Page 82 - Photoreactive Organic Thin Films

2, ULTRAFAST DYNAMICS IN THE EXCITED STATES OF AZO COMPOUNDS £ j

frequencies of molecular vibration modes in the excited (A) and in the ground
(B) states. They were calculated by integrating the TFD along the frequency
axis to study the correlation between them. Figure 2.6A, which corresponds
to the modulation power spectrum of the instantaneous frequency in the
excited state, shows the modulation period of the two vibration modes in the
excited state. There exists a relatively good coincidence and strong correla-
tion between the two modes of C-N and N=N stretching. Figure 2.6B which
corresponds to the ground states, shows weaker correlation between the
modulations of the N=N and C-N stretching frequencies.
It is to be stressed that the sub-5fs real-time spectroscopy enabled by the
extremely short pulse allows us to make a detailed analysis of the dynamical
mechanism of the chemical reaction. We can also gain information about
geometrical change in the excited state by studying the changes of frequency
and phases of molecular vibrations relevant to the chemical reaction. This
can be done only with the use of a laser with a sufficiently short and stable
pulse to determine the change in the molecular vibration period brought
about by modulations as small as 5% of the period corresponding to ~lfs.
38
The first part, Section 2.3.1, was published elsewhere, and the second
part, Section 2.3.2, has been submitted for publication. 39

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