Zero-phonon Hole Research Articles

Primary Charge-Separation Rate at 5 K in Isolated Photosystem II Reaction Centers Containing Five and Six Chlorophyll a Molecules

Although the primary charge-separation rate in the photosystem II (PSII) reaction center (RC) has been the subject of many time- and frequency-domain experiments, its value is still an unsettled issue, especially at or near room temperature. As a first step toward understanding its temperature dependence, it is important to have a reliable value for the rate at liquid helium temperatures. Presented are results from triplet bottleneck hole-burning (TBHB) experiments at 5.0 K on RCs isolated from spinach that contain either six or five chlorophyll a (Chl) molecules per two pheophytin (Pheo) molecules (referred to as RC-6 and RC-5). RC-5 possesses only one of the two peripheral Chls of the RC. The triplet state that serves as the population bottleneck is formed by charge recombination of the radical ion pair P680 + Pheo1- , where P680 is the primary electron donor and Pheo1 is the acceptor on the active (D1) branch. In the TBHB experiments, the laser burn intensity was varied over 2 orders of magnitude to assess the contribution from fluence broadening to the width of the zero-phonon hole (ZPH) burned into the P680 absorption band. The widths of the ZPH were also corrected for interference from a weak and sharp hole contribution from a state(s) that is unlikely to be involved in efficient charge-separation. The corrected ZPH widths for RC-5 and RC-6 were the same (2.3 ± 0.2 cm-1) and correspond to a charge-separation rate of (4.6 ± 0.4 ps)-1, in good agreement with a value recently measured for RC-6 by femtosecond pump−probe spectroscopy at 7 K (Greenfield, S. R.; Seibert, M.; Wasielewski, M. R. J. Phys. Chem. B 1999, 103, 8364). It appears that the removal of a peripheral Chl does not lead to structural changes in the core region of the RC that affect primary charge separation.

Carcinoma and SV40-Transfected Normal Ovarian Surface Epithelial Cell Comparison by Nonphotochemical Hole Burning

Results are presented of nonphotochemical-hole-burning experiments on the mitochondrial specific dye rhodamine 800 incubated with two human ovarian surface epithelial cell lines: OSE(tsT)-14 normal cells and OV167 carcinoma cells. This dye is selective for the plasma and inner membranes of the mitochondria, as shown by confocal microscopy images. Dispersive hole-growth kinetics of zero-phonon holes are analyzed with theoretical fits, indicating that subcellular structural heterogeneity of the carcinoma cell line is lower relative to the analogous normal cell line. Broadening of holes in the presence of an applied electric field (Stark effect) was used to determine the permanent dipole moment change for the S 0→S 1 transition in the two cell lines. For the carcinoma cell line, the permanent dipole moment change value is a factor of 1.5 higher than for the normal cell line. It is speculated that this difference may be related to differences in mitochondrial membrane potentials in the two cell lines.

Ohmic Brownian Oscillator Approach to Hole-Burning and Photon-Echo Spectroscopies

The multimode Brownian oscillator (MBO) model has been at the forefront in interpreting the subsystem−bath interaction manifestations in optical spectroscopy for probing homogeneous structure of chromophores in crystals and amorphous solids. The spectroscopic consequences of employing the underdamped MBO model with Ohmic dissipation in linear absorption, photon-echo, and hole-burning data of chromophores in solid hosts at low temperatures are investigated. The zero-phonon line (ZPL) in homogeneous linear absorption spectrum, the slow-decay component (due to ZPL) in photon-echo signal, and the zero-phonon hole (ZPH) in hole-burned spectra in host molecular solids at low temperatures are usually resolved from the multiphonon-transitions structure. In the MBO model, the harmonic vibrations (Brownian oscillators) are linearly coupled to bath modes. This coupling, with Ohmic dissipation, results in a maximum contribution of the bath modes to the ZPL region. This contribution affects the width of the ZPL profil...

