Complete List of Papers with Abstract

Preprints

Publications

  1. Subhas Ghosal, Richard J. Doyle, Christiane P. Koch, and Jeremy M. Hutson
    Stimulating the production of deeply bound RbCs molecules with laser pulses: the role of spin-orbit coupling in forming ultracold molecules
    accepted for publication in New J. Phys. 11 (2009)
    arXiv:0810.5703

    We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85Rb and 133Cs atoms colliding on the X1Sigma+ potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S) + Cs(6P1/2) asymptote. We explore the nature of the Ω=0+ levels in this region, which have mixed A1Σ+ and b3Π character. We then study the quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A wavepacket is formed by exciting a few vibronic levels and is allowed to propagate on the coupled electronic potential energy curves. For a detuning of 7.5 cm-1, the wavepacket for RbCs reaches the short-range region in about 13 ps, which is significantly faster than for the homonuclear Rb2 system; this is mostly because of the absence of an R-3 long-range tail in the excited-state potential curves for heteronuclear systems. We give a simple semiclassical formula that relates the time taken to the long-range potential parameters. For RbCs, in contrast to Rb2, the excited-state wavepacket shows a substantial peak in singlet density near the inner turning point, and this produces a significant probability of deexcitation to form ground-state molecules bound by up to 1500 cm-1. Our analysis of the role of spin-orbit coupling concerns the character of the mixed states in general and is important for both photoassociation and stimulated Raman deexcitation.
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  2. Christiane P. Koch, Mamadou Ndong and Ronnie Kosloff
    Two-photon coherent control of femtosecond photoassociation
    accepted for publication in Faraday Discussions 142
    arXiv:0810.5738

    Photoassociation with short laser pulses has been proposed as a technique to create ultracold ground state molecules. A broad-band excitation seems the natural choice to drive the series of excitation and deexcitation steps required to form a molecule in its vibronic ground state from two scattering atoms. First attempts at femtosecond photoassociation were, however, hampered by the requirement to eliminate the atomic excitation leading to trap depletion. On the other hand, molecular levels very close to the atomic transition are to be excited. The broad bandwidth of a femtosecond laser then appears to be rather an obstacle. To overcome the ostensible conflict of driving a narrow transition by a broad-band laser, we suggest a two-photon photoassociation scheme. In the weak-field regime, a spectral phase pattern can be employed to eliminate the atomic line. When the excitation is carried out by more than one photon, different pathways in the field can be interfered constructively or destructively. In the strong-field regime, a temporal phase can be applied to control dynamic Stark shifts. The atomic transition is suppressed by choosing a phase which keeps the levels out of resonance. We derive analytical solutions for atomic two-photon dark states in both the weak-field and strong-field regime. Two-photon excitation may thus pave the way toward coherent control of photoassociation. Ultimately, the success of such a scheme will depend on the details of the excited electronic states and transition dipole moments. We explore the possibility of two-photon femtosecond photoassociation for alkali and alkaline-earth metal dimers and present a detailed study for the example of calcium.
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  3. Mamadou Ndong, Hillel Tal-Ezer, Ronnie Kosloff, and Christiane P. Koch
    A Chebychev propagator for inhomogeneous Schrödinger equations
    J. Chem. Phys. 130, 124108 (2009) (arXiv:0812.4428)

    We present a propagation scheme for time-dependent inhomogeneous Schrödinger equations which occur for example in optimal control theory or in reactive scattering calculations. A formal solution based on a polynomial expansion of the inhomogeneous term is derived. It is subjected to an approximation in terms of Chebychev polynomials. Different variants for the inhomogeneous propagator are demonstrated and applied to two examples from optimal control theory. Convergence behavior and numerical efficiency are analyzed.
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  4. Christiane P. Koch
    Perspectives for coherent optical formation of strontium molecules in their electronic ground state
    Phys. Rev. A 78, 063411 (2008) (arXiv:0811.0015)

