T. Manaka et al., OBSERVATION OF THE LOWEST A(G) EXCITED-STATE LYING IN THE OPTICAL GAPIN POLYDIACETYLENE BY MEANS OF 2ND-HARMONIC GENERATION SPECTROSCOPY, Synthetic metals, 95(2), 1998, pp. 155-158
We have observed second-harmonic generation (SHG) spectra in polydiace
tylene (poly [5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], PDA-4U2 fo
r short) films of red and blue forms. In addition to resonant peaks lo
cated at the high-energy side of the main absorption peak, a new reson
ant peak was observed at the lower-energy side of the absorption peak
in blue-form PDA-4U2. In contrast, in red-form PDA-4U2, the SHG resona
nce peak position in the lower-energy side coincides with the absorpti
on peak position. The resonance enhancement of SHG originates from the
electric-quadrupole transition to the dipole-forbidden state. The low
est resonance peak observed in the blue-form PDA-4U2 strongly suggests
the existence of the lowest A(g), excited state lying below the lowes
t B-u, excited state. (C) 1998 Elsevier Science S.A. All rights reserv
ed.