Spectroscopy

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Spectroscopy

• Valeriia Starovoitova

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Outline

• Introduction
• Bohr Model
• Examples
• X-ray spectroscopy
• Raman spectroscopy
• Fluorescent spectroscopy
• Nuclear Resonance Vibrational Spectroscopy

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What Is Spectroscopy?

• It is a study of the interaction between
  radiation and matter as a function of wavelength
  (?) or frequency (f)

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Spectrum

• A plot of the response as a function of
  wavelength, or frequency, or energy is referred
  to as a spectrum.

E hf
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What Do We Measure?

• Depends on the type of spectroscopy
• - Electromagnetic spectroscopy (intensity)
• - Electron spectroscopy (intensity)
• - Mass spectrometry (mass)
• - Acoustic spectroscopy (intensity)
• - Dielectric spectroscopy (permittivity)
• - Mechanical spectroscopy (stretch, torsion)

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Spectroscopic Methods
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History
Two of the earliest explanations of the optical
spectrum came from Isaac Newton, when he wrote
his Opticks, and from Goethe, in his Theory of
Colours, although earlier observations had been
made by Roger Bacon who first recognized the
visible spectrum in a glass of water, four
centuries before Newton discovered that prisms
could disassemble and reassemble white light.
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Visible spectrum
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Atomic spectra

• Joseph von Fraunhofer 1814
• the spectrum of sunlight is crossed by dark
  lines some wavelengths are missing from the
  light that reaches us from the Sun!

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Absorption and Emission Spectra
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Johann Balmer, 1885

• Four visible lines in hydrogen spectrum have been
  measured by 1885.
• Balmer found the measurements to fit the formula

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Is it a coincidence?
l 397 nm - Violet edge of the spectrum
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Generalized Formula

• Johannes Rydberg

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Atomic Stability

• Classical electromagnetic theory
• An accelerating charge should emit
  electromagnetic waves!
• All electrons should collapse in about 10-11s!

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Bohrs Explanation of Atomic Stability

• Not all electron orbits are allowed!
• A certain discrete set stationary orbits can
  exist.
• This results in a discrete set of energies.
• An electron in an allowed state would not emit
  radiation.
• Radiation is emitted once there is a transition
  from one state to another this explains spectra!

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Transitions
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The Bohr Model of Hydrogen Atom
Classical Mechanics
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The Bohr Model of Hydrogen Atom
Quantum Mechanics
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The Bohr Model of Hydrogen Atom
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The Bohr Model of Hydrogen Atom

• Knowing possible radii of the electrons orbit,
  we can find possible energies

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The Bohr Model of Hydrogen Atom

• Rydberg constant is in the perfect agreement
  with the value obtained by Balmer

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Properties of Bohr Atom
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Example 1

• Helium atom has 2 stationary states 2s and 3p
  with energies 20.6 eV and 23.1 eV (from the
  ground state) correspondingly. What is a
  wavelength of a photon emitted as a result of
  such a transition?

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Example 2

• What is the diameter of a hydrogen atom with
  n100?
• Atoms with this high value of n can exist only
  in a good vacuum (interatomic spacing at normal
  pressure is about 3 nm)

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Diffraction

• Diffraction and interference are similar
• phenomena.
• Interference is the effect of superposition
• of 2 coherent waves.
• Diffraction is the superposition of many
• coherent waves.

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Spectroscopy. Part II

• Outline
• 1. Diffraction
• 2. X-rays
• 3. X-ray spectroscopy

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Double Slit Experiment
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Double Slit Experiment

• 2dsin? n?

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Diffraction Grating

• Consists of a flat barrier which contains many
  parallel slits separated by a short distance d.
• A parallel monochromatic light beam passing
  through the grating is diffracted by an angle ?
  2dsin? n? similar to two slit interference.
• However, the intensity of the diffracted light is
  higher and the peaks are much narrower.

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Compact Disk

• The closely spaced dots act like a diffraction
  grating.

