Kristen A. Peterson

Electronic excitation transport on isolated, flexible polymer chains in the amorphous solid state randomly tagged or end tagged with chromophores

The transport of electronic excitations among chromophores which are randomly tagged or end tagged on finite flexible polymer chains in the amorphous solid state is described. First, the previously reported first order cumulant expansion treatment of excitation transport among chromophores randomly distributed in solution is extended to treat finite volume systems with nonrandom chromophore distributions. The method is demonstrated by considering chromophores randomly distributed in a finite sphere. The results are in good agreement with a previous treatment of this problem using a density expansion. The probability of finding the initially excited chromophore, still excited at time t, Gs(t), is calculated. Gs(t) is directly related to fluorescence depolarization and other observables. Gs(t) is then calculated for a finite flexible polymer chain in a polymer blend which is randomly tagged in low concentration with chromophores. The method permits any form of the polymer pair correlation function to be use...

Dispersive excitation transport at elevated temperatures (50–298 K): Experiments and theory

Time‐resolved fluorescence depolarization has been used to measure electronic excitation transport among naphthyl chromophores in polymeric glasses. 2‐ethylnaphthalene randomly distributed in PMMA and 2‐vinylnaphthalene/methyl methacrylate copolymer in PMMA were studied. It was found that excitation transport is dispersive at all temperatures studied, from 50 K to room temperature, i.e., the extent of transfer depends on the excitation wavelength within the S0–S1 absorption band. A theory based on the nondispersive, Forster mechanism for excitation transfer has been developed to describe dispersive transport. Good agreement between the theoretical and experimental results are achieved without resorting to adjustable parameters. Both the theory and experiment show that, for the observable used here, excitation at a certain wavelength, called the ‘‘magic wavelength,’’ results in a time dependence that is identical to the Forster nondispersive result, i.e., dispersive transport appears to vanish.

Dispersive electronic excitation transport in polymeric solids at and near room temperature

Abstract Time-resolved fluorescence depolarization is used to examine electronic excitation transport among naphthalene chromophores in a polymeric solid as a function of excitation wavelength between 300 and 50 K. The characteristics of the wavelength and temperature dependences reveal, for the first time, that dispersive transport occurs at and near room temperature. The rate of excitation transport depends on the wavelength of excitation. A theoretical treatment is able to reproduce the essential features of the wavelength and temperature dependences without recourse to adjustable parameters by avoiding the complexities of the relationship between the difference in energy of a pair of molecules and the pairwise transfer rate.

Demonstration of complex-conjugate-resolved harmonic Fourier-domain optical coherence tomography imaging of biological samples

Complex-conjugate-resolved Fourier-domain optical coherence tomography, where the quadrature components of the interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is applied to multisurface test targets and biological samples. The method provides efficient suppression of the complex-conjugate, dc, and autocorrelation artifacts. A complex-conjugate rejection ratio as high as 70 dB is achieved.

Vibrational relaxation of carbon monoxide in model heme compounds. 6-coordinate metalloporphyrins (M = Fe, Ru, OS)

Abstract Vibrational relaxation of carbon monoxide bound to a series of metalloporphyrin complexes (M-(coproporphyrinate-I tetraisopropyl ester)(CO)(pyridine); M = Fe, Ru, Os) was measured using picosecond infrared pump-probe experiments. The vibrational relaxation rates ((1–2.5) × 10 10 s −1 ) increased with increasing mass of the metal ion. This effect is opposite that predicted by through-σ-bond models and is interpreted as arising from a through-π-bond coupling between the CO vibrational fundamental and porphyrin vibrations.

