CHEM 156 Lecture Notes - Lecture 17: Molecular Electronic Transition, Emission Spectrum, Stokes Shift

• School: UCLA
• Department: Chemistry and Biochemistry
• Course: CHEM 156
• Professor: Yeates Todd

Document Summary

Molecules can exist only in discrete energy states and transitions between states can be driven by absorption or emission of electromagnetic radiation (photons) if energy of photon matches energy difference of transition. Elationship (cid:271)et(cid:449)een f(cid:396)e(cid:395)uen(cid:272)y o(cid:396) (cid:449)a(cid:448)elength of (cid:396)adiation and ene(cid:396)gy is e=h(cid:448)=h(cid:272)/ . Simple molecules like single atoms typically show very sharp absorption/emission bands: undergo transitions at only very narrow wavelengths, discreteness of their spectral properties reflects simplicity of their allowable energy states. Complex molecules have complex spectra: presence of multiple atoms introduces dependence of energy on nuclear positions, nuclear motions give rise to vibrational energy states, energy differences between vibrational states are smaller than those between electronic states. Acc. to boltzmann equation, probability of molecule in excited electronic state rather than ground electronic state is essentially zero. At ordinary temperatures and unless excited, molecules generally populate almost exclusively the lowest vibrational state of lowest electronic states: high probability transition requires the initial energy state to be well-populated.