Knowledge Bank

The absorption spectra of several aromatic hydrocarbons in the crystalline form and in solid-solutions of lucite have been measured as a function of compression in a high-pressure optical cell modeled after the one evolved by $Drickameretal.1$ It is found that with increasing pressure in the 0--30 kbar range the absorption bands of linear and angular polyacenes, as well as of pericondensed polycyclics, show pronounced red shifts, increases in bandwidth, and slight variations in intensity. The shifts in lucite solutions appear to depend quadratically on density and linearly on oscillator strength, in conformity with theoretical prediction. The larger rate of red shifts per given density interval observed for the poly-crystalline compounds can be qualitatively interpreted as resulting from exchange interactions in the molecular crystals. A blue shift observed in the $Lb$ band of the pseudoaromatic compound azulene can be interpreted on the basis of its polarity in the two states involved in the transition.