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arXiv:1411.7860 · New Journal of Physics

Rotational state detection of electrically trapped polyatomic molecules

Rosa Glöckner, Alexander Prehn, Gerhard Rempe, Martin Zeppenfeld

Published May 22, 2015• 10 citations
arXivHTMLPDF

Abstract

Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multiphoton ionization or laser-induced fluorescense do not work. As this is the case for most polyatomic molecule species, in this paper we investigate an alternative based on state-selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge, as the frequency differences between various transitions are easily substantially less than the Stark broadening in an electric trap. However, by using a unique trap design that provides homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M -substate.

DOI
10.1088/1367-2630/17/5/055022

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Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multiphoton ionization or laser-induced fluorescense do not work. As this is the case for most polyatomic molecule species, in this paper we investigate an alternative based on state-selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge, as the frequency differences between various transitions are easily substantially less than the Stark broadening in an electric trap. However, by using a unique trap design that provides homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M -substate.
Rapid progress in cooling and trapping of molecules has enabled first experiments on high-resolution spectroscopy of trapped diatomic molecules, promising unprecedented precision. Extending this work to polyatomic molecules provides unique opportunities due to more complex geometries and additional internal degrees of freedom. Here, this is achieved by combining a homogeneous-field microstructured electric trap, rotational transitions with minimal Stark broadening at a"magic" offset electric field, and optoelectrical Sisyphus cooling of molecules to the low millikelvin temperature regime. We thereby reduce Stark broadening on the J=5←4 (K=3) transition of formaldehyde at 364 GHz to well below 1 kHz, observe Doppler-limited linewidths down to 3.8 kHz, and determine the magic-field line position with an uncertainty below 100 Hz. Our approach opens a multitude of possibilities for investigating diverse polyatomic molecule species.
Trapped molecular ions in pure rovibronic states are desirable in experiments ranging from cold chemistry to searches for physics beyond the Standard Model. Resonance-enhanced multiphoton ionisation (REMPI) can be used to prepare molecular ions in specific internal states with high fidelities. However, in the presence of electric fields, ionisation spectra exhibit frequency shifts and the ionisation thresholds are broadened. For this reason, REMPI studies are normally conducted in low and highly homogeneous electric fields, whereas the operating principle of rf ion traps requires electric fields that vary in space and time. In order to investigate the impact of this on the state-selectivity of REMPI in ion traps, we have simulated the expected broadening of the ionisation threshold under various operating conditions of a typical linear Paul trap. In many cases, the width of the ionisation threshold exceeds the separation between rotational energy levels, preventing state-selective ionisation. Careful choice of the trapping and laser parameters during loading can reduce this broadening, enabling state-selective ionisation in some instances. Where this strategy is not sufficient, the broadening can be reduced further by rapidly switching the trapping voltages off and on again during loading. This has been demonstrated experimentally for a Coulomb crystal of [Formula: see text] ions without descrystallising it.
Polyatomic molecular ions contain many desirable attributes of a useful quantum system, including rich internal degrees of freedom and highly controllable coupling to the environment. To date, the vast majority of state-specific experimental work on molecular ions has concentrated on diatomic species. The ability to prepare and read out polyatomic molecules in single quantum states would enable diverse experimental avenues not available with diatomics, including new applications in precision measurement, sensitive chemical and chiral analysis at the single-molecule level, and precise studies of Hz-level molecular tunneling dynamics. While cooling the motional state of a polyatomic ion via sympathetic cooling with a laser-cooled atomic ion is straightforward, coupling this motional state to the internal state of the molecule has proven challenging. Here we propose a method for readout and projective measurement of the internal state of a trapped polyatomic ion. The method exploits the rich manifold of technically accessible rotational states in the molecule to realize robust state preparation and readout with far less stringent engineering than quantum logic methods recently demonstrated on diatomic molecules. The method can be applied to any reasonably small $(\ensuremath{\lesssim}10$ atoms) polyatomic ion with an anisotropic polarizability.
Polyatomic molecules offer many applications in quantum science once the wealth of internal and external degrees of freedom can be controlled. A widely applicable approach for controlling the rotational states of trapped polyatomic molecules is presented in this thesis. It includes the development of a rotational-state detection method, rotational-state cooling via optical pumping, and the preparation of a motionally cold (~30 mK) ensemble of 10 6 trapped CH 3 F molecules in a single rotational state.
Polar molecules in selected quantum states can be guided, decelerated, and trapped using electric fields created by microstructured electrodes on a chip, as shown on p. 1799 by G. Santambrogio et al. Transitions between two of these quantum states can be induced while the molecules are on the chip. CO (a3Π1, v=0) molecules, prepared at the J=1 rotational level, are trapped and guided to the center of the chip. There, they undergo the J=2←J=1 rotational transition driven by narrow-band sub-THz (mm-wave) radiation and, after being guided off the chip, are state-selectively detected.
Polar molecules in selected quantum states can be guided, decelerated and trapped using electric fields created by microstructured electrodes on a chip. Here we demonstrate that transitions between two vibrational quantum states can be induced while the molecules are trapped above the chip. We use CO molecules, prepared in the J = 1 rotational level of the a 3Π1, v = 0 state and induce the transition to either the J = 1 or the J = 2 level in the vibrationally excited a 3Π1, v = 1 state with pulsed, narrowband IR radiation. First, the vibrational excitation is studied using CO molecules in a freely propagating molecular beam, in a well-defined homogeneous electric field. Then, we demonstrate that the IR radiation can be coupled to CO molecules that are trapped less than 50 µm above the chip by guiding molecules in the v = 0 level to the center of the chip where they are pumped to the v = 1 level. The molecules remain trapped and are then guided off the chip and state-selectively detected.