J. Konradi

Jacobs University, Bremen, Bremen, Germany

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Publications (12)27.04 Total impact

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    ABSTRACT: The use of four‐wave mixing techniques in femtosecond time‐resolved spectroscopy has considerable advantages. Due to the many degrees of freedom offered e.g. by coherent anti‐Stokes Raman scattering (CARS), the dynamics even of complex systems can be analyzed in detail. Using pulse shaping techniques in combination with a self‐learning loop approach, molecular mode excitation can be controlled very efficiently in a multi‐photon excitation process. Results obtained from the optimal control of CARS on β‐carotene are discussed.
    No preview · Article · Nov 2008
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    ABSTRACT: Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) gives access to ultrafast molecular dynamics. However, femtosecond laser pulses are spectrally broad and therefore coherently excite several molecular modes. While the temporal resolution is high, usually no mode-selective excitation is possible. This paper demonstrates the feasibility of selectively exciting specific molecular vibrations in solution phase with shaped fs laser excitation using a feedback-controlled optimization technique guided by an evolutionary algorithm. This approach is also used to obtain molecule-specific CARS spectra from a mixture of different substances. The optimized phase structures of the fs pulses are characterized to get insight into the control process. Possible applications of the spectrum control are discussed.
    No preview · Article · Nov 2008 · AIP Conference Proceedings
  • A. Scaria · J. Konradi · V. Namboodiri · A. Materny
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    ABSTRACT: A feedback-controlled optimization in a femtosecond coherent-anti Stokes Raman scattering (CARS) process is applied to selectively excite or suppress vibrational modes in the gas and liquid phase. The optimal control experiments are performed on carbon disulfide and toluene molecules. Here our aim is to understand whether the interaction of the molecules with the surrounding medium affects the optimization process. The CARS excitation was chosen to be not in resonance with an electronic transition in the molecule but to excite different vibrational modes coherently. A pure phase modulation of the Stokes pulse resulted in changes of the ratio of the Raman lines observed in the nonlinear scattering spectrum. This could also be achieved when no temporal shift between the pump and Stokes laser resulted in a simple change of the Raman resonances. The relative intensities of the Raman lines could be changed more effectively in the liquid phase than in the gas phase. The higher density in the condensed matter, which hinders free rotation and makes interactions between molecules an important factor, obviously seems to influence the control mechanism. Copyright © 2008 John Wiley & Sons, Ltd.
    No preview · Article · Jun 2008 · Journal of Raman Spectroscopy
  • J Konradi · A Gaal · A Scaria · V Namboodiri · A Materny
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    ABSTRACT: Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) gives access to ultrafast molecular dynamics. However, the gain of the temporal resolution entails a poor spectral resolution due to the inherent spectral width of the femtosecond excitation pulses. Modifications of the phase shape of one of the exciting pulses results in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. A feedback-controlled optimization of specific modes making use of phase and/or amplitude modulation of the pump laser pulse is applied to selectively influence the anti-Stokes signal spectrum. The optimization experiments are performed under electronically nonresonant and resonant conditions. The results are compared and the role of electronic resonances is analyzed. It can be clearly demonstrated that these resonances are of importance for a selective excitation by means of phase and amplitude modulation. The mode selective excitation under nonresonant conditions is determined mainly by the variation of the spectral phase of the laser pulse. Here, the modulation of the spectral amplitudes only has little influence on the mode ratios. In contrast to this, the phase as well as amplitude modulation contributes considerably to the control process under resonant conditions. A careful analysis of the experimental results reveals information about the mechanisms of the mode control, which partially involve molecular dynamics in the electronic states.
    No preview · Article · Mar 2008 · The Journal of Physical Chemistry A
  • A Scaria · V Namboodiri · J Konradi · A Materny
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    ABSTRACT: By using a combination of an initial pump pulse and a degenerate four-wave mixing process, we show that an interrogation of the vibrational dynamics occurring in different electronic states of molecules is possible. The technique is applied to iodine. The initial pump pulse is used to populate the B((3)Pi) state of molecular iodine in the gas phase. Now, by using an internal time delay in the DFWM process, which is resonant with the transition between the B state and a higher lying ion-pair state, the vibrational dynamics of the B state and the ion-pair state could be observed. States of even symmetry are investigated, which are accessed by a one photon transition from the B state. By a proper choice of the wavelengths used for the pump and DFWM beams, the dynamics of ion-pair states belonging to two different tiers are monitored.
    No preview · Article · Mar 2008 · Physical Chemistry Chemical Physics
  • A Scaria · V Namboodiri · J Konradi · A Materny
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    ABSTRACT: Femtosecond degenerate four-wave-mixing spectroscopy following an initial pump laser pulse was used to observe the wave packet dynamics in excited electronic states of gas phase iodine. The focus of the investigation was on the ion pair states belonging to the first tier dissociating into the two ions I-(1S) + I+(3P2). By a proper choice of the wavelengths of the initial pump and degenerate four-wave-mixing pulses, we were able to observe the vibrational dynamics of the B (3)Pi(u) (+) state of molecular iodine as well as the ion pair states accessible from there by a one-photon transition. The method proves to be a valuable tool for exploring higher lying states that cannot be directly accessed from the ground state due to selection rule exclusion or unfavorable Franck-Condon overlap.