    Optical Feshbach resonances [Phys. Rev. Lett. 94, 193001 (2005)] and pump-dump photoassociation with short laser pulses [Phys. Rev. A 73, 033408 (2006)] have been proposed as means to coherently form stable ultracold alkali dimer molecules. In an optical Feshbach resonance, the intensity and possibly frequency of a cw laser are ramped up linearly followed by a sudden switch-off of the laser. This is applicable to tightly trapped atom pairs. In short-pulse photoassociation, the pump pulse forms a wave-packet in an electronically excited state. The ensuing dynamics carry the wave-packet to shorter internuclear distances where, after half a vibrational period, it can be deexcited to the electronic ground state by the dump pulse. Short-pulse photoassociation is suited for both shallow and tight traps. The applicability of these two means to produce ultracold molecules is investigated here for 88Sr. Dipole-allowed transitions proceeding via the B1Σu+ excited state as well as transitions near the intercombination line are studied.
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  5. Christiane P. Koch and Robert Moszynski
    Engineering an all-optical route to ultracold molecules in their vibronic ground state
    Phys. Rev. A 78, 043417 (2008) (arXiv:0810.0179)

    We propose an improved photoassociation scheme to produce ultracold molecules in their vibronic ground state for the generic case where non-adiabatic effects facilitating transfer to deeply bound levels are absent. Formation of molecules is achieved by short laser pulses in a Raman-like pump-dump process where an additional near-infrared laser field couples the excited state to an auxiliary state. The coupling due to the additional field effectively changes the shape of the excited state potential and allows for efficient population transfer to low-lying vibrational levels of the electronic ground state. Repetition of many pump-dump sequences together with collisional relaxation allows for accumulation of molecules in v=0.
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  6. José P. Palao, Ronnie Kosloff, and Christiane P. Koch
    Protecting coherence in Optimal Control Theory: State dependent constraint approach
    Phys. Rev. A 77, 063412 (2008) (arXiv:0707.2401)

    Optimal control theory is developed for the task of obtaining a primary objective in a subspace of the Hilbert space while avoiding other subspaces of the Hilbert space. The primary objective can be a state-to-state transition or a unitary transformation. A new optimization functional is introduced which leads to monotonic convergence of the algorithm. This approach becomes necessary for molecular systems subject to processes implying loss of coherence such as predissociation or ionization. In these subspaces controllability is hampered or even completely lost. Avoiding the lossy channels is achieved via a functional constraint which depends on the state of the system at each instant in time. We outline the resulting new algorithm, discuss its convergence properties and demonstrate its functionality for the example of a state-to-state transition and of a unitary transformation for a model of cold Rb2.
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  7. H. K. Pechkis, D. Wang, Y. Huang, E. E. Eyler, P. L. Gould, W. C. Stwalley, C. P. Koch
    Enhancement of the formation of ultracold 85Rb2 molecules due to resonant coupling
    Phys. Rev. A 76, 022504 (2007) (arXiv:0707.2401)

    We have studied the effect of resonant electronic state coupling on the formation of ultracold ground-state 85Rb2. Ultracold Rb2 molecules are formed by photoassociation (PA) to a coupled pair of 0u+ states, 0u+(P1/2) and 0u+(P3/2), in the region below the 5S+5P1/2 limit. Subsequent radiative decay produces high vibrational levels of the ground state, X1Σg+. The population distribution of these X state vibrational levels is monitored by resonance-enhanced two-photon ionization through the 21Σu+ state. We find that the populations of vibrational levels v''=112-116 are far larger than can be accounted for by the Franck-Condon factors for 0u+(P1/2) → X1Σg+ transitions with the 0u+(P1/2) state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of 0u+ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two 0u+ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.
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  8. Christiane P. Koch, Ronnie Kosloff, Eliane Luc-Koenig, Françoise Masnou-Seeuws, and Anne Crubellier
    Photoassociation with chirped laser pulses : Calculation of the absolute number of molecules per pulse
    J. Phys. B 39, S1017 (2006)

    The total number of molecules produced in a pulsed photoassociation of ultracold atoms is a crucial link between theory and experiment. A calculation based on first principles can determine the experimental feasibility of a pulsed photoassociation scheme. The calculation method considers an initial thermal ensemble of atoms. This ensemble is first decomposed into a representation of partial spherical waves. The photoassociation dynamics is calculated by solving the multichannel time-dependent Schrödinger equation on a mapped grid. The molecules are primarily assembled in a finite region of internuclear distances, the 'photoassociation window'. The ensemble average was calculated by adding the contributions from initial scattering states confined to a finite volume. These states are Boltzmann averaged where the partition function is summed numerically. Convergence is obtained for sufficiently large volume. The results are compared to a thermal averaging procedure based on scaling laws which leads to a single representative initial partial wave which is sufficient to represent the density in the 'photoassociation window'. For completeness a third high-temperature thermal averaging procedure is described which is based on random phase thermal Gaussian initial states. The absolute number of molecules in the two first calculation methods agree to within experimental error for photoassociation with picosecond pulses for a thermal ensemble of rubidium or caesium atoms in ultracold conditions.
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  9. Ulrich Poschinger, Wenzel Salzmann, Roland Wester, Matthias Weidemüller, Christiane P. Koch, Ronnie Kosloff
    Theoretical model for ultracold molecule formation via adaptive feedback control
    J. Phys. B 39, S1001 (2006) (physics/0604140)

