Stoichiometric analysis of competing intermolecular hydrogen bonds using infrared spectroscopy.

We quantitatively analyze multiple hydrogen bonds in mixtures of two monomers: urethane dimethacrylate (UDMA) and triethylene glycol-divinylbenzyl ether (TEG-DVBE). The carbonyl stretching band in infrared (IR) absorption spectra is deconvoluted into free and hydrogen-bonded carbonyl groups. The amounts of the sub-components are determined for 21 mixture compositions and initially analyzed using a simple stoichiometric model (based on one dominant hydrogen acceptor group per monomer species) for the equilibrium state of hydrogen bond formation. However, our in-depth stoichiometric analysis suggests that at least two UDMA acceptor groups (carbonyl and alkoxy oxygens) and one TEG-DVBE acceptor group (ether oxygen) contribute to intermolecular hydrogen bonding interactions. This finding is further supported by a quantitative analysis of the hydrogen bonding effect on the N-H stretching band. Moreover, the equilibrium constants of these hydrogen bond formations confirm that the interassociation between UDMA and TEG-DVBE is non-negligible in comparison to the UDMA selfassociations. Such quantitative information on intermolecular interactions provides insight into the effect of hydrogen bonding on the copolymerization kinetics of these monomer mixtures.

Beam scanning for rapid coherent Raman hyperspectral imaging.

Coherent Raman imaging requires high-peak power laser pulses to maximize the nonlinear multiphoton signal generation, but accompanying photo-induced sample damage often poses a challenge to microscopic imaging studies. We demonstrate that beam scanning by a 3.5-kHz resonant mirror in a broadband coherent anti-Stokes Raman scattering (BCARS) imaging system can reduce photo-induced damage without compromising signal intensity. Additionally, beam scanning enables slit acquisition, in which spectra from a thin line of sample illumination are acquired in parallel during a single charge-coupled device exposure. Reflective mirrors are employed in the beam-scanning assembly to minimize chromatic aberration and temporal dispersion. The combined approach of beam scanning and slit acquisition is compared with the sample-scanning mode in terms of spatial resolution, photo-induced damage, and imaging speed at the maximum laser power below the sample-damage threshold. We show that the beam-scanning BCARS imaging method can reduce photodamage probability in biological cells and tissues, enabling faster imaging speed by using a higher excitation laser power than could be achieved without beam scanning.

Role of electronic-vibrational mixing in enhancing vibrational coherences in the ground electronic states of photosynthetic bacterial reaction center.

We describe polarization controlled two-color coherence photon echo studies of the reaction center complex from a purple bacterium Rhodobacter sphaeroides. Long-lived oscillatory signals that persist up to 2 ps are observed in neutral, oxidized, and mutant (lacking the special pair) reaction centers, for both (0°,0°,0°,0°) and (45°,-45°,90°,0°) polarization sequences. We show that the long-lived signals arise via vibronic coupling of the bacteriopheophytin (H) and accessory bacteriochlorophyll (B) pigments that leads to vibrational wavepackets in the B ground electronic state. Fourier analysis of the data suggests that the 685 cm(-1) mode of B may play a key role in the H to B energy transfer.

Pseudo-rephasing and pseudo-free-induction-decay mechanism in two-color three-pulse photon echo of a binary system.

We investigate the two-color three-pulse photon echo peak shift in a (left-right) binary system, where each component consists of a heterodimer. On the basis of the model, we find that the effect of the excitonic asymmetry between two components leads to an additional factor in the peak shift. A pseudo-rephasing and pseudo-free-induction-decay mechanism is proposed to explain the resultant negative peak shift, when the differences between the two left/right components have the opposite sign. In such a case, estimates of the electronic coupling strength via two- and one-color peak shift experiments lead to an underestimate of the coupling magnitude.