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  • 1
    Electronic Resource
    Electronic Resource
    Direct observation of the excited-state cis–trans photoisomerization of bacteriorhodopsin: Multilevel line shape theory for femtosecond dynamic hole burning and its application (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 199-208 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456658
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  • 2
    Electronic Resource
    Electronic Resource
    Excited state structure and femtosecond ring-opening dynamics of 1,3-cyclohexadiene from absolute resonance Raman intensities (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4274-4281 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonance Raman spectra of 1,3-cyclohexadiene have been obtained with excitation wavelengths ranging from 300 to 253 nm. The absolute excitation profiles were analyzed to determine the initial dynamics that occur following photoexcitation in the lowest energy 260 nm 1B2 absorption band. The initial atomic displacements parallel the Woodward–Hoffmann, symmetry-allowed, ring-opening pathway towards all-cis-hexatriene. The potential surface displacements in the optically prepared 1B2 excited state are, however, a small fraction of the total distance along the reaction coordinate. The low absolute scattering cross sections indicate an excited state dephasing time of 10−14 s, and the fluorescence quantum yield of 2×10−6 shows that the total dephasing rate is dominated by ultrafast internal conversion. The Gaussian homogeneous line shape indicates a nonexponential excited electronic state population decay. We attribute these observations to an allowed crossing of the 1B2 state with a second excited 2A1 state that is responsible for the subsequent excited-state ring-opening reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455784
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  • 3
    Electronic Resource
    Electronic Resource
    Classical theory for real-time femtosecond probing of the NaI* photodissociation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6146-6150 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Femtosecond pump-probe experiments [Rosker et al., Chem. Phys. Lett. 146, 175 (1988)] on the dissociation of NaI* are modelled classically to obtain the absorption transients as a function of the pump and probe wavelengths. The initial ground state, the optically pumped predissociating state, and the third surface that is coupled by the probe pulse are explicitly included. The classical model can almost quantitatively explain all the features of the experimental results. The oscillations in the transients are shown to be due to molecules trapped in the adiabatic well, with most of the intensity coming from the covalent region of the well before the crossing point rx. The rising troughs in the transients are due to successive leaks out of the well into the covalent region after the crossing point rx, leading to dissociation. The difference in the absorption transient between on-resonance 589 nm probing as compared to off-resonance 612 nm and 580 nm probing is shown to arise from the difference in Lorentzian absorption widths between the trapped (500 cm−1) and dissociating (100 cm−1) covalent molecules, with the transition dipole moment to the third surface having about the same magnitude over the range of the covalent potential.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456330
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  • 4
    Electronic Resource
    Electronic Resource
    Wave packet theory of dynamic absorption spectra in femtosecond pump–probe experiments (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4012-4029 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The large spectral width of ultrashort optical pulses makes it possible to measure the complete time-resolved absorption spectrum of a sample with a single pulse, offering simultaneously high resolution in both the time and frequency domains. To quantitatively interpret these experiments, we start with the usual perturbative density matrix theory for the third-order susceptibility of a multilevel system. However, the theory is formulated in terms of four-time correlation functions which are interpreted as the time-dependent overlap of bra and ket vibrational wave packets propagating independently on the ground and excited electronic state potential surfaces. This approach captures the critical distinction between electronic population decay and pure dephasing processes, while retaining the intuitive physical picture offered by the time-dependent wave packet theories of molecular spectroscopy. A useful simplification is achieved by considering the absorption of the probe pulse as the first-order spectroscopy of the nonstationary state created by the pump pulse. In this case, the dynamic spectrum is obtained through the Fourier transform of the time-dependent overlap of the initial wave packet propagating on its potential surface and a second wave packet, created by the probe pulse, which evolves simultaneously on the final surface. Calculations for model systems using harmonic surfaces and δ-function pulses are presented to illustrate the application of this theory and to clarify the unique spectral behavior of the nonstationary states created in femtosecond pump–probe experiments. Finally, we demonstrate the practical application of the theory for anharmonic surfaces and finite pulses by analyzing the dynamic spectroscopy of the excited state torsional isomerization of the bacteriorhodopsin chromophore.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457815
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  • 5
    Electronic Resource
    Electronic Resource
    Simple interpretation of dephasing in absorption and resonance Raman theory (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3744-3748 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the effects of "pure dephasing'' in vibronic absorption and resonance Raman scattering may be simply derived by beginning with the Fermi Golden Rule. The medium introduces electronic dephasing by modulating the electronic energy gap of the molecule. In both absorption and Raman scattering the cross section depends upon a medium correlation function φ(t) whose form is identical to one that appears in vibrational relaxation theory. Explicit equations for the resonance Raman cross section are derived for an exponential medium correlation function. Also, φ(t) is derived for medium models which consist of a collection of classical and quantum mechanical oscillators.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450947
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  • 6
    Electronic Resource
    Electronic Resource
