Photochemistry Flashcards
What does photo-induced chemistry require?
absorption of electromagnetic radiation
What does a strong absorption involve
interaction of electric field (epsilon) of electromagnetic radiation with trasition dipole moment (u) of atom or molecule
- electric dipole transition
How much weaker are electric quadrupole and magnetic dipole transitions compared to electric dipole transitions?
~ 6 orders of magnitude
What occurs during an electric dipole transition?
In the presence of electromagnetic radiation a molecule experiences a perturbation described by the dipolar interaction
What is the condition for two states Y1 and Y2 being connected by an electric dipole allowed transition?
If the matrix element integral does not equal zero
What assumptions are used (generally) in photochemistry? (2)
- molecular dimensions are far smaller than wavelength of the photon inducing the transition
- Interaction is weak (first order perturbation theory)
Within the BO approximation, total wavefunction be written as a product of what 2 components?
electronic and vibrational parts
Which part of the total wavefunction does the transtion dipole operate on?
Electric part
Electronic transitions are often accompanied by…
a change in vibrational state
What is the Franck-Condon factor?
square of vibrational overlap integrals
What does the vibrational structure accompanying an electronic transition depend on?
Franck-Condon factor
Which two types of electronic transitions in molecules are very weak, appearing near-UV?
Non-bonding to anti-bonding transitions
Which transitions in molecules are strongly allowed and cause intense absorptions in deep UV?
bonding to anti-bonding
What are Rydberg orbitals
Rydberg orbitals are spatially diffuse ‘atomic’ like orbitals with high energies which form hydrogenic-like series converging to ionisation limit
What is B12
B12 is the Einstein B-coefficient for absorption and for a given transition is related to molar absorption coefficient
What is A21
A21 is the Einstein A-coefficient for spontaneous emission
What is B21
B21 is the Einstein B-coefficient for stimulated emission
What is the requirement for stimulated emission
Photon of correct frequency to match energy gap of transition; two photons of the same frequency and directionality emitted for conversion of energy
What are all of the possible decay routes for an electronically excited state (8)
- Fluorescence (delta S = 0) (radiative)
- Phosphorescence (delta S =/ 0) (radiative)
- Stimulated emission
- Internal conversion (delta S = 0, radiationless)
- Intersystem crossing (radiationless, delta S =/ 0)
- Isomerisation
- Dissociation
- Collisional relaxation / quenching
Describe direct photodissociation
Excited molecule dissociates in <1 vibrational period giving a Gaussian like absorption spectrum
What is predissociation
When a molecule is photoexcited to a state that is bound with respect to excited state products but is at energy above lowest dissociation limit, the excited molecule undergoes 1+ vibration in bound excited state before radiationless transfer to repulsive potential. This gives a structured absorption spectrum
What does the appearance of a spectrum involving predissociation depend on?
Rate/lifetime
How does lifetime broadening occur?
From energy-time form of uncertainty principle, uncertainty gives an estimate of FWHM linewidth
What is Doppler Broadening?
Doppler Broadening arises as a result of rotational fine structure and this makes a bigger contribution to line width than lifetime broadening
What is the requirement for lasing
population inversion which ensures gain (amplification) not loss (absorption) of radiation. Also requires optical feedback achieved by placing mirrors (an optical cavity) around lasing medium
What is an excimer laser?
an excimer laser is a gas laser employing molecules whose ground state is unbound but have strongly bound excited states with essentially ionic bonding eg Ar+ F-
-excited molecules are formed in electric discharge
Give two examples of rare gas-halogen mixtures supporting lasing action at UV Vis and give their wavelengths
XeCl (308 nm)
KrF (248 nm)
Why are dye lasers good
They have tuneable radiation
How do dye lasers work
- Dye molecules contain conjugated chains which absorb in the UV-Vis and emit at Stokes shifted (red/longer) wavelengths over a limited range
- If you use 25 dyes you have a 360 to 1000 nm range
- Dye solution housed in cavity with one mirror being a diffraction grating
- excite with frequency doubled Nd-YAG or excimer laser
- S1 S0 fluorescence or in cavity stimulated emission (LASING)
- solvent induced quenching to S0 (low v) ie 4 levels
- vary wavelength with angle of diffraction grating
What is a Ti-sapphire laser good for and why?
ultrafast femtosecond experiments because the emission spectrum of Ti3+ is very broad and gives laser action over a wide range of wavelengths.
