Refereed Publications
- Jun Kubota, Koji Kusafuka, Akihide Wada, Kazunari Domen,
and Satoru S. Kano, "Time-Resolved Sum-Frequency
Generation Spectroscopy of Methoxy and Deuterated Methoxy
on Ni(111) Using Near-Infrared Laser Pulses," J.
Phys. Chem. B. vol. 110, pp. 10785-10791, 2006.
Abstract - Methoxy (CH3O-)
and deuterated (d-) methoxy (CD3O-) species on Ni(111)
are investigated by sum-frequency generation (SFG) spectroscopy.
Methoxy adsorbed on the Ni(111) surface is confirmed by
SFG spectroscopy to be oriented normal to the surface.
Two resonant peaks produced by methoxy, at 2921 and 2821
cm-1, are assigned to Fermi resonance between
the CH symmetric stretching and overtone modes. Deuterated
methoxy exhibits a single strong peak at 2051 cm-1
assigned to the CD symmetric stretching mode. Investigation
of the sub-nanosecond transient behavior of methoxy and
d-methoxy species on Ni(111) under short-pulse laser pumping
at 1064 nm reveals a clear weakening and recovery of the
SFG peaks upon heating. The observed temporal profile
is reproduced by simulation assuming that the original
methoxy in the ground state is in chemical equilibrium
with a new state produced by instantaneous heating. The
dependence of the SFG spectra on the initial substrate
temperature is also reproduced by the simulation. The
simulation suggests a temperature jump of 250 K upon laser
pumping, inducing a change in the molecular orientation
or adsorption site of methoxy on the Ni(111) surface without
decomposition of methoxy to adsorbed CO and hydrogen,
which occurs under normal heating at 200 K.
- I. Otake, S. S. Kano, and A. Wada, "Pulse shaping
effect on two-photon excitation efficiency of alpha-perylene
crystals and perylene in chloroform solution," J.
Chem. Phys, vol. 124, 014501:1-5, 2006.
Abstract - We demonstrated
that the two-photon excitation efficiency of perylene
in chloroform solution as well as that of crystalline
perylene was dramatically increased by optimizing the
shape of intense femtosecond laser pulses of a regenerative
amplifier output. The efficiency was three times higher
than for an unshaped single femtosecond pulse with the
pulse width of shorter than 50 fs. The pulse shape optimized
for the solution sample was a pulse train with a repetition
frequency of about 340 cm-1, and the pulse
shape optimized for crystalline perylene was very similar.
These results supported our previous findings on alpha-perylene
crystals using weak femtosecond pulses from a mode-locked
laser oscillator. Furthermore, it was confirmed that the
shaped pulse optimized for the liquid sample could also
increase the two-photon excitation efficiency of alpha-perylene
crystals and vice versa. We concluded that the mechanism
for the increase in excitation efficiency of perylene
in chloroform was almost the same as that for alpha-perylene
crystal, and that the efficiency increased mainly through
intramolecular dynamical processes. Processes involving
intermolecular interactions and/or energy states delocalized
over the crystal cannot play the major role.
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