Abstract
Reflection spectroscopy and Kramers-Kronig analysis are used to locate the intense electronic transition of internal charge transfer character in 4-nitro-pyridine- n-oxyde (NPO) single crystals at 388 nm ( c-polarized) and at 339 nm ( a-polarized). The large excitonic splitting of 3860 cm −1 is attributed to dipole-dipole interactions. The observed polarization ratios cannot be explained with an oriented gas model but suggest the contribution of intermolecular charge transfer states. The spectral width of the c-component at 2 K (500 cm −1) is attributed to the rapid decay to a lower electronic state. Such a state is located in transmission spectra which reveal a weak, sharp, and structured absorption assigned to a nπ* transition with an out-of-plane polarized 0-0 line at 429 nm. The implications of these observations for the optimization of nonlinear organic crystals are discussed.
Published Version
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