Introduction to spectroscopy

Introduction to spectroscopy

1. study of absorption & emission of electromagnetic radiation by atoms & molecules

2. Uses

   • Determination of molecular structure
   • Identification of unknown molecules
   • Detection of known molecules
   • Measurement of concentration

3. Electromagnetic radiation

   • form of E produced by oscillating electric & magnetic disturbance
   • electric & magnetic waves travel perpendicular to each other
   • General properties
     • travel through empty space
     • speed of light always constant
     • wavelength measured btw distance of crest & troughs
   • Amplitude
     • distance from the maximum vertical displacement of the wave to the middle of wave
     • measure the magnitude of oscillation of a particular wave
     • larger amplitude, larger energy
   • Wavelength, λ
     • distance of one full cycle of the oscillation
     • shorter wavelength, higher frequency, greater energy,
     • c = λv
       • c = 3.0x10^8 m/s or 3.0x10^10cm/s
       • v : 1/s
   • Period, T
     • amount of time a wave takes to travel one wavelength, in seconds

4. Photon

   • quantum of electromagnetic E
   • E = hv
     • h: planck constant, 6.62607 x 10^-34 Js

5. Atomic Spectroscopy Techniques

   • Atomic Emission Spectroscopy, AES
     • require electrons to be excited b4 they emit
     • selective excitation may occur
   • Atomic Absorption Spectroscopy, AAS
     • produce spectra whose lines reveal spacing btw E levels in electronic structure of atoms
     • produce high level of noise
     • maps the ground state, so there will always plenty of electrons available to absorb photons with right frequency

6. Energy level & Transitions

   • Energy level: internal E in atoms & molecules is quantized
   • E(photon) = E(upper) - E(lower) = hv
     • Absorption of photons
       • upward transition to higher level
       • increase in internal E
       • Excitation
     • Emission of photons
       • downward transition to lower E level
       • decrease in internal E
       • Spontaneous emission
         • electrons drop from excited state to lower E & photon released
       • Stimulated emission
         • emission triggered by incoming photon of same frequency

7. Transition dipole moment, μ(fi)

   • μ(fi) = ∫φ(f)μφ(i)d
     • μ: electric dipole moment operator
     • φ(f) & φ(i) : state level initial & final
   • when μ ≠ 0, electric dipole is allowed
   • when μ =0, electric dipole forbidden
   • when a 1s electron become 2s electron, no net μ
   • when a 1s electron become 2p electron , hv net μ

8. Selection Rules

   • define which transition are allowed / forbidden
   • Gross selection rules
     • permanent electric dipole moment
   • Specific selection rules
     • quantum number changes

9. Intensity of spectral lines

   • Factors
     • Rate of transition
       • more intense electromagnetic field, greater rate of transition induced, stronger absorption
     • Population of initial E level
   • Beer-Lambert Law
     • Transmittance, T=I/I'
       • I : Intensity of transmitted light
       • I' : Intensity of incident light
     • I=I_0 x 〖10〗^(-ε[J]L)
       • ε : molar absorption coeficient
         • depend on frequency of incidnet radiation & greatest when absorption is most intense
       • L : length of sample
       • [J] : molar concentration
     • Absorbance, A =ε[J]L
       • ε[J]L : optical of sample
       • A =-log T

10. Factors of broadening, ∆v

    • Natural linewidth
      • ∆E∆t = h/2π
      • electrons in excited state remain for average time, ∆t b4 decay
    • Collisional broadening
      • ∆v(total) = ∆v + v(collision)/π
      • at excited state, collision occur, loss E, larger at higher density
    • Thermal broadening
      • ∆v(D) = v_0/c x √(2kT/m)
      • larger at higher T
      • at excited, T increase, molecules decay, emitted E decrease

11. absorption probability

    • Intensity of incident radiation at absorption frequency
    • square of magnitude of μ(fi)

12. Emission probability

    • Intensity of emitted radiation
    • square of magnitude of μ(fi)