Homogeneous linewidth of the S₁←S₀ transition of free-base porphyrin in an n-octane crystal as studied by photochemical hole-burning

The homogeneous linewidth of the 0-0 band of the S1←S 0 transition of free-base porphyrin (H2P) in an n-octane matrix and its temperature dependence (T = 1.5-4.2 K) have been measured by means of photochemical hole-burning. The linewidth extrapolated to T = 0 is ∼9 MHz for the two types of sites which have been investigated, and it appears to be determined entirely by the decay time of the emitting state S 1 (17 ns). The temperature dependence of the linewidth is strongest for the thermodynamically less stable sites. The increase in linewidth with temperature may arise from a relaxation process involving a low frequency resonance mode (∼5 cm-1) localized at the H2P molecules. At 77 K the homogeneous linewidths are comparable to the inhomogeneous widths of 3-4 cm-1; hence, hole-burning is no longer observed. Copyright © 1977 American Institute of Physics.