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

Abstract

The homogeneous linewidth of the 0–0 band of the S₁←S₀ 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 S₁ (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.