Spectroscopy. Radiation and Matter Spectroscopic Methods. Luís Santos

Transkript

1 Spectroscopy Radiation and Matter Spectroscopic Methods

2 Spectroscopy Spectroscopy studies the way electromagnetic radiation (light) interacts with matter as a function of frequency, thus, it studies the energy exchange between matter and radiation.

3 What is light? According to Maxwell, light is an electromagnetic wave, caracterized by a frequency ν or a waveleght λ, where νλ=c and by a polarization.

4 What is light? Light waves are fluctuations of the electric and magnetic field. These fields are orthogonal among themselves and orthogonal to the propagation direction.

5 What is light? Light has characteristics of a wave, but also of a particle. λ=h/mv Louis de Broglie (1925)

6 What is light? The light particle is called photon and its energy (E) is given by: E=hν=hc/λ E [J] h [J.s] ν [Hz or s -1 ] c [ms -1 ] λ [m]

7 What is light? Fenómeno Waves Particles Reflection Refraction Interference Difraction Polarization Photoelectric Ef.

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9 Nuclear transitions Electronic transitions (Rγ) λ 0.1 Å camadas fundamentais (RX) camadas intermédias (Far UV) camadas de valência (Near UV/Vis) o o A 1000 A Molecular transitions vibrational (NIR+ rotational (FIR) MIR) 1µm 100µm Spin orientations electrical spin (µwaves+ radio waves) magnetic spin(radio waves) 1mm 100m

10 Radiation photons electrons ions neutrons... Matter electron { core valence nucleus molecules Magnetic field...

11 Radiation Matter Cross section Population Time Selection rules

12 Cross Section Interaction between radiation and matter has an efective area of interaction, called cross section (σ). The higher the interaction between radiation and matter, the higher the cross section. High cross section processes present higher interaction and small differences are more clearly observed.

13 Population If both energy levels, the upper and the lower present the same population, then absorption and emission have the same probability and any spectroscopic transition is not seen. The ratio between the population of any two energy levels separated by E is given by the Boltzmann distribution: N upper /N lower = exp (- E/kT) E [J] k [JK -1 ] T [K]

14 Time Excitation time s Relaxation time - UV/Vis: s IV: s NMR: s Fluorescence: < 10-8 s Phosphorescence: > 10-8 s

15 Selection rules Intrinsic to the quantum theory. Indicate which transitions are allowed and which transitions are forbidden. IV: ν = ± 1 (ν=0 ν=1)

16 Introduction to Spectroscopy The four most common spectroscopic methods used in organic analysis are: Method Abbrev. Energy used Units Ultraviolet-Visible Spectroscopy UV-Vis ultraviolet-visible nm Infrared Spectroscopy IR infrared µm or cm -1 Nuclear Magnetic Resonance NMR radio frequencies Hz or δ Mass Spectroscopy Mass Spec electron volts amu Question: What actually happens to the sample during an analysis? {How do the sample and energy interact?}

17 Matter/Energy Interactions What happens when a sample absorbs UV/Vis energy? π* excitation of ground state electrons UV/Vis (typically π and n electrons) sample ( 200 nm) E electronic increases momentarily π π π* transition What happens when a sample absorbs IR energy? stretching and bending of bonds (typically covalent bonds) E vibration increases momentarily -O-H IR -O H ( 3500 cm -1 )

18 Radiation Matter Interaction What happens when light interacts with a molecule? Absorption Transmission Emission Scattering Reflection Refraction

19 Radiation Matter Interaction Absorption: Transition from a lower energy level to a higher energy level by energy transfer from the incident radiation. Emission: Transition from a higher energy level to a lower energy level by energy emission from the molecule. Scattering: Light dispersion due to interaction with matter. May occur energy transfer.

20 Absorption

21 Radiation Matter Interaction Transmission: Light fraction not absorbed, reflected, scattered or refracted. A+R+D+T = 1 Absorption: Transition from a lower energy level to a higher energy level by energy transfer from the incident radiation. T (%) = (I/I 0 ) * 100 A = - log 10 (I/I 0 ) = - log 10 (T) A = log 10 (1/T) (A - Absorbance)

22 Radiation Matter Interaction Scattering: Change of light direction due to matter interaction. Energy transfer can occur. Rayleigh λ -4 (Ø << λ) Mie λ -2 (Ø ~ λ) Tyndall λ (Ø >> λ)

23 Radiation Matter Interaction Emission: Transition from a higher energy level to a lower energy level by energy emission from the molecule. hν hν Absorption / Emission

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25 XPS X-Ray X-Ray Auger e - Absorption Fluorescence e - Auger FL FL FL FL

26 E (J) UV/Vis IV Raman E=1 E=0

27 Absorption Beer`s law (I) Transmission 100% 50% C 100% 25% 2C 100% 100% 50% 25% 3C 12.5% 100% 4C 6.75% 100% 5C 3.125% Concentration

28 Absorption Beer`s law (II) T = I/I 0 = e -αx A = log 1/T = log I o /I = εcx α absorption coefficient [cm -1 ] ε molar absortivity [dm 3 mol -1 cm -1 ] c concentration of absorbent specie [mol dm -3 ] x optic path [cm]

29 Absorption Beer`s law (III)

30 Two-Component Mixture Example of a two-component mixture with little spectral overlap

31 Two-Component Mixture Example of a two-component mixture with significant spectral overlap

32 Beer`s law The absorbance at any λ of a mixture, is the sum of the absorbencies of each component at that λ. Usually one measure the absorbencies at a number of λ = to the number of components, at the λ of maximum absorbance of each component. Ex: A A λ 1 ( x+ y) = A λ 1 x + A λ 1 y λ 1 x l λ cx y 1 =ε +ε λ 2 λ x y A 2 λ x A 2 λ 2 λ y x lc 2 ( + ) = + =ε x+ε y After calibration with concentration standards of the pure components, at the selected λ, one can determine c x e c y. lc lc y y