Electronic Transitions

7 Key excerpts on "Electronic Transitions"

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  Photobiology

  • Elli Kohen, Rene Santus, Joseph G. Hirschberg(Authors)
  • 1995(Publication Date)
  • Academic Press

    (Publisher)

  As a result chemical changes can occur during light absorption. Bibliography Birks, J. B. (1970). Photophysics of Aromatic Molecules, pp. 1-28. Wiley (Interscience), London. Bom, M., and Wolf, E. (1970). Principles of Optics, p. xxi. Pergamon, Oxford. Debus, A. G., ed. (1968). World Who's Who in Science, p. 523. Marqis-Who's Who Inc., Chicago, de Fermat, P. (1679) (post). Varia Opera Mathematica. Pathways of Molecular Excitation and Deactivation 2.1 Electronic Transitions The possible electron distributions defining the molecular orbitals and energy levels of molecules in their ground and excited states are given by the approximate solutions of a generalized Schrodinger equation. The latter takes into account electrostatic attractions and repulsions of pro-tons and electrons, internuclear vibrations, and the rotational movement of molecules as well as magnetic interactions due to electron and nuclear spins and orbital motion. Molecular orbitals can contain no more than two electrons. A transition between two states of a molecule corresponds to the movement of one electron from one orbital to another. For instance, an electron in a TT molecular orbital can be transfered to an excited TT orbital (TT*) and the corresponding transition will be noted TT -^ TT*. Simi-larly, a a electron can be promoted to a a* orbital and the transition will be noted a -^ a^. These transitions can be induced by absorption of radiation, provided the energy of the incident light quanta (given by the 23 24 Chapter 2 Pathways of Molecular Excitation and Deactivation Planck equation; see Chapter 1) is equal to the energy difference between two electronic levels. A molecule will interact with the electromagnetic field and absorb (or create) a photon of frequency v only if it possesses, at least transiently, an electric dipole oscillating at this frequency. The molecular transient polarization can be explained as follows.

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  It is important to note that figures 4.13, 4.14 and 4.15 show only the visible portions of the absorption and emission spectra. Electronic Transitions also take place in regions of the electromagnetic spectrum that the human eye cannot detect (as shown in figure 4.17). Instruments allow scientists to observe electromagnetic radiation in these regions. 146 Chemistry FIGURE 4.16 (a) A ball on a staircase shows some of the properties of quantised energy levels. (b) Quantised energy levels can be depicted using an energy level diagram. (c) Electronic Transitions occur between quantised energy levels through either absorption or emission of photons. ball on a staircase 5 4 3 2 1 gain of energy loss of energy electron in atom 5 4 3 2 1 energy level diagram ΔE = E 3 ‒ E 5 ΔE = E 5 ‒ E 1 emission of photon absorption of photon (a) (b) (c) FIGURE 4.17 Energy levels for the hydrogen atom and some of the transitions that occur between levels, as well as the resulting emission spectrum. Upward arrows represent absorption transitions, and downward arrows represent emission transitions. 1 2 3 656 nm 486 nm 434 nm 410 nm infrared (> 780 nm) ultraviolet (< 380 nm) 4 5 n ∞ 6 E n = n = a positive integer 2.18 × 10 ‒18 J n 2 E n absorptions emissions ‒ CHAPTER 4 Atomic energy levels 147 Elements other than hydrogen also have quantised energy levels, but as we will see later in this chapter they cannot be so simply described because they have more than one electron. In these cases, scientists use experimental values for observed absorption and emission lines to calculate the allowed energy levels for each different element. WORKED EXAMPLE 4.5 Energy level diagrams Ruby lasers use crystals of Al 2 O 3 that contain small amounts of Cr 3+ ions, which absorb light between 400 and 560 nm. The excited-state ions lose some energy as heat. After losing heat, the Cr 3+ ions return to the ground state by emitting red light with a wavelength of 694 nm.

