6. really basic optics sample prep instrument out put signal (data) select light sample interaction...

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

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

6. Really Basic Optics y Sin=opp/hyp A 90o phase angle /2 radian phase angle /2 3 /2 22

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 e E hi E lo Frequency 1/s

Theoretical width of an atomic spectral line

Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure 4.Electric and magnetic fields Lifetime of an excited state is typically 1x10 -8 s Natural Line Widths frequency

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

Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure 4.Electric and magnetic fields

Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure 4.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. FWHM = full width half maximum o is the peak center in frequency units We use pressure broadening On purpose to get a large Line width in AA for some Forms of background correction

Line spectra occur when radiating species are atomic particles which Experience no near neighbor interactions Overlapping line spectra lead to band emission Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure 4.Electric and magnetic fields Line events Can lie on top Of band events

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

Wiens Law Stefan-Boltzmann Law = Energy density of radiation h= Plancks constant C= speed of light k= Boltzmann constant T=Temperature in Kelvin = frequency 1.As (until effect of exp takes over) 2.As T ,exp, Plancks Blackbody Law

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

Begin 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 1. Laser 2. FT instrument 3. Can be used to obtain information about distances 4. Interference filter. 5. Can be used to select wavelengths END: Key Definitions

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, 6 th Ed Photons can stimulate Emission just as much As they can stimulate Absorption (idea behind LASERs Stimulated Emission) * o Stimulated Emission The rate of stimulated event is described by : Is the energy density of radiation already present at the frequency of the transition B = empirical constant known as the Einstein coefficient for stimulated absorption or emission N* and N o are the populations of upper state and lower states Where w =rate of stimulated emission or absorption The more perturbing photons the greater the Stimulated emission Light Amplification by Stimulated Emission of Radiation

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

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 pumpE

Multiple directions, Multiple phase lags Stimulated emission 1.Single phase 2.Along same path =Constructive Interference Coherent radiation Incoherent radiation Radiation not along the Path is lost mirror Constructive/Destructive interference 1. Laser 2. FT instrument 3. Can be used to select wavelengths 4.Can be used to obtain information about distances 5.Holographic Interference filter.

FTIR Instrument Constructive/Destructive interference 1. Laser 2. FT instrument 3. Can be used to select wavelengths 4.Can be used to obtain information about distances 5.Holographic Interference filter.

Time Domain: 2 frequencies 1 beat cycle

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

-2 0 +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.

FOURIER TRANSFORMS Advantages 1.Jaquinot or through-put little photon loss; little loss of source intensity 2.Large number of wavelengths allows for ensemble averaging (waveform averaging) 3. This leads to Fellget or multiplex advantage multiple spectra in little time implies?

DIFFRACTION Huygens principle = individual propagating waves combine to form a new wave front Can get coherent radiation if the slit is narrow enough. Coherent = all in one phase Constructive/Destructive interference 1. Laser 2. FT instrument 3. Can be used to select wavelengths 4.Can be used to obtain information about distances 5.Holographic Interference filter.

June 19, 2008, Iowa Flood Katrina Levee break

Fraunhaufer diffraction at a single slit

From which we conclude C B F F D E d W L O

The complete equation for a slit is Width of the line depends upon The slit width!! Therefore resolution depends On slit width Also see This spectra leak of Our hard won intensity B D E d W L b=W/2

The base (I =0) occurs whenever sin =0 Which occurs when The smaller the Slit width the Smaller The line width, Which leads To greater Spectral Resolution Remember R is Inversely proportional To the width of The Gaussian base

SLIT IMAGE 1 2 3 Image 1 2 3 4 5 Position number Slit When edge AB atDetector Sees Position 10% power Position 250% power Position 3100% power Position 450% power Position 50 % power Detector output: Triangle results when Effective bandpass = image To resolve two images that are apart requires Implies want a narrower slit A B

Essentially, Narrow slit widths Are generally better

GRATINGS GratingsGroves/mm UV/Vis300/2000 IR10/20 Points: 1.Master grating formed by diamond tip under ground 1.Or more recently formed from holographic processes 2.Copy gratings formed from resins

q q + -

EXAMPLE Calculate for a grating which has i=45 2000 groves per mm 1)Get d 2) Use grating equation to solve for

