S. Völker

Thermal broadening of optical homogeneous linewidths in organic glasses and polymers studied via photochemical hole-burning

Abstract Photochemical hole-burning was used to study optical dephasing in the S1 ← So OO transitions of dimethyl-s-tetrazine, chlorin and free-base porphin in several organic glasses and polymers. The homogeneous width (Γhom) seems to follow a T1.3±0.1 law between 0.4 and 20 K for all guest—host pairs, and extrapolates the lifetime-limited values of each guest for T→0. For a given temperature, Γhom, appears to be related to the number of OH groups in the glass, and to the size of side-groups on the polymer chain.

Energy transfer in the B800–850 antenna complex of purple bacteria Rhodobacter sphaeroides: A study by spectral hole-burning

Abstract The energy-transfer process within the isolated B800–850 pigment-protein complex of the purple bacterium Rhodobacter sphaeroides has been studied by means of spectral hole-burning. The band at 800 nm is inhomogeneously broadened because holes could be burnt into it. The widths of these permanent holes are independent of wavelength and temperature between 1.2 and 30 K. The BChl 800→BChl 850 energy-transfer time deduced from these widths is 2.3±0.4 ps for the isolated complex, and also for chromatophores at 1.2 K.

Optical linewidths and dephasing of organic amorphous and semi-crystalline solids studied by hole burning

Abstract Optical homogeneous linewidths Γ hom of organic molecules in glassy hosts can be determined by means of hole burning. This process may either be the result of photochemistry (photochemical hole burning) or of a relative reorientation of the guest-host system (nonphotochemical hole burning) on laser excitation. The experiments described here have been performed on the 0−0 S 1 ←S 0 transitions of a large variety of amorphous and semi-crystalline systems at temperatures between 0.3 and 20 K. It was found that, independent of the hole-burning mechanism, Γ hom follows a T 1.3 temperature law, and extrapolates to or in fact attains the fluorescence lifetime-limited value of the guest for T → 0. From a study of the holewidth dependence on laser power, burning time, sample preparation and detection method, conditions could be established for the determination of Γ hom and the study of spectral diffusion. Semi-crystalline materials, as opposed to amorphous systems, have a much steeper Γ hom dependence on temperature than T 1.3 which varies with the degree of crystallinity of the polymer host. The results show that hole burning is a very sensitive technique to probe the amount of disorder of the environment directly surrounding the guest molecule. An interpretation of the experiments in terms of various theoretical models for dephasing in glasses is presented.

Photochemical hole-burning down to 0.3 K in organic amorphous systems

Abstract The homogeneous linewidth (Γhom) of the OO S1 ← S0 transition of free-base porphin (H2P) in polyethylene and diglycerol has been studied via photochemical hole-burning down to 0.3 K. On lowering the temperature the Γhom α T1.3 relation previously observed goes over to a linear dependence at the point where Γhom ≈ 60 MHz. At ≈ 0.3 K Γhom in H2P in polyethylene reaches the lifetime-limited value of ≈ 10 MHz. Two recently proposed models that attempt to explain these results are discussed.

Optical relaxation in organic disordered systems submitted to photochemical and non-photochemical hole-burning

Abstract The temperature dependence of the homogeneous linewidth (Γhom) of S1 ← S0 0-0 transitions of organic amorphous systems undergoing either photochemical or non-photochemical hole-burning from 4.2 down to 0.3 K is presented. In all cases Γhom follows a T1.3 dependence, and extrapolates to or actually reaches the lifetime-limited value of the guest. An estimate of the relative guest-host coupling strengths is made.

Does non-photochemical hole-burning reflect optical dephasing processes in amorphous materials? pentacene in polymethylmethacrylate as an affirmative example

Abstract Non-photochemical hole-burning is a reliable technique for the study of optical dephasing in organic glasses. This is proved by an analysis of the holewidth dependence on sample preparation, optical density, temperature, burning time and laser power. Holes were detected simultaneously by fluorescence excitation and transmission, and conditions for the occurrence of slow relaxation processes in the glass were found.