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Visible light

• Gratings used to disperse ultraviolet (UV) and
  visible light usually contain between 300 and
  3000 grooves per millimeter, so the distance
  between adjacent grooves is on the order of one
  micron.
• l 400 - 700 nm 0.4 0.7 micron

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EM Spectrum
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X-ray Spectroscopy

• Originally discovered by Wilhelm Roentgen in the
  nineteenth century, X-rays have become one of the
  most useful applications of spectroscopy in both
  science and medicine.

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X-rays

• X-rays are a form of electromagnetic radiation
  with a much higher degree of energy than UV
  radiation. This extra energy allows X-rays to be
  absorbed by core electrons within atoms. Also,
  X-rays can penetrate crystal structures more than
  other forms of EM radiation, having a wavelength
  on the same order of magnitude as interatomic
  distances. This allows the X-rays to be
  diffracted, producing diffraction patterns of the
  crystal.

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Three Basic Applications of X-rays

• 1. Diffraction of X-rays on crystalline materials
  to obtain their crystal structure
• 2. Measurement of the energy of emitted X-rays
  (X-ray Fluorescence )
• 3. Using X-rays to knock out core electrons of
  atoms to provide surface chemical information
  from samples (X-ray Photoelectron Spectroscopy)

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1. Diffraction

• Braggs law
• 2dsin? n?
• Visible light the diffraction of sunlight
  through a bird's feather was first reported by
  James Gregory in the later 17th century.

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1. X-ray Diffraction

• The idea that crystals could be used as a
  diffraction grating for X-rays arose in 1912 in a
  conversation between Paul Peter Ewald and Max von
  Laue.

They shined a beam of X-rays through a sphalerite
crystal and record its diffraction on a
photographic plate.
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1. Simple Inorganic Crystals

• Although diamonds and graphite are identical in
  chemical composition being both pure carbon
  X-ray crystallography revealed the arrangement of
  their atoms.

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1. X-ray Sources

• Synchrotrons (Argonne, Grenoble, etc)
• X-ray tubes

X-rays are made monochromatic and collimated to a
single direction before they are allowed to
strike the crystal.
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1. Mounting the Crystal

• Goniometer - is an instrument that allows an
  object to be rotated to a precise angular
  position.

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1. Recording the Reflections

• Photographic film
• Area detector
• Charge Couple Device (CCD) sensor
• One image of spots is insufficient to
  reconstruct the whole crystal to collect all the
  necessary information, the crystal must be
  rotated step-by-step through 180.

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2. X-ray Fluorescence

• X-ray fluorescence is the emission of
  characteristic "secondary" (or fluorescent)
  X-rays from a material that has been excited by
  bombarding with high-energy X-rays or gamma rays.
  The phenomenon is widely used for elemental
  analysis and chemical analysis, particularly in
  the investigation of metals, glass, ceramics and
  building materials, and for research in
  geochemistry, forensic science and archaeology.

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2. Electronic orbitals
Each element has electronic orbitals of
characteristic energy. Following removal of an
inner electron by an energetic photon provided by
a primary radiation source, an electron from an
outer shell drops into its place.
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2. X-ray Fluorescence Spectrum

• X-ray fluorescence spectrum showing the presence
  of different elements.

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3. Basic Principles of XPS
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3. X-ray Photoelectron Spectroscopy

• Uses soft x-ray (200-2000 eV) radiation to
  examine core-level electrons.

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3. Analyzing Surface with XPS

• Fracturing
• Cutting or scraping in air or UHV to expose the
  bulk chemistry
• Ion beam etching
• Changes due to heating
• Changes due to exposure to reactive gases or
  solutions
• Changes due to exposure to UV

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3. Electron Spectroscopy for Chemical Analysis

• XPS detects all elements with 3ltZlt103
• Detection limits for most of the elements are in
  the parts per thousand range.
• XPS is routinely used to analyze inorganic
  compounds, metal alloys, semiconductors,
  polymers, glasses, ceramics, bio-materials,
  viscous oils, glues, ion modified materials and
  many others.

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XPS measures

• elemental composition of the surface
• empirical formula of pure materials
• elements that contaminate a surface
• chemical or electronic state of each element in
  the surface
• uniformity of elemental composition across the
  top surface
• uniformity of elemental composition as a function
  of ion beam etching

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3. Typical XPS spectrum
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• The End