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Physical > Photochemistry > Flashcards

Photochemistry Flashcards

1
Q

What does photo-induced chemistry require?

A

absorption of electromagnetic radiation

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2
Q

What does a strong absorption involve

A

interaction of electric field (epsilon) of electromagnetic radiation with trasition dipole moment (u) of atom or molecule
- electric dipole transition

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3
Q

How much weaker are electric quadrupole and magnetic dipole transitions compared to electric dipole transitions?

A

~ 6 orders of magnitude

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4
Q

What occurs during an electric dipole transition?

A

In the presence of electromagnetic radiation a molecule experiences a perturbation described by the dipolar interaction

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5
Q

What is the condition for two states Y1 and Y2 being connected by an electric dipole allowed transition?

A

If the matrix element integral does not equal zero

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6
Q

What assumptions are used (generally) in photochemistry? (2)

A
  • molecular dimensions are far smaller than wavelength of the photon inducing the transition
  • Interaction is weak (first order perturbation theory)
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7
Q

Within the BO approximation, total wavefunction be written as a product of what 2 components?

A

electronic and vibrational parts

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8
Q

Which part of the total wavefunction does the transtion dipole operate on?

A

Electric part

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9
Q

Electronic transitions are often accompanied by…

A

a change in vibrational state

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10
Q

What is the Franck-Condon factor?

A

square of vibrational overlap integrals

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11
Q

What does the vibrational structure accompanying an electronic transition depend on?

A

Franck-Condon factor

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12
Q

Which two types of electronic transitions in molecules are very weak, appearing near-UV?

A

Non-bonding to anti-bonding transitions

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13
Q

Which transitions in molecules are strongly allowed and cause intense absorptions in deep UV?

A

bonding to anti-bonding

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14
Q

What are Rydberg orbitals

A

Rydberg orbitals are spatially diffuse ‘atomic’ like orbitals with high energies which form hydrogenic-like series converging to ionisation limit

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15
Q

What is B12

A

B12 is the Einstein B-coefficient for absorption and for a given transition is related to molar absorption coefficient

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16
Q

What is A21

A

A21 is the Einstein A-coefficient for spontaneous emission

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17
Q

What is B21

A

B21 is the Einstein B-coefficient for stimulated emission

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18
Q

What is the requirement for stimulated emission

A

Photon of correct frequency to match energy gap of transition; two photons of the same frequency and directionality emitted for conversion of energy

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19
Q

What are all of the possible decay routes for an electronically excited state (8)

A
  • 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
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20
Q

Describe direct photodissociation

A

Excited molecule dissociates in <1 vibrational period giving a Gaussian like absorption spectrum

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21
Q

What is predissociation

A

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

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22
Q

What does the appearance of a spectrum involving predissociation depend on?

A

Rate/lifetime

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23
Q

How does lifetime broadening occur?

A

From energy-time form of uncertainty principle, uncertainty gives an estimate of FWHM linewidth

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24
Q

What is Doppler Broadening?

A

Doppler Broadening arises as a result of rotational fine structure and this makes a bigger contribution to line width than lifetime broadening

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25
Q

What is the requirement for lasing

A

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

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26
Q

What is an excimer laser?

A

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

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27
Q

Give two examples of rare gas-halogen mixtures supporting lasing action at UV Vis and give their wavelengths

A

XeCl (308 nm)

KrF (248 nm)

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28
Q

Why are dye lasers good

A

They have tuneable radiation

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29
Q

How do dye lasers work

A
  • 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
30
Q

What is a Ti-sapphire laser good for and why?

A

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

31
Q

What is a Ti-sapphire laser composed of?

A

Low concentration of Ti3+ ions doped in sapphire, Al2O3

32
Q

What is a Kerr medium and give an example of one

A

A Kerr medium is a material whose refractive index changes when exposed to intense laser pulse, sapphire is one

33
Q

Why does a Ti-sapphire work?

A

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.

