chap6 instr - CHAPTER6 EM RADIATION AND MATTER @665 cm...

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Unformatted text preview: CHAPTER6 EM RADIATION AND MATTER @665 cm Kama? Virtually all our knowle ge about matt r comes from the interaction Of matter (moleculos, atogng 'ons,...) Withrelectromagij‘é" c (EM) radiation (light, X-rays, radio waves, microwaves,...). r ‘I ’ radio waves; erowaves,...) with matter (meleeules, atoms, ions,...). 70%: 7a 1594:? ccording to Maxwell light is a propagating electromagnetic wave ' 1831—1879. -_ characterized by a wavelength )L or frequency v, related by WL= c, and a polarization. firm» 5'. 65am;rm; 0n ééoeez flooé . . . E Electric field / I‘— A —>| Propagation direction Ex H \ Magnetic field _ ‘ L. A single-frequency EM wave exhibits a sinusoidal 1331487 ‘ variation of electric and magnetic fields in space. t EM radiation is a transverse wave because the electric (E) and magnetic (H) fields are perpendicular to each other, and both are perpendicular to the direction (E x H) of propagation. 45 QM» S. @nmym; CHEM 4369 E Electric field Electric field / Propagation direction EXH \ Magnetic field Magnetic field - An EM wave, unlike a wave on a string or a sound wave, does not require a medium in which to propagate. EM wave can travel through a vacuum or a material substance, since electric and magnetic fields can exist in either one. 44; fimu S. @nmym; CHEM 4369 756% 7d a adaptation... ight is the oscillating EM field that interacts with the electrically charged nuclei and electrons of a molecule and results in energy transfer to the molecule. %MM 5'. Egrzmym; Wavelength = distance per cycle A1 = 2 = 4 its crest and Frequency Trough —-i 12 l"— The wave nature of EM radiation is described by two intwdqoendent variables: —-i"-si-— ' Wavelength (A) is the horizontal distance between two successive crests, two successive troughs, or any two successive equivalent points on the wave. Puma“ S. figmaexaflm; Wavelength = dlstance per cycle Wavelength Wavele ngth of? St and Frequency _..i 12 |___ Trough The wave nature of EM radiation is described by two intwdqomdmt variables: «new v1 :12 v2 = 11'4 «'3 Frequency = cycles per second ° Frequency (1/) is the number of times per second that a crest passes a given point on the propagation axis. Note that as 2! decreases, 1/ increases, and vice versa. 43 23mm» 5'. fignmflmz CHEM 4369 Wavelength = dlstance per cycle Wavelength Wavele ngth 7’1 x Frequency Ht! —“‘ = Velocity v1 = 1.12 ‘\'2 =1I4N'3 vi = 02 = :23 Frequency = cycles per second - The product of frequency and wavelength of the EM wave is a constant that defines the EM radiation velocity (distance per sec). This constant is medium dependent!!! 440 25mm“ 5'. €5r:m;em; CHEM 4369 Wow cm 57% Pediatrician... . . - Higher Lower . The amplitude amplitude is related to the (brighter) (dimmer) amplitude of the wave. «- The amplitude of the EM wave is a measure of the strength of its electric and magnetic fields. Wavelength, it 0 Light of particular shade of red, for instance, always has the same frequency and wavelength, but it can be dim (low amplitude) or bright (high amplitude). 4-11 firm”: 5'. @nmym; CHEM 4369 The Speed of Light - All EM radiation has a fixed characteristic speed when measured in vacuum, 0 = speed of "9“ .0 = 299 792 453 ms-1 5 : Qifl/ z 3.00x108 nas—l Note: in any other medium, the velocity of light = speed of lightl refractive index of the medium o The fixed speed of light dictates that... o light of higher frequency will have shorter wavelength - light of lower frequency will have longer wavelength %me 5'. @ezMflW; CHEM 4369 Light Velocity and Medium - In any other medium, the EM waves travel at a speed that is less than c. Refiactive index 7) = / ofthe mediune - When light passes from one medium to another of different 11, the value of 1/ does not change, but the value of A changes to accommodate the different value of v. QM» S. @nmym; Light Wavelength and Medium From A: DIV: CIVI'V, we can derive = 7’1 [772 since... - Frequency, v, is determined by the radiation source and is independent of the medium. The wavelength, A, and velocity, 7), change with the medium 27. Elm» S. egezmym; )1. = 589.0 nm 1; = 3.00x1o‘3 ms-1 1; = 3.0mm8 ms“ v=5.09x1o” Hz v=5.09x1014 Hz All“, 111 =1.0 Air, 731 = 1.0 A = (539.0 nm) x (1.0115) = 392.7 nm 1!:(3.00x103 ms-‘)/(1.5) = 2.0x103 ms“ 7; v: (3.00x108 ms'1)I(589.0x10'9 m) = 5.09x1o14 Hz - Radiation wavelength shortens as the light passes into medium of higher refractive index. 