1. 1.1 What’s electromagnetic radiation
Chapter 6 An Introduction to Spectrometric Methods 1. Wave properties of Electromagnetic Radiation
1.1 What’s electromagnetic radiation
- 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
Both.
Wave-particle duality!

2. 1.2 Wave properties of electromagnetic radiation
(considering electric field only since it’s responsible for spectroscopy including transmission, reflection, refraction, and absorption)
: wavelength, linear distance between two equivalent points on successive waves.
A: amplitude, the length of electric vector at a maximum
: frequency, the number of oscillations occurred per sec.
T: period, time for 1  to pass a fixed point, =1/ 
y = A sin(t + ), with time as variable
: angular velocity =2,
: phase angle
Coherent: a set of waves with identical  and difference in phase angle  remains constant
y = A sin(t + ),
y’ = A’sin(t + ’),  - ’ = constant
Fig. 6-1 (p.133)

3. 1.2.2 Reflection and refraction
1.2.1 Transmission
velocity of wave propagation (m/s) = (m) x  (s-1)
- In a vacuum: electromagnetic wave travels at the speed of light, c = 3.00 x108 m/s
- In other media,  remains constant,  and thus v decreases
v = c/n,
n: the medium refractive index 1.00
1.2.2 Reflection and refraction
The fraction of reflection:
The extent of refraction:

4. Diffraction
Parallel electromagnetic wave can be bend when passing through a narrow opening (width   ).
Fig. 6-7 (p.138)

5. Two diffracted rays from two slits will have interference.
Fig. 6-7 (p.138)
Fig. 6-8 (p.139)
Two diffracted rays from two slits will have interference.
Constructive interference (intense band) can be observed when the difference in path length from two slits is equal to wavelength (first order interference), or 2, 3 … corresponding to difference between two phase angles = 2n, n is an integral 1,2,3…

6. 2. Particle Description of Radiation
2.1 Particle Properties
According to Photoelectric Effect experiment (p )
energy of a photon can be related to its frequency
E (J) = h
h: Planck's constant, x10-34 Js
 =c/  E = hc/
energy is inversely proportional to the wavelength

7. wavelength units vary with the spectral region
2.2 Some Commonly Used Units
wavelength units vary with the spectral region
X-ray and short UV: Å = m
UV/Visible range: nm = 10-9 m
m = 10-6 m
Infrared range: m
wavenumber (cm-1):
Photon energy
X-ray region: eV 1J = x1018 eV
Visible region: kJ/mol kJ/mol = J/photon x6.02 x1023 photon/mol X10-3 kJ/J

8. 2.2 Range of wavelength/frequencies
Fig. 6-3 and Table 6-1 (p.135)

9. 3 Interaction with Matter
3.1 Postulates of Quantum Mechanics
Atoms, ions and molecules exist in discrete energy states only -- quantized
E0: ground
E1, E2, E3 … : excited states
Excitation can be electronic, vibrational or rotational
Energy levels of atoms, ions or molecules are all different,
Measuring energy levels gives means of identification of chemical species – spectroscopy
When an atom, ion or molecule changes energy state, it absorbs or emits radiation with energy equal to the energy difference
E = E1 - E0
The wavelength or frequency of radiation absorbed or emitted during a transition

10. 3.2 Emission Spectra from Excited States
Fig (p.147) Sample is excited by the application of thermal, electrical or chemical energy
Fig (p.151)

11. Fig (p.153)
Measurement of the emitted radiation as a function of wavelength

12. 3.3 Absorption Spectra Atoms
Just as in emission spectra an atom, ion or molecule can only absorb radiation if energy matches separation between two energy states.
Atoms
No vibrational or rotation energy levels – sharp line spectra with few features
Na 3s3p , nm (yellow),
For valence excitation, visible energy
For core(inner) excitation, UV and X-ray energy

13. Fig (p.148) For absorption to occur, the energy of incident beam must be correspond to one of the energy difference
3s3p 589.0, nm
Fig (p.153)
Measurement of the amount of light absorbed as a function of wavelength

14. Electronic, vibrational and rotational energy levels all involved –
Molecules
Electronic, vibrational and rotational energy levels all involved –
Each electronic state – many vibratioanl states
Each vibrational states – many rotational states
E = Eelec + Evib + Erot
 broad band spectra with many features.
Fig (p.153)

15. Lifetime of excited state is short (fsms) – relaxation processes
Nonradiative relaxation
loss of energy by collisions, happens in a series of small steps.
Tiny temperature rise of surrounding species
Radiative relaxation (emission)
Fluorescence (<10-5s)
Stokes shift: emission has a lower frequency than the radiation (due to vibrational relaxation occurs before fluorescence).

16. Fast vibrational relaxation
E2+e4”-E1
Stokes shift:
emission has a lower frequency than the radiation  due to vibrational relaxation occurs before fluorescence.
E2-E1
Fig (p.154)

17. 3.5 Quantitative aspects of spectrochemical measurements
Assuming blank signal is already corrected for
Emission Spectra
S = kc
Absorption Spectra
Transmittance expressed as percent:
T% = P/P0 x 100%
Absorbance: A = -log10 T = log(P0/P)
Beer’s Law
A = bc
: molar absorptivity (Lmol-1cm-1)
b: path length of absorption (cm-1)
C: molar concentration (mol L-1)
Fig (p.158)