Electronic Spectroscopy of Molecules

1
Electronic Spectroscopy of Molecules
2
(No Transcript)
3
Whenever an electronic transition occurs in a
molecule the nuclei are subjected to a change in
the Coulomb force as a result of the
redistribution of electronic charge that
accompanies the transition. The nuclei can break
into a more or less vigorous vibration and the
absorption or emission spectra will show a
structure characteristic of the vibrational
energy levels of the molecule VIBRONIC transitions
4
(No Transcript)
5
Quantum mechanically The transition occurs from
the ground vibrational state of the lower
electronic state to the vibrational state that it
most resembles in the upper electronic state. The
vibrational wave function undergoes least change
- preservation of dynamical state of the nuclei
(FC) Transition Dipole Moment
electrons
nuclei
6
BO Approximation vibronic state is described by
the wavefunction
Dipole transition moment
Transition moment is approximately independent of
the locations of the nuclei - as long as they are
not displaced by a large amount from equilibrium
overlap integral between the 2 vib states
where
7
Usually several vibrational states have similar
values of S and so transitions occur to all of
them. A progression of lines is stimulated and a
series of lines is observed in the electronic
spectrum. The relative intensities are
proportional to the Franck-Condon factors
8
(No Transcript)
9
For every vibronic transition there are also
rotational transitions.
DL 0, 1 giving rise to P,
Q, and R branches
Because the rotational constants of the upper and
lower electronic states can be quite different,
head formation occurs
10
Dissociation
Predissociation
11
Fluorescence and Phosphorescence
12
Photoelectron Spectroscopy
13
Femtochemistry
Ugo Fano The ultimate goal of all low energy
gas phase physics is the full understanding of
the dynamics of chemical reactions in wave
mechanical terms
Technical GoalSelective making and breaking of
molecular bonds and control of chemical reactions
by light
Conventional Laser Chemistry? Internal energy
flow faster than duration of laser pulse? Photon
energy rapidly dissipated?Unspecific
heating FST? Laser parameters chosen to control
ultrafst flow of energy and particle motion
within reactants in real time? Specific reaction
channels opened before dissipation occurs.
14
Nobel Prize for Chemistry 1999
Ahmed Zewail Caltech
för hans studier av kemiska reaktioners
övergångstillstånd med femtosekundspektroskopi
15
(No Transcript)
16
Within the last few years fs lasers have become
much more user friendly and reliable a tool
instead of a full-time job!
17
time and length scalessecond (1s) femtosecond
(1fs)

• in a time interval of 1 sa light pulse travels
  300000km, i.e. the distance from earth to moon
• in 1 psa light pulse travels 0.3 mm, i.e. the
  thickness of cardboard
• in 1 fslight travels 300 nanometer, less than
  the wave length of visible light and a typical
  dimension of present day electronic circuit
  elements

300 000 km
18
time scales
stopclock
molecular rotation/vibration
electronic motion
chemical reaction(explosion)
fast camerashutter
fast digitalelectronics
primary processes in photosynthesis
time
10 -12 secpico
10 -15 sec femto
1 sec
10 -6 secmicro
10 -9 secnano
10 3 secmilli
19
How are fs pulses made?
Mode-locking
Typically, many longitudinal modes can oscillate
in the laser resonator standing waves (integer
number of half wavelengths)
Number of modes present depends on emission
profile of active medium
20
Modes with statistical, independently fluctuating
phases of the modes
The total intensity is just the square of the sum
of the individual mode amplitudes
21
Modes with a fixed phase relationship
The individual modes can interfere and at the
position where there is constructive interference
there are maxima in the intensity
22
The maxima become more pronounced as the number
of interfering modes increases
Pulse distance T2L/c(round-trip time for light
in the resonator)
Pulse duration DT2L/Nc 1/Dn Dnfrequency
spread of laser output
A single pulse is produced whose duration is
shorter, the broader is the amplification profile
23
Ar Laser 200 ps pulses NdYAG 5
ps Dye 10 fs
Titanium doped sapphire crystal 3 fs
Short pulses implies high power (J/s)
24
Power of energy sources and of energy consumers
short pulse power
lighning
bomb
flashlight
high field laser
nuclear power station
electric bulb
world
car
electric cooker
small city
Germany
sun
continuous power
25
High field lasers worldwide
26
Applications of fs Lasers
Material structuring
100 fs
3 ps
Al2O3
Medical Applications nanometer control, low
damage
Mass Spectrometry.Efficient (ca. 100)
ionisation. Quantitative analysis
Time resolved X-ray diffraction, electron
diffraction
27
Femtochemistry
Coherence and control/study of non-statistical
behaviour
Early gas-phase set-up (Zewail)
Pump-probe technique Put energy into the
molecule (coherently) at t0, wait for the system
to develop for a certain time fs-ps. Probe the
products with a second ultrashort pulse (normally
from same laser)
28
(No Transcript)
29
Femtosecond Photoelectron Spectroscopy, Na2
30
I2 vibrational and rotational motion
31
NaI