Means of Excitation Light Sources

1
Means of Excitation Light Sources
Hanh Le Devon Johnson Justin Gwilt Jing
Zhang Bao-Tram Khuc
2
Means of Excitation Light Sources

• Infrared radiation
• Lasers
• Hollow Cathode Lamps
• Electrodeless Discharge Lamps
• Applications

3
What is Infrared Radiation?
radio
infrared
visible
UV
Xray
gamma

• 750nm-1mm

4
More About Infrared

• Infrared is divided into 3 spectral regions
  near, mid and far-infrared
• Near Infrared Radiation 2,500 to 750 nm (nearest
  to visible light) -- A common source for near
  infra-red spectrum light is a diode laser useful
  in probing bulk material with little or no sample
  preparation
• Mid-infrared 2500 10,000 nm -- Hot objects
  radiate strongly in this range absorbed by
  molecular vibrations
• Far-infrared 10,000nm-1mm -- transmitted
  (noncontact) through most plastics, paper, and
  materials other than metals

5
Infrared Radiation Emission

• Infrared radiation is emitted by any object that
  has a temperature (ie radiates heat).
• The wavelength at which an object radiates
  depends on its temperature.
• Some infrared wavelengths are better suited for
  studying certain objects than others.

6
Infrared Absorption

• Compounds with covalent bonds absorb frequencies
  of in the infrared region
• molecules are excited to higher energy state
  when they absorb infrared radiation
• absorption quantized, only select freqs are
  absorbed by a molecule
• absorption of infrared corresponds to ?E of 8-40
  kJ/mole corresponds to range of stretching and
  bending vibrational frequencies of bonds
• freqs of infrared radiation that match natural
  vibrational freqs of molecules are absorbed and
  the E absorbed increases the amplitude of
  vibrational motions of the bonds in the molecule

7
General Use of Infrared Spectrum

• Every type of bond has a dif natural freq of
  vibration
• Environment can affect the freq of vibration
• Thus, no two molecules of dif structure have
  exactly the same infrared absorption pattern or
  infrared spectrum ? fingerprint
• Structural information

8
A beam of infra-red light is produced and split
into two separate beams. One is passed through
the sample, the other passed through a reference
which is often the substance the sample is
dissolved in. The beams are both reflected back
towards a detector, however first they pass
through a splitter which quickly alternates which
of the two beams enters the detector. The two
signals are then compared and a printout is
obtained. A reference is used for two
reasons This prevents fluctuations in the output
of the source affecting the data This allows the
effects of the solvent to be cancelled out (the
reference is usually a pure form of the solvent
the sample is in)
9
Far-Infrared

• Far-infrared (far-IR) or terahertz (THz)
  radiation used to detect biological and chemical
  agents, explosives, and drugs/narcotics inside
  sealed, nonmetallic containers
• Advantage the only known method for detecting
  and identifying amounts of toxic chemicals or
  explosives in sealed containers made of plastics,
  paper, and materials other than metals
• Limitation medium

10
Lasers

• Invented in 1960
• Light Amplification by Stimulated Emission of
  Radiation
• -device that controls the way energized
  atoms release photons
• Various uses including
• industrial and commercial (e.g. supermarket
  scanners, laser printers, CD players)
• medical (e.g. LASIK, cosmetic surgery)
• military (illuminator, laser sight
• SCIENTIFIC (spectroscopy, photochemistry,
  nuclear fusion)

11
How do they work? Part 1

• Several features of lasers that make it different
  from white light. Most light sources spread out
  as they travel. Lasers more or less dont spread
  out and travel in a parallel beam.
• Also-lasers monochromatic and coherent.
  ?Monochromaticity and coherency make lasers ideal
  for recording data on CDs.
• So, 3 properties monochromaticity, coherence,
  organization

12
Differences
13
How do they work? Part 2

• Generation of laser light depends on rates
  atoms/molecules return to base rate of
  excitation.
• Atoms in constant motion, range in states of
  excitation (different energies). Applying a lot
  of energy to an atom brings it to an excited
  state, leaving ground state. Energy level
  dependent on energy applied by head light or
  electricity.
• Excited electron --gt moves to higher energy orbit
  and wants to return to ground state --gt release
  of a photon (light particle)

