Salient Results Decay Times of Excited States

5 Salient Results Decay Times of Excited States 

While DFWM and other NLO measurement techniques yield information on response and decay times as well, as some of the data cited above indicate, additional data are obtainable from such techniques as pump-and-probe ps or fs transient absorption spectroscopy. Some of the first such data were collected by the Shank group at the erstwhile Bell Labs [524]. Fig. 12-31 shows their data for a 2.0 eV pump for cis-and trans-P(Ac), whilst Fig. 12-32 shows photoinduced absorption, at 1.4 eV, of P(cis-Ac) on a ps time scale. In the latter case, the rise time is less than 150 fs, the 

In an interesting smdy on P(3-dodecyl thiopene). Pang and Prasad [527] showed the importance of the laser pump pulsewidth in the measured 3rd-order NLO response. They found that with 60 fs pulses at 620 nm, this parameter is sixfold smaller than with 400 fs pulses, since the relaxation time of the photogenerated excitation responsible for the optical nonlinearity is comparable to the pulsewidth. 

Photoinduced difference spectra at 0.5 and 200 psec (a) for trans-polyacetylene and (b) for cw-polyacetylene, excited at 2.0 eV. After Reference [524], reproduced with permission. 

Apart from P(Ac), magnetic susceptibility measiurements have been carried out for very few CPs. Values for the paramagnetic susceptibilities, xipora) of cis- and trans-P(Ac), typically ca. 10 and 3 X 10 emu/mol respectively, [458], may be compared with that for a marginal semiconductor (PVC, irradiated, ca. 10 emu/mol), and the diamagnetic susceptibility x(dia) for Si, ca. -3 X lO . Undoped Shirakawa trans-P(Ac) shows adherence to the Curie law down to ca. 1.5 K, with ca. 10 spins/C-atom, whilst Durham trans-P(Ac) shows such behavior only to ca. 30 K, due to impurities. 

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Salient Results Decay Times of Excited States. . 361... 

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