On B800→B800 energy transfer in the LH2 complex of purple bacteria

The B800 Q y -states of the light harvesting 2 (LH2) photosynthetic complex of purple bacteria exhibit two relaxation channels. One is B800→B850 excitation energy transfer that occurs in ∼2 ps in low-temperature limit. This channel is dominant for excitation wavelengths located to lower energy of the B800 absorption band maximum. The second “additional” channel sets in for excitation on the high-energy side of the maximum. The mechanism for the additional channel has been controversial. That it is due to relaxation of Bacteriochlorophyll a (Bchl a) vibrational modes has already been excluded. This left intra-band B800→B800 energy transfer and relaxation of mixed B800–B850 exciton levels as possible mechanisms. High-pressure hole burning results for LH2 of Rhodopseudomonas acidophila are presented, which, together with excitonic calculations, indicate that the latter mechanism is relatively unimportant. Intra-band B800→B800 excitation energy transfer is considered from a probabilistic viewpoint. It appears that the B800 zero-phonon hole action spectrum corresponds to those of Bchl a sites on the C 9 ring that cannot undergo downward energy transfer to a nearest-neighbor site. One-third of the sites fall into this category. It is concluded that the additional decay channel is most likely due to above intra-band transfer.

Nonphotochemical hole burning spectroscopy of a mitochondrial selective rhodamine dye molecule in normal and cancerous ovarian surface epithelial cells

Results are presented of nonphotochemical hole burning experiments on the mitochondrial dye rhodamine 800 (MitoFluor Far Red 680, Molecular Probes) incubated with two human ovarian surface epithelial cell lines: OSE(tsT)-14 normal cells and OV167 carcinoma (cancer) cells. This dye is believed to be selective for the plasma and inner membranes of the mitochondria. Importantly, the dispersive (distributed) growth kinetics of zero-phonon holes were found to be significantly different with the OV167 line exhibiting the higher burn efficiency. This may reflect greater structural heterogeneity of that line. Equally interesting is that the permanent dipole moment change (Δ μ) for the S 0→S 1 transition of the dye is markedly different for the two cell lines, the Δ μ value for the carcinoma cell line being a factor of 1.5 higher. Discussion of this finding in terms of membrane potentials is given.

Monte Carlo simulation of energy transfer rates: application to downward energy transfer within the 825 nm absorption band of the FMO complex of Prosthechochloris aestuarii

Splitting of the individual subunit absorptions of the FMO complex of Prosthechochloris aestuarii due to disorder is determined by Monte Carlo simulation. The simulations accurately reproduce the 825 nm absorption band and indicate an upper limit for the inter-subunit coupling of 10 cm −1. From the simulations, rates for downward energy transfer between subunits can be determined using the spectral density determined from hole burning (J. Phys. Chem. B 104 (2000) 9564). On average the splitting between adjacent energy levels is ∼25 cm −1 and the energy transfer rate for relaxation between such levels is equal. Relaxation from the top to lowest level is significantly slower. This is in contrast with the experimental finding that relaxation from the top to bottom levels occurs at about twice the rate for the other two relaxations. Further, the rates determined do not agree with the observed variation of zero-phonon hole width and photon echo dephasing times with frequency. The possible involvement of Herzberg–Teller-like dark modes with a peak near 50 cm −1 is discussed.

A Model of Inhomogeneous Broadening and Pressure Induced Hole Shifts in the Optical Spectra of Organic Chromophores in Glasses

