum along x to  p x –As the uncertainty in x increases (  x  ), that of p x decreases (  x  ), and vice versa

26. Spectra and Spectrometers  Spectra are usually plotted as frequency vs. amplitude –Instead of frequency, wavelength or energy can also be used –The choice of x- and y-axes is often dependent on the particular technique, its history, etc… –In most techniques, a key parameter is the frequency/energy/wavelength resolution  Spectrometers: instruments that measure the interaction of radiation with matter, so the properties of such interactions can be studied

27. Spectroscopy in Analytical Chemistry  Widely used approach for characterizing systems ranging from chemical physics to biology, from individual atoms to the largest molecules  Some of the most common techniques are: –UV-Visible spectroscopy –Fluorescence spectroscopy –IR spectroscopy –Raman spectroscopy –X-ray spectroscopy –NMR spectroscopy –EPR spectroscopy

28. Further Reading P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3 rd Ed. Oxford University Press, New York (2003). R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading, MA (1977). M. Fox, Optical Properties of Solids, Oxford University Press, New York (2010). Physics textbooks often contain good discussions of basic spectroscopic phenomena.