34
Q

Give 8 important photochemical applications

A
  • photosynthesis
  • vision
  • therapy
  • lithography
  • storage of solar energy
  • synthetic: [2+2] cycloaddition, isomerisation
  • atmospheric chemistry
  • initiating reactions, generating reactants for bimolecular reaction dynamics
35
Q

In what 2 big ways does photochemistry contribute to atmospheric chemistry

A

ozone depletion, smog formation

36
Q

What are the 2 key photodissociation steps in the Chapman cycle

A

photodissociation of O2 –> O(3P) + O(1D)

photodissociation of O3 –> O2 + O (wavelength under 242 nm)

37
Q

Where does the vibrational structure converge to a dissociation limit and what else is this known as

A

wavelength ~ 176 nm

long wavelength threshold for O(3P) and O(1D) photoproducts

38
Q

How does the ion imaging study of photodissociation of O2 work

A
  • 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
39
Q

where do fast products (large v) recoil to

A

fast products with large v recoil approximately parallel to epsilon (field)

40
Q

where do intensity peaks arise for fast products in a recoil velocity distribution

A

at the poles

41
Q

where do slow products recoil to

A

approximately perpendicular to epsilon (field)

42
Q

where do intensity peaks arise for slow products in a recoil velocity distribution

A

at the equator

43
Q

What does recoil anisotropy tell us about

A

Recoil anisotropy is a signature of the correlation between the field and dipole

44
Q

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

A

those with a dipole parallel to the field

this gives rise to an anisotropic ensemble of photoexcited molecules

45
Q

What is axial recoil

A

Axial recoil is where product atoms recoil parallel to the breaking bond

46
Q

What is the primary source of tropospheric OH

A

reactions of water and O(1D)

47
Q

How does time-dependent fluorescence spectroscopy work

A
  • 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
48
Q

What do we measure in femtochemistry

A

Loss of parent and build up of products using a pump and a probe laser

49
Q

What two detection techniques are used for ICN photolysis

A

REMPI

LIF

50
Q

How does a wavepacket arise

A

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

51
Q

How does a wavepacket behave

A

the centre of a wavepacket evolves like a particle under influence of a potential; the wavepacket itself behaves quantum mechanically

52
Q

Give 3 advantages of using a laser

A
  • high resolution
  • high sensitivity
  • long path lengths (by multi-passing)
53
Q

Give 4 limits of direct absorption measurements

A
  • 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
54
Q

How does cavity enhanced absorption spectroscopy work

A

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

55
Q

How does cavity ring down spectroscopy work

A

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

56
Q

What are the pros of cavity ring-down spectroscopy

A

Quantitive, delta intensity minimum measured is 10^-8 therefore much more sensitive

57
Q

What is the method of choice for electronic spectroscopy

A

laser induced fluorescence

58
Q

How does LIF work

A
  • 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
59
Q

Give 3 limitations of LIF

A
  • 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
60
Q

Give 2 cool applications of LIF

A
  • jet-cooled molecular species in supersonic beams

- spatially resolved species detection and concentration profiling

61
Q

How does single molecule spectroscopy work

A

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

62
Q

Give 2 variants of single molecule spectroscopy

A

Confocal fluorescence microscopy and wide-field epi illumination

63
Q

How does wide-field epi illumination work

A
  • 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
64
Q

Why is single molecule spectroscopy useful

A

It allows study of energy transfer within or between isolated molecules as opposed to an emsemble

65
Q

Explain observation of S1 blinking in an individual molecule

A
  • Chromophore is a single quantum system therefore digital behaviour
  • Excite S1
66
Q

Give 1 limitation to single molecule spectroscopy

A

photobleaching (light induced destruction of pi-system on repeat excitation) –> higher states, excited state absorption

67
Q

Explain how fluorescence confocal microscopy works

A
  • 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)
68
Q

How does super resolution fluorescence microscopy work

A
  • 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
69
Q

What is Forster Resonant Energy Transfer?

A
  • 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
70
Q

In FRET, what does the energy transfer efficiency E depend on (2)

A
  • Overlap of D emission and A absorption spectra

- relative orientations of transition dipole moments of D emission and A absorption spectra

71
Q

What is a really good use of FRET

A

Proteins: physical interactions where relative proximity of molecules must be determined more precisely than possible with traditional diffusion limited imaging methods

72
Q

What is the condition for FRET observation

A

A and D must be close in space

Physical (4 decks)

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