Profiles

Markus Kowalewski

Markus Kowalewski

Biträdande lektor

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Arbetar vid Fysikum
Telefon 08-553 786 61
E-post markus.kowalewski@fysik.su.se
Besöksadress Roslagstullsbacken 21
Rum C4:3041
Postadress Fysikum 106 91 Stockholm

Om mig

The group works on a wide variety of topics involving photo chemistry, coherent control, polaritonic chemistry, ultrafast spectroscopy, and numerical methods related to quantum dynamics.

Polaritonic Chemistry and Light-Matter Materials

Gaining detailed control over chemical reactions has always been a chemists dream. Quantum coherent control has been pursuing this dream by using specially tailored light fields to control chemical reactions on an atomistic level. With the advancement of cavity quantum electrodynamics and its recent application to molecules, using the quantum properties of light to control photo-chemistry has come into reach. Recent,
ground breaking experiments have show that one can utilize the vacuum field of an optical nano-resonator to significantly modify the potential energy landscape and thus its photo-chemistry. The underlying effect is the formation of so called "dressed states", which are created when the quantized radiation field mode couples to a molecular electronic transition. In the resulting coupled light-matter system the molecular and the photonic degrees of freedom are heavily mixed. While this effect is well understood for atomic samples, it is not yet fully understood for molecules. The introduction of the nuclear degrees of freedom requires new theoretical frameworks. This effect can be used to modify reaction pathways of chemical and photo-chemical reactions. This opens a wide range of possibilities to engineer novel types of light driven catalysts. We are looking at the underlying mechanisms and are working on building a suitable tool chest for numerical simulations. With the new insight and tools we want to propose new photo-chemical applications.
 

Ultrafast X-Ray Spectroscopy of Conical Intersections

Conical intersections (CoIns) so far have eluded direct experimental observations. The evidence for their existence is based on ultra fast relaxation rates and other indirect signatures. The rapidly varying energy gap in the vicinity of a C oIn poses a main obstacle for their direct detection. The required extreme combination of temporal and spectral resolution is not available in conventional optical femtosecond experiments. Ultra short laser pulses in the extreme ultraviolet and X-ray laser regime, as they are provided by free electron laser and high harmonic generation sources, fulfill the spectral and temporal requirements to resolve the coupled nuclear+electronic dynamics in the vicinity of CoIns. Ultrafast hard X-ray sources make time-resolved diffraction experiments possible, paving the way to capture the nuclear dynamics of molecules in time as well as in space, with the "molecular movie" of a CoIn as the ultimate goal.

We theoretically investigate novel, X-Ray baseed experimental techniques for spectroscopic detection of CoIns with ultra short X-ray pulses. Simulation strategies for non-linear X-ray spectra and diffraction schemes are applied to molecular system of increasing complexity.

Publikationer

I urval från Stockholms universitets publikationsdatabas
  • 2020. Eric Davidsson, Markus Kowalewski. Journal of Physical Chemistry A 124 (23), 4672-4677

    Strong light-matter coupling can modify the photochemistry of molecular systems. The collective dynamics of an ensemble of molecules coupled to the light field plays a crucial role in experimental observations. However, the theory of polaritonic chemistry is primarily understood in terms of single molecules, since even in small molecular ensembles the collective dynamics becomes difficult to disentangle. Understanding of the underlying ensemble mechanisms is key to a conceptual understanding and interpretation of experiments. We present a model system that simplifies the problem by mixing two-level Mg atoms with a single MgH+ molecule and investigate its collective dynamics. Our focus is on the modified chemical properties of a single diatomic molecule in the presence of an ensemble of resonant atoms as well as the structure of the major and intermediate polariton states. We present quantum dynamics simulations of the coupled vibronic-photonic system for a variable size of the atomic ensemble. Special attention is given to dissociative the dynamics of the MgH+ molecule.

  • 2019. András Csehi (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 100 (5)

    Coherent control experiments in molecules are often done with shaped laser fields. The electric field is described classically and control over the time evolution of the system is achieved by shaping the laser pulses in the time or frequency domain. Moving on from a classical to a quantum description of the light field allows one to engineer the quantum state of light to steer chemical processes. The quantum field description of the photon mode allows one to manipulate the light-matter interaction directly in phase space. In this paper we demonstrate the basic principle of coherent control with quantum light on the avoided crossing in lithium fluoride. Using a quantum description of light together with the nonadiabatic couplings and vibronic degrees of freedoms opens up alternative perspective on quantum control. We show the deviations from control with purely classical light field and how back-action of the light field becomes important in a few-photon regime.

  • 2019. Markus Kowalewski, Kochise Bennett, Shaul Mukamel. EPJ Web of Conferences 205

    We theoretically examine time-resolved diffraction from molecules which undergo non-adiabatic dynamics and identify contributions from inelastic scattering that indicate the presence of an avoided crossing and the corresponding nuclear configuration.

  • 2019. András Csehi (et al.). New Journal of Physics 21

    Nonadiabatic effects appear due to avoided crossings or conical intersections (CIs) that are either intrinsic properties in field-free space or induced by a classical laser field in a molecule. It was demonstrated that avoided crossings in diatomics can also be created in an optical cavity. Here, the quantized radiation field mixes the nuclear and electronic degrees of freedom creating hybrid field-matter states called polaritons. In the present theoretical study we go further and create CIs in diatomics by means of a radiation field in the framework of cavity quantum electrodynamics. By treating all degrees of freedom, that is the rotational, vibrational, electronic and photonic degrees of freedom on an equal footing we can control the nonadiabatic quantum light-induced dynamics by means of CIs. First, the pronounced difference between the the quantum light-induced avoided crossing and the CI with respect to the nonadiabatic dynamics of the molecule is demonstrated. Second, we discuss the similarities and differences between the classical and the quantum field description of the light for the studied scenario.

  • 2019. Daeheum Cho (et al.). Philosophical Transactions. Series A 377 (2145)

    X-ray diffraction signals from the time-evolving molecular charge density induced by selective core excitation of chemically inequivalent carbon atoms are calculated. A narrowband X-ray pulse selectively excites the carbon K-edge of the –CH3 or –CH2F groups in fluoroethane (CH3–CH2F). Each excitation creates a distinct core coherence which depends on the character of the electronic transition. Direct propagation of the reduced single-electron density matrix, using real-time time-dependent density functional theory, provides the time-evolving charge density following interactions with external fields. The interplay between partially filled valence molecular orbitals upon core excitation induces characteristic femtosecond charge migration which depends on the core–valence coherence, and is monitored by the sum-frequency generation diffraction signal.

Visa alla publikationer av Markus Kowalewski vid Stockholms universitet

Senast uppdaterad: 22 oktober 2020

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