|
|
Relaxation of photo-excited exciton states
- From band structure models for quasi-one-dimensional materials follows:
The absorbing state
a lies energetically above the indirectly emitting state e.
- Primary processes:
1. Pump (excitation into absorbing state a by short pulse)
2. Ultrafast exciton relaxation into emitting state e
3. Probe: A delayed pulse causes excited state absorption (ESA) from e into
higher lying states Sn.
|
|
 |
|
|
|
|
|
|
Excited state absorption
- For interpretation of pump-probe measurements, nature of higher excited states must be known
- Measurements of broad band transient spectra (by pump probe) are combined with quantum
chemical studies (cooperation with Karin Schmidt, Atlanta)
- First step is investigation of isolated molecules
- For matrix isolated molecules of MePTCDI and PTCDA, transient spectra are now fully explained
by comparison with quantum chemistry (see figure)
|
|
|
|
|
|
|
|
|
Ultrafast relaxation processes
- For both materials MePTCDI and PTCDA, ultrafast relaxation is detected
- Pump-probe scheme as in Figure above
- Pump pulse into lowest absorption peak (absorption spectrum dash-dotted in Panel (a)
and (c))
- Transient absorption spectrum shown by solid line in panel (a) and (c)
- Probe pulse shown by shaded area (energetically below linear absorption spectrum),
within strong transient absorption range)
- Pump-Probe signal vs. delay in panels (b) and (d), in comparison with time resolution
(dashed line)
- Relaxation process clearly time resolved as delayed rise of pump-probe signal
(65 fs for MePTCDI and 100 fs for PTCDA)
|
|
|
|
|
|
|
|
|
Luminescence and Pump-Probe anisotropy
- Two nonequivalent molecules in the unit cells of MePTCDI and PTCDA lead to Davydov
splitting of the exciton bands from the one-dimensional models
- Orientation of the molecules and polarization ratio is very different for
our two model materials
- Investigation of luminesence and pump-probe anisotropy gives information about
relaxation between different Davydov components
|
|
|
|
|
|
|
|
|
