Abstract
Nonlinear absorption spectra of single $\ensuremath{\alpha}$-perylene nanocrystals of 150--300 nm in size were measured at low temperature by multichannel phase-sensitive pump-probe microspectroscopy. For nanocrystals, free exciton (FE) absorption blueshifts while self-trapped exciton (STE) emission redshifts and FE emission is intense, compared with bulk crystals. The mechanism of the redshift is suggested to be lattice softening from a redshift in space-resolved STE emission at the edge of bulk crystals. The FE absorption band of several single nanocrystals shows a double-peak structure, suggesting that the blueshift is due to ${B}_{u}$ excitons, which are much stronger in oscillator strength than ${A}_{u}$ excitons, yet difficult to observe in bulk crystals.
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