Zero phonon holes were burned in the S1rS0 absorption bands of organic compounds in ethanol glass at 6 K. The application of He gas pressure (P) leads to the shift of holes with a frequency coefficient d O/dP. The linear dependence of dO/dP on hole position (O) within the inhomogeneous band is of considerable interest, since it can provide the S1rS0 energies of nonsolvated chromophores and the (local) compressibility of the surrounding matrix. However, the extrapolated frequencies O0(P) at which pressure shift should vanish deviate from the actual 0-0 energies of free solutes in a vacuum. The slope (a) of the plot of dO/dP vs O varies between 1.5 10 -5 and 6.2 10 -5 bar -1 for different dyes, and therefore cannot be directly associated with the isothermal compressibility of the matrix. To account for these facts, a model was developed based on the assumption that the solvent shift is a superposition of repulsive, dispersive, electrostatic, and other possible interactions. Each interaction has a specific intermolecular distance ( r) dependence (e.g. r -6 for dispersive and r -12 for repulsive interactions) and is assigned a Gaussian frequency distribution function. It follows from the model that the observed slope a depends on, in addition to the volume compressibility of the matrix, the widths of the constituent Gaussians and the respective power coefficients of r. Both the measured bandwidth and the slope a are also sensitive to the type of correlation between different solvent shift mechanisms. In rhodamine dyes a dipole moment change between the ground and the excited state is responsible for a strong broadening and a shallow slope a. The conformational flexibility of a dicarbocyanine dye HIDCI has similar consequences. The mechanisms leading to a matrix polarity dependent hypsochromism in open chain cyanine dyes, free base tetrapyrroles, and s-tetrazine correspond to a distance dependence of r -6 and are deemed to be of multipolar nature. The magnitude of the hypsochromic solvent shift owing to repulsive forces is probably small, although their contribution to the slope a and the inhomogeneous bandwidth may be relatively large.

Non-Lorentzian zero-phonon holes and new insights on nonphotochemical hole burning: Al-phthalocyanine in hyperquenched glassy water

Zero-phonon hole (ZPH) profiles and hole spectra that span about eight decades of burn fluence are reported for Al-phthalocyanine tetrasulphonate in hyperquenched glassy water (HGW) films at 5.0 K. The profiles of unsaturated zero-phonon holes (low burn fluence stage) are more sharply tipped than a Lorentzian. It is shown that the non-Lorentzian behavior is a natural consequence of the interplay between the three distributions that govern the dispersive kinetics of nonphotochemical hole growth. They are denoted by λ, α, and ω, where λ is the tunnel parameter associated with nonphotochemical hole burning (NPHB) and α is the angle between the transition dipole and the laser polarization. The ω distribution stems from off-resonant absorption of the zero-phonon line (ZPL). The single site absorption spectrum used in the calculations included the ZPL and the phonon sideband. The contribution of a distribution of homogeneous ZPL widths to the non-Lorentzian behavior was assessed and found to be negligible compared to that of the above distributions. The burn fluence dependence of the hole spectra, which include the ZPH, phonon sideband, and antihole structures, leads to new insights on the mechanism of NPHB, ones that necessitate modification of the Shu–Small mechanism [L. Shu and G. J. Small, J. Opt. Soc. Am. B 9, 724 (1992)]. Although that mechanism recognizes the importance of coupling between the intrinsic and extrinsic two-level systems (TLSint,TLSext) of the chromophore/glass system and diffusion of excess free volume triggered by optical excitation, it does not adequately account for the effects of multiple excitations of redshifted (relative to the burn frequency ωB) preburn and antihole sites. The results show that multiple excitations ultimately lead to the entire antihole being blueshifted. A “second channel” of hole burning becomes apparent at sufficiently high burn fluences. A model for this channel based on a distribution of extrinsic multilevel systems is proposed.

