JABLONSKI DIAGRAM INTERACTIONS BETWEEN LIGHT AND MATTER LIGHT AS A WAVE LIGHT AS A PARTICLE 2/1/16. Photoelectric effect Absorp<on Emission ScaDering

Transcription

1 INTERACTIONS BETWEEN LIGHT AND MATTER LIGHT AS A WAVE Diffrac<on Refrac<on Transmission Reflec<on ScaDering Polariza<on LIGHT AS A PARTICLE Photoelectric effect Absorp<on Emission ScaDering JABLONSKI DIAGRAM 1

2 JABLONSKI DIAGRAM TRANSITIONS Electronic excita-on- promo-on of an electron to an excited state (electronic, vibra-onal, rota-onal). S 0 à S 1 Nonradia-ve decay (vibra-onal relaxa-on)- vibra-onal energy transferred to other molecules through collisions. Very fast. Excited state à S 1 ground vibra-onal state Fluorescence- emission of photon to return to S 0. S 1 às 0 +hν Internal conversion- radia-onless transi-on to an extremely vibra-onally excited state of S 0 without a change in energy. S 1 às 0 Intersystem crossing- radia-onless transi-on from S 1 to T 1 with no change in energy. Change of electron spin. S 1 àt 1 Phosphorescence- emission of photon to return to S 0. T 1 às 0 +hν A SIMPLE ABSORPTION EXPERIMENT Beer s Law T= transmission P 0 = incident power P= transmided power A= absorbance ε= molar absorp<vity b= path length C= analyte concentra<on Concentra<on rela<ve to mixing direc<ons

3 SOURCES OF NONLINEARITY OF BEER S LAW 1. Solu<on factors 2. Non-monochroma<c light 3. Not analyzing at λ max 4. Stray light 5. Mismatched cuvedes 6. Instrument noise Too much or too lidle absorp<on Absorbance (arb) [Kool-aid] DERIVATION OF BEER S LAW T= transmission P 0 = incident power P= transmided power A= absorbance ε= molar absorp<vity b= path length C= analyte concentra<on 3

4 COMPONENTS OF OPTICAL INSTRUMENTS CHEM 314 SKOOG N HOLLER CH 7 OBJECTIVES State the components and phenomena that can be probed with op<cal instruments. Recall the methods of wavelength isola<on Diagram, label, describe, and compare prism- vs diffrac<on-based monochromators State and be able to perform calcula<ons related to mono performance characteris<cs and λ dispersion. Recall UV-Vis detectors Diagram, label, describe, and compare the following detectors: Vacuum phototube, PMT, silicon diode 4

5 OPTICAL INSTRUMENTATION Phenomena probed Absorp<on Luminescence Emission ScaDering Components 1. Stable radia<on source 2. Transparent sample holder 3. Wavelength isola<on 4. Detector 5. Signal processing BUILDING A SPECTROSCOPIC INSTRUMENT 5

6 BUILDING A SPECTROSCOPIC INSTRUMENT Components 1. Stable radia<on source 2. Wavelength isola-on 3. Transparent sample holder/ op<cs 4. Detector 5. Signal processing This lecture will focus on common components of instruments for atomic and molecular spectroscopies SOURCES Why does this chart differen<ate between line and con<nuum sources? When would you use a line rather than con<nuum source? 6

7 OPTICS SAMPLE CUVETTES Absorbance Quartz or plas-c? 4 Quartz Plas<c Wavelength (nm) 7

8 BUILDING A SPECTROSCOPIC INSTRUMENT Components 1. Stable radia<on source 2. Wavelength isola-on 3. Transparent sample holder/ op<cs 4. Detector 5. Signal processing WAVELENGTH SELECTION 8

9 MONOCHROMATOR BANDWIDTH Mono slit width determines spread of λ incident on sample (bandwidth) Wide slits allow More light (higher throughput) More λ (larger bandwidth) Image incident on mono exit plane No such thing as a free lunch BANDWIDTH MEASUREMENTS 9