    No preview · Article · Nov 2007 · The Journal of Chemical Physics
  • J. Konradi · A. Scaria · V. Namboodiri · A. Materny
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    ABSTRACT: Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) results in spectra that, as a function of the probe delay time, yield information about the dynamics of the coherently excited vibrational modes. A change of the shape of the exciting laser pulses has a dramatic influence on the spectral response. A feedback-controlled optimization of specific modes making use of phase and amplitude modulation of the Stokes laser pulse is applied to selectively influence the anti-Stokes signal spectrum. The role of phase and amplitude changes of the frequency components of the ultrashort pulse is analyzed. It can be demonstrated that the optimization process is clearly dominated by the effect of timing of the dispersed pulse segments. The modulation of the spectral amplitudes has only a small influence on the mode ratios. We conclude that mode focusing in time domain CARS spectroscopy can be achieved only by correctly setting the phases of the spectral pulse components (here demonstrated for the Stokes laser). Copyright © 2007 John Wiley & Sons, Ltd.
    No preview · Article · Aug 2007 · Journal of Raman Spectroscopy
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    ABSTRACT: We present a comparison of results obtained by Raman and fs coherent anti-Stokes Raman scattering (CARS) spectroscopy on four molecules with puckered rings, analyzing the ring puckering band progressions. The dynamics are probed in gas and liquid phase. For the gas phase a rich beating structure is observed in the anti-Stokes signal over many picoseconds while the liquid phase transients rapidly decay. Therefore, fast Fourier analysis yields highly resolved lines for the molecules in gas phase, which can be assigned to the Raman modes. No effect due to rotational orientation changes could be observed.
    Full-text · Article · Dec 2006 · Chemical Physics Letters
  • J. Konradi · A.K. Singh · A. Materny
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    ABSTRACT: Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) gives access to ultrafast molecular dynamics. Due to the spectrally broad laser pulses, usually poorly resolved spectra result from this spectroscopy. However, it can be demonstrated that by shaping the femtosecond pulses a selective excitation of specific vibrational modes is possible. We demonstrate that using a feedback-controlled optimization technique, molecule-specific CARS spectra can be obtained from a mixture of different substances. A careful analysis of the experimental results points to a nontrivial control of the vibrational mode dynamics in the electronic ground state of the molecules as underlying mechanism.
    No preview · Article · Jun 2006 · Journal of Photochemistry and Photobiology A Chemistry
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    ABSTRACT: Femtosecond laser pulses are spectrally broad and therefore coherently excite several molecular modes. While the temporal resolution is high, usually no mode-selective excitation is possible. In this work, we demonstrate that, by phase shaping one of the two pulses, which excite the vibrational modes of β-carotene within a coherent anti-Stokes Raman scattering (CARS) process, specific modes can be enhanced or suppressed. Using the pulse shaper setup in a feedback-controlled closed-loop optimizes this mode selection. Here, the ratio of signal intensities observed in the CARS spectrum serves as a feedback function for an evolutionary algorithm responsible for the optimization. The optimized phase structure of the femtosecond pulse is characterized using the frequency resolved optical gating (FROG) technique to get an insight into the optimization process. Furthermore, it is shown that the temporal resolution after optimization is still high enough to investigate the ultrafast molecular dynamics. The suppression or relative enhancement of vibrational modes persists over the entire coherence time. Copyright © 2006 John Wiley & Sons, Ltd.
    No preview · Article · Jun 2006 · Journal of Raman Spectroscopy
  • J. Konradi · A.V. Scaria · A.K. Singh · A. Materny
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    ABSTRACT: Femtosecond (fs) pulses are spectrally broad, and therefore, a selective excitation of specific vibrational modes of molecules is usually not achieved in ultrafast spectroscopy. This chapter provides an introduction to two experiments: answering questions in the context of spectrum control in fs time-resolved spectroscopy. Coherent control techniques are applied to a variety of systems to influence the multimode dynamics. The shaping of the fs laser pulses by variation of the phases and/or amplitudes of their different frequency components prove to have considerable influence on the excitation of the molecular modes. The fact that a prediction of the pulse shape required for a desired control of molecular or reaction dynamics becomes nearly impossible for complex systems leads to the introduction of self-learning optimization techniques. The experimental result serves as a feedback signal for an optimization algorithm.
    No preview · Article · Jan 2006
  • J Konradi · A K Singh · A Materny
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    ABSTRACT: The feasibility of mode-selective excitation with broadband femtosecond laser pulses is demonstrated for toluene in liquid phase. A learning-loop optimal control scheme was applied to a stimulated Raman excitation process. Modifications of the phase shape of one of the exciting pulses resulted in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. An evolutionary algorithm guided the coherent excitation process to a selective enhancement or suppression of one or more vibrational modes over the complete coherence lifetime spanning several picoseconds. New ways of spectral filtering as well as exciting possibilities of mode-selective studying of chemical reaction dynamics are indicated.
    No preview · Article · Nov 2005 · Physical Chemistry Chemical Physics