New Insights on Persistent Nonphotochemical Hole Burning and Its Application to Photosynthetic Complexes

Nonphotochemical hole burning (NPHB) at low temperatures of the electronic absorption bands of molecular chromophores imbedded in amorphous solids (glasses and polymers) and in proteins is a striking manifestation of configurational tunneling triggered by electronic excitation. The current mechanism of NPHB has it due to a hierarchy of relaxation events that begin in the outer shell and involve the intrinsic two-level systems (TLSint) of the glass and terminate in the inner shell where the rate determining step involving the extrinsic TLS (TLSext) occurs. The TLS correspond to asymmetric intermolecular double well potentials. The TLSext are associated with the chromophore and the inner shell of solvent molecules. The TLSint are intimately associated with the excess free volume of glasses. Their tunneling leads to diffusion of excess free volume. Results for Al-phthalocyanine tetrasulfonate (APT) in hyperquenched glassy water (HGW) and ethanol (HGE) films that provide strong support for the critical role of excess free volume are discussed. Hole spectra of APT/HGW obtained over eight decades of burn fluence reveal that the current mechanism needs to be modified to include multilevel extrinsic systems (MLSext) in order to explain why the antihole (“photoproduct” absorption) lies to the blue of the burn frequency for sufficiently high burn fluences, an intriguing up-conversion process. The spectra also reveal, for the first time, that the zero-phonon hole (ZPH) profile is non-Lorentzian. This is shown to be a natural consequence of the interplay between the three distributions that result in dispersive hole growth kinetics. They are associated with the tunnel parameter λ of the TLSext, the angle α between the laser polarization and transition dipole, and off-resonant absorption of the zero-phonon line (the ω distribution). Theoretical simulations of hole growth data for APT/HGW obtained over six decades of burn fluence show that the λ distribution is of primary importance, describing well the first 80% of the saturated burn. The paper ends with an application of NPHB combined with high pressure and external electric (Stark) fields to the critically important “red” antenna states of photosystem I. The addition of pressure and Stark fields enhances the already impressive selectivity of NPHB. The results show that the linear pressure shift, permanent dipole moment change, and linear electron−phonon coupling are correlated. Of particular importance is that these properties can be used to identify states which involve interacting chlorophyll molecules that possess significant charge transfer character because of electron-exchange coupling. The results also show that the site distribution functions of the antenna states are largely uncorrelated, consistent with the findings for previously studied complexes. This is important because the absence of correlation means that the electronic energy gaps of donor and acceptor states are distributed which, in turn, means that the kinetics can be dispersive under certain conditions.

Modeling of dispersive nonphotochemical hole growth kinetics data: Al-phthalocyanine tetrasulphonate in hyperquenched glassy water

Zero-phonon hole (ZPH) growth kinetics data that span six decades of burn fluence are reported for Al-phthalocyanine tetrasulphonate (APT) in hyperquenched glassy water (HGW) at 5.0 K. The kinetics are highly dispersive. The hole growth equation used for analysis of the dispersion incorporates three distributions (λ, α, and ω) where λ is the tunnel parameter associated with nonphotochemical hole burning (NPHB), α is the angle between the transition dipole and the laser polarization and the ω-distribution stems from off-resonant absorption of the zero-phonon line (ZPL). The single site absorption profile used includes the phonon sideband as well as the ZPL. The homogeneous width of the ZPL and shape of the phonon sideband were determined from experiment. Eight models, which include the possible combinations of the above distributions, were used to fit the data. As in previous works the λ-distribution was taken to be a Gaussian peaked at λ=λ0 with a standard deviation of σλ. The results show that the contribution to the dispersive kinetics from the λ-distribution is of primary importance. It provides a good fit to the data over the first three decades of burn fluence (∼80% of the saturated ZPH depth). The intrinsic contributions from the α- and ω-distributions become important for the last ∼20% of the burn. These two distributions by themselves or in combination yielded poor fits to the data. The three distributions in combination (λαω-model) provided a good fit over the first five decades of burn fluence. Importantly, the λ0 and σλ values of 8.3 and 0.95 from the λ-distribution alone are nearly the same as those from the λαω-distribution. The above findings for APT/HGW should be widely applicable since previous studies of other NPHB systems led to σλ values ≳1. It is emphasized that APT/HGW is an ideal system for hole growth studies because of its very narrow ZPL and weak electron-phonon coupling (S∼0.2) and because it satisfies the homogeneity condition, i.e., all sites are burnable.

