Ultrafast Spectroscopy PowerPoint Presentation
Ultrafast examples:. Photosynthesis. : energy transfer in <200 . fs. Vision. : . isomerization. of retinal in 200 . fs. Dynamics. : ring opening reaction in ~100s . fs. Transition states. : Fe(CO). ID: 599078Embed code:
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Presentations text content in Ultrafast Spectroscopy
: energy transfer in <200
of retinal in 200
: ring opening reaction in ~100s
exchange in <1
: properties of liquid
How can we measure things this fast?
Level empties fast!
Pump energy source
Method of creating pulsed output
Broadband laser pulse
What we need for ultrashort pulse generation:Slide6
Ultrafast Laser Overview
Three types of Luminescence methods are:
In each, molecules of the
are excited to give a species whose emission spectrum provides information for qualitative or quantitative analysis. The methods are known collectively as
absorption of photon, short-lived excited state (singlet), emission of photon.
absorption of photon, long-lived excited state (triplet), emission of photon.
no excitation source – chemical reaction provides energy to excite molecule, emission of photon.
Luminescence: High sensitivity
strong signal against a dark background.
Used as detectors for HPLC &
THEORY OF FLUORESCENCE
Types of Fluorescence:
= excitation ; e.g., AF)
Stokes shift (emitted > excitation ; e.g., molecular fluorescence)
Electron spin and excited statesExcited, paired = excited singlet state fluorescenceExcited, unpaired = excited triplet state phosphorescenceSlide11
Process by which an excited molecule returns to the ground state
Minimizing lifetime of electronic state is preferred (i.e., the deactivation process with the faster rate constant will predominate)
Without emission of a photon (i.e., without radiation)
TERMS FROM ENERGY-LEVEL DIAGRAM Term: Absorption Effect: Excite Process: Analyte molecule absorbs photon (very fast ~ 10-14 – 10-15 s); electron is promoted to higher energy state. Slightly different wavelength excitation into different vibrational energy levels. Term: Vibrational Relaxation Effect: Deactivate, Radiationless Process: Collisions of excited state analyte molecules with other molecules loss of excess vibrational energy and relaxation to lower vibrational levels (within the excited electronic state)Slide14
Term: Internal conversion Effect: Deactivate, Radiationless Process: Molecule passes to a lower energy state – vibrational energy levels of the two electronic states overlap (see diagram) and molecules passes from one electronic state to the other. Term: Fluorescence Effect: Deactivate, Emission of h Process: Emission of a photon via a singlet to singlet transition (short – lived excited state ~10-7 – 10-9 s).Slide15
Term: Intersystem Crossing Effect: Deactivate, Radiationless Process: Spin of electron is reversed leading to change from singlet to triplet state. Occurs more readily if vibrational levels of the two states overlap. Common in molecules with heavy atoms (e.g., I or Br)Slide16
Term: External Conversion Effect: Deactivate, Radiationless Process: Collisions of excited state analyte molecules with other molecules molecule relaxes to the ground state without emission of a photon. Term: Phosphorescence Effect: Deactivate, Emission of h Process: Emission of a photon via a triplet to single transition (long–lived excited state ~ 10-4 – 101s)Slide17
Quantum Yield The quantum yield or quantum efficiency for fluorescence or phosphorescence is the ratio of the number of molecules that luminesce to the total number of excited molecule. Gives a measure of how efficient a fluorophore (i.e., fluorescing molecule) is.A quantum yield = 1 means that every excited molecules deactivates by emitting a photon – such a molecule is considered a very good fluorophore.Can express quantum yield as a function of rate constantsSlide18
Hg lamp (254 nm)
Xe lamp (300 – 1300 nm)
Isolate emission from excitation
Usually PMT: very low light levels are measured.Slide19
- chemical reaction yields an electronically excited species that emits light as it returns to ground state. - relatively new, few examples A + B C* C + hnExamples of Chemical Systems giving off light:
- phenyl oxalate ester (glow sticks
Luminol (used to detect blood)Slide20
Luciferase (Firefly enzyme)
Luciferase gene cloned into plants
Determination of nitrogen monoxide NO + O3 → NO2* + O2 NO2* + → NO2 + h ( = 600 – 2800 nm)
Determination of sulfur
* + 4H
* → S
( = 384 and 394 nm)