4-15 fixed» 5'. @nmng; CHEM 4369 740w 70ml d Ddéemma... In order to explain... blaCkbOdy radiation — the light of changing intensity and wavelength emitted from hot, dense objects the photoelectric effect — the emission of electrons from the surface of metals when light strikes it :2) atomic SpGCtra — the individual colors emitted from thermally or electrically excited atoms of elemenrtvs ...an additional property of light MP»: must be Introduced. §ma S. @nmym; CHEM 4369 BIaokbody Radiation and the Quantization of Energy {According to Planck hot, glowing objects could emit (or absorb) only certain quantities of energy (quanta): y . . Radiation frequan E = n h "V Quantum \ number Proportionality “Max Planck 1 3 . . . constant «3358—1947 ‘ ( ’ 2’ ) ‘ fima S. @nmym; So, Wéez‘ 7e (fie Paint? - The hot object’s radiation is emitted by the atoms contained within it. 0 If the atom can emit only certain quantities of energy, it follows that the atom itself can only have certain quantities of energy. 0 Thus, the energy of an atom is quantized: it exists only in certain fixed quantities, rather than being continuous. - Planck believed that this quantization applied only to the absorption and emission of energy by matter, not to electromagnetic waves themselves. . However, it turned out to be much more Manama t ‘ general than he could have imagined. ‘43 858—1947 fima S. @nmym; CHEM 4369 flgam, Wéet w 159%? j cording to Einstein light of frequency v is a collection . of discrete packets of energy (photon: or quanta), Albert Einstein '\ 187 9—1 955 each packet (photon) containing an amount of energy E that is 4 fim» S. @nmym; directly determined by v. CHEM 4369 The Photoelectric Effect and the Photon Theory of Light Ephoton = hv = My II The frequency of light must match the threshold value for emission of the electron. Evidence for the existence of photons comes from the photoelectric effect, the ejection of electrons from the surface of metals by high-frequency electromagnetic radiation. fima S. @nmym; Incoming Ifi‘t/ l1ght .\_\ Evacuated "a '16-) Electron — freed from metal surface Light- # . sensitlve fl metal plate Positive // electrode ++ —+ Current / meter Battery I Figure 6.13 CHEM 4369 Energy of a Photon Plank’s constant h = 6.626x10*34js Speed of light \ / E = hfv = hell. = hch / /" /" Frequency Wavelength Wavenumber J The higher the frequency (shorter wavelength) of the light, the larger the photon energy. J A {tiny source of light emits relatively few photons. J A bright source of light emits a dense stream of photons. 4-21 fireman S. @nmirm; CHEM 4369 756% 7d a eeeeect‘c'ae... In considering the interaction of My radiation with matter, the smallest quantity _ of energy that may be involved is a single quantum. J Energy changes may occur only by absorption or emission of whole quanta, i.e., AE = niw, where n is an integer. 4-22 fixed» 5'. fignmxeflw; CHEM 4369 U13” .' Relationship Symbol Frequently Used Units AV: 6 A = wavelength nm (nanometer, 10'9 m) A (Angstrom, 10‘10 m) ,u (micron, 10'6 m) my (millimicron, 10‘9 m) v = frequency Hz (hertz, 1 Hz 2 1 oscillation per second), 5'1 17: wavenumber cm‘1 E = energy J (1 joule = 1 kg m2 5'2) cal (calorie, 1 cal: 4.184 J) erg (1 erg = 10‘7 J) at (1 electron volt = 1.6022 x 10-19 J) h = Planck's constant 6.662 x 10—34 J 5 4-23 $54M»: 5'. @nmirm; CHEM 4369 - Monochromatic radiation — consists of a single frequency (e.g., lasers). - Polychromatie radiation — contains a variety of frequencies (e.g., sun, light bulb). o Electromagnetic spectrum — a continuum of frequencies over a wide range. 