Ultrafast Spectroscopy
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
How can we measure things this fast?
–6
Timescale (seconds)
10
–9
10
Electronics
–12
10
Optics
–15
10
1960
1970
1980
Year
1990
2000
Laser Basics
Four-level
system
•Population inversion
•Pump energy source
Fast decay
Pump
Transition
•Lasing transition
Laser
Transition
Fast decay
Level
empties
fast!
What we need for ultrashort pulse
generation:
• Method of creating pulsed output
• Compressed output
• Broadband laser pulse
Ultrafast Laser Overview
pump
Laser
oscillator
Amplifier
medium
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.
• Fluorescence: absorption of photon, short-lived
excited state (singlet), emission of photon.
• Phosphorescence: absorption of photon, longlived 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.
THEORY OF FLUORESCENCE
AND PHOSPHORESCENCE
Types of Fluorescence:
• Resonance (emitted  = excitation ;
e.g., AF)
• Stokes shift (emitted  > excitation ;
e.g., molecular fluorescence)
Electron spin and excited states
• Excited, paired = excited singlet state 
fluorescence
• Excited, unpaired = excited triplet state 
phosphorescence
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)
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)
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).
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)
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)
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
total # luminescing molecules
Quantum Yield,  =
total # of excited molecules
kf

[ k = rate constant]
kf  ki  kec  kic  kpd  kd
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.
Chemiluminescence
- chemical reaction yields an electronically excited species that emits
light as it returns to ground state.
- relatively new, few examples
A + B  C*  C + h
Examples of Chemical Systems giving off light:
Luminol (used to detect blood)
NH2
O
NH2
C
COONH
O2/OH-
+ h + N2 + H2O
NH
C
COO-
O
- phenyl oxalate ester (glow sticks)
Biological systems
Luciferase (Firefly enzyme)
O
O
C
C
R2
Luciferase
Luciferin + O2
O
R2
Spontaneous
CO2 +
O
Light
C*
R1
1
R
N
S
HO
“Glowing” Plants
Luciferase gene cloned into plants
S
N
O
Luciferin (firefly)
HO
Other Applications
Determination of nitrogen monoxide
NO + O3 → NO2* + O2
NO2* + → NO2 + h ( = 600 – 2800 nm)
Determination of sulfur
4H2 + 2SO2 → S2* + 4H2O
S2* → S2 + h ( = 384 and 394 nm)