Stark Deceleration Research Articles

Static Trapping of Polar Molecules in a Traveling Wave Decelerator

We present experiments on decelerating and trapping ammonia molecules using a combination of a Stark decelerator and a traveling wave decelerator. In the traveling wave decelerator, a moving potential is created by a series of ring-shaped electrodes to which oscillating high voltages (HV) are applied. By lowering the frequency of the applied voltages, the molecules confined in the moving trap are decelerated and brought to a standstill. As the molecules are confined in a true 3D well, this kind of deceleration has practically no losses, resulting in a great improvement on the usual Stark deceleration techniques. The necessary voltages are generated by amplifying the output of an arbitrary wave generator using fast HV amplifiers, giving us great control over the trapped molecules. We illustrate this by experiments in which we adiabatically cool trapped NH3 and ND3 molecules and resonantly excite their motion.

Motional, isotope and quadratic Stark effects in Rydberg–Stark deceleration and electric trapping of H and D

Hydrogen and deuterium Rydberg atoms, initially moving at velocities of 600 and 560 m s−1, respectively, in pulsed supersonic beams, have been decelerated and electrostatically trapped following adiabatic 90° deflection from their initial axis of propagation to minimize collisions with the trailing edge of the gas pulses. The time evolution of the potential energy surfaces, over which the atoms undergoing deceleration travel during the trap-loading process, is analogous to that of a moving electrodynamic trap. It has been studied in the laboratory-fixed frame of reference and in the continuously moving frame of reference defined by the instantaneous position of the electric-field minimum around which the atoms are located. The importance of the quadratic Stark effect in the deceleration of samples in Rydberg states with principal quantum numbers above 35 has also been investigated by comparison of experimental results with predictions resulting from the numerical calculation of particle trajectories. The data presented for deuterium atoms represent the first application of Rydberg–Stark deceleration and trapping for this atom. Comparison of the rate of loss of n = 30 H and D atoms from the trap enables one to conclude that it is not affected by the particle dynamics during deceleration and trap loading.

Dependences of slowing results on both decelerator parameters and the new operating mode: taking ND3 molecules as an example

We experimentally test our homemade electrostatic Stark decelerator with 99 slowing stages with a supersonic ND3 molecular beam. Relationships between the performance of the decelerator and various parameters, including the kinetic energy loss per stage versus the synchronous phase angle ϕ0, the final central velocity of a slowed molecular packet versus both the phase angle ϕ0 and the decelerating stage number and the relative slowing efficiency versus the phase angle ϕ0, are studied both theoretically and experimentally. With our homemade Stark decelerator, a supersonic ND3 beam is successfully decelerated from 330 to 24 m s−1, producing a molecular packet with a translational temperature of ∼36 mK. Experimental results are in good agreement with their simulated and theoretically calculated counterparts. In addition, we propose and study a new mode to operate a Stark decelerator that allows one to prepare a slow, number-density-enhanced, high-energy-resolution molecular beam, which is essentially a complementary mode to the scheme proposed by Parazzoli et al (2009 New J. Phys. 11 055031) in a low velocity regime and promises important applications in the research of cold collisions and cold chemistry.

A Forty-Segment Molecular Synchrotron

We present a synchrotron for polar molecules consisting of 40 straight hexapoles arranged in a circle with a 50 cm diameter. The mechanical design and alignment procedure as well as the trigger scheme used to switch the voltages applied to the hexapoles are described in detail. The stability of the synchrotron is demonstrated by measurements in which multiple packets are stored for over 13 s, during which they have completed over 1000 round trips and traveled a distance of over one mile. Furthermore, we demonstrate the simultaneous trapping of 26 packets; 13 revolving clock-wise and 13 counter clock-wise, that are injected into the synchrotron by two Stark decelerator beamlines. We discuss the opportunities for using the synchrotron as a low-energy collider.

Vibrational enhancement of electron emission in CO (a3Π) quenching at a clean metal surface

We have combined a Stark decelerator with a molecular beam-surface scattering setup to accurately measure the absolute electron emission yield, γ, of CO (a(3)Π) quenching at a Au(111) surface with quantum state resolution. We clearly observe an enhanced probability of electron emission when scattering vibrationally excited states of CO from the surface.