4-24 QM»: S. @nmym; CHEM 4369 Electromagnetic Spectrum 8.... (in magnetic field) lnner Shell (K, L} Molecular Electrons (EPFi) Nuclear Electrons Vismle HOiatiGnS Nuclei (NM Ft) r—A—\ r—‘-—\ . r—H r—-—4—-s Cosmic Rays 3: —Fiav5 X —Flay3 Ultraviolet l I Infrared Microwave Radio Ir.th 1.GDx1El“2 ‘l.DEIx1El"° 1.00::10'“ [3.30 iconic-l 3.oox10-" 1.00 \'(HZ) 3.oox10zfl 3001:10la 3.00110": {7.89 3.54}x10” 1.ovox10‘2 3.00on21 E(Jirnoie] 1.2;:10“ 1.2x109 1.2x10" rat 1.531105 4.00 TRANSITION or ENERGY INVOLVED menone Fundamental 1.000.000 SUDUU 25.300 12.800 3.333 33 33.3 a; fl—a a—/ +4 Middte Shell Valence Molecular Molecular Electrons Electrons Vibrations Rotations OPTICAL SPECTROSCOPY deals with a central portion of the electromagnetic spectrum, Spanning the infrared {IR}, visible (VIS) and ultraviolet ( UV) wavelengths a) instrumental requirements are similar in) materials used for dispersing, focusing and directing the radiation are conventionaiopticai materials (glass, quartz. or alkali crystals} ATOMIC SPECTROSCOPY MOLECULAR SPECTROSCOPY deals with spectcchemical phenomena involving deals with optical rneasureme nts of molecular free atomic species, usually in the vapor state species, in the vapor, solution or solid states am“ 5. agemgam; CHEM 4369 Interconverting Wavelength and Frequency PROBLEM: A dental hygienist uses X—rays (it = 1.00 A) to take a series of dental radiographs while the patient listens to a radio station (it = 325 cm) and looks out the window at the blue sky (A = 473 nm). What is the frequency (in 3'1) of the electro- magnetic radiation from each source? (Assume that the radiation travels at the speed of light, 3. 00x1 03 mis.) PLAN: Use c = )w SOLUTION: 3 108 / Eyelength in units at; 1.00 A: v=1JS1—Orq(:n E3X1018 . x ‘ 1A=10‘1°m 1 cm =10'2m 1nm=10‘9m 3 ‘lOBmlS I 325 cm: v: x—_2 E923x‘lO7I-H Eavelength in! 325X10 m v=CDL 8 3x10 m/s . = — 14 Equenmsa orig 473nm. v 473X1 O_9m [6.34MB rag firm“ 5. @HMfiW; CHEM 4369 Calculating the Energy of Radiation from Its Frequency PROBLEM: A laser emits blue and red radiation with a frequency of 6.4x1014 Hz and 4.4x1014 Hz, respectively. What are the energies of one photon ofthese blue and red radiations? PLAN: We can use the photon energy equation directly. SOLUTION: E = IN E: (6.626x10-34J.s)(6.4x1014s—1) [ammo-ii E: (6.626x10-34J.s)(4.4x10143—1) [2.9x10-1i "were is more energy in one photon of the blue light than in one photon of the red light. Ham» 5. @hmfiw; CHEM 4369 Calculating the Energy of Radiation from Its Wavelength PROBLEM: A cook uses a microwave oven to heat a meal. The wavelength of the radiation is 1.20 cm. What is the energy of one photon of this microwave radiation? PLAN: After converting cm to m, we can use the energy equation, E = [W combined with v: c/A to find the energy. SOLUTION: E = hall 6.626x10-34J.s 3x108m/s . = -<___A_>. [mm—2; 1.20x10-2m am“ 5. agnmgaw; CHEM 4369 Calculating the Number °f Ph°t°"s Emitted amps .. «It by the Laser Pointer PROBLEM: In 1.0 s, the He-Ne laser pointer gives out 1.0 mJ of energy (or 1.0 mW output power) in the form of red beam of light of wavelength 632 nm. How many photons of red light does the laser generate in 1.0 s? SOLUTION: Divide the output power by the energy of one photon (E = hem. (E:‘:.626><1O‘34 J-s/photon)(3.00x108 mls) 6.32x10-7 rn 1.010‘3J- /1.0 - # of photons = w)- E_2x1015 -_=-.i:. 1 .345x10‘19 J/photon E = 1.345X10‘19J/photon This means that when I turn on my laser pointer, it generates more than 1015 photons of red light each second. fime s. ggfimjfw; CHEM 4369 What happens when light interacts with a molecule? Scattering and photoluminescence Absorption along radiation beam Transmission ( as Absorption as Emission (photoluminescence) as Scattering 4.30 am“ 5'. @nmme, CHEM 4359 Energy Transfer Diagrams EXCITED a molecule is said to be in an excited state STATE { when it occupies other (higher) energy |evel(s) TRANSITION a transition is said to occur when MOMENT { the energy of a molecule changes from one state to another the state corresponding to the lowest possible energy level which the system .. .. .'