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arXiv:1809.06776 · New Journal of Physics

Ultrafast infrared spectroscopy with single molecular ions

Philipp Schindler

Published Jul 25, 2019• 3 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/1367-2630/ab3549

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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.
Absorption spectroscopy is a fundamental tool for probing molecular structure. However, performing absorption spectroscopy on individual molecules is challenging because of the low signal-to-noise ratio. Here we report on non-destructive 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 by the momentum transfer from the absorbed photon onto the molecule. This recoil signal is amplifed using a non-classical state of motion of the two-ion crystal and subsequently read out by 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 spectrum obtained for the vibrational transition with single-photon sensitivity. This method can provide a way of performing non-destructive state detection of complex polyatomic molecules, for preparation and measurement of the quantum state of a wide range of molecular species. This research was funded by Austrian science fund FWF (10.55776/COE1, 10.55776/PIN3213524), the Austrian Research Promotion Agency (FFG) under the project ‘HDCode’ (FO999921407), and the ERC Horizon 2020 project ERC-2020-STG 948893. This research is funded in part by the Gordon and Betty Moore Foundation through grant GBMF12992. V.Š. was supported by funding from the MEYS of Czech Republic under the project CZ.02.01.01/00/22 008/0004649. M.G. and M.T. acknowledge the National Science Centre, Poland (grant no. 2021/43/B/ST4/03326) for financial support and the high-performance computing infrastructure PLGrid of Poland (HPC centres: ACK Cyfronet AGH) for providing computer facilities and support (computational grant no. PLG/2024/017527). Open access funding provided by University of Innsbruck and Medical University of Innsbruck.
Recently developed techniques for quantum computation using trapped ions allow precise coherent control of the internal and external states of single atoms. Here we report how these techniques can be employed to perform precision spectroscopy of atomic and molecular ions that lack accessible transitions for laser cooling and detection. Furthermore, we discuss how quantum logic can be used to laser‐cool molecules to near their rotational and vibrational ground state by avoiding detrimental spontaneous emission of photons from the molecule.
We present theoretical and experimental progress towards a new approach for the precision spectroscopy, coherent manipulation and state-to-state chemistry of single isolated molecular ions in the gas phase. Our method uses a molecular beam for creating packets of rotationally cold neutrals from which a single molecule is state-selectively ionized and trapped inside a radiofrequency ion trap. In addition to the molecular ion, a single co-trapped atomic ion is used to cool the molecular external degrees of freedom to the ground state of the trap and to detect the molecular state using state-selective coherent motional excitation from a modulated optical-dipole force acting on the molecule. We present a detailed discussion and theoretical characterization of the present approach. We simulate the molecular signal experimentally using a single atomic ion, indicating that different rovibronic molecular states can be resolved and individually detected with our method. The present approach for the coherent control and non-destructive detection of the quantum state of a single molecular ion opens up new perspectives for precision spectroscopies relevant for, e.g., tests of fundamental physical theories and the development of new types of clocks based on molecular vibrational transitions. It will also enable the observation and control of chemical reactions of single particles on the quantum level. While focusing on N2+ as a prototypical example in the present work, our method is applicable to a wide range of diatomic and polyatomic molecules.
Laser spectroscopy is a versatile tool to investigate the internal structure of atoms, molecules and nuclei.High precision spectroscopy requires laser cooling to reduce Doppler broadening and Doppler shifts.Sympathetic cooling extends these advantages to species which do not have an appropriate transition for laser cooling and allows precision spectroscopy of previously inaccessible species using the quantum logic technique.The original implementation of quantum logic spectroscopy relies on the long lifetime of the excited spectroscopy state to realize a transfer sequence.Precision spectroscopy of transitions with a short-lived excited state is typically performed using laser induced uorescence or laser absorption spectroscopy.Neither of these methods has reached the fundamental quantum projection noise limit and are rather limited by photon shot noise.For these situations, we develop the technique of photon recoil spectroscopy as an extension of quantum logic spectroscopy to broad transitions.The common motion of a sideband-cooled two-ion crystal is excited by photon recoil during photon absorption on the spectroscopically investigated ion.This motional excitation is transferred to an internal state excitation on the co-trapped ion of a dierent species and detected with high eciency.We demonstrate the technique on the 2 S 1/2 ↔ 2 P 1/2 transition of the 40 Ca + ion and resolve the line center to 1/300 of its observed linewidth.The simplicity and versatility of the technique promises applications to other previously inaccessible species.A variation of the technique is proposed for rotational state preparation of molecular ions.A moving optical lattice is employed to exert a state-dependent force on the molecule.This would allow the detection of the rotational state of the molecular ion and therefore enable precision spectroscopy of specic rovibrational transitions in the molecule.
Abstract. Preparing molecules at rest and in a highly pure quantum state is a long standing dream in chemistry and physics, so far achieved only for a select set of molecules in dedicated experimental setups. Here, a quantum-limited combination of mass spectrometry and Raman spectroscopy is proposed that should be applicable to a wide range of molecular ions. Excitation of electrons in the molecule followed by uncontrolled decay and branching into several lower energy states is avoided. Instead, the molecule is always confined to rotational states within the electronic and vibrational ground-state manifold, while a co-trapped atomic ion provides efficient entropy removal and allows for extraction of information on the molecule. The outlined techniques might enable preparation, manipulation and measurement of a large multitude of molecular ion species with the same instrument, with applications including, but not limited to, precise determination of molecular properties and fundamental tests of physics. 1.