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  Foundations for Nanoscience and Nanotechnology

  • Nils O. Petersen(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  C H A P T E R 15 Molecular spectroscopy The previous chapter focused on the quantum mechanical description of the interaction of light with matter with particular attention to electronic transi- tions. In this chapter, we will explore some of the consequences and utilities of Electronic Transitions in probing properties of molecules through various forms of spectroscopy and then show how these can be applied to study nanoscale phenomena. The emphasis will be on the concept of fluorescence spectroscopy since this has proven particularly useful in studies in the life sciences with applications in nanobiotechnology and bionanotechnology. Some distinguish the concept of nanobiotechnology as the applica- tion of nanoscale tools to understand biological systems from the concept of bionanotechnology as the application of biological understanding to nanotechnology systems. An example of the former is the use of electron microscopes to study biological structures and an example of the latter is the developing of self-assembly of nanostructures based on our under- standing the biological macromolecule assembly. 15.1 THE OPTICAL TRANSITIONS The Electronic Transitions in many molecules and in some nanomaterials occur in the optical part of the electromagnetic spectrum. Consider the case of conjugated electronic systems described in section 5.1.1, where the energy needed to excite an electron from the ground electronic state to the first excited electronic state corresponded to wavelengths ranging from 200 to 300 nm, that is, in the ultra-violet part of the electromagnetic spectrum. In many organic molecules the conjugations extend the “particle-in-the-box” so that the energy separations decrease and the wavelengths extend to the 400-700 nm range, which is the visible part of the electromagnetic spectrum. Since the electrons are associated with bonds within the molecular struc- ture, the energies are affected by the vibrational state of the bond so that 239

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  Electronic Properties of Crystalline Solids

  An Introduction to Fundamentals

  • Richard Bube(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  k diagram. Direct transitions are first-order processes; indirect transitions are second-order processes. F o r most elec-tronic transitions in crystalline solids conservation of energy a n d electron wavevector between initial a n d final states is required. 401 402 11 Optical Absorption CP ο Fig. 11.1 Schematic display of different types of optical absorption typically found in crystals. (1) Transitions to high-lying bands, (2) excitons, (3) absorption edge for valence-band-to-conduction-band transition, (4) imperfection absorption, (5) free-carrier absorption, and (6) Reststrahlen absorption. Metals are distinguished by their high reflectivity and opacity except in very thin films. These optical properties are associated primarily with the absorption of electromagnetic radiation by the free electrons in the metal. Indirect optical transitions are involved in such absorption processes. These transitions m a y be reasonably adequately described by a quasi-classical approach, although a q u a n t u m approach is required for a m o r e general treatment. W e confine our discussion here to that of the quasi-classical view in the interest of simplicity and brevity. Semiconductors m a y also be highly reflecting and o p a q u e over certain ranges of wavelength, and yet appreciably transmitting over others. All of the absorption processes pictured in Fig. 11.1 can be detected in semi-conductors, including the free-electron absorption characteristic of metals provided that the free-carrier density is sufficiently large. T h e d o m i n a n t absorption process in semiconductors is usually the excitation of an electron from the valence b a n d across the forbidden gap to the conduction band. This process can occur via either a direct or an indirect transition. T h e dependence of the absorption constant on the p h o t o n energy can be calcu-lated for these two kinds of process by the use of time-dependent perturba-tion theory.

