Gerald J. Small

Non-photochemical hole burning and impurity site relaxation processes in organic glasses

Abstract Laser-induced holes in the inhomogeneously broadened absorption spectra of aromatic hydrocarbons in frozen glasses are reported. The holes are shown to be of non-photochemical origin. Mechanisms for excited state site interconversion based on a metastable distribution of sites in asymmetric double well potentials are presented. Methods of eliminating non-photochemical holes and of distinguishing between photochemical and non-photochemical holes are described.

Herzberg–Teller Vibronic Coupling and the Duschinsky Effect

The assumption that the difference in composition, in terms of symmetry coordinates, between the normal vibrational motions of the ground and first excited state (Duschinsky effect) can be neglected in the Herzberg–Teller theory of absorption and fluorescence vibronic intensities is critically examined. A theory of the Duschinsky effect is described which involves expansion of the potential energy of the fluorescent state in terms of the ground state normal coordinate displacements. Off‐diagonal quadratic contributions arise in the expansion whenever there is a coupling between the crude Born–Oppenheimer electronic wavefunctions of the fluorescent and other excited states through two or more ground state normal motions of the same symmetry. The importance of the Duschinsky effect depends on the magnitudes of the Herzberg–Teller vibronic matrix elements and the frequency differences between the fundamentals which effect vibronic perturbations. Inclusion of the Duschinsky effect in the Herzberg–Teller theor...

Theory for spectral hole burning of the primary electron donor state of photosynthetic reaction centers

A theory for solid state spectral hole burning is developed. The theory is valid for arbitrarily strong linear electron-phonon coupling within the Condon approximation which is accurate for strongly allowed optical transitions. Model calculations show that, from the dependence of the hole profile on the burn frequency, the contribution to the absorption linewidth from site inhomogeneity and the linear coupling parameters can be determined. The theory is used to analyze recent hole-burning data on the primary electron-donor states of isolated reaction centers of Rhodopseudomonas viridis, Rhodobacter sphaeroides and Photosystem I. The results show that all of these states are characterized by strong linear electron-phonon coupling (Huang-Rhys factor S ≳ 4) and large site inhomogeneous line broadening.

Applications of spectral hole burning spectroscopies to antenna and reaction center complexes.

The underlying principles of spectral hole burning spectroscopies and the theory for hole profiles are reviewed and illustrated with calculated spectra. The methodology by which the dependence of the overall hole profile on burn wavelength can be used to reveal the contributions from site inhomogeneous broadening and various homogeneous broadening contributions to the broad Qy-absorption bands of cofactors is emphasized. Applications to the primary electron donor states of the reaction centers of purple bacteria and Photosystems I and II of green plants are discussed. The antenna (light harvesting) complexes considered include B800–B850 and B875 of Rhodobacter sphaeroides and the base-plate complex of Prosthecochloris aestuarii with particular attention being given to excitonic interactions and level structure. The data presented show that spectral hole burning is a generally applicable low temperature approach for the study of excited state electronic and vibrational (intramolecular, phonon) structures, structural heterogeneity and excited state lifetimes.

Red antenna states of photosystem I from cyanobacterium Synechococcus elongatus: a spectral hole burning study

Abstract The existence of at least three low-energy chlorophyll a (Chl a ) antenna states is shown for photosystem I (PS I) of Synechococcus elongatus by the use of nonphotochemical hole burning (NPHB) spectroscopy. In addition to the previously reported states at 708 and 719 nm, it is demonstrated that there is a third state at 715 nm. The responsible Chl a molecules are referred to as C-708, C-715 and C-719. For both Synechococcus and Synechocystis , the lowest energy state is shown to be characterized by strong electron–phonon coupling (large Huang-Rhys factor S ), large permanent dipole moment change ( f · Δμ ), and large linear pressure shift rates attributable to electron exchange of dimeric Chl a . The lowest energy state of Synechocystis is at 714 nm. The properties of the 714 and 719 nm states are very similar, suggesting that their dimer structures are also similar. Although the other red antenna states of these cyanobacteria have smaller values for S , f · Δμ , and linear pressure shift rate, these are still larger than typically measured for monomeric antenna Chl a . Possible assignments of red absorption bands to particular chlorophyll dimers or trimer are discussed.

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

Abstract 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 Forstertype. The B850 vibronic hole structure allows for an identification of bacteriochlorophyll a modes that participate in Forster transfer.

On the mechanism of nonphotochemical hole burning of optical transitions in amorphous solids

Abstract New temperature-dependent nonphotochemical hole burning results are reported for cresyl violet perchlorate in polyvinyl alcohol which indicate that the simple model for spectral hole production based on a static distribution of two-level systems (TLS) provides an inadequate description. One of the key observations reported is noted to have been made for a variety of other systems. A new mechanism, based on an “outside-in” hierarchy of constrained configurational tunneling events, is proposed in which the time evolution of free volumes plays an important role. The model is consistent with the available data and lends itself to testing by further experiments.