This is important for short pulse generation
10 fs pulse bandwidth > 530 /cm
delta wavelength @ 780 nm ~ 30 nm
What is a Ti-sapphire laser composed of?
Low concentration of Ti3+ ions doped in sapphire, Al2O3
What is a Kerr medium and give an example of one
A Kerr medium is a material whose refractive index changes when exposed to intense laser pulse, sapphire is one
Why does a Ti-sapphire work?
Sapphire is a Kerr medium. This results in passive mode-locking where only the most intense part of light pulse oscillates back and forth along the laser cavity and gets amplified
The system has many closely spaced vibronic levels in the excited state, 2E and in the ground state, 2T2.
Give 8 important photochemical applications
- photosynthesis
- vision
- therapy
- lithography
- storage of solar energy
- synthetic: [2+2] cycloaddition, isomerisation
- atmospheric chemistry
- initiating reactions, generating reactants for bimolecular reaction dynamics
In what 2 big ways does photochemistry contribute to atmospheric chemistry
ozone depletion, smog formation
What are the 2 key photodissociation steps in the Chapman cycle
photodissociation of O2 –> O(3P) + O(1D)
photodissociation of O3 –> O2 + O (wavelength under 242 nm)
Where does the vibrational structure converge to a dissociation limit and what else is this known as
wavelength ~ 176 nm
long wavelength threshold for O(3P) and O(1D) photoproducts
How does the ion imaging study of photodissociation of O2 work
- 157 nm photodissociation of O2
- O atoms created at defined time and position and therefore defined velocities from conservation of energy
- O(1D) products ionised by REMPI at point of creation
- 2+1 resonance enhanced multiphoton ionisation detection
O(1D) –> O** –> O+ + e- - O+ ions recoil & simultaneously accelerated by carefully designed electric fields –> impact on a time and position sensitive vector
- radius r is proportional to velocity v and movo=movo
- from TKER and conservation of E can calculate energies
where do fast products (large v) recoil to
fast products with large v recoil approximately parallel to epsilon (field)
where do intensity peaks arise for fast products in a recoil velocity distribution
at the poles
where do slow products recoil to
approximately perpendicular to epsilon (field)
where do intensity peaks arise for slow products in a recoil velocity distribution
at the equator
What does recoil anisotropy tell us about
Recoil anisotropy is a signature of the correlation between the field and dipole
In a gas, where dipoles point in all directions, which molecules does linearly polarised light preferentially interact with
and what does this give rise to
those with a dipole parallel to the field
this gives rise to an anisotropic ensemble of photoexcited molecules
What is axial recoil
Axial recoil is where product atoms recoil parallel to the breaking bond
What is the primary source of tropospheric OH
reactions of water and O(1D)
How does time-dependent fluorescence spectroscopy work
- A laser excites molecules from S0 wavefunction to excited state at time t0
- Fragmentation occurs and excited molecule can fluoresce to different vibrational levels of S0
- Emission spectrum: series of broadened bands separated according to S0 vibrational spacing
What do we measure in femtochemistry
Loss of parent and build up of products using a pump and a probe laser
What two detection techniques are used for ICN photolysis
REMPI
LIF
How does a wavepacket arise
Excitation with a femtosecond pulse can lead to simultaneous population of several vibrational levels which results in a coherent superposition of eigenstates i.e. a wavepacket
How does a wavepacket behave
the centre of a wavepacket evolves like a particle under influence of a potential; the wavepacket itself behaves quantum mechanically
Give 3 advantages of using a laser
- high resolution
- high sensitivity
- long path lengths (by multi-passing)
Give 4 limits of direct absorption measurements
- detector noise
- fluctuations in I0 (limiting factor)
- difficult to measure small attenuations in I0
- Minimum detectable change in I0 is around 10^-4
- relatively insensitive
How does cavity enhanced absorption spectroscopy work
It boosts sensitivity by building a cavity around the sample and then multipassing the laser pulse through it and detecting rate of loss of light
With the sample light is lost faster than without sample
How does cavity ring down spectroscopy work
