High-pressure studies of second harmonic generation in organic materials

The effect of pressure to 90 kbar has been measured on the change of second harmonic generation (SHG) efficiency with pressure for six organic materials as powders and as crystals. Increases of as much as 30 or more (4-aminobenzophenone) and decreases as large as a factor of 40 (urea) are observed. Evidence for both first-order and higher-order phase transitions is seen. For methylnitroaniline, the shift of the absorption maximum parallels the change of SHG. For N-isopropylcarbazole, the change in luminescence intensity reflects the same events as the SHG signal. For all materials except N-isopropylcarbazole, the effects are reversible.