1 Spectroscopy Introduction Lecturer: Somsak Sirichai Chemistry department, Burapha University

2 Spectroscopy involve primarily in measuring emitted or absorbed electromagnetic radiation (ER) by analyte concern with the interaction of electro- magnetic radiation (ER) and matter

3 What is ER? type of energy that is transmitted through space at enormous velocities

4 Wavelength: the physical distance in the direction of propogation over which the electric and magnetic fields of a photon make one complete oscillation.

5 The relationship between the energy of photon, wavelength, and frequency: E = the energy of photon h = Plank ’ s constant v = the frequency ( number of flips, or oscillations, that occur in one second ) (1)

6 Frequency,, is related to wavelength, and velocity of propagation, c by the expression (2) (in air or vacuum)

7 (1)+(2): (3) (4) = wavenumber (cm -1 ) (5)

8 Units for wavelength 1 nm = m = cm = 10 A 1  m = m

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10 Type spectroscopy Usual wavelength range* Usual wavenumber range, cm -1 Type of Quantum Transition

11 What is the matter? Matter is composed of molecules or atoms. Physical forms: gas liquid solid

12 Absorption Spectroscopy

13 Terms employed in absorption spectroscopy Transmittance (T) Absorbance (A) Molar absorptivity  Absorptivity (a)

14 b P0P0 P Absorbing solution of concentration c

15 Transmittance (T) (6)

16 Absorbance (A) The absorbance (A) of a solution is defined by the equation: (7)

17 Absorptivity (a) and Molar absorptivity  Absorbance is directly proportional to the path length the solution and the concentration c of the absorbing species. (8) a = constant called the absorptivity

18 Units of absorptivity b (cm) & c (g/L)  a (L g -1 cm -1 ) b (cm) & c (mol/L)   (L mol -1 cm -1 )  = molar absorptivity

19 Relationship between A and %T A = 2 log %T Derive:

20 Practice questions 1: ความเข้มข้นของลำแสงอันหนึ่งลดลง 20% เมื่อผ่าน absorbing medium ที่มีความหนาของเซล 1.00 cm ความเข้มข้นของแสงจะลดลงกี่เปอร์เซ็นต์ ถ้าผ่าน แสดงนี้ไปยัง absorbing medium ชนิดเดียวกัน โดย มีความหนาของเซลเป็น 5.00 cm

21 Practice questions 2: ค่า molar absorptivity ของสารตัวหนึ่งมีค่า 1.10 x 10 4 จงคำนวณค่า A (absorbance) และ %T เมื่อผ่านจากเซลที่มีความหนา 0.50 cm ของสารละลายที่มีความเข้มข้น 3.00 x M

22 Next lecture: Quantitative Law of Radiation Absorption Lambert ’ s Law Beer ’ s Law Molecular Ultraviolet-Visible Spectroscopy