Multiple wavelengths Are observed At a single angle Of reflection!! You get light of 674.9 nm ; 1/3; 1/4; 1/5; etc. Czerny-Turner construction 440.3 220.1 146.8 88 73 All come through

Physical Dimensions: 89.1 mm x 63.3 mm x 34.4 mm Weight: 190 grams Detector:Sony ILX511 linear silicon CCD array Detector range: 200-1100 nm Pixels:2048 pixels Pixel size:14 m x 200 m Pixel well depth:~62,500 electrons Sensitivity: 75 photons/count at 400 nm; 41 photons/count at 600 nm Design: f/4, Symmetrical crossed Czerny-Turner Focal length: 42 mm input; 68 mm output Entrance aperture: 5, 10, 25, 50, 100 or 200 m wide slits or fiber (no slit) Grating options: 14 different gratings, UV through Shortwave NIR Detector collection lens option: Yes, L2 OFLV filter options: OFLV-200-850; OFLV-350-1000 Other bench filter options: Longpass OF-1 filters Collimating and focusing mirrors: Standard or SAG+ UV enhanced window: Yes, UV2 Fiber optic connector: SMA 905 to 0.22 numerical aperture single-strand optical fiber Spectroscopic Wavelength range: Grating dependent Optical resolution: ~0.3-10.0 nm FWHM Signal-to-noise ratio: 250:1 (at full signal) A/D resolution: 12 bit Dark noise: 3.2 RMS counts Dynamic range: 2 x 10^8 (system); 1300:1 for a single acquisition Integration time: 3 ms to 65 seconds Stray light:

PHOTONS AS PARTICLES The photoelectric effect: The experiment: 1. Current, I, flows when E kinetic > E repulsive 2. E repulsive is proportional to the applied voltage, V 3. Therefore the photocurrent, I, is proportional to the applied voltage 4. Define V o as the voltage at which the photocurrent goes to zero = measure of the maximum kinetic energy of the electrons 5. Vary the frequency of the photons, measure V o, = E kinetic,max Energy of Ejected electron Frequency of impinging photon (related to photon energy) Work function=minimum energy binding an Electron in the metal

To convert photons to electrons that we can measure with an electrical circuit use A metal foil with a low work function (binding energy of electrons)

DETECTORS Ideal Properties 1.High sensitivity 2.Large S/N 3.Constant parameters with wavelength Where k is some large constant k d is the dark current Classes of Detectors Namecomment Photoemissivesingle photon events Photoconductive (UV, Vis, near IR) Heataverage photon flux Want low dark current

1.Capture all simultaneously = multiplex advantage 2. Generally less sensitive Rock to Get different wavelengths Very sensitive detector

Sensitivity of photoemissive Surface is variable Ga/As is a good one As it is more or less consistent Over the full spectral range

Diode array detectors -Great in getting -A spectra all at once! Background current (Noise) comes from? One major problem -Not very sensitive -So must be used -With methods in -Which there is a large -signal

Photodiodes Photomultiplier tube The AA experiment

Charge-Coupled Device (CCD detectors) 1. Are miniature therefore do not need to slide the image across a single detector (can be used in arrays to get a Fellget advantage) 2. Are nearly as sensitive as a photomultiplier tube +V 3.Apply greater voltage 4.Move charge to gate And Count, 5.move next bin of charge and keep on counting 6. Difference is charge in One bin 1.Set device to accumulate charge for some period of time. (increase sensitivity) 2.Charge accumulated near electrode Requires special cooling, Why? The fluorescence experiment

END 6. Really Basic Optics

Since polarizability of the electrons in the material also controls the dielectric Constant you can find a form of the C-M equation with allows you to compute The dielectric constant from the polarizability of electrons in any atom/bond N = density of dipoles = polarizability (microscopic (chemical) property) r = relative dielectric constant Frequency dependent Just as the refractive index is Typically reported Point of this slide: polarizability of electrons in a molecule is related to the Relative dielectric constant

Grating 2 nd order 1 st order Angle of reflection i=45

2 nd order 1 st order Angle of reflection i=45

6. really basic optics sample prep instrument out put signal (data) select light sample interaction...

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large line width