The CP43 Core Antenna Complex of Photosystem II Possesses Two Quasi-Degenerate and Weakly Coupled Qy-Trap States

The CP43 chlorophyll a-core protein complex plays an important role in funneling excitation energy absorbed by more peripheral antenna complexes of photosystem II (PSII) to the reaction center (RC). Identification and characterization of the lowest energy Qy-states of CP43 is important for understanding the kinetics of excitation energy transfer (EET) from CP43 to the RC. We report the results of several types of spectroscopic experiments performed at liquid He temperatures on the isolated CP43 complex from spinach. Nonphotochemical hole burning (NPHB) and triplet bottleneck hole burning spectroscopies as well as zero-phonon hole (ZPH) action and Stark hole burning spectroscopies were employed. Two quasi-degenerate trap states at 682.9 nm (B state) and 683.3 nm (A state) are identified. The widths of their mainly inhomogeneously broadened Qy-absorption bands are 45 and 120 cm-1, respectively. The uncorrelated site excitation distribution functions (SDF) of the two states are nearly the same as their absorption bands since the electron−phonon coupling is weak (optical reorganization energies of ∼6 cm-1). The NPHB spectra establish that the B state is the primary trap for EET from higher energy Qy-states. The permanent dipole moment change (Δμ) of the S0 → Qy transition for both the B and A states is small, f·Δμ = 0.25 ± 0.05 and 0.47 ± 0.05, respectively, where f is the local field correction factor. These values, together with the weak electron−phonon coupling and other results, indicate that both states are highly localized on a single Chl a molecule. Holewidth measurements led to the remarkable finding that the rates of A → B and B → A EET processes are extremely slow, ∼(6 ns)-1. This suggests that the Chl a molecules of the two states belong to different layers of Chl a molecules located at opposite sides of the membrane. The intriguing question of why CP43 possesses two quasi-degenerate trap states that are so weakly coupled is addressed. The possibility that they play a role in the photoinhibitory and photoregulatory processes is raised.

Low-temperature absorption off-resonant hole-burning spectroscopy of pheophytine a in photosystem II reaction center

Chlorophylls, pheophytines and carotenes are main photosynthetic pigments. They play a crucial role in primary photosynthetic processes (excited energy transfer and electron transport). In this contribution we report on both resonant and off-resonant hole-burning low-temperature absorption spectra. The resonant narrow zero-phonon holes together with broad (≈84 cm −1) off-resonant holes were burnt into Q y absorption bands, while very broad (≈250 cm −1) off-resonant satellite holes were observed in Q x absorption transitions. This technique enabled one to distinguish between satellite holes of chlorophyll a and pheophytine a and consequently to localize spectral bands of both pheophytines a in the complex Q y absorption spectrum of reaction center of photosystem II containing six chlorophylls a, two pheophytines a and one β-carotene.

Assignment of the lowest Qy-state and spectral dynamics of the CP29 chlorophyll a/b antenna complex of green plants: a hole-burning study.

Low-temperature absorption, fluorescence and persistent non-photochemical hole-burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption-origin band of the lowest Qy-state lies at 678.2 nm and carries a width of approximately 130 cm-1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest-energy state is highly localized on a single Chl a molecule. The electron-phonon coupling of the 678.2 nm state is weak with a Huang-Rhys factor S of 0.5 and a peak phonon frequency (omega m) of approximately 20 cm-1. These values give a Stokes shift (2S omega m) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence-origin band at 679.1 nm. Zero-phonon holes associated with the lowest state have a width of approximately 0.05 cm-1 at 4.2 K, corresponding to a total effective dephasing time of approximately 400 ps. The temperature dependence of the zero-phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass-like two-level systems of the protein. Zero-phonon hole-widths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 +/- 150 fs and 4.2 +/- 0.3 ps, respectively, for the Chl b-->Chl a energy-transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 +/- 0.6 ps at 4.2 K.

