CHM 4130_Ch8&9_Spring11

CHM 4130_Ch8&9_Spring11 - INTRODUCTION TO OPTICAL...

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1 INTRODUCTION TO OPTICAL ATOMIC SPECTROSCOPY Atomic spectroscopy techniques : Optical spectrometry Mass spectrometry X-Ray spectrometry Optical spectrometry : Elements in the sample are atomized before analysis. Atomization : Elements present in the sample are converted to gaseous atoms or elementary ions. It occurs in the atomizer (see Table 8- 1). Optical spectroscopy techniques : Atomic Absorption Spectroscopy (AAS) Atomic Emission spectroscopy (AES) Atomic Fluorescence Spectroscopy (AFS)
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2 Optical Atomic Spectra Figure 8-1a shows the energy level diagram for sodium. A value of zero electron volts (eV) is arbitrarily assigned to orbital 3s. The scale extends up to 5.14eV, the energy required to remove the single 3s electron to produce a sodium ion. 5.14eV is the ionization energy . A horizontal line represents the energy of and atomic orbital. “p” orbitals are split into two levels which differ slightly in energy: 3s → 3p: λ = 5896Å or 5890Å 3s → 4p: λ = 3303Å or 3302Å 3s → 5p: λ = 2853.0Å or 2852.8Å There are similar differences in the d and f orbitals, but their magnitudes are usually so small that are undetectable, thus only a single level is shown for orbitals d. Spin-orbit coupling
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3 Multiplicity: number of possible orientations of the resultant spin angular momentum = 2S +1
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4 Atomic Line Widths Widths of atomic lines are quite important in atomic spectroscopy. Narrow lines in atomic and emission spectra reduce the possibility of interference due to overlapping lines. Atomic absorption and emission lines consists of a symmetric distribution of wavelengths that centers on a mean wavelength ( λ 0) which is the wavelength of maximum absorption or maximum intensity for emitted radiation. The energy associated with λ 0 is equal to the exact energy difference between two quantum states responsible for absorption or emission. A transition between two discrete, single- valued energy states should be a line with line-width equal to zero. However, several phenomena cause line broadening in such a way that all atomic lines have finite widths. Line width or effective line width ( ∆λ 1/2) of an atomic absorption or emission line is defined as its width in wavelength units when measured at one half the maximum signal. Sources of broadening: (1) Uncertainty effect (2) Doppler effect (3) Pressure effects due to collisions (4) Electric and magnetic field effects
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5 Uncertainty Effect It results from the uncertainty principle postulated in 1927 by Werner Heisenberg . One of several ways of formulating the Heisenberg uncertainty principle is shown in the following equation: t x E = h The meaning in words of this equation is as follows: if the energy E of a particle or system of particles photons, electrons, neutrons or protons – is measured for an exactly known period of time t, then this energy is uncertain by at least h/ t. Therefore, the energy of a particle can be
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This note was uploaded on 11/07/2011 for the course CHM 4130 taught by Professor Staff during the Spring '11 term at University of Central Florida.

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CHM 4130_Ch8&9_Spring11 - INTRODUCTION TO OPTICAL...

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