... Gg-I-oAl-Jrlén may occupy and in which more and more molecules are found as the temperature is Exc'tatlon Deactwatlon reduced towards absolute zero induced spontaneous (stimulated) or stimulated quantum jump quantum jump - A particular molecule can exist in a variety of electronic, vibrational, rotational, etc, energy states and can move from one state to another only by sudden jump involving a finite amount of energy (quantization of the energy). 4-31 fima S. @nmym; CHEM 4369 Nonradiational Excitation Emission or hv Chemiluminescence first excited state ’1" *V\\//'/v ’1" .heat W Sample ' electrical energy r / \ I} the p 05983 ht; -chemical reaction EMISSION { ofphoron . being emitted ground state Emission spectroscopy Chemiluminescence refers to spectral information that results is the emission that results from nonraaiationai activation process from species excited by (e.g., from heat, electrical discharge, etc). chemicai reactions. s Q Spectrochemical information is provided by measuring the amount of EM radiation emitted by the species as it returns to the ground state from an excited level ( relaxation processes & excited state lifetimes—kinetic informtion) 4-32 fimu S. @nmym; CHEM 4369 Flame tests Strontium 38Sr '1: ' _- 1” 1 .Fireworks ¢ Some elements emit light of a distinctive color when their compounds are heated in a flame or their vapors are exposed to an electric discharge (a storm of electrons and ions passing between to electrodes). fime S. @nmym; CHEM 4369 The Line Spectra of Elements $31.1 rm 4 ID.1 nm 486.1nm 656 3 nm - The series H of emission 400 ' 5a ' ' 'aao at different wavelengths is called a Gas discharge line mm spectrum. A Visrble l',._ mm] . . , : , . - — . -. I . . 400 600 550 The line spectra of different elements contain different lines. : . I . . 450 500 550 '3. 700 750 mm QM» S. @nmym; CHEM 4369 Emission Spectrum of Atomic Hydrogen Three series of spectral lines of atomic hydrogen Ultraviolet Visible Infrared series series series A (‘13 r A J I ‘ . I 0 200 400 600 800 1000 1200 1400 1600 1800 2000 nm The wavelength of the emission can be calculated by the equation: i A Rydberg equation for the visible series, :11 = 2 and n2 = 3, 4, 5, R is the Rydberg constant = 1.096776x107 m’1 Johah' ydberg' 1854—1919 am» 5. agemgam; CHEM 4369 0 Lines: arise from individual atoms in a gas phase (e.g., a Bseilp electronic transition at «630 nm in Na) 9 Bands: gaseous radicals or small molecules (vibrational structure of the electronic transition) 9 Continuum: atomic and molecular oscillations excited in the condensed solid (black-body radiation: heated solids are important sources ofIR, Vis, and UV ‘ ' ' ' ' ‘ ' ' ' excitation radiation) 4-36 fime S. @flméffmg CHEM 4369 Absorption of Light AE = h Incident W V” Transmitted light light Sample - A photon of the right energy can be absorbed by the molecule causing a jump from one energy level to another. The energy is thereby transferred to the molecule. - This results in a reduction in the intensity of the electro- magnetic radiation transmitted by the sample (e.g., UV-Vts, IR, microwave spectroscopy). - Deactivation of excited molecule may (photoluminescence) or may not produce emission (heat release; kinetic energy). 