Hole-burning study of exciton migration and pigment–protein interaction in photosynthetic systems

Spectral hole burning enabled to determine excited state lifetimes T 1 calculated from widths δ ZPH of resonant zero phonon holes. Values of δ ZPH reflected ultra-fast exciton migration within the photosystem II reaction centers containing (5 and 6) chlorophylls a, (2) pheophytines a and (1) β-carotene. This technique moreover provided Debye–Waller factor α and mean frequency of protein phonons Ω, which characterized the pigment–protein interactions. The resonant holes were observed together with their vibronic satellites burnt both into absorption and fluorescence spectra of the reaction centers. The phenomenon of laser induced hole-filling of the primary hole after the burning of the secondary holes at different wavelengths, was used to study the role of fast de-excitation processes in hole burning.

Analysis of phonon structure in line-narrowed optical spectra

Abstract The complexities involved in interpretation of the phonon structure in hole burned (HB) and fluorescence line narrowed (FLN) spectra are discussed by analyzing the four contributions to the spectra, one of which is the zero-phonon hole (line) and the remaining three due to multi-phonon transitions. It is concluded that HB spectra allow for a more reliable qualitative assessment of the electron–phonon coupling. In addition, related HB experiments provide good first estimates of the Huang–Rhys factor S, the mean phonon frequency ωm and Γinh. An application is given for the LHC II chlorophyll a/b antenna complex of photosystem II.

Phonon sidebands of electronic transitions in Li-doped CdS

Electronic Raman scattering has been measured at low temperatures from Li-doped CdS, where Li is incorporated as a shallow acceptor. The spectra are interpreted in terms of a LO-phonon mode bound to an impurity site, and in terms of the electronic transitions between bound hole states and the phonon sidebands of these transitions. A strong Fano-type effect accompanied by a low-energy shift is observed in the LO-phonon sideband of the $1\stackrel{\ensuremath{\rightarrow}}{s}2s$ transition due to its overlap with a continuum corresponding to the zero-phonon hole transitions from the $1s$ state to the valance band. A calculation of the self energy in second-order perturbation theory with the Fr\"ohlich interaction shows good agreement with the observed shift. The phonon sideband is also found to resonate with the incident laser energy near the energy gap between the conduction band and the $2s$ acceptor state. This resonance behavior is interpreted in terms of a many-body theory given earlier for the phonon sideband in Li-doped ZnTe, which shows good agreement with experimental results.

Qy-Level Structure and Dynamics of Solubilized Light-Harvesting Complex II of Green Plants: Pressure and Hole Burning Studies

Nonphotochemical hole burning and pressure-dependent absorption and hole-burning results are presented for the isolated (disaggregated) chlorophyll a/b light-harvesting II trimer antenna complex of green plants. Analysis of the 4.2 K burn-fluence dependent hole spectra and zero-phonon hole action spectra indicates that the three lowest energy states (Qy) lie at 677.1, 678.4 and 679.8 nm. Their combined absorption intensity is equivalent to that of three Chl a molecules. The inhomogeneous broadening of their absorption bands is 70 cm-1. It is argued that these states, separated by 30 cm-1, are associated with the lowest energy state of the trimer subunit with the 30 cm-1 separations due to the indigenous structural heterogeneity of protein complexes. The linear electron−phonon coupling of the 679.8 nm state is weak and characterized, in part, by a mean phonon frequency of ωm = 18 cm-1 and Huang−Rhys factor of Sm = 0.8, values which yield the correct Stokes shift for fluorescence from the 679.8 nm state at 4.2 K. The temperature dependence of the zero-phonon hole (ZPH) width for that state is consistent with optical dynamics due to coupling with glasslike two-level systems of the protein. The ZPH width at 1.9 K is 0.037 cm-1. Satellite hole structure produced by burning in the above three states as well as their low linear pressures shift rates (about − 0.08 cm-1/MPa) indicate that the Chl a molecule of the subunit associated with them is weakly coupled to other Chl molecules. The linear pressure shift rates for the main Qy-absorption bands are also low. The shift rates appear to be dictated by protein−Chl interactions rather than excitonic couplings. Holes burned into the 650 nm absorption band reveal energy transfer times of 1 ps and ∼100 fs which are discussed in terms of time domain measurements of the Chl b → Chl a transfer rates (Connelly et al. J. Phys. Chem. B 1997, 101, 1902). The holewidths associated with burning into the 676 nm absorption band lead to Chl a → Chl a transfer times in the 6−10 ps range, in good agreement with the time domain values (Savikhin et al. Biophys. J. 1994, 66, 1597).

