CHM 4130_Ch6&7_Spring11

CHM 4130_Ch6&7_Spring11 - Introduction to...

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1 Introduction to Spectroscopy (Chapter 6) and Components of Optical Instruments (Chapter 7) Before even attempting to understand spectrochemical analysis, one needs to have a basic understanding of electromagnetic radiation and radiation and matter interactions Sample Electromagnetic radiation Spectrum How is electromagnetic radiation generated? How is electromagnetic radiation detected? How is a spectrum generated? Before even attempting to understand spectrochemical analysis, one needs to have a basic understanding of instrumentation
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2 Electromagnetic radiation Electromagnetic radiation can be described by two models: the wave model and the particle model. The two models are not exclusive but, rather, complementary. Wave properties of electromagnetic (EM) radiation An EM wave can be represented as electric and magnetic fields that undergo in-phase sinusoidall oscillations at right angles to each other and to the direction of propagation. Both fields can be represented as vectors perpendicular to each other and to the direction of propagation The term plane-polarized implies that all oscillations of either the electric or the magnetic fields lie within a single plane The term monochromatic implies that the EM radiation is composed only by one wavelength Wave of EM radiation Polarizer Plane-polarized EM radiation Plane-polarized EM radiation Wavelength selector Monochromatic plane-polarized radiation
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3 Representation of a beam of monochromatic, plane-polarized radiation: Interaction of the electric field and the magnetic field of an EM wave on an orbit about the nucleus We will focus our discussion on the electric field of the EM wave
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4 Velocity = wavelength x frequency
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6 The Electromagnetic Spectrum
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8 6-19
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9 6-22
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11 6-23
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14 14 Multiplicity of an electronic state The multiplicity (M) of an electronic state is related to the total spin quantum number, i.e.: M = 2S + 1 The total spin quantum number is equal to: S = ∑si, where si = +1/2 or -1/2. Molecules in the ground state have a total spin quantum number equal to zero. As a consequence, the multiplicity of the ground state is equal to 1. Electronic states with multiplicity equal to 1 are called “singlet” states and denoted by “S”. When one of the two electrons of opposite spins, belonging to a molecular orbital of a molecule in the ground state, is promoted to a molecular orbital of higher energy, its spin is - in general - unchanged. Because the total spin quantum number does not change, the multiplicity of the ground and excited state do not change and the transition is called a singlet-singlet transition: S0  S1; S0  S2; S1  S2; etc. S0 denotes the ground state.
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CHM 4130_Ch6&7_Spring11 - Introduction to...

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