10 EFFECTIVE BANDWIDTH EFFECT OF SLIT WIDTH ON SPECTRAL RESOLUTION 10

11 FILTERS FILTERS 11

12 MONOCHROMATORS 1. Entrance slit- provides rectangular op<cal image 2. Collima<ng lens or mirror- makes light beams parallel 3. Dispersive element- disperses light into component wavelengths 4. Focusing element- reforms rectangular op<cal image focused on focal plane 5. Exit slit- on focal plane, selects desired bandwidth MONOCHROMATOR: PRISMS VS GRATINGS Refrac<on Reflec<on Consider the figures, is or the longer and why. 12

13 MONOCHROMATORS: PRISMS VS GRATINGS When might a prism be beder than a diffrac<on mono? PRISMS WORK BY REFRACTION Snell s law Refrac<ve index 13

14 BUNSEN PRISM LEARNING CHECK Calculate the angle of devia-on of 350, 500 and 650 nm light as it passes through a prism. n 350 = n 500 = n 650 =

15 LEARNING CHECK Calculate the angle of devia-on of 350, 500, and 650 nm light as it passes through a prism. n 350 = n 500 = n 650 = Calculate the distance between these three wavelengths of light on an exit plane placed 4 cm away from the prism. REFRACTIVE INDEX OF GLASS AS A FUNCTION OF WAVELENGTH 15

16 OTHER PRISM GEOMETRIES Cornu Prism LiDrow Prism REFLECTION GRATING MONOCHROMATOR hdps://encrypted-tbn0.gsta<c.com/images?q=tbn:and9gcs53if5b18udb7pvw7texat3q63kqm1qmwvo1pbt5r-uv1axefg0-t4hl0 16

17 ECHELLETTE- DIFFRACTION LONG EDGE LEARNING CHECK 17

18 ECHELLE GRATING LEARNING CHECK Calculate the angle at which the 350, 500, and 650 nm light are reflected off the surface of a diffrac-on gra-ng with 1400 grooves per mm. The incident angle is 20 degrees Compare your results with the prism calcula-ons 18

19 ECHELLE GRATING ECHELLE MONOCHROMATOR 19

20 MONOCHROMATOR PERFORMANCE CHARACTERISTICS 1. Spectral purity 2. Dispersion of gra-ng (D) Reciprocal linear dispersion (D -1 ) 3. Resolving power (R= λ/δλ) 4. Effec-ve bandwidth (Δλ eff ) 5. Light gathering power (F) Focal length (f) 20

21 BUILDING A SPECTROSCOPIC INSTRUMENT Components 1. Stable radia<on source 2. Wavelength isola<on 3. Transparent sample holder/ op<cs 4. Detector 5. Signal processing IDEAL DETECTORS 1. High sensi<vity 2. High signal to noise 3. Constant detector response as a func<on of λ 4. Fast response <me 5. No dark current 6. Signal propor<onal to radiant power 7. Rugged, cheap, simple S = kp + k d 21

22 DETECTORS DETECTORS Figure 7-27 CdS PMT Se/SeO CdSe GaS PbS Si photodiode Thermocouple Golay cell 22

23 DETECTORS Lytle, 1974 DETECTORS 23

24 BARRIER-LAYER PHOTOVOLTAIC CELL VACUUM PHOTOTUBE 1. Photon hits cathode 2. Cathode emits e- that travels through vacuum to the anode 3. Generates a current 4. Converted to a measureable voltage 24

25 PHOTOMULTIPLIER TUBE (PMT) PN JUNCTIONS 25

26 SILICON PHOTODIODE MULTICHANNEL SI-BASED DETECTORS Photodiode array (PDA) Charge Injec<on Device (CID) Charge Coupled Device (CCD) 26

27 MULTICHANNEL SI-BASED DETECTORS Photodiode array (PDA) Charge Injec<on Device (CID) Charge Coupled Device (CCD) MULTICHANNEL SI-BASED DETECTORS Photodiode array Charge Injec<on Device (CID) Charge Coupled Device (CCD) 27

28 COMPARING DETECTOR SENSITIVITY detector λ 1 s 10 s 100 s PMT UV Vis PDA UV Vis CCD UV Vis Harris, Table 19-2 LOOKING AHEAD Monday (Feb 1)- Instrument components (Ch 7) Tuesday (Feb 2)- Experiment 1 Metals Standard Addi-on Calcs Thursday (Feb 4)- Experiment 1 Metals Atomic Spectroscopy Standard Addi-on Due Prelab 2, Experiment 1 Due 28