Electronic Spectroscopy 1

Optical Electronic Spectroscopy 1 Lecture Date: January 23rd, 2008 The Electromagnetic Spectrum  UV-Visible  X-ray 1 What is Electronic Spectroscopy?  Spectroscopy of the electrons surrounding an atom or a molecule: electron energy-level transitions Atoms: electrons are in hydrogen-like orbitals (s, p, d, f) From http://education.jlab.org (The Bohr model for nitrogen) Molecules: electrons are in molecular orbitals (HOMO, LUMO, …) (The LUMO of benzene) Optical Electronic Spectroscopy  Definition: Spectroscopy in the optical (UV-Visible) range involvingelectronic energy levels excited by electromagnetic radiation (often valence electrons).  This lecture is related to the “high-energy” (“non-optical”) electron spectroscopy covered in the X-ray lecture  Methods: – Atomic absorption – Atomic emission (e.g ICP-OES) – Molecular UV-Visible absorption – Luminescence, Fluorescence, Phosphorescence 2 Definitions of Electronic Processes  Emission: radiation produced by excited molecules, ions, or atoms as they relax to lower energy levels.  Absorption: radiation selectively absorbed by molecules, ions, or atoms, accompanied by their excitation (or promotion) to a more energetic state.  Luminescence: radiationproduced by a chemical reaction or internal electronic process, possibly following absorption. More Electronic Processes  Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity – Occursabout 10-5 to 10-8 seconds after photon absorption  Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity – Occursabout 10 to 10-5 seconds after photon absorption 3 What is Emission?  Atoms/molecules are driven to excited states (in this case electronic states), which can relax by emission of radiation. M + heat M* Higher energy ΔE = hn Lower energy  Other process can be active, such as “non-radiative” relaxation (e.g. transfer of energy by random collisions). M* M + heat  OES = Optical Emission Spectroscopy What is Absorption?  Electromagnetic radiation travels fastest in a vacuum. – WhenEM radiation travelsthrough a substance, it can be slowed by propagation “interactions” that do not cause frequency (energy) changes: 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  Absorption does involve frequency/energy changes, since the energy of EM radiation is transferred to a substance, usually at specific frequencies corresponding to natural atomic or molecular energies – Absorption occurring at optical frequenciesinvolveslow to mid-energy electronic transitions. 4 Absorption and Transmission  Transmittance: T = P/P0 P0 P  Absorbance: A= -log10 T = log10 P0/P b Ais linear vs. b! (Apreferred overT) Graphs fromhttp://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm The Beer-Lambert Law  The Beer-Lambert Law (a.k.a. Beer’s Law): A= ebc Wherethe absorbanceAhas no units, sinceA= log10 P0 / P e is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)  Beer’s law can be derived from a model that considers infinitesimal portions of a “block” absorbing photons in their cross-sections, and integration over the entire block – Beer’s law is derived under the assumption that the fractionof the light absorbed by each thin cross-section of solution is the same – See pp. 302-303 of Skoog, et al. for details 5 ... - tailieumienphi.vn