Hole-burning Study Research Articles

Exciton transfer and dephasing in polysilanes at low temperature

We report on time-correlated single photon counting studies of the sigma conjugated polysilane, poly(di-n-hexylsilane), at 2 K. The emission processes of Raman and dephased flourescence are distinguished. It is shown that the risetime of flourescence and the decay time of the Raman polarization are similar and that energy transfer occurs on a tens of picosecond timescale. Simultaneous depolarization of the emission suggests energy transfer along a twisted chain. The energy transfer time is several times longer than the (T1) dephasing time derived from hole burning studies of the polymer.

Temperature-dependent spectral hole-burning study of dye–surface and mixed matrix–dye–surface systems

Temperature-dependent persistent spectral hole-burning measurements are reported on the dye–surface system Quinizarin/γ-alumina (Q/γ-Al2O3) and on the mixed matrix–dye–surface systems 3-methylpentane/Q/γ-Al2O3, methylene chloride/Q/γ-Al2O3, and n-heptane/Q/γ-Al2O3. In all cases the temperature dependence of the hole-burning linewidth ΓHB could be best fitted to a combination of a Tα law and a contribution from an exponentially activated process, indicating that the dynamics of the mixed systems are similar to Q/γ-Al2O3. For Q/γ-Al2O3 the power-law term shows clearly the glasslike nature of the γ-Al2O3 surface. The temperature dependence of ΓHB yielded a large coupling constant for the exponentially activated term for n-heptane/Q/γ-Al2O3, indicating a higher degree of crystallinity for the maxtrix n-heptane than for methylene chloride. The mixed matrix–dye–surface systems generally exhibit a smaller contribution of two-level-system modes to exponentially activated processes than does Q/γ-Al2O3. The distance from the zero-phonon line peak to the phonon side-hole peak ν˜0 differs by a factor of 3, demonstrating a distinct dependence of the dominant low-frequency mode on the chromophore environment. A very large ν˜0 of ≈50 cm−1 was found for the dye–surface system Q/γ-Al2O3. ν˜0 does not correlate to the frequency of the exponentially activated process. For Q/γ-Al2O3 stable holes could be burned even at 77 K. The Debye–Waller factor was measured for this system over an extended temperature range.

Energy transfer dynamics of the B800—B850 antenna complex of Rhodobacter sphaeroides: a hole burning study

Hole burned spectra for the B800—B850 bacteriochlorophyll a antenna complex of Rb. sphaeroides chromatophores have been obtained for burn wavelengths ranging from the high-energy side of the B800 absorption profile to the low-energy side of B850 profile. The results indicate that B800—B800 energy transfer at 1.6 K is slow relative to B800 to B850 transfer which occurs in 2.4 ps. Whereas B800 is largely site inhomogeneously broadened, B850 is found to exhibit a homogeneous linewidth of about 220 cm −1 at low temperatures. The latter results is used to explain the temperature independence of the B800 to B850 energy transfer rate. The homogeneous linewidth of 220 cm −1 is argued to be a measure of the exciton bandwidth of B850. It is concluded that B800 and B850 are weakly exciton-coupled and that the mechanism for B800 to B850 energy transfer is most likely of the Förstertype. The B850 vibronic hole structure allows for an identification of bacteriochlorophyll a modes that participate in Förster transfer.

Temperature dependent hole-burning study of dye molecules absorbed on metal oxide powders

We report on the investigation of quinizarin adsorbed to the surface of γ-alumina powder with persistent spectral hole-burning. In this particular material the chromophore is strongly bound (chemisorbed) to the surface, and it was shown that the holewidth at 1.6 K is narrower than for other dye—surface systems. The temperature dependence of the hole-burning linewidth between 1.6 and 10 K was best fitted by a combination of a T α contribution referring to the glass-like nature of the disordered γ-alumina surface.

Effects of detergent on the excited state structure and relaxation dynamics of the photosystem II reaction center: A high resolution hole burning study

Low temperature (4.2 K) absorption and hole burned spectra are reported for a stabilized preparation (no excess detergent) of the photosystem II reaction center complex. The complex was studied in glasses to which detergent had and had not been added. Triton X-100 (but not dodecyl maltoside) detergent was found to significantly affect the absorption and persistent hole spectra and to disrupt energy transfer from the accessory chlorophyll a to the active pheophytin a. However, Triton X-100 does not significantly affect the transient hole spectrum and lifetime (1.9 ps at 4.2 K) of the primary donor state, P680(*). Data are presented which indicate that the disruptive effects of Triton X-100 are not due to extraction of pigments from the reaction center, leaving structural perturbations as the most plausible explanation. In the absence of detergent the high resolution persistent hole spectra yield an energy transfer decay time for the accessory Chl a QY-state at 1.6 K of 12 ps, which is about three orders of magnitude longer than the corresponding time for the bacterial RC. In the presence of Triton X-100 the Chl a QY-state decay time is increased by at least a factor of 50.