4-3? fireman S. @nmym; CHEM 4369 UV—V 5 Absorption Spectrum s....,,....w absorption spectra la) Na vu or 0 UV and Vis radiation has p ” sufficient energy to cause I I_ A I transitions of the valence W 3:13“ mp0?” electrons only. 9 In atomic spectra, excitation can only occur by an electronic process (e.g., a doublet at 589.0 and 589.6 nm of Na vapor arises from excitation ofthe 35 electron to two 3 p states that dtffer only slightly in energy). I 8 In molecular spectra, excitation may involve electronic, vibrational, and a rotational transitions. ' 1:3...c....,..h. iii? 4-38 am“ 5'. @nmym; CHEM 4369 Absorhancc Photoluminescence The emission of light from Transmitted excited states produced by ' light absorption of photons — radiational excitation Emitted light - the frequency of the emitted photon may be the same as the frequency of the incident photon (a) (a) mo = hvg W W Resonance fluores C8115 CB radiationless decay coiiisions Kinetic energy am» 5. @HMflW; CHEM 4369 PilUiUillilliilajlj. . The emission of light from Incident Transmitted excited states produced by light light absorption of photons — radiational excitation Emitted light - the frequency of the emitted photon may be the same as the frequency of the incident photon (a), or it may be different (b). (a mo = hve hl’o i ) (b) : collisions 6) Fluorescence W W ® Phosphorence radiationless decay commons The type depends on the excited state. Kinetic energy fima S. @nmym; CHEM 4369 Three Types of Photoluminescence Spectra 0 Excitation Spectrum: 6 E=excitation is obtained by measuring 0 Q P fluorescence luminescence intensity at ' fixed it while the excitation h is varied (~ absorption spectrum). phenanthrene AE<AF<AP 9 Fluorescence Spectrum: excitation is brought about by absorption of photons, and the electronic transition responsible ' ' J ’I’ for fluorescence do not involve mu m sun a change in electron spin wa“"'““”"“"‘ Relative intensin 9 ' : excitation is brought about by absorption of photons, and a change in electron spin accompanies phosphorescence 4.41 am» 5'. @Hmym; CHEM 4369 Scattering of Light John William'strutt '- ' ‘ I :fLord Rayleigh _:- C. V. Raman '._ 1842—1919 ' - 1842—1919 - Redirection of light due to its interaction with matter. May or may not occur with transfer of energy, i.e., the scattered radiation has a different or the same frequency. 4-42 fima S. @nmpm; CHEM 4369 Scattering of Light ELASTIC no change in the frequency of the incident photon; change in direction only RAYLEIGH ,— I . RAD'AT'ON qnnn William‘stmtt' particle size < AD v :;Lol;d4gayée§h othenvise: reflection or refraction " ' - Redirection of light due to its interaction with matter. May or may not occur with transfer of energy, i.e., the scattered radiation has a different or the same frequency. 4-43 fima S. @nmpm; CHEM 4369 Most popular: hw=hw INELASTIC (RAMAN) the frequency of the scattered light is modified by molecular energies hvo > has STOKES 1)) ANTISTOKES RADIATION RADIATION ' c. v. Raman temperature controiied 18427191 9 ° Redireton of light due to its interaction with matter. May or may not occur with transfer of energy, i.e., the scattered radiation has a different or the same frequency. 4-44 fixed“ 5'. @nmym; CHEM 4369 CHEM 4369 A Scattering (Raman) Spectrum 0 Rayleigh Radiation: CC14 @ 433-0 11m = 20,492 cm‘l excitation photons neither gain nor lose energy after interaction Stokes With CCI4 molecules. Raman 9 Stokes Radiation: excitation photons lose quanta of Vibrational energy to CCI4 molecules in the interaction process. ant i—Stokes Raman Intensity —» Rayleigh (attenuated) 6 Anti-Stokes Radiation: excitation photons gain quanta of vibrational energy from CCI4 . l . I molecules in the interaction '20“ 0 20" process Raman shift (cm 1) Intensity: Rayleigh >>> Stokes Raman > anti—Stokes Raman 4.45 am“ 5'. @«mam, CHEM 4359 Reflection and Refraction of Light hv/’ lg: do not involve a change in the frequency of the incident photon Refraction Reflection [[33 particle size >> A CHEM 4369 The path of light through the solution is invisible (right), while the beam is easily visible \. - through the colloid (left) as - . - the particles reflect (scatter) 182°“ ‘ light (the Tyndall effect). We 5'. @Hmdyade; CHEM 4369 ...
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chap6 instr - CHAPTER6 EM RADIATION AND MATTER @665 cm...

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