Ultrafast Spectroscopy PowerPoint Presentation

Ultrafast Spectroscopy PowerPoint Presentation

2017-10-24 63K 63 0 0

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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: 599078

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Presentations text content in Ultrafast Spectroscopy

Slide1

Ultrafast Spectroscopy

Slide2

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)

5

ligand

exchange in <1

ps

High intensity

: properties of liquid

carbon

Slide3

How can we measure things this fast?

1960

1970

1980

1990

2000

10

–6

10

–9

10

–12

10

–15

Timescale (seconds)

Year

Electronics

Optics

Slide4

Laser Basics

Level empties fast!

Four-level system

Laser Transition

Pump Transition

Fast decay

Fast decay

Population inversion

Pump energy source

Lasing transition

Slide5

Method of creating pulsed output

Compressed output

Broadband laser pulse

What we need for ultrashort pulse generation:

Slide6

Ultrafast Laser Overview

Laser oscillator

Amplifier medium

pump

Slide7

Luminescence Spectrometry

Three types of Luminescence methods are:

(

i

)

Molecular

fluorescence

(ii)

Phosphorescence

(iii)

Chemiluminescence

In each, molecules of the

analyte

are excited to give a species whose emission spectrum provides information for qualitative or quantitative analysis. The methods are known collectively as

molecular luminescence

procedures.

Slide8

Fluorescence:

absorption of photon, short-lived excited state (singlet), emission of photon.

Phosphorescence:

absorption of photon, long-lived excited state (triplet), emission of photon.

Chemiluminescence

:

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 &

Capillary Electrophoresis.

Slide9

THEORY OF FLUORESCENCE

AND PHOSPHORESCENCE

Types of Fluorescence:

Resonance (emitted

 = excitation ; e.g., AF)

Stokes shift (emitted  > excitation ; e.g., molecular fluorescence)

Slide10

Electron spin and excited statesExcited, paired = excited singlet state  fluorescenceExcited, unpaired = excited triplet state  phosphorescence

Slide11

Deactivation

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)

Radiationless Deactivation

Without emission of a photon (i.e., without radiation)

Slide12

Slide13

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 constants

Slide18

INSTRUMENTATION

Sources

Hg lamp (254 nm)

Xe lamp (300 – 1300 nm)

Filter/monochromator

Isolate excitation

Scan excitation 

Isolate emission  from excitation 

Scan emission 

Detector

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:

Chemiluminescence

- phenyl oxalate ester (glow sticks

)

Luminol (used to detect blood)

Slide20

Luciferase (Firefly enzyme)

Luciferin (firefly)

“Glowing” Plants

Luciferase gene cloned into plants

Biological systems

Slide21

Determination of nitrogen monoxide NO + O3 → NO2* + O2 NO2* + → NO2 + h ( = 600 – 2800 nm)

Other Applications

Determination of sulfur

4H

2

+ 2SO

2

→ S

2

* + 4H

2

O

S

2

* → S

2

+ h

 ( = 384 and 394 nm)


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