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Femtosecond pump-probe spectroscopy for single trapped molecular ions

Markus Kowalewski, S. Kahra, Günther Leschhorn, T. Schätz, Regina de Vivie‐Riedle

Published Jan 1, 2013• 2 citations
Publisher

Abstract

The vibrational dynamics of single molecular ions confined in a coulomb crystal are followed by 4 fs ultraviolet pulses. Theoretical calculations predicted the optimal laser parameters for a successful experiment under background free conditions. The ions are subject to internal heating, which has to be treated in the theory and handled in the experiment. Beyond the experiment the pump-probe scheme is extended by a coherent control scheme designed to suppress unwanted vibrational heating.

DOI
10.1051/epjconf/20134102028

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Dec 22, 2008•1 citation
The vibrational dynamics of single molecular ions confined in a coulomb crystal are followed by 4 fs ultraviolet pulses. Theoretical calculations predicted the optimal laser parameters for a successful experiment under background free conditions. The ions are subject to internal heating, which has to be treated in the theory and handled in the experiment. Beyond the experiment the pump-probe scheme is extended by a coherent control scheme designed to suppress unwanted vibrational heating.
Abstract We propose a method to investigate the vibrational dynamics of single polyatomic molecular ions confined in a Paul trap. Quantum logic techniques are employed to detect the recoil of single photon absorption events on the molecule via a co-trapped atomic ion. In particular, the recoil is mapped onto the electronic state of the atom which can be read out with high fidelity. This recoil detection serves as the basis for a pump-probe scheme to investigate ultrafast molecular dynamics, such as intra-molecular vibrational redistribution. The total recoil from the interaction with a sequence of ultrafast laser pulses with the molecular vibration is measured. This work discusses the experimental requirements and expected performance for multiple molecular ions with masses ranging from 17 to 165 Dalton.
Absorption spectroscopy is a fundamental tool for probing molecular structure. However, performing absorption spectroscopy on individual molecules is challenging due to the low signal-to-noise ratio. Here, we report on a nondestructive absorption spectroscopy on a mid-infrared vibrational transition in a single molecular ion that is co-trapped with an atomic ion. The absorption of a single photon is detected via the momentum transfer from the absorbed photon onto the molecule. This recoil signal is amplified using a non-classical state of motion of the two-ion crystal and subsequently read out via the atomic ion. We characterize the recoil detection method and use it to investigate the interaction between femtosecond laser pulses and the O-H stretching vibration in individual CaOH+ molecular ions. Furthermore, we present the single-photon absorption spectrum obtained for the vibrational transition. This method represents a milestone towards quantum non-demolition measurements of complex polyatomic molecules, providing high-fidelity methods for preparation and measurement of the quantum state of a wide range of molecular species.
Intense, few-femtosecond pulse technology has enabled studies of the fastest vibrational relaxation processes. The hydrogen group vibrations can be imaged and manipulated using intense infrared pulses. Through numerical simulation, we demonstrate an example of ultrafast coherent control that could be effected with current experimental facilities, and observed using high-resolution time-of-flight spectroscopy. The proposal is a pump-probe-type technique to manipulate the D 2 + ion with ultrashort pulse sequences. The simulations presented show that vibrational selection can be achieved through pulse delay. We find that the vibrational system can be purified to a two-level system thus realizing a vibrational qubit. A novel scheme for the selective transfer of population between these two levels, based on a Raman process and conditioned upon the delay time of a second control-pulse is outlined, and may enable quantum encoding with this system.
The thesis involves three different aspects of quantum dynamics in cold and isolated molecular systems that are investigated theoretically with methods ranging from wavepacket dynamics to density matrix propagation. In the first part the efficiency of cavity sideband cooling of trapped molecules is theoretically investigated for the case in which the infrared transition between two ro-vibrational states is used as a cycling transition. The molecules are assumed to be trapped either by a radio frequency or optical trapping potential, depending on whether they are charged or neutral, and confined inside a high-finesse optical resonator that enhances radiative emission into the cavity mode. Using realistic experimental parameters and Carbonyl sulfide as a molecular representative, we show that in this setup, cooling to the trap ground state is feasible. The second part investigates femtosecond pump-probe spectroscopy of single MgH+ ions confined in an ion trap. The molecular ions are embedded in a coulomb crystal of atomic magnesium ions which are laser cooled to a few millikelvin. Single molecules are addressed by femtosecond ultraviolet laserpulses and the induced molecular processes are investigated. The simulations of the wave packet motion and dissociation behavior were used to predict the experimental parameter regime and are compared directly to the laboratory results. For this purpose a multiscale model is developed which involves the wave packet motion as well as the problem of vibrational heating occurring on a millisecond time scale under laboratory conditions. The third part investigates the gas phase SN2 collision reaction of chloride and methyl iodine. Motivated by the experimental results of Roland Wester and co-workers [Mikosch et al., Science 319, 183 (2008)], remaining questions were addressed. The collision reaction is simulated by solving the time dependent Schrödinger equation on ab initio potential energy surfaces. With the chosen reactive coordinates it is possible to reproduce the basic features of the immediate collision reaction. The internal molecular coordinates are transformed into reaction path based model which allows for an efficient numerical description of the nuclear dynamics. The energy transfer in the system is investigated and compared to the experimental results. From the new insight into the process, an intuitive concept of a dynamical barrier can be derived. Moreover the role of the spectator mode can be clarified.
Trapped laser-cooled atomic ions are a new tool for understanding cold molecular ions. The atomic ions not only sympathetically cool the molecular ions to millikelvin temperatures, but the bright atomic ion fluorescence can also serve as a detector of both molecular reactions and molecular spectra. We are working towards the detection of single molecular ion spectra by sympathetic heating spectroscopy. Sympathetic heating spectroscopy uses the coupled motion of two trapped ions to measure the spectra of one ion by observing changes in the fluorescence of the other ion. Sympathetic heating spectroscopy is a generalization of quantum logic spectroscopy, but does not require ions in the motional ground state or coherent control of the ion internal states. We have recently demonstrated this technique using two isotopes of Ca+ [Phys. Rev. A, 81, 043428 (2010)]. Limits of the method and potential applications for molecular spectroscopy are discussed.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.