hatcher content part 10

what are the 3 primary processes of photoinduced energy and electron transfer?

A* + B → A + B* (PEnT)

A* + B → A+ + B- (PET)

A* + B → A- + B+ (PET)

can be intra or intermolecular

what are the two types of donor systems for PEnT?

Förster - dipole dipole interaction

Dexter - through bond interaction

what are pump probe measurements?

require pulsed pump (excitation) and probe (measurement) pulses that are synchronised to arrive in a specific time sequence

sample is irradiated by a pump pulse and a fraction of the molecules are promoted to an electronically excited state

the sample is then measured by a probe pulse that arrives after a time delay (change in t)

vary t and make measurements at each step, a series of time resolved snapshots are collected that can be faster than the normal time resolution of the set up

what is TA spectroscopy?

transient absorption spectroscopy

time resolved absorption measurements

a pump-probe spectroscopic technique used to
measure the photogenerated excited state absorption energies
and associated lifetime
nanoseconds (ns) to femtoseconds (fs)

“pump” light is usually a monochromatic laser pulse, while
the “probe” is a pulsed white light source

in situ measurements

what can you measure with femtosecond scale transient absorption spectroscopy?

C-O bond cleavage

transition states/primary photoproducts

relaxation pathway from transition state

how are TA spectra normally presented?

difference absorption spectrum

excited state spectrum - ground state spectrum

useful because typically only a fraction of the molecules are excited

includes any absorption change as a function of time and wavelength

what effects does a TA spectrum reflect the sum of?

ground state bleaching (negative)

further excitation to higher excited states (positive)

stimulated emission (negative)

product absorption (positive)

how do you calculate the A(t)?

Ao x e ^ (kt)

A(t) is absorbance at time t

Ao is initial absorbance

k is decay constant

how do you calculate the half life?

ln(2)/k

how do you calculate the lifetime?

1/k

k is decay constant

how do you calculate the mean lifetime?

A(t) = Ao x e ^ (t/lifetime)