    Quantitation of homogeneous and inhomogeneous broadening mechanisms in trans-stilbene using absolute resonance Raman intensities (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5000-5006 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The sensitivity of resonance Raman cross sections to the excited state homogeneous width is exploited to separate the homogeneous and inhomogeneous contributions to the optical absorption linewidth in trans-stilbene. Absolute Raman cross sections have been measured using excitation at 356, 299, 282, and 266 nm, and calculations have been performed to model the intensities of the eleven strongly Raman-active modes as well as the absorption spectrum. A simple analysis neglecting explicit temperature effects requires a homogeneous Lorentzian linewidth (Γ) of 310 cm−1 and an inhomogeneous Gaussian distribution of zero–zero energies with a standard deviation (θ) of 500 cm−1. Consideration of thermal excitation in the lowest-frequency single-bond torsional mode significantly improves the fit to the experimental absorption by broadening the spectrum asymmetrically to higher energies, while allowing Γ and θ to be reduced to 120 and 450 cm−1, respectively. The spectral broadening generated by thermal excitation of ground-state torsional modes thus contributes significantly to the homogeneous linewidth in trans-stilbene. The relatively large effective homogeneous width at room temperature is consistent with the low Raman intensity in the ∼200 cm−1 C=C–φ bend compared with the prominence of this vibration in resolved low-temperature electronic spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449765
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  • 7
    Electronic Resource
    Electronic Resource
    Excited-state structure and photochemical ring-opening dynamics of 1,3,5-cyclo-octatriene from absolute resonance Raman intensities (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2492-2504 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absolute resonance Raman cross sections are measured for 1,3,5-cyclo-octatriene (COT) in cyclohexane with excitation from 325 to 200 nm. These intensities and the absorption spectrum are modeled using a fully thermalized time-correlator theory to quantitate the excited-state equilibrium geometry displacements along 19 Raman-active normal modes. The resonance Raman spectra show significant intensity in low-frequency modes corresponding to planarization of the eight-membered ring. The 140 cm−1 twist-boat planarization (Δ=4.6) and the 339 cm−1 ring deformation (Δ=1.6) are particularly strong. However, no intensity is observed in modes which project onto the predicted disrotatory ring-opening motion, such as the nontotally symmetric CH2 twist fundamental or its overtone. Analysis of the fluorescence quantum yield (φF=2×10−6) gives an excited state lifetime on the order of ∼30 fs. These results show that ring planarization is the first step in the disrotatory ring opening of COT followed by rapid depopulation of the initially prepared state to a lower-lying excited electronic state upon which the actual ring opening occurs. Comparison of these results with the excited-state dynamics of other pericyclic systems suggests that pericyclic rearrangements occur only once a planar structure is established and that the bond rearrangement occurs predominantly on a low-lying, optically forbidden excited state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466498
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  • 8
    Electronic Resource
    Electronic Resource
    Excited-state structure and electronic dephasing time of Nile blue from absolute resonance Raman intensities (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 8037-8045 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absolute resonance Raman cross sections are measured for Nile blue 690 perchlorate dissolved in ethylene glycol with excitation at 514, 531, and 568 nm. These values and the absorption spectrum are modeled using a time-dependent wave packet formalism. The excited-state equilibrium geometry changes are quantitated for 40 resonance Raman active modes, seven of which (590, 1141, 1351, 1429, 1492, 1544, and 1640 cm−1 ) carry 70% of the total resonance Raman intensity. This demonstrates that in addition to the prominent 590 and 1640 cm−1 modes, a large number of vibrational degrees of freedom are Franck–Condon coupled to the electronic transition. After exposure of the explicit vibrational progressions, the residual absorption linewidth is separated into its homogeneous [350 cm−1 half-width at half-maximum (HWHM)] and inhomogeneous (313 cm−1 HWHM) components through an analysis of the absolute Raman cross sections. The value of the electronic dephasing time derived from this study (25 fs) compares well to previously published results. These data should be valuable in multimode modeling of femtosecond experiments on Nile blue.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462355
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  • 9
    Electronic Resource
    Electronic Resource
    A 77-K cold stage for Raman microprobes and optical microscopy (1989)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 2628-2630 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 77-K cold stage has been developed for spectroscopic measurements in an optical microscope. This stage eliminates fogging of the optical windows observed in an earlier design by adding vacuum jackets around the liquid nitrogen transfer lines and by maximizing the sample-to-environment distance without sacrificing optical imaging power. Increased maneuverability of the cold stage is achieved by using flexible stainless-steel liquid-nitrogen transfer lines. In our application, the sample in the cold stage is illuminated with a focused laser beam and vibrational resonance Raman scattering spectra are recorded. We have used this Raman microprobe system to measure the Raman scattering from a variety of individual rod photoreceptor cells. This work has provided new information about the mechanism of wavelength regulation in color vision.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140682
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  • 10
    Electronic Resource
    Electronic Resource
    Laser-excited confocal-fluorescence gel scanner (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 807-812 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A high-sensitivity, laser-excited, confocal-fluorescence scanner has been developed for the detection of fluorescently labeled nucleic acids separated on slab gels. The gel is placed on a motor-driven, two-dimensional scan stage and raster scanned past the optical detection system. The 488-nm argon ion laser beam is introduced into the confocal optical system at a long-pass dichroic beam splitter and focused within the gel to an ∼2 μm diameter spot by a high-numerical aperture microscope objective. The resulting fluorescence is gathered by the objective, passed back through the first long-pass beam splitter, and relayed to a second dichroic beam splitter that separates the red and green emissions. The fluorescence is then focused on confocal spatial filters to reduce stray and scattered light, passed through spectral filters, and detected with photomultipliers. The resulting signals are amplified, filtered, and digitized for display on a computer. This system can detect as little as 5×10−12 M fluorescein, the resolution as operated is 160 μm, and it can scan a 6 cm×6 cm gel using a scan rate of 4 cm/s in 12 min. The detection of DNA on slab gels, two-color DNA fragment sizing, and microtiter plate scanning are presented to illustrate some of the possible applications of this apparatus.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144905
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