Instrumental Analytics 3

Front 1

Spectroscopy

Back 1

Interaction of nuclei, atoms, ions, or molecules with electromagnetic radiation.

Front 2

Electromagnetic radiation (EM)

Back 2

- a sinusoidal electric and magnetic wave traveling through the space
- a discrete series of “particles” that have a specific energy but have no mass, photons

Front 3

Wave Properties of EM Radiation: wavelength

Back 3

Linear distance between two equivalent points on successive waves.

Front 4

Wave Properties of EM Radiation: amplitude (A)

Back 4

The length of electric vector at a maximum

Front 5

Wave Properties of EM Radiation: frequency

Back 5

The number of oscillations occurred per sec.

Front 6

Wave Properties of EM Radiation: period (T)

Back 6

Time for 1 wavelength to pass a fixed point; =1/freq

Front 7

Wave Properties of EM Radiation: velocity (V)

Back 7

vi=wavelentghi * freq (c=2.99792x10^8 m/s)

Front 8

Wave Properties of EM Radiation: Diffraction

Back 8

Parallel electromagnetic wave can be bend when passing through a narrow opening (width ~ wavelength).

Front 9

Wave Properties of EM Radiation: Constructive interference (intense band)

Back 9

can be observed when the difference in path length from two slits is equal to wavelength (first order interference), or 2wavelength, 3 wavelength... corresponding to difference between two phase angles = 2nPi, n is an integral 1,2,3...

Front 10

Wave Properties of EM Radiation: Transmission

Back 10

Velocity of wave propagation (m/s) = wavelength(m) x frequency (s^-1).
- In a vacuum: electromagnetic wave travels at the speed of light, c = 3.00 x108 m/s
- In other media, frequency remains constant, wavelength and thus v decreases
vi=c/ni ni: the medium refractive index >=1 (liquids 1.3-1.8, solids >=1.3)

see lecture 3!

Front 11

Particle Description of Radiation: Particle Properties

Back 11

According to Photoelectric Effect experiment:
- energy of a photon can be related to its frequency

E (J) = h * freq where h is Planck's constant 6.6254x10^-34 J*s

energy is inversely proportional to the wavelength
freq= c/wavelength -> E=hc/wavelength

Front 12

Commonly Used Units (Wavelength units vary with spectral region)

Back 12

X-ray & short UV: Angstrom = 10^-10m

UV/Visible range: nm=10^-9m µm=10^-6

Infrared range: µm

Photon energy:
- X-ray region: eV 1J = 6.24x10^18 eV
- Visible region: kJ/mol = J/photon x 6.02x10^23 photon/mol x10^-3 kJ/J

Front 13

Interaction with Matter: Postulates of Quantum Mechanics

Back 13

Atoms, ions and molecules exist in discrete energy states only -- quantized.
E0: ground; E1 E2 E3 exited states.
Exitation can be electronic, vibrational or rotational.

Enegery levels of atoms, ions or molecules all different -> measuring energy levels gives means of identification of chemical species - spectroscopy

Front 14

Interaction with Matter: Absorption spectra - Atoms

(Just as in emission spectra an atom, ion or molecule can only absorb radioation if energy matches separation between two energy states)

Back 14

Atoms: No vibrational or rotation energy levels - sharp line spectra with few features
Na 3s->3p 589.0, 589.6 nm (yellow)

For valence excitation, visible energy.
For core (inner) excitation, UV and X-ray energy.

For absorption to occur, the energy of incident beam must correspond to one of the energy difference.
Measurement of the amount of light absorbed as a function of wavelength.

Front 15

Interaction with Matter: Absorption spectra - Molecules

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Electronic, vibrational and rotational energy levels all involved.

Each electronic state – many vibrational states
Each vibrational states – many rotational states

E = Eelec + Evib + Erot

-> broad band spectra with many features

Front 16

Interactions with Matter: Photoluminescence Spectra

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- Photoluminescence methods (fluorescence and phosphorescence)

- Fluorescence: prompt emission

- Phosphorescence: delayed emission

Front 17

Interactions with Matter: Relaxation Process

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Lifetime of excited state is short (fs->ms)

Nonradiative relaxation: loss of energy by collisions, happpens in a series of small steps. Tiny temperature rise of surrounding species.

Radiative relaxation (emission): Fluorescence (<10^-5s)

Front 18

Stokes shift:

Back 18

Emission has a lower frequency than the radiation (due to vibrational relaxation occurs before fluorescence)

Front 19

Major Classes of Spectrochemical Methods

see lecture!

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- Emission (Method: Atomic emission)

- Luminescence (Method: Atomic and molecular fluorescence, phosphorescence and chemiluminescence)

- Scattering (Method: Raman scattering, turbidimetry, and particle sizing)

- Absorption (Method: Atomic and molecular absorption)