Really Basic Optics Instrument Sample Sample Prep Instrument Out put Signal (Data) Polychromatic light Selected light Turn off/diminish intensity Sample interaction Select light Turn on different wavelength select source detect

Really Basic Optics Key definitions Phase angle Atomic lines vs molecular bands Atomic Line widths (effective; natural) Doppler broadening Molecular bands Continuum sources Blackbody radiators Coherent vs incoherent radiation

Really Basic Optics A Sin=opp/hyp  y  /2 3/2  2 90o phase angle /2 radian phase angle

Emission of Photons Electromagnetic radiation is emitted when electrons relax from excited states. A photon of the energy equivalent to the difference in electronic states Is emitted Ehi e Elo Frequency 1/s

Really Basic Optics Key definitions Phase angle Atomic lines vs molecular bands Atomic Line widths (effective; natural) Doppler broadening Molecular bands Continuum sources Blackbody radiators Coherent vs incoherent radiation

Theoretical width of an atomic spectral line

Natural Line Widths frequency Line broadens due Uncertainty Doppler effect Pressure Electric and magnetic fields frequency Lifetime of an excited state is typically 1x10-8 s

Typical natural line widths are 10-5 nm Example: 253.7 nm Typical natural line widths are 10-5 nm

Line broadens due Uncertainty Doppler effect Pressure Electric and magnetic fields

Line broadens due Uncertainty Doppler effect Pressure Electric and magnetic fields The lifetime of a spectral event is 1x10-8 s When an excited state atom is hit with another high energy atom energy is transferred which changes the energy of the excited state and, hence, the energy of the photon emitted. This results in linewidth broadening. The broadening is Lorentzian in shape. We use pressure broadening On purpose to get a large Line width in AA for some Forms of background correction FWHM = full width half maximum o is the peak “center” in frequency units

Line spectra – occur when radiating species are atomic particles which Experience no near neighbor interactions Line broadens due Uncertainty Doppler effect Pressure Electric and magnetic fields Line events Can lie on top Of band events Overlapping line spectra lead to band emission

Continuum emission – an extreme example of electric and magnetic effects on broadening of multiple wavelengths High temperature solids emit Black Body Radiation many over lapping line and band emissions influenced by near neighbors

Stefan-Boltzmann Law Planck’s Blackbody Law = Energy density of radiation h= Planck’s constant C= speed of light k= Boltzmann constant T=Temperature in Kelvin = frequency Wien’s Law As  ↓(until effect of exp takes over) As T,exp↓, 

Really Basic Optics Key definitions Phase angle Atomic lines vs molecular bands Atomic Line widths (effective; natural) Doppler broadening Molecular bands Continuum sources Blackbody radiators Coherent vs incoherent radiation

Incoherent radiation The Multitude of emitters, even if they emit The same frequency, do not emit at the Same time A B Frequency,, is the Same but wave from particle B lags behind A by the Phase angle 

Begin END: Key Definitions Using Constructive and Destructive Interference patterns based on phase lag By manipulating the path length can cause an originally coherent beam (all in phase, same frequency) to come out of phase can accomplish Many of the tasks we need to control light for our instruments Constructive/Destructive interference Laser FT instrument Can be used to obtain information about distances Interference filter. Can be used to select wavelengths

More Intense Radiation can be obtained by Coherent Radiation Lasers Beam exiting the cavity is in phase (Coherent) and therefore enhanced In amplitude

Argument on the size of signals that follows is from Atkins, Phys. Chem. p. 459, 6th Ed Stimulated Emission Light Amplification by Stimulated Emission of Radiation Photons can stimulate Emission just as much As they can stimulate Absorption (idea behind LASERs Stimulated Emission) * o The rate of stimulated event is described by : Where w =rate of stimulated emission or absorption Is the energy density of radiation already present at the frequency of the transition The more perturbing photons the greater the Stimulated emission B= empirical constant known as the Einstein coefficient for stimulated absorption or emission N* and No are the populations of upper state and lower states

can be described by the Planck equation for black body radiation at some T frequency In order to measure absorption it is required that the Rate of stimulated absorption is greater than the Rate of stimulated emission If the populations of * and o are the same the net absorption is zero as a photon is Absorbed and one is emitted

Need to get a larger population in the excited state Compared to the ground state (population inversion) Degeneracies of the different energy levels Special types of materials have larger excited state degeneracies Which allow for the formation of the excited state population inversion Serves to “trap” electrons in the excited State, which allows for a population inversion E pump

Constructive/Destructive interference Laser FT instrument Can be used to select wavelengths Can be used to obtain information about distances Holographic Interference filter. Radiation not along the Path is lost mirror mirror Stimulated emission Single phase Along same path =Constructive Interference Coherent radiation Multiple directions, Multiple phase lags Incoherent radiation

FTIR Instrument Constructive/Destructive interference Laser FT instrument Can be used to select wavelengths Can be used to obtain information about distances Holographic Interference filter.

Time Domain: 2 frequencies 1 “beat” cycle

B Fixed mirror A C Moving mirror Beam splitter IR source detector Constructive interference occurs when

-2 -1 +1

INTERFEROGRAMS Remember that: Frequency of light

An interferometer detects a periodic wave with a frequency of 1000 Hz when moving at a velocity of 1 mm/s. What is the frequency of light impinging on the detector?

No need to SELECT Wavelength by using Mirror, fiber optics, Gratings, etc.