Chapter 12 Optical spectroscopy and photobiology Spectroscopy electromagnetic radiation the study of absorption emission and scattering of Vibrational spectra report on molecular vibrations excited by the absorption or scattering of electromagnetic radiation Ultraviolet and visible spectra probe the electronic distribution in a molecule and result from the absorption or emission of ultraviolet and visible radiation General features of spectroscopy Three varieties of spectroscopy o Emission spectroscopy o Absorption spectroscopy a molecule undergoes a transition from a state of high energy to a state of lower energy and emits the excess energy as a photon the frequency of the radiation is swept over a range passed through the sample and the frequencies present in the radiation scattered by the sample are recorded an intense monochromatic incident beam is the absorption of radiation is monitored as o Raman spectroscopy photons that collide with the molecules give up some of Anti Stokes radiation Stokes radiation their energy and scatter emerge at a higher frequency direction without change of frequency Rayleigh radiation photons that collect energy from molecules and the component of radiation scattered into the forward 12 1 The intensities of spectroscopic transitions empirical aspects a The Beer Lambert law Beer Lambert law an empirical law that relates the intensity of absorption of radiation at a particular wavelength passing through a uniform sample to the concentration of the absorbing species radiation depends on the wavelength of the incident Molar absorption coefficient b The determination of concentration We can observe the appearance or depletion of a species during a reaction by monitoring changes in the absorbance of the reaction mixture Isosbestic wavelength coefficient of the two species are equal Isosbestic points the wavelength at which the molar extinction invariant points in the absorption spectrum o The observation of an isosbestic point or at least not more than one such point is compelling evidence that a solution consists of only two solutes in equilibrium with each other with no intermediates 12 2 The intensities of transitions theoretical aspects The intensity of a spectroscopic transition depends on a variety of factors including the form of the wavefunctions of the initial and final states of the molecule and the population of the initial energy levels a The transition dipole moment Transition dipole moment determines whether or not an absorption band has a large integrated absorption coefficient and can be driven by the surrounding electromagnetic field In order for a molecule to be able to interact with the electromagnetic field and absorb or create a photon of frequency v it must possess a dipole oscillating at that frequency A transition will be active only if the accompanying charge redistribution is The intensity of the transition is proportional to the square of the transition a statement about when the transition dipole can be non Gross selection rule specifies the general features a molecule must have if it dipolar dipole moment Selection rule zero is to have a spectrum of a given kind Specific selection rule number may occur in a transition a statement about which changes in quantum o Allowed o Forbidden a transition that is permitted by a specific selection rule a transition that is disallowed by a specific selection rule Sometimes occur weakly because the selection rule is based on approximation b Stimulated and spontaneous transitions a transition from a low energy state to one of higher Stimulated absorption energy that is driven by the electromagnetic field oscillating at the transition frequency o If the Einstein coefficient of stimulated absorption is large then the given intensity of incident radiation will induce transitions strongly and the sample will be strongly absorbing Stimulated emission state to undergo a transition to the lower state and hence to generate a photon of frequency v radiation is able to induce the molecule in the upper o Only radiation of the same frequency as the transition can stimulate an excited state to fall to a lower state Spontaneous emission generate radiation and return to the lower state another way by which the excited state could o The coefficients of stimulated absorption and emission are equal o Spontaneous emission can be ignored at the low frequencies of vibrational transitions c Populations and intensities Intensity of a spectroscopic transition depends on the number of molecules that are in the initial state Almost all vibrational absorptions and all electronic absorptions occur from the ground state of a molecule d Linewidths The width of an electron transition results from the simultaneous excitation The shorter the lifetime of a state the less well defined its energy the energy spread inherent to the states of systems of molecular vibrations Lifetime broadening that have finite lifetimes Only if the lifetime is infinite can the energy of a state be specified exactly The shorter the lifetimes of the states the broader the spectral lines Two processes responsible for the finite lifetimes of excited states arises from collisions between molecules o Collisional deactivation o Spontaneous emission Natural linewidth the lifetime broadening resulting from spontaneous emission