Ultrafast infrared spectroscopy in biomolecules: Active site dynamics of heme proteins

Rapid advances in the generation of intense tunable ultrashort mid-infrared (IR) laser pulses allow the use of ultrafast IR pump-probe and vibrational echo experiments to investigate the dynamics of the fundamental vibrational transition of CO bound to the active site of heme proteins. The studies were performed using a free-electron laser (FEL) and an experimental set up at the Stanford University FEL Center. These novel techniques are discussed in some detail. Pump-probe experiments on myoglobin-CO (MbCO) measure CO vibrational relaxation (VR). The VR process involves loss of vibrational excitation from CO to the protein and solvent. Infrared vibrational echoes measure CO vibrational dephasing. The quantum mechanical treatment of the force-correlation function description of vibrational dynamics in condensed phases is described briefly. A quantum mechanical treatment is needed to explain the temperature dependence of VR in Mb-CO from 10 to 300 K. A molecular-level description including elements of heme protein structure in the treatment of vibrational dynamics is also discussed. Vibrational relaxation of CO in Mb occurs on the 10−11-s time scale. VR was studied in proteins with single-site mutations, proteins from different species, and model heme compounds. A roughly linear relationship between carbonyl stretching frequency and VR rate has been observed. The dominant VR pathway is shown to involve anharmonic coupling from CO through the π-bonded network of the porphyrin, to porphyrin vibrations with frequencies > 400 cm−1. The heme protein influences VR of bound ligands at the active site primarily via altering the through π-bond coupling between CO and heme. Preliminary vibrational echo studies of the effects of protein conformational relaxation dynamics on ligand dephasing are also reported.

Complex-conjugate-resolved imaging using two-harmonic FD-OCT

The two-harmonic FD-OCT method, where the quadrature components of the spectral interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is used for complex-conjugate- resolved imaging of biological samples. The method is implemented using sampling of the phase modulated interferogram with an integrating detector array followed by digital demodulation at the first and second harmonics. A complex conjugate rejection ratio as high as 70 dB is achieved.

Real-time video-rate harmonically detected Fourier domain optical coherence tomography

Real-time video-rate imaging using harmonically detected Fourier domain OCT is demonstrated using an 800 nm light source and a silicon line scan camera. At an imaging rate of 11.7 B-scans (1024 pixels × 256 pixels) per second, the measured complex conjugate artifact suppression is 30-35 dB, the sensitivity is 121 dB, and the dynamic range is about 60 dB.

Simultaneous acquisition of the real and imaginary components in Fourier domain optical coherence tomography using harmonic detection

Fourier domain optical coherence tomography (FD-OCT) is an interferometric imaging technique that allows imaging to depths of a few mm in scattering biological tissues with high resolution of the order of 1-10 μm. However, the usefulness of FD-OCT is limited by background and autocorrelation interference terms that reduce the sensitivity and by phase ambiguity that halves the useful imaging depth range. These limitations can be overcome by obtaining the full, complex spectral interferogram. Simultaneous detection of the imaginary and real terms is obtained by phase modulating the reference arm of the interferometer and detecting at the first and second harmonics. A mathematical derivation of harmonically detected FD-OCT and experimental measurements showing that phase ambiguity artifacts can be suppressed by up to 70 dB are presented. The method provides efficient suppression of the complex conjugate, dc, and autocorrelation artifacts and has low sensitivity to phase noise. Beyond the removal of artifacts, the ability to obtain the full, complex interferogram is key to the development of spectrally resolved FD-OCT which would add depth-resolved spectroscopic detail to the structural information.

Picosecond infrared studies of protein vibrational modes: the amide I mode of myoglobin

The population lifetime of the amide I vibration (v10 fundamental, ca. 1650 cm-1) in the protein myoglobin in D2O has been determined by picosecond infrared pump- probe spectroscopy using the Stanford mid-infrared free electron laser to be 1.3 +/- 0.2 ps. In a glass forming mixture of deuterated glycerol and D2O, the vibrational lifetime was found to increase from 1.3 +/- 0.2 ps at 310 K to 1.8 +/- 0.2 ps at 10 K. In addition to determining the time-scale of vibrational relaxation, we also observed multi-level vibrational excitation which has implications regarding the anharmonicity and homogeneous linewidth of the mode.