    We investigate pump-dump photoassociation of ultracold molecules with amplitude- and phase-modulated femtosecond laser pulses. For this purpose a perturbative model for the light-matter interaction is developed and combined with a genetic algorithm for adaptive feedback control of the laser pulse shapes. The model is applied to the formation of 85Rb2 molecules in a magneto-optical trap. We find for optimized pulse shapes an improvement for the formation of ground state molecules by more than a factor of 10 compared to unshaped pulses at the same pump-dump delay time, and by 40% compared to unshaped pulses at the respective optimal pump-dump delay time. Since our model yields directly the spectral amplitudes and phases of the optimized pulses, the results are directly applicable in pulse shaping experiments.
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  10. Christiane P. Koch, Ronnie Kosloff, Françoise Masnou-Seeuws
    Short-pulse photoassociation in rubidium below the D1 line
    Phys. Rev. A 73, 043409 (2006) (physics/0511235)

    Photoassociation of two ultracold rubidium atoms and the subsequent ground state molecule formation is investigated theoretically. The method employs laser pulses inducing transitions via excited states correlated to the 5S+5P1/2 asymptote. Weakly bound ground state molecules can be created by a single pulse while the formation of more deeply bound molecules requires a two-color pump-dump scenario. Deeply bound ground state molecules can be produced only if efficient mechanisms for both pump and dump steps exist. While long-range 1/R3-potentials allow for efficient photoassociation, stabilization is facilitated by the resonant spin-orbit coupling of the 0u+ states. Molecules in the singlet ground state bound by a few wave numbers can thus be formed. This provides a promising first step toward ground state molecules which are ultracold in both translational and vibrational degrees of freedom.
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  11. Christiane P. Koch, Eliane Luc-Koenig, Françoise Masnou-Seeuws
    Making ultracold molecules in a two color pump-dump photoassociation scheme using chirped pulses
    Phys. Rev. A 73, 033408 (2006) (physics/0508090)

    This theoretical paper investigates the formation of ground state molecules from ultracold cesium atoms in a two-color scheme. Following previous work on photoassociation with chirped picosecond pulses [Luc-Koenig et al., Phys. Rev. A 70, 033414 (2004)], we investigate stabilization by a second (dump) pulse. By appropriately choosing the dump pulse parameters and time delay with respect to the photoassociation pulse, we show that a large number of deeply bound molecules are created in the ground triplet state. We discuss (i) broad-bandwidth dump pulses which maximize the probability to form molecules while creating a broad vibrational distribution as well as (ii) narrow-bandwidth pulses populating a single vibrational ground state level, bound by 113 cm-1. The use of chirped pulses makes the two-color scheme robust, simple and efficient.
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  12. Sören Dittrich, Hans-Joachim Freund, Christiane P. Koch, Ronnie Kosloff, and Thorsten Klüner
    Two-dimensional surrogate Hamiltonian investigation of laser-induced desorption of NO/NiO(100)
    J. Chem. Phys. 124, 024702 (2006)

    The photodesorption of NO from NiO(100) is studied from first principles, with electronic relaxation treated by the use of the surrogate Hamiltonian approach. Two nuclear degrees of freedom of the adsorbate-substrate system are taken into account. To perform the quantum dynamical wave-packet calculations, a massively parallel implementation with a one-dimensional data decomposition had to be introduced. The calculated desorption probabilities and velocity distributions are in qualitative agreement with experimental data. The results are compared to those of stochastic wave-packet calculations where a sufficiently large number of quantum trajectories is propagated within a jumping wave-packet scenario.
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  13. Christiane P. Koch, Françoise Masnou-Seeuws, Ronnie Kosloff
    Creating Ground State Molecules with Optical Feshbach Resonances in Tight Traps
    Phys. Rev. Lett. 94, 193001 (2005) (quant-ph/0412166)