Decelerating a pulsed subsonic molecular beam by a quasi-cw optical lattice: 3D Monte-Carlo simulations

We propose a practical scheme to realize the deceleration of a pulsed subsonic molecular beam by using a multistage optical Stark decelerator (i.e., a 1D quasi-cw, cavity-enhanced optical lattice), which is composed of two nearly counter-propagating, time-varying, red-detuned, light fields with an intensity of ∼108Wcm−2 in a folded ring resonator. The dependences of the molecular slowing effects on the synchronous phase angle, the deceleration-stage number and the initial central velocity of incident molecular beam as well as the cavity enhancement factor and its cavity length are investigated by the 3D Monte-Carlo method. Our study shows that the proposed decelerator cannot only be used to slow a pulsed subsonic beam from 240m/s to standstill, but also to obtain a ultracold molecular packet with a temperature of a few μK due to the bunching effect of our multistage optical Stark decelerator, and the corresponding fraction of cold molecules is 10−4–10−6, which strongly depends on the synchronous phase angle, the cavity enhancement factor and the initial central velocity of incident molecular beam, etc..

Communication: Magnetic dipole transitions in the OH A 2Σ+ ← X 2Π system

We report on the observation of magnetic dipole allowed transitions in the well-characterized A (2)Σ(+) - X (2)Π band system of the OH radical. A Stark decelerator in combination with microwave Rabi spectroscopy is used to control the populations in selected hyperfine levels of both Λ-doublet components of the X (2)Π(3/2), v = 0, J = 3/2 ground state. Theoretical calculations presented in this Communication predict that the magnetic dipole transitions in the A (2)Σ(+), v = 1 ← X (2)Π, v = 0 band are weaker than the electric dipole transitions by a factor of 2.58 × 10(3) only, i.e., much less than commonly believed. Our experimental data confirm this prediction.

Deceleration and trapping of heavy diatomic molecules using a ring-decelerator

We present an analysis of the deceleration and trapping of heavy diatomic molecules in low-field seeking states by a moving electric potential. This moving potential is created by a “ring-decelerator”, which consists of a series of ring-shaped electrodes to which oscillating high voltages are applied. Particle trajectory simulations have been used to analyze the deceleration and trapping efficiency for a group of molecules that is of special interest for precision measurements of fundamental discrete symmetries. For the typical case of the SrF molecule in the (N, M) = (2, 0) state, the ring-decelerator is shown to outperform traditional and alternate-gradient Stark decelerators by at least an order of magnitude. If further cooled by a stage of laser cooling, the decelerated molecules allow for a sensitivity gain in a parity violation measurement, compared to a cryogenic molecular beam experiment, of almost two orders of magnitude.

Traveling-wave deceleration of heavy polar molecules in low-field-seeking states

We demonstrate the deceleration of heavy polar molecules in low-field seeking states by combining a cryogenic source and a travelling-wave Stark decelerator. The cryogenic source provides a high intensity beam with low speed and temperature, and the travelling-wave decelerator provides large deceleration forces and high phase-space acceptance. We prove these techniques using YbF molecules and find the experimental data to be in excellent agreement with numerical simulations. These methods extend the scope of Stark deceleration to a very wide range of molecules.

State purity of decelerated molecular beams

Cold, velocity-controlled molecular beams consisting of a single quantum state are a powerful tool for exploring molecular interactions. Here, we explore the state purity and resulting dynamics of a Stark-decelerated beam of ammonia molecules where numerous rotational states are initially populated. Under these circumstances, Stark deceleration is shown to be ineffective at producing a molecular beam consisting of a single quantum state. Therefore, quantum state purity must be carefully considered when using Stark decelerated beams and analogous techniques, particularly in collision experiments where contributions from all quantum states must be addressed.

A Multistage Optical Stark Decelerator for use with a Pulsed Molecular Beam with an Electrostatic Storage Ring

We propose a promising scheme to repeatedly decelerate a pulsed molecular beam using a red-detuned quasi-cw semi-Gaussian laser beam (SGB) and an electrostatic storage ring. Using the Monte-Carlo simulation method, we demonstrate that this promising optical Stark decelerator can be used to efficiently slow a pulsed ND3 molecular beam extracted from a Stark decelerator or a cryogenic reservoir by using a single SGB. The deceleration effect of this scheme on the intensity of the SGB is discussed in detail.

Decelerating a pulsed subsonic molecular beam by a quasi-cw traveling optical lattice

We propose a promising scheme to realize the deceleration of a pulsed subsonic molecular beam by using a multistage optical Stark decelerator (i.e., a 1D quasi-cw traveling optical lattice), which is composed of two nearly counter-propagating, time-varying, red-detuned light fields with an intensity of ~10⁷Wcm⁻² and a fixed frequency difference between them. We also study the influence of the velocity reduced amount of the traveling lattice, the lattice power, the synchronous phase angle, the deceleration-stage number and the temporal profile of laser pulses on the molecular slowing results by using 3D Monte-Carlo method. Our study shows that the proposed decelerator cannot only be used to slow a pulsed subsonic beam from 240m/