Introduction to Spectroscopy Lecture Date: January 18th, 2007 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 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 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 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 Introduction to Spectroscopy The electromagnetic spectrum Each color you see is a specific (narrow) range of frequencies in this spectrum Figures from NASA (www.nasa.gov) The Electromagnetic Spectrum Modern life (not just analytical spectroscopy) revolves around the EM spectrum! Properties of Electromagnetic Radiation Wave/particle duality Perpendicular E and B Note – this figure shows polarized radiation! components – E = electric field – B = magnetic field Wave properties: – Wavelength (frequency) – Amplitude – Phase 1 0.5 Long wavelength (low frequency) 1 2 3 4 5 1 2 3 4 5 -0.5 -1 c 1 0.5 Short wavelength (high frequency) -0.5 c = the speed of light (~3.00 x 108 m/s) = the frequency in cycles/second (Hz) = the wavelength in meters/cycle -1 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 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 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 Transmission and Refraction The refractive index (ni) of a medium is given by: ni c i c = the speed of light (~3.00 x 108 m/s) i = the velocity of the radiation in the medium in m/s ni = the refractive index at the frequency i Refractive index measures the degree of interaction between the radiation and the medium – Liquids: ni ~ 1.3 to 1.8 – Solids: ni ~ 1.3 to 2.5 Refractive index can be used to identify pure liquid substances Refraction When radiation passes through an interface between two media with different refractive indices, it can abruptly change direction Snell’s law: sin 1 n1 v2 sin 2 n2 v1 1 = the velocity of the radiation in medium 1 in m/s n1 = 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 Medium 1 Medium 2 2 Diffraction Fraunhofer diffraction: – Also known as far-field diffraction, parallel beam diffraction – Important in optical microscopy Fresnel diffraction – Also known as near-field diffraction Diffraction Bragg diffraction – multiple slit Fraunhofer diffraction: – Important for instrument design, crystallography n 2d sin 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 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) scattering 1 4 Mie scattering: applies to large particles, involves scattering in different directions. – Practical use in particle size analysis Polarization Polarization of EM radiation – a simple classical picture: Figure from Sears, et al., “University Physics”, 7th Ed., 1988 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 “crosscorelated” 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) 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) 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 h 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 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 Higher energy Absorption Lower energy Emission E = h Energy Levels Several types of quantum-mechanical energy levels occur in nature: – Electronic – Rotational – Vibrational (including phonons and heat) – Nuclear 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) 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”: Et 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: xpx 12 – In other words, if you know the position of a particle to within x, then you can specify its momentum along x to px – As the uncertainty in x increases (x ), that of px decreases (x ), and vice versa Spectra and Spectrometers Spectra are usually plotted as frequency vs. amplitude – Instead of energy, wavelength or energy (related properties) can also be used – The choice of x- and y-axes is often dependent on the particular technique, its history, etc… – Key parameter is frequency/energy/wavelength resolution Spectrometers: instruments that measure the interaction of radiation with matter, so the properties of such interactions can be studied 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 – – – – IR spectroscopy Raman spectroscopy X-ray spectroscopy NMR spectroscopy – EPR spectroscopy Further Reading P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd Ed. Oxford, New York (2003) R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading MA (1977) Any good physics text!