Page 78 - Photoreactive Organic Thin Films

2, ULTRAFAST DYNAMICS IN THE EXCITED STATES OF AZO COMPOUNDS 57

o 0.0
400 500 600 700
Wavelength (nm)

FIG. 2.3 Absorption spectra of DMAAB/DMSO at 0.2 (solid curve) and 0.01 (dashed curve) mol
3
dm" . The spectrum of NOPA (noncollinear optical parametric amplifier) output (dotted curve) was
used for both pump and probe pulses.

pump-probe experiments. The pulse spectrum has a small but finite overlap
with the sample's n-n* absorption band tail. The absorption coefficients at
1 2
420nm(n-jc*) and 600nm(n-7t*) are 30,000 and ZSOmol- dm , respectively.
By separating the absorption spectrum into n-n* and n-7t*, the fraction of the
former excitation is estimated as 70%.

2.3.2.1 Wave Packet in the Ground and Excited States
It is well known that a coherent wave packet can be generated both in
the ground and excited states and that the ratio between the amounts of the
wave packets excited on the two potential curves is sensitive to the chirp
28 30
direction. " Positively chirped (PC) and Fourier-transform limited (TL)
pulses excite a wave packet in the excited state much more efficiently than in
the ground state, whereas negatively (NC) chirped pulses with an appropriate
rate induces a wave packet more efficiently in the ground state. This is
because the damping of the wave packet photogene rated by the leading edge
of the pulse takes place by the trailing edge of the NC pulse in cases where
the chirp rate is close to the lowering rate of the molecular system's potential
energy in the excited state. We prepared NC, PC, and TL pulses for sample
excitation by changing the insertion amount of a prism pair used as a pulse
compressor in NOPA. The second-harmonic generation frequency-resolved
31 32
optical gating (SHG-FROG) measurement ' was used for the pulse charac-
terization. The quadratic phase terms <l>"(v} of the NC and PC pulses were
2
determined from the FROG traces to be -930 and 990 fs , respectively. The
TL pulse is slightly down-chirped, but the absolute value of the residual chirp
2
is smaller than 330 fs in the main part of the pulse. The full width at half
maximum of the NC, PC, and TL pulses are 9.6, 8.4, and 7.4fs, respectively.