Electronic dephasing of APT in glassy films of water from 5 to 100 K: Implications for H‐bonding liquids

Nonphotochemical hole burning was used to characterize the linear electron–phonon coupling and measure the temperature dependence of the pure electronic dephasing of Al‐phthalocyanine tetrasulphonate (APT) in unannealed and annealed hyperquenched glassy films of water (HGW). Below about 10 K, the dephasing is dominated by coupling to the intrinsic two‐level systems (TLSint) of HGW. This dephasing is a factor of 5× faster for unannealed HGW due to its higher TLSint number density. For annealed HGW, the pure electronic dephasing time (i.e., that dephasing associated with the zero‐phonon line), T2*, is 6.4 ns at 5 K, the slowest dephasing time yet reported for a molecular glassy system at this temperature. At higher temperatures, dephasing due to exchange coupling with pseudolocalized modes at 50 and 180 cm−1, which correlate well with the transverse and longitudinal acoustic modes of water, becomes dominant. The exchange coupling mechanism is based on diagonal quadratic electron–phonon coupling. At 100 K, for example, the pure electronic dephasing times (T2*) are close to 1 ps in value for both types of film. Whereas the transverse acoustic mode is Franck–Condon active (S∼0.5), the 180 cm−1 mode is silent (S<0.02). The determination of the electron–phonon coupling parameters and static inhomogeneous broadening (400 cm−1) of APT’s origin band allowed for simulation of the burn temperature dependence of the overall hole profile (zero‐phonon hole plus phonon‐sideband holes). Comparison with experimental profiles shows that the hole profile theory of Hayes et al. [J. Phys. Chem. 98, 7337 (1994)] captures the main features of the temperature dependence.The S value of the transverse acoustic mode is used to quantitatively explain the burn temperature dependence of the saturated intensity of the zero‐phonon hole and its demise at temperatures just above 100 K (due to Franck–Condon forbiddeness). In view of the essentially complete understanding of the electron–phonon coupling and pure electronic dephasing of APT in HGW attained in this work, the data are used for extrapolation to ice (0° C) and water (at temperatures not far above 0° C) in order to connect with recent photon echo studies of optical coherence loss of dye molecules in liquids. The extrapolation predicts an ‘‘average’’ T1‐dephasing time of ∼0.1 ps due to multiphonon (Brownian oscillator) transitions associated with the transverse acoustic mode and subpicosecond pure electronic dephasing due to exchange coupling with the longitudinal mode. It is suggested that the marriage of hole burning and photon echo techniques in studies of glass forming liquids should be a powerful approach to understanding optical coherence loss in liquids.Nonphotochemical hole burning was used to characterize the linear electron–phonon coupling and measure the temperature dependence of the pure electronic dephasing of Al‐phthalocyanine tetrasulphonate (APT) in unannealed and annealed hyperquenched glassy films of water (HGW). Below about 10 K, the dephasing is dominated by coupling to the intrinsic two‐level systems (TLSint) of HGW. This dephasing is a factor of 5× faster for unannealed HGW due to its higher TLSint number density. For annealed HGW, the pure electronic dephasing time (i.e., that dephasing associated with the zero‐phonon line), T2*, is 6.4 ns at 5 K, the slowest dephasing time yet reported for a molecular glassy system at this temperature. At higher temperatures, dephasing due to exchange coupling with pseudolocalized modes at 50 and 180 cm−1, which correlate well with the transverse and longitudinal acoustic modes of water, becomes dominant. The exchange coupling mechanism is based on diagonal quadratic electron–phonon coupling. At 100 K, f...

Mechanism of nonphotochemical hole burning: Cresyl Violet in polyvinyl alcohol films

Polarized nonphotochemical hole-burned spectra of Cresyl Violet in polyvinyl alcohol films are presented for various burn temperatures (TB). For TB = 15 K, but not for TB = 2.2 K, a significant rotation of Cresyl Violet is indicated. The broad, tailing antihole is observed to depend on TB. The quantum efficiency distributions for the zero-phonon hole and pseudo-phonon sideband are quite similar, proving that the phonons created by excitation of the phonon sideband are not important for hole burning. The quantum efficiency is independent of TB over the range studied, 1.6–15 K. Over this range essentially 100% of the zero-phonon lines can be burned. The results show that the standard two-level-system (extrinsic) model for nonphotochemical hole burning is inadequate. The results are discussed in terms of an outside-in hierarchy of configurational relaxation events model [ Chem. Phys.141, 447 ( 1990)] as well as other models.

Mechanisms of non-photochemical hole-burning in organic glasses

Abstract From observation of the temperature dependence of hole formation and disappearance and of an anomalous temperature variation in zero-phonon fluorescence of tetracene in alcoholic glasses, conclusions regarding hole-burning mechanisms are made. At temperature above ≈10 K the homogeneous hole width allows determination of the rate of site interconversion.