18.1Photons Photon energies - Interactions of light with matter. Ionizing radiationRadiation damagex-rays Compton effectX-ray diffractionPhoton EnergyFind the energy of a photon with a wavelength of 500 nm. Use units of electron volts (1eV= 1.60x10-19 J)cEhfh==λ834 1993x10 m/ s6.63x10 Js 4.0x10 J500x10 m−−−⎛⎞==⎜⎟⎝⎠An electron volt is the energy change in moving an electron across a potential of 1 volt.A few electron volts is the energy of electrons inmolecules. This is why visible light is absorbed bymolecules.19194x10 JE 2.5eV1.6x10 J / eV−−==Photon Energies (eV)E=hf =hc/λ1 10310610-310-610-9eVuvirPhoton conceptEphoton=hfLight energy is has specific energies i.e. is quantized in the interaction of light with matter.Photons of different wavelengths, interact with different molecular properties- either in the generation of the photon or the absorption of the photon.sourceabsorber∆E=hf∆E=hfPhotonsX-ray imagingX-ray diffractionElectrons scattering, tightly bound electrons104eV1 nmX-raysRadiation damageSkin cancerbonding electrons4-5 eV200-300 nmUltravioletVision PhotosynthesisPhotographyLow energy electrons (pigments)2-3 eV400-700 nmVisible Heat lampMolecular vibrations10-1 eV10-5InfraredMicrowave oven cell phoneMolecular rotations10-4 eV10-2microwavesNMR imagingnuclear magnetic 10-7 eV10radioapplicationsMolecular interactionsTypical Photon energy(eV)Typical Wavelength(m)lightMicrowave Ovenf =10GHzhf =4x10-5 eVHeats by exciting molecular rotations andvibrations. Cell phones use the same frequency rangebut at much lower intensity.2• UV radiation – 4-5 eV photons cause radiation damage by altering DNA• Ozone in the atmosphere protects by absorbing UV radiation.Radiation damage to DNAUV light Altered DNALong exposure to low intensity uv can be harmful. e-hfhfhftungsten lampfluorescent bulblight emitting diodeblack body radiation of a heated metal filamentHgemission of excited atomsemission by electronacross an energy gapin a semiconductorsemiconductorSources of visible lightFluorescent bulbs and LEDs are more efficient sources of light thanincandescent lamps.The tungsten lamp is inefficient as a light source. Most of light is in the infrared regionvisirX-raysX-rays are produced by electrons accelerated throughhigh voltages V~ 104 VPhoton energies ~ 104eVGeneration of x-rays by electrons3X-ray source Rotating anodee-X-raysX-raysmaxminhceV hf∆= =λA Quantum Effect.The maximum photon energy is ~ equal to the kinetic energy of the electron. X-ray spectrum producedby 35 keV electrons hitting amolybdenum target.λminQuestionFind the minimum x-ray wavelength for a 35 kev electron.e-KE= 35x103eV x (1.6x10-19J/eV) = 5.6x10-15JhcEhf KE== =λhcKEλ=34 811156.63x10 Js(3.0x10 m/ s)3.6x10 m5.6x10 J−−−==hf=KE(neglect work function-it is small compared tox-ray energies)KEphotonE=hf0.036 nmx-ray imagingx-ray photographof Wolfgang Roentgen’swife’s hand.x-rays penetrate soft tissue (light atoms) but are absorbed by heavymetal atoms. eg.Calcium, GoldThe wavelength of a photon scattered from an electron is shifted due to loss of photon energy.Compton scattering of x-rays.High energy photons knock electrons out light atoms such as carbon.oeh(1 cos )mc∆λ = λ − λ = − θөThe Compton effect shows the particle natureof light.Incident photonelectronScattered electronScattered photonX-ray diffraction• X-rays have wavelengths close to atomic dimensions• Crystalline solids have an ordered array of atoms that scatter x-rays much like a three-dimensional diffraction grating• The x-ray diffraction pattern from crystals of molecules can be used to determine the density of scattering electrons (i.e. the electron density) and thus the molecular structure.4NaCl Crystal – an ordered array of atoms0.56 n Fig. 27-11, p.883Diffraction of x-rays from a crystal.Each atom acts as a wave source.X-ray diffraction pattern of NaClCondition for diffraction of x-rays from planesof atoms in a crystalBragg’s Law2dsinө = mλm=1, 2, 3 ........X-ray diffraction pattern from a crystalline fiber of DNA. WatsonAnd Crick used this data to deduce the structure of the DNA moleculeDNA structure determined by x-ray