Stark hole-burning spectroscopy of a photosynthetic complex: LH2 of purple bacteria

Results from the first Stark hole-burning spectroscopic studies of photosynthetic complexes are reported. External electric field effects were determined as a function of laser burn frequency and polarization for the B800 band and the B870 band of Rhodobacter sphaeroides, where B870 is the lowest exciton level of the strongly coupled bacteriochlorophyll a molecules of the B850 ring. For either parallel or perpendicular polarization of the burn laser relative to the external Stark field, only broadening of the zero-phonon holes which is linear in field strength was observed. Near the center of the B870 and B800 bands, f(Δ μ ‖, Δ μ ⊥)=(1.1, 1.4) and (1.1, 0.8) Debye, respectively, where f is the local field correction factor. However, a dependence of the Δ μ-values on location of the hole within the inhomogeneously broadened absorption bands is observed. For B870 they increase by 10–20% from the blue to red edge while for B800 an opposite trend is observed. Results for B870 of Rhodopseudomonas acidophila and Rhodospirillum molischianum are similar. A detailed discussion of the results and a comparison of them with those from classical Stark modulation spectroscopic studies of the entire B800 and B850 absorption bands are given. It is concluded for the B850 ring that mixing of charge transfer states with the Q y-exciton levels is of minor importance. The hole burning data, together with consideration of the C n -symmetry and the effects of energy disorder from structural heterogeneity, suggest that localization effects from energy disorder are probably important for understanding the effects of external electric field on the B850 band which is comprised of several components.

Stark Hole Burning of Aluminum Phthalocyanine Tetrasulfonate in Normal and Cancer Cells

The results of Stark hole burning of aluminum phthalocyanine tetrasulfonate (APT) in MCF-10F (normal) and MCF-7 (cancer) cells are reported. Hole broadening was observed for zero-phonon holes (ZPH) for both cell lines when laser polarization was parallel and perpendicular to an applied electric (Stark) field. Changes in dipole moment suggest that dye molecules are located near the cell membrane and, in normal cells, experience some degree of ordering not found in the cancer cells.

Non-Linear Excitation Effect in Persistent Hole-Burning of Linear Frenkel Exciton System with Large Coherence Length

Excitonic structure of J-bands of pseudo-isocyanine bromide J-aggregates in WEG glasses at 4.7 K has been investigated by persistent spectral hole burning. Clear crossover of hole line shape was observed in the intensity dependence of pulse laser burning (7ns, 10-Hz repetition). With decreasing the intensity the line shape changes from non-resonant red-shifted (∼3 cm−1 from the laser frequency ωB) broad hole to resonant sharp hole, which is similar to those obtained by cw-laser. With cw-laser light for burning, clear zero-phonon hole was formed with the width of 0.36 cm−1. The hole profile seems strongly dominated by the peak intensity of the burning light. We proposed the existence of the crossover is due to switching from the |1-ex) state resonance by one-photon absorption to the |2-ex) state resonance by two-photon absorption using real dipole-allowed |1-ex) state. The model provides the offset of the hole is equal to half of the difference of k = 2 and k = 1 exciton energy, from which we can deduce a coherence length N = 56 as typical order of magnitude. Further experiments are described, which basically support the interpretation.