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  Physical Chemistry

  • Robert J. Silbey, Robert A. Alberty, George A. Papadantonakis, Moungi G. Bawendi(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  14.10. Electrons in orbitals on the ligands can then be excited into an unfilled d orbital on the metal, giving a charge transfer transition that is usually very intense. In addition, electrons can be excited from one set of d orbitals to the other, also giving rise to absorption in the visible. These latter transitions are weaker than the charge transfer bands because d → d transitions are forbidden unless some perturbation occurs, such as a distortion of the octahedron (as shown in Fig. 14.11) to a lower symmetry with no inversion center of symmetry. d d transition e g t 2g ← FIGURE 14.10 The d orbital states in an octahedral complex and the meaning of d ← d transitions. 512 CHAPTER 14 Electronic Spectroscopy of Molecules FIGURE 14.11 Vibrational motion or distortion in an octahedral complex that leads to the destruction of the center of symmetry, thereby making d ← d transitions allowed. 14.7 CONJUGATED MOLECULES: FREE-ELECTRON MODEL For molecules with conjugated systems of double bonds [i.e., R(CH=CH) n R ′ ], it is found that the electronic absorption bands shift to longer wavelengths as the number of conjugated double bonds is increased. ∗ Approximate quantitative calculations of the absorption frequencies may be made on the basis of the free-electron model for the  electrons of these molecules. The energy for the lowest electronic transition is that required to raise an electron from the highest filled level to the lowest unfilled level. In a system of conjugated double bonds each carbon atom has three  bonds that lie in a plane, and each  bond involves one outer electron of that carbon atom. Above and below this plane are the  orbital systems. Each carbon atom contributes one electron to this  system, but these electrons are free to move the entire length of the series of  orbitals and are not localized at a given carbon atom.

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  Advances in High Temperature Chemistry

  Volume 1

  • Leroy Eyring(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  The Spectroscopy of Diatomic Transition Element Molecules C. J. Cheetham and R. F. Barrow PHYSICAL CHEMISTRY LABORATORY, OXFORD UNIVERSITY, OXFORD, ENGLAND I. Introduction II. Experimental Techniques A. Optical Spectroscopy in the Gas Phase B. Determination of Low-Lying Energy States . C. Matrix Isolation Spectroscopy D. Dissociation Energies III. Molecular Constants IV. Electronic Structure of the Ground States A. Oxides and Halides B. Hydrides V. Conclusions References 7 8 8 11 11 12 13 27 27 32 36 36 I. Introduction The low-lying molecular energy levels of simple diatomic molecules of the lighter atoms such as HF, CO, and N 2 are well documented. In recent years detailed molecular orbital calculations on molecules of this order of com-plexity have begun to give results not only of qualitative but also of quantita-tive importance, so that the basic principles of bonding with s and p electrons may be said to be understood for the most part at least in principle, if not always (especially in open-shell configurations) in detail. In contrast, the contributions of d electrons to the chemical bond have been studied mainly in those systems where their overlap with ligand orbitals may be taken to be zero (as in the crystal field theory) or at most small (as in the ligand field theory). Diatomic molecules containing at least one transition metal atom are therefore of interest because they represent the simplest systems in which to study the effects of d electrons on bonding. As in the chemistry of complex compounds, a successful theory would predict not only the character of the ground state, but also the nature of the lower excited states and, thus, of the 7 8 C. J. Cheetham and R. F. Barrow spectroscopic properties. Here, as elsewhere, spectroscopic and thermody-namic studies prove to be complementary.

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  Physical Chemistry

  • David Ball(Author)
  • 2014(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Atomic orbitals from atoms are used to determine molecular orbital energies by defining the integrals H xy and S xy in a fashion similar to that just presented for the p electrons. Although similar in principle, it requires larger matrices because all valence electrons are treated. Other concerns preclude a detailed discussion here, but other references can be consulted for details. † † (like J. P. Lowe, Quantum Chemistry, 2nd ed., Academic Press, Boston, 1993) Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Unless otherwise noted, all art on this page is © Cengage Learning 2014. 15.11 | Fluorescence and Phosphorescence 561 15.11 Fluorescence and Phosphorescence In a perfect molecule, Electronic Transitions would go like this: Absorption of a photon excites a molecule from initial (usually ground) state to excited state; excited state emits a photon having the same energy/frequency/wavelength and molecule goes from excited state to previous initial ground state. The first process, excitation, would be followed by the exact opposite process, called de-excitation or decay. Such processes would follow quantum-mechanical selec-tion rules strictly. In reality, Electronic Transitions stray somewhat from the ideal selection rules. In particular, when an excited electronic state decays to a lower electronic state, a photon having the same energy as the excitation photon might not be emitted. Instead, the molecule may de-excite by transferring the extra energy into various vibrational, rotational, or solid-state vibrational (called “phonon”) modes of the sample.

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