14
How do they work? Part 3

• The medium is pumped to get the atoms into
  excited mode. Majority of atoms need to be in
  excited state with high energy electrons so that
  laser works efficiently.
• Emitted energy comes in form of photon. Photon
  emitted has specific wavelength dependent on
  electrons energy level
• To make mono., coherence, and org. work,
  stimulated emission needs to take place
• Stimulated emission-when an excited electron
  comes into contact with similarly excited
  electron
• First photon induces atomic emission, 2nd atom
  vibrates with same wavelength (coherence)

15
How do they work? Part 4

• Laser uses spontaneous emission to emit light
  waves.
• Spontaneous emission- electrons absorb energy ?
  excited state ? gives off excess energy
  spontaneous emission
• When a wave of energy (photon) from one atom
  strikes another excited one, causes that atom to
  release too. Multiple hits cause amplification of
  light. Interaction with unexcited atom lost
  amplification.

16
How do they work? Part 5

• Last component
• Mirrors flanking medium. Photons with specific
  wavelength reflect off mirrors and bounce back
  and forth
• More photons can be generated
• One end half slivered-lets some light through and
  reflects the rest
• Light that gets through is laser light!

17
A Visual..
18
Types of Lasers

• Solid State
• Gas
• Liquid
• Semiconductor

19
Solid Laser

• Ruby Laser
• A flash tube,
• a ruby rod and
• two mirrors
• First laser built

20
Gas Lasers

• Use ultraviolet light, electron beams, electric
  current, or chemical reactions
• Gases used
• Neon, Argon, Krypton

21
Liquid Lasers

• Employ a wide range of materials including
  copper, chromium, dyes, metallic salts
• Have a very broad emission spectra

22
Hollow Cathode Lamp

• type of discharge lamp that produces narrow
  emission from atomic species
• The cathode is made from the element of interest
• Expensive and produce low intensity light

23
How do Hollow-cathode lamps work?

• The electric discharge ionizes rare gas atoms,
  which are accelerated into the cathode and
  sputter metal atoms into the gas phase
• Collisions with gas atoms or electrons excite the
  metal atoms to higher energy levels, which decay
  to lower levels by emitting light.

24
Applications of Hollow-Cathode Lamps

• HCLs are often used in Atomic-absorption (AA)
  spectroscopy to provide the light source (the
  lamp is made of the element being analyzed).
• This process uses the absorption of light to
  measure the concentration of gas-phase atoms
• Most samples are solid or liquid and are
  vaporized.
• The vaporized atoms absorb light and make
  transitions to higher electronic energy levels
• The analyte concentration is determined from the
  amount of absorption.

25
Electrodeless discharge lamp

• The lamp has a quartz tube containing argon gas
  and a metal of interest.
• An intense radio frequency or microwave field is
  applied to the sealed quartz tube.

26
How EDLs Work

• The Ar gas in the tube is ionized when the RF or
  microwave field is applied and gains kinetic
  energy from the field.
• Energy is then transferred to the metal upon
  collision
• When the excited metal returns to ground state,
  light is emitted.

27
Advantages and Disadvantages of Electrodeless
discharge lamps

• Advantages
• Most used source
• 10 times more intense than a hollow cathode lamp
• Disadvantages
• Unstable output, hard to produce
• Only available for about 15 elements

28
Infrared Microspectroscopy (IMS) and F rensic
Science

• What is IMS?
• One instrument spectroscopy microscopy
• Microscope uses visible light / Spectroscope uses
  IR

• How does IMS Work?
• use reflective optics instead of refractive
• IR radiation reflected from the IMS reflective
  surfaces ? the resultant spectrum reveals the
  functional groups present on the surface.

29
Infrared Microspectroscopy (IMS) and F rensic
Science

• Cases solved using IMS
• Abduction and murder
• Cocaine abuse
• Police assault/murder

30
Infrared Microspectroscopy (IMS)

• TAKING IMS TO THE CRIME SCENE?
• smaller, more portable units with easier sample
  prep
• quick and accurate characterization of unknown
  organic chemicals
• provide fast answers to questions involving
  flammability, toxicity, and disposal.
• With portable IMS, fire fighters would know
  immediately whether they were dealing with
  something they could simply flush away with a
  fire hose.