Optical dephasing in organic glasses between 0.3 and 20 K. A hole-burning study of resorufin and free-base porphin

Abstract Spectral holes have been burnt in the S 1 ←S 0 0-0 transition of resorufin in polymethylmethacrylate (PMMA) and glycerol, and of free-base porphin (H 2 P) in PMMA and polyethylene (PE). The holewidths follow a T 1.3 dependence over almost two orders of magnitude in temperature and extrapolate to the fluorescence lifetime-limited value of each guest when T →0. The discrepancy between the holewidths of resorufin in glycerol presented here and those reported in the literature are attributed to burning fluences. Photon-echo and hole-burning results are compared. Optical dephasing data are interpreted in terms of low-frequency localized phonon modes and are critically discussed.

Site-selection spectroscopy and hole-burning of ionic dyes in amorphous hosts at low temperature

Abstract The hole-burning (HB) mechanism of the ionic dyes resorufin and cresyl violet was investigated in various glasses and polymers at liquid helium temperature. By means of hole-filling experiments the spectral distance of the photoproduct absorption maximum to that of the original molecule δ max was determined. It was observed that δ max and the electron-phonon coupling strength, S , increase with host polarity, Z , in the ionic dyes studied, whereas δ max and S are both independent of Z in the neutral free-base porphin molecule. A linear relation between δ max and S was found for all cases. The results suggest that these ionic dye systems undergo intermolecular photochemical hole-burning with high HB efficiency. Furthermore, two differently solvated species of resorufin in alcoholic glasses (ethanol and glycerol) were identified, and the vibrational frequencies of ground and first excited singlet states were obtained for both species. It is inferred that one of the species is hydrogen bonded to the host, whereas the other is a “free” resorufin molecule. Although hydrogen-bonding solvents enhance the HB efficiency, they are not necessary for the hole-burning process to occur.

Spectral Distributions of “Trap” Pigments in the RC, CP47, and CP47−RC Complexes of Photosystem II at Low Temperature: A Fluorescence Line-Narrowing and Hole-Burning Study

Broad-band absorption and fluorescence, fluorescence line-narrowing (FLN), and spectral hole-burning experiments have been performed on the Qy-band of three subcore reaction-center complexes of photosystem II between 1.2 and 4.2 K:  the isolated reaction center (RC), the inner core antenna CP47, and the CP47−RC complex. In the RC, fluorescence line-narrowing (FLN) is observed for excitation wavelengths λexc ≥ 676 nm, whereas in CP47, this occurs for λexc ≥ 680 nm. The FLN spectra of CP47−RC appear to be the sum of the individual spectra of the RC and CP47, an indication that this complex has two “traps”. This has been confirmed from the spectral distributions obtained by measuring the hole depth (at constant hole width) as a function of λexc, as previously done for the RC [Groot, M. L., et al. J. Phys. Chem. 100, 1996, 11488]. The maxima of these distributions are at ∼682 nm for the RC “trap” and at ∼690 nm for the CP47 “trap” within the CP47−RC complex. Further support that the two distributions of pigme...

In search of spectral diffusion in glasses. A time-resolved transient hole-burning study of porphins in polyethylene

Abstract A transient hole-burning (THB) experiment with millisecond time resolution is described. The hole-broadening kinetics as a function of burning power is the same for free-base porphin (H 2 P) in crystalline n -octane and in amorphous polyethylene (PE). Optical dephasing experiments on the S 1 ←S 0 0–0 transition of H 2 P in PE between 0.3 and 4 K yielded identical results on time scales of 10 −3 and 10 2 s. The pure dephasing time seems to be linearly dependent on the fluorescence lifetime for at least six porphins in PE. The holewidths are not influenced by spectral diffusion.