    We propose to create ultracold ground state molecules in an atomic Bose-Einstein condensate by adiabatic crossing of an optical Feshbach resonance. We envision a scheme where the laser intensity and possibly also frequency are linearly ramped over the resonance. Our calculations for 87Rb show that for sufficiently tight traps it is possible to avoid spontaneous emission while retaining adiabaticity, and conversion efficiencies of up to 50% can be expected.
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  14. Christiane P. Koch, José P. Palao, Ronnie Kosloff, Françoise Masnou-Seeuws
    Stabilization of Ultracold Molecules Using Optimal Control Theory
    Phys. Rev. A 70, 013402 (2004) (quant-ph/0402066)

    In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by photoassociation, Feshbach resonances, and three-body recombination. The created molecules are translationally cold, but vibrationally highly excited. This will eventually lead them to be lost from the trap due to collisions. We propose shaped laser pulses to transfer these highly excited molecules to their ground vibrational level. Optimal control theory is employed to find the light field that will carry out this task with minimum intensity. We present results for the sodium dimer. The final target can be reached to within 99% provided the initial guess field is physically motivated. We find that the optimal fields contain the transition frequencies required by a good Franck-Condon pumping scheme. The analysis identifies the ranges of intensity and pulse duration which are able to achieve this task before any other competing processes take place. Such a scheme could produce stable ultracold molecular samples or even stable molecular Bose-Einstein condensates.
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  15. David Gelman, Christiane P. Koch, Ronnie Kosloff
    Dissipative quantum dynamics with the Surrogate Hamiltonian approach. A comparison between spin and harmonic baths
    J. Chem. Phys. 121, 661 (2004) (quant-ph/0402144)

    The dissipative quantum dynamics of an anharmonic oscillator coupled to a bath is studied with the purpose of elucidating the differences between the relaxation to a spin bath and to a harmonic bath. Converged results are obtained for the spin bath by the surrogate Hamiltonian approach. This method is based on constructing a system?bath Hamiltonian, with a finite but large number of spin bath modes, that mimics exactly a bath with an infinite number of modes for a finite time interval. Convergence with respect to the number of simultaneous excitations of bath modes can be checked. The results are compared to calculations that include a finite number of harmonic modes carried out by using the multiconfiguration time-dependent Hartree method of Nest and Meyer [J. Chem. Phys. 119, 24 (2003)]. In the weak coupling regime, at zero temperature and for small excitations of the primary system, both methods converge to the Markovian limit. When initially the primary system is significantly excited, the spin bath can saturate restricting the energy acceptance. An interaction term between bath modes that spreads the excitation eliminates the saturation. The loss of phase between two cat states has been analyzed and the results for the spin and harmonic baths are almost identical. For stronger couplings, the dynamics induced by the two types of baths deviate. The accumulation and degree of entanglement between the bath modes have been characterized. Only in the spin bath the dynamics generate entanglement between the bath modes.
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  16. Christiane P. Koch, Thorsten Klüner, Hans-Joachim Freund and Ronnie Kosloff
    Surrogate Hamiltonian Study of of electronic relaxation in the femtosecond laser induced desorption of NO/NiO(100)
    J. Chem. Phys. 119, 1750-1765 (2003)

    A microscopic model for electronic quenching in the photodesorption of NO from NiO(100) is developed. The quenching is caused by the interaction of the excited adsorbate-substrate complex with electron hole pairs (O2p->Ni3d states) in the surface. The electron hole pairs are described as a bath of two level systems (TLS) which are characterized by an excitation energy and a dipole charge. The parameters are connected to estimates from photoemission spectroscopy and configuration interaction (CI) calculations. Due to the localized electronic structure of NiO a direct optical excitation mechanism can be assumed, and a reliable potential energy surface for the excited state is available. Thus a treatment of all steps in the photodesorption event from first principles becomes possible for the first time. The Surrogate Hamiltonian method which allows to monitor convergence is employed to calculate the desorption dynamics. Desorption probabilities of the right order of magnitude and velocities in the experimentally observed range are obtained.
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  17. Christiane P. Koch, Thorsten Klüner, Hans-Joachim Freund and Ronnie Kosloff
    Femtosecond Photodesorption of Small Molecules from Surfaces: A Theoretical Investigation from First Principles
    Phys. Rev. Lett. 90, 117601 (2003)