It measures the change in ring-down rate as a function of excitation wavelength to give an absorption spectrum
“ring-down time”: time for light to decay to 1/e of initial intensity
What are the pros of cavity ring-down spectroscopy
Quantitive, delta intensity minimum measured is 10^-8 therefore much more sensitive
What is the method of choice for electronic spectroscopy
laser induced fluorescence
How does LIF work
- LIF improves sensitivity by measuring the consequence of absorption of photon not the absorption itself
- delta intensity ~ 10^-17 measured
- fluorescence from species excited by a laser tuned to one of its absorption lines detected very sensitively
- off-resonant signals very smalls
- inidividual atoms detectable
Give 3 limitations of LIF
- fluorescence quantum yield can’t be too low; atomic therefore sensitive to competition from (pre)dissociation, internal conversion, intersystem crossing, collisional quenching etc
- less useful if fluorescence decay rate is slow; IR transitions not studied
- relative rather than absolute number densities therefore calibration is required
Give 2 cool applications of LIF
- jet-cooled molecular species in supersonic beams
- spatially resolved species detection and concentration profiling
How does single molecule spectroscopy work
Using a small area with 1 or few dye molecules, biomolecules or polymers tagged with biomolecules, as v dilute samples. Illuminate samples, resulting redshifted LIF collected
Use microscope objective lens
Give 2 variants of single molecule spectroscopy
Confocal fluorescence microscopy and wide-field epi illumination
How does wide-field epi illumination work
- Excite large area
- Image resulting LIF
- Intensities and shapes give information on orientation
- Apparent ‘size’ of molecules limited by optical aperture; molecules are much smaller
Why is single molecule spectroscopy useful
It allows study of energy transfer within or between isolated molecules as opposed to an emsemble
Explain observation of S1 blinking in an individual molecule
- Chromophore is a single quantum system therefore digital behaviour
- Excite S1
Give 1 limitation to single molecule spectroscopy
photobleaching (light induced destruction of pi-system on repeat excitation) –> higher states, excited state absorption
Explain how fluorescence confocal microscopy works
- sample doped with dye molecules that absorb strongly at pump wavelength and have high fluorescence quantum yields
- excited dye molecules emit at stokes shifted wavelength
- use filters to ensure only fluorescence reaches the detector
- allows visualisation of features in for example living cells and tissues
- pinhole aperture in front of detector ensures good depth resolution
- illuminated voxel is a diffraction limited spot (diffraction limit D is proportional to wavelength)
How does super resolution fluorescence microscopy work
- size of fluorescence focal spot reduced by selectively inhibiting fluorescence from its periphery; by stimulating emission from that region
- smaller spot –> greater spatial resolution
- use pair of synchronised sub picosecond laser pulses
- excitation pulse tuned to absorption maximum of dye and focussed on a sample to give ordinary diffraction limited spot
- immediately followed by stimulated emission depletion (STED) pulses with red shifted wavelength with respect to emission spectrum of dye so it acts only on dye molecules quenching them to S0
- Arrange STED pulse in doughnut shape: selectively dump excited molecules at periphery of focal spot so they do not contribute to overall fluorescence
What is Forster Resonant Energy Transfer?
- Within 2 chromophores, (donor D and acceptor A), energy transfer can occur from excited D to A by dipole-dipole coupling in a non-radiative process
- energy transfer efficiency is proportional to separation ^-6
- —> FRET is extremely sensitive fluorescence technique for small distances on the scales of few nm
In FRET, what does the energy transfer efficiency E depend on (2)
- Overlap of D emission and A absorption spectra
- relative orientations of transition dipole moments of D emission and A absorption spectra
What is a really good use of FRET
Proteins: physical interactions where relative proximity of molecules must be determined more precisely than possible with traditional diffusion limited imaging methods
What is the condition for FRET observation
A and D must be close in space
