Physics 552: Optical Spectroscopy - Lecture Topics and Text Recommendation - Prof. Robert , Lab Reports of Optics

Download Physics 552: Optical Spectroscopy - Lecture Topics and Text Recommendation - Prof. Robert and more Lab Reports Optics in PDF only on Docsity! 1 Physics552 Optical Spectroscopy – fall 2008 Lectures and lab Lectures: Tu&Th 10:30 am -12 noon Loomis 144 Lab: Th 1:00 pm – 4:00 pm Th 5:00 pm – 8:00 pm Fr 12 noon – 3:00 pm • 6106 Eng Sci B, Loomis 387 Course director & Lecturer: R. Clegg email*: rclegg; Room 301A Loomis; 244-8143 office hours: any afternoons (call ahead) Discussion/Lab TA: Yi-Chun Chan e-mail: ychen52* Room 387 Loomis 244-8143 office hours: TBA Discission/Lab Instructor: Ulai Noomnarm e-mail: unoomna2* Room 387 Loomis 244-8143 office hours: TBA http://online.physics.uiuc.edu/courses/phys498OS/fall07/ See web for: • Lectures (deposited before lecture - hopefully) • Course description (lectures and lab) • Lab schedule and sign-up • Suggested books (list on web) – suggested test Text: “Molecular Fluorescence: principles and applications”, B. Valeur, Wiley- VCH, Weinheim, 2002, pp. 387. You can get this book from Amazon. • Literature (journal papers) will be deposited on the web as these different topics are covered in the lecture. Homework will be according to topics as they are covered. 2 Lecture topics for optical spectroscopy – Physics 552 1. 1 Lecture: General introduction and overview of spectroscopy and transitions – the examples will almost exclusively involve biophysical measurements except for the beginning examples 2. 1 Lecture: Properties of electromagnetic radiation and basic interactions of E&M radiation with matter - 3. 2 Lectures: Classical electron theory of absorption and emission spectroscopy; polarizabilities & molecular properties 4. 2 Lectures: Short skirmish with Quantum – a short review of important concepts – take our time 4.1. Schrödinger Equation 4.2. Particle in a box and harmonic oscillator 4.3. free electron model of spectroscopic transitions 4.4. Perturbation theory 4.5. What are molecular orbitals? 4.6. Spins 4.7. Time-dependent perturbation theory 4.8. Derivation of the Fermi Golden Rule & introduction to its simple uses in describing kinetic effects 5. 2 Lectures: Semi-classical interaction of radiation with matter. Topics include: 5.1. Einstein coefficients and their relation to absorption and emission: Radiative decay rates; Spontaneous emission decay rate; Stimulated decay of excited systems; Nonradiative decay rates of an excited atom (molecule) 5.2. Emission linewidth due to radiative decay 5.3. Line widths 5.4. Homogeneous and Inhomogeneous broadening 5.5. Electric dipole radiation 5.6. Born-Oppenheimer approximation. 5.7. Frank-Condon approximation and Frank-Condon factor 5.8. Oscillator strength (QM) – comparison to classical model 5.9. Selection rules for electric dipole transitions in atoms and molecules – simple examples 5.10. Polarizabilities and molecular spectroscopy (relation of α to spectroscopic transitions; Kuhn Thomas sum rule, etc.) 6. 1 Lecture: Absorption electronic spectroscopy 6.1. Fundamentals and connection to experiment. 6.2. Electronic absorption of proteins(aromatic side chains, amide backbone, exiciton interactions, secondary structure effects), nucleic acids (purines, pyrimidines, hypochroism), dye molecules, inorganic ion complexes. 6.3. Linear and circular dichroism and optical rotation - connection to secondary structure – use classical model. 7. 1 Lecture: IR spectroscopy and Raman spectroscopy – this will be short and partly homework – examples from proteins and nucleic acids. 5 The birth of a name Luminescenz = “phenomena of light not solely conditioned by the rise in temperature” (Eilhardt Wiedermann, 1888). Cold light, as opposed to hot light (black body, fire, ovens). There is a nice history of luminescence, from its beginnings to modern times by Harvey Newton. A bit of early history: Milestones in the history of fluorescence and phosphorescence: Tab. 1.3. Milestones in the history of fluorescence and phosphorescence during the first half of the twentieth century” Year Scientists Observation or achievement 1905, E. L. Nichols and First Huorescence excitation spectrum of a dye 1910 E. Merrit 1907 E.L. Nichols and Mirror symmetry between absorption and fluorescence E. Merrit spectra 1918 J. Perrin Photochemical theory of dye fluorescence 1919 Stern and Volmer _ Relation for fluorescence quenching 1920 F. Weigert Discovery of the polarization of the fluorescence emitted by dye solutions 1922 S. J. Vavilov Excitation-wavelength independence of the fluorescence quantum yield 1923 S. J. Vavilov and First study of the fluorescence polarization of dye solutions W. L. Levshin 1924 S. J. Vavilov First determination of fluorescence yield of dye solutions 1924 F. Perrin. Quantitative description of static quenching (active sphere model 1924 F. Perrin. First observation of alpha phosphorescence (E-type delayed fluorescence) 1925 F. Perrin Theory of fluorescence polarization (influence of viscosity) 1925 L. Levshin Theory of polarized fluorescence and phosphorescence 1925 J. Perrin Introduction of the term delayed fluorescence Prediction of long-range energy transfer 1926 E. Gaviola First direct measurement of nanosecond lifetimes by phase fluorometry (instrument built in Pringsheim’s laboratory) 1926 F. Perrin Theory of fluorescence polarization (sphere). Perrin's equation Indirect determination of lifetimes in solution. Comparison with radiative lifetimes 1927 E. Gaviola and Demonstration of resonance energy transfer in solutions P. Pringsheim 1928 E. Jette and W. First photoelectric fluorometer West 1929 F. Perrin Discussion on Jean Perrin's diagram for the explanation of the delayed fluorescence by the intermediate passage through a metastable state First qualitative theory of fluorescence depolarization by resonance energy transfer 1929 J. Perrin and Sensitized dye fluorescence due to energy transfer Choucroun 1932 F, Perrin Quantum mechanical theory of long-range energy transfer between atoms 1934 F, Perrin Theory of fluorescence polarization (ellipsoid) 1935 A. Jablonski Jablonskis diagram 1944 Lewis and Kasha