    A microscopic model for the excitation and relaxation processes in photochemistry at surfaces is developed. Our study is based on ab initio calculations and the surrogate Hamiltonian method treating surface electron-hole pairs as a bath of two-level systems. Desorption probabilities and velocities in the experimentally observed range are obtained. The excited state lifetime is calculated, and a dependence of observables on pulse length is predicted.
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  18. Christiane P. Koch, Thorsten Klüner and Ronnie Kosloff
    A complete quantum description of an ultrafast pump-probe charge transfer event in condensed phase
    J. Chem. Phys. 116, 7983-7996 (2002)

    An ultrafast photoinduced charge transfer event in condensed phase is simulated. The interaction with the field is treated explicitly within a time-dependent framework. The description of the interaction of the system with its environment is based on the Surrogate Hamiltonian method where the infinite number of degrees of freedom of the environment is approximated by a finite set of two-level modes for a limited time. This method is well suited to ultrafast events, since it is not limited by weak coupling between system and environment. Moreover, the influence of the external field on the system-bath coupling is included naturally. The Surrogate Hamiltonian method is generalized to incorporate two electronic states including all possible system-bath interactions. The method is applied to a description of a pump-probe experiment where every step of the cycle is treated consistently. Dynamical variables are considered which go beyond rates of charge transfer such as the transient absorption spectrum. The parameters of the model are chosen to mimic the mixed valence system (NH3)5RuNCRu(CN)5-.
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  19. Christiane Koch and Bernd Esser
    Spin-boson Hamiltonian and optical absorption of molecular dimers
    Phys. Rev. A 61, 022508 (2000) (quant-ph/9911042)

    An analysis of the eigenstates of a symmetry-broken spin-boson Hamiltonian is performed by computing Bloch and Husimi projections. The eigenstate analysis is combined with the calculation of absorption bands of asymmetric dimer configurations constituted by monomers with nonidentical excitation energies and optical transition matrix elements. Absorption bands with regular and irregular fine structures are obtained and related to the transition from the coexistence to a mixing of adiabatic branches in the spectrum. It is shown that correlations between spin states allow for an interpolation between absorption bands for different optical asymmetries.
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  20. Christiane Koch and Bernd Esser
    Spectrum, lifetime distributions and relaxation in a dimer with strong excitonic-vibronic coupling
    J. Lumin. 81 (1999) 171-181

    The fine structure of the complex quantum spectrum of a dimer constituted by monomers with a finite lifetime in the excited states and a strong excitonic-vibronic coupling has been investigated in detail. Lifetime distributions of the spectrum are analysed for different system parameter sets. It is shown that in case of an asymmetric configuration the spectrum may be characterised by a broad distribution of the lifetimes of the eigenstates. This can give rise to a strongly varying relaxation behaviour, which is due to the mixing of the monomer spectra with two different excitonic lifetimes in the dimer spectrum.
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Conference proceedings

Book contributions

  • A. Lindinger, V. Bonačić-Koutecký, R. Mitrić, D. Tannor, C. P. Koch, V. Engel, T. M. Bernhardt, J. Jortner, A. Mirabal, L. Wöste.
    Analysis and control of small isolated molecular systems
    In: Analysis and control of ultrafast photoinduced reactions. Springer Series in Chemical Physics Vol. 87
    Eds. O. Kühn and L. Wöste, Springer Berlin 2007.
  • Christiane P. Koch, David Gelman, Ronnie Kosloff, Thorsten Klüner.
    Irreversibilität in Quantensystemen mittels der Methode des Surrogate Hamiltonian
    In: Physik Irreversibler Prozesse und Selbstorganisation. Eds. T. Pöschel, L. Schimansky-Geier und H. Malchow. Logos Verlag Berlin 2006.

    Die Methode des Surrogate Hamiltonian stellt einen neuartigen Ansatz dar, quantendissipative Systeme zu behandeln: Es wird ein 'Ersatz'-Hamilton-Operator konstruiert, der für begrenzte Zeiten dieselbe Dynamik generiert wie der echte Hamilton-Operator. Die dissipative Zeitentwicklung erhält man dann über die Lösung der zeitabhängigen Schrödingergleichung für das Gesamtsystem und anschliessender Spurbildung über die Umgebungsfreiheitsgrade, d.h. eine Dichtematrixpropagation wird vermieden. Einfache Beispiele zur Illustration der Methode werden vorgestellt und weitergehende Anwendungen diskutiert.
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