Excited-state structure and energy-transfer dynamics of two different preparations of the reaction center of photosystem II: a hole-burning study

Two distinctly different preparations of the D{sub 1}-D{sub 2}-cytochrome b-559 photosystem II reaction center (RC) complex are shown to exhibit very similar 4.2 K absorption and hole-burned spectra in glasses. Absorption profiles for P680 and the active pheophytin are fully characterized with the mean zero-point energy of P680{sup *} lying {approx}25 cm{sup {minus}1} lower in energy than the Q{sub y} state of the latter. Decay of the accessory chlorophyll and pheophytin Q{sub y} states due to energy transfer is shown to be about 3 orders of magnitude slower than in bacterial RC.

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

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 2P) in crystalline n-octane and in amorphous polyethylene (PE). Optical dephasing experiments on the S 1←S 0 0–0 transition of H 2P 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.

A hole burning study of excitonic states of chain molecules in glasses

Thermal broadening of spectral holes burnt into excitonic states of long chain molecular aggregates of pseudoisocyanine iodide is investigated over a temperature range from 350 mK to 80 K. The results differ very much from the usual behavior of small molecules in glasses. We found that extended states are almost completely decoupled from the amorphous host lattice and spectral diffusion effects play a minor role. The homogeneous linewidth is independent of temperature below 10 K. Above, thermal broadening occurs in two steps: there is a weak onset around 15 K and a strong onset around 65 K. The data can be fitted by a superposition of two exponentials.

Direct observation of the excited-state c i s–t r a n s photoisomerization of bacteriorhodopsin: Multilevel line shape theory for femtosecond dynamic hole burning and its application

A dynamic hole-burning study of light-adapted bacteriorhodopsin (BR568) using 6 fs optical pulses has recently been reported [R. A. Mathies, C. H. Brito Cruz, W. T. Pollard, and C. V. Shank, Science 240, 777 (1988)]. The temporal evolution of the excited state absorption and emission spectra after excitation with 60 fs pulses provides a direct observation of the C13=C14 torsional isomerization of the retinal chromophore on the excited state potential surface. Here, we present a more detailed discussion of these spectra. The transient hole line shapes are then calculated by solving the density matrix equations for the third-order susceptibility of a multilevel system. The resulting equations are written without reference to the individual vibronic transitions by using the absorption correlation function 〈i‖i(t)〉. The calculations show that the sharp features seen at short delays arise from coherence coupling effects which occur when the pump and probe pulses overlap in time. This analysis demonstrates that the hole seen at 60 fs is consistent with the broad homogeneous absorption line shape for BR568 originally predicted from resonance Raman intensities, and points out the utility of 〈i‖i(t)〉, derived from resonance Raman intensity analysis, in understanding femtosecond dynamic hole-burning experiments.

Solute-solvent dynamics and interactions in glassy media: Photon echo and optical hole burning studies of cresyl violet in ethanol glass

Photon echo and non-photochemical hole burning experiments on cresyl violet in ethanol glass are reported. At 1.5 K the optical dephasing time measured by the picosecond echo is eight times longer than the time measured by hole burning because of the difference in the time scales associated with the measurements. The observed temperature dependence arises from the glass's two-level systems and from a pseudolocal phonon mode at higher temperatures. Comparing the ratios of the echo to hole burning dephasing times for cresyl violet and resornfin, suggests that their dephasing is influenced by the nature of their ethanol solvation shells in addition to the dynamics of the bulk ethanol.

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

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 2P) 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.

An investigation of the mechanism of non-photochemical hole burning of resorufin in ethanol glass

Non-photochemical hole burning (NPHB) studies of resorufin in glasses composed of mixtures of ethanol/ethanol- d (hydroxyl hydrogen deuterated) are reported. The relative hole burning efficiencies, determined from hole area per photon absorbed, are found to show an essentially quadratic dependence on the concentration of ethanol. The mechanism of NPHB is discussed in terms of a model of photoinduced concerted rearrangement of intermolecular hydrogen bonds between two ethanol molecules and a resorufin molecule.