Triplet state 1948 Th. Forster Quantum mechanical theory of dipole-dipole energy transfer  7 Short Introduction to the basic principles and definitions of optical spectroscopy. Some very simple things (Overview - details come later): Optical spectroscopy involves electromagnetic radiation ( cλν = ) throughout the broad range from gamma rays to long radio waves. See the table below. In this class we will concentrate on the range from infra-red (vibrations – IR and Raman spectroscopy) to the UV (absorbance and luminescence); that is, we will not discuss molecular rotations directly. The energy of a quantum is E h hcν λ= = ergs. 276.024 10 erg sech −= × , 10 -13 10 cm secc = × . The frequency is in sec-1. Avogadro’s number (the number of molecules in a mole) is 236.023 10AN = × . The definition of an einstein is a mole of photons of a certain frequency. This is used in photochemistry because in a photochemical reaction the absorbance of one einstein (small “e”) can cause the reaction of one mole of absorbing reactant. Also the table below shows the energy involved in an einstein of the different frequencies of radiation. Often the unit of ferequency of E&M radiation is given as a wavenumber, which is the number of waves per unit length; this is of course ( )( ) 1-1 1 -1 -1 sec cm sec cm cmcν λ ν− −= ≡ , where of course in order to use 1ν λ−= we have to give the wavelength in cm ( 710cm nm= ). 10 We can learn a lot about the molecules and their environment from just looking at the spectrum. Upon absorbing a quantum, the molecule enters an excited electronic state, usually the S1 state (first electronically excited state) but depending on the actual energy absorbed the molecule will enter this electronic state into one of the excited vibronic states (called a Frank-Condon State – see figure below). This will depend on the vibrational overlap, which we will cover later. Following the initial absorption process (in about 10-15 seconds), the energy very rapidly decays to the lowest (lower) vibrational state of S1 (this takes about 10-12 seconds). This is called “vibrational relaxation” (next page). When the molecule arrives at the lowest vibrational level of the S1 electronic state it is metastable, and remains there usually for from 10-10 to 10-8 seconds (it is from this state that fluorescence is generated). Note that if the molecule is excited into a higher electronic state, it also (almost always) retires immediately (in 10-12 seconds) to the S1 electronic excited state (“internal conversion”). This is where the energy passes from a vibrational state of a higher electronic state to a higher vibrational state of a lower electronic state. During the time the molecule is in the S1 state, several things can happen – it is from the S1 state that the exploitation of fluorescence is derived. We will spend a lot of time analyzing happenings from this state. There are several pathways by which the molecules can leave the excited S1 state. Some of these are depicted in the diagram on the next page, along with the fate of molecules that transfer to the “triplet” state. The passage from the singlet to the triplet state (or back) is termed “intersystem crossing” (next page). Frank-Condon state transitions. This is based on the fact that light is absorbed in 10-15 seconds, and the nuclei cannot move in this time. The electrons find a new spatial distribution in this time, but this is not the minimum energy configuration of the excited state. The shape of the absorption and the emission curves can be derived from this simple diagram. Where the spectra have low and high slopes corresponds to sharp and broad parts of the spectrum.. Mirror symmetry sometimes. The potential curves, are actually surfaces, and they are multidimensional. 11 Energy difference of the 0-0 transitions. See the figure two pages previously. Why the emission spectrum, including the 0-0 transition, is often shifted farther to the red of the absorption spectrum than you might expect from just the normal Stokes shift. This gives valuable information of the surrounding solvent, or “molecular cage” The electron distribution is often very different in the excited state from that in the ground state. The permanent dipole moment or the polarizability of the excited molecule is often more pronounced than that of the ground state (the electrons in the excited state are more spatially extended). Before the excited stated emits the “polar” solvent may have time to reorient, and the solvent molecules will orient such that the energy of the excited molecule is lowered. That is, the cusp of the potential becomes lower. This change in energy is different than the transition to the Frank-Condon state with the subsequent (very fast) vibrational relaxation to the lowest vibrational state of the excited molecule. At low temperatures, or high viscosity (high rigidity of the solvent molecules) the solvent molecules do not have time to reorient before emission of a photon. So there is not so much 0-0 spliting in these conditions. If the temperature is high, then there is not so much orientation of the solvent molecules, so again the 0-0 splitting is not so much. There is a maximum 0-0 splitting at some intermediate temperature, and this has been observed. There is a relationship between the 0-0 band shift, the polarity of the solvent and the dipole moment of the solute upon excitation – this is the Lippert equation. E. Lippert, Z. Naturforsch. 10a (1955) 541; Z. Electrochem., 61 (1957) 962. 12 The approximate time scales are indicated on the following diagram. Other pathways will be discussed later, such as energy transfer and photochemical reactions. Kinetics of transitions: As you can see from the diagram the different pathways compete with each other, and there is a “rate” for each process for leaving the excited state (we are not now considering “stimulated” emission – see later). The rates of the different pathways and the way these pathways are affected by the environment and neighboring molecules, dictates the fate of the excited molecules, determines the fraction of molecules following the different pathways, and provides a lot of information about the molecular environment.