Site-selective spectroscopy and level ordering in C-phycocyanine

We present a combined fluorescence and hole-burning study of the biliprotein C-phycocyanin. Sharp zero-phonon holes compare with a broad structureless fluorescence. This finding is rationalized in terms of the special level structure in this pigment, the fast energy-transfer processes and a lack of correlation of the energies of the emissive states.

Optical dynamics of the reaction center of photosystem II. A hole-burning and photon-echo study

Photon-echo and transient hole-burning experiments on the P-band of the reaction center of photosystem II are reported which show that the P-band exhibits a large homogeneous width. A photon-echo signal with a decay time of 500 ps is also observed in the same spectral region and assigned to extraneous chlorophyll. These observations are discussed with reference to the bacterial reaction center and photosystem I.

Picosecond photon echo and optical hole burning studies of chromophores in organic glasses

The range of relaxation rates present in amorphous systems is shown to affect the results of optical dephasing measurements. The interpretation of these experiments in terms of four-time correlation functions is introduced. Photon echo and hole burning results are shown for resorufin in three glass hosts: ethanol, d-ethanol, and glycerol. The hole burning data are shown to be affected by slow relaxation processes whereas the echo results are not. The implications of a slowest relaxation rate in amorphous systems are also considered.

Zeeman effects in amorphous solids: A hole-burning study of free-base porphin in polyethylene

Zeeman shifts of the order of 10 −4 to 10 −3 of the inhomogeneous spectral linewidth have been observed in the 0-0 transition of free-base porphin embedded in amorphous polyethylene at 4.2 K by means of optical hole burning. From fits of the experimental results to calculated line shapes as a function of frequency and field strength, the value of the change in the magnetic susceptibility of the molecule in a magnetic field on excitation to the first excited singlet state, 1 2 χ z′,z′ = 60.6 MHz/T 2, was determined. This is in good agreement with the value previously found for the same molecule in a crystalline host.

A spectral hole-burning study of long-wavelength chlorophyll a forms in greening leaves at 5 K

Persistent holes have been burnt in the spectra of post-etiolated maize leaves in the spectral interval 700–745 nm at 5 K. Narrow (< 1 cm −1) holes detectable both in fluorescence and excitation spectra correspond to the zero-phonon purely electronic S 1S 0 transition of chlorophyll a rather than to the S 1←S 0 vibronic or S 1 ←S 0 higher singlet transitions. The spectral composition of the inhomogeneously broadened absorption and fluorescence contours of long-wavelength chlorophyll a forms in plants is discussed in terms of the contributions of phonon-wings, vibronic satellites, purely electronic S 1S 0 transitions and its higher excitonic components.

Optical dynamics of condensed molecular aggregates: An accumulated photon-echo and hole-burning study of the J-aggregate

Accumulated photon-echo and hole-burning experiments on the well-known J-bands of pseudoisocyanine bromide in an aqueous ethylene glycol glass are reported. From the low-temperature photon-echo decay of ≈14 ps and the assumption that this lifetime is purely radiative, the mean number of molecules in the aggregates is calculated to be about 500. It is shown that the temperature-activated optical dynamics of the excitonic J-bands can best be explained by invoking the presence of a pseudo-localized degree of freedom of ≈9 cm −1. It is concluded that photoionization is the primary process that causes hole burning of the J-bands and that the charge-separated state forms the bottleneck in the optical pumping cycle.

Hole-burning study of zero-phonon linewidths in organic glasses

Methods, models, and experimental results on the homogeneous broadening of zero-phonon lines in the spectra of impurity molecules in organic glass-like matrices at low and superlow temperatures are reviewed.

Evidence for a very early intermediate in bacterial photosynthesis. A photon-echo and hole-burning study of the primary donor band in Rhodopseudomonas sphaeroides

Two coherent spectroscopic methods, accumulated photon echo and population bottleneck hole-burning, have been employed in a study of the decay rate of the primary donor (P) of Rhodopseudomonas sphaeroides at 1.5 K. The decay rate is instrument-limited in the photon-echo experiment, implying a population relaxation time <100 fs. The hole-burning study revealed the P absorption at 900 nm to be largely homogeneously broadened, from which a decay time of = 25 fs was inferred. Comparison of these data with a photon-echo study of the bacteriochlorophyll a monomer suggests that this ultrafast process is not due to vibrational relaxation within P *, but to an excited state electronic decay mechanism. It is suggested that the initial event after excitation in P is a very rapid charge separation within the dimer pair, prior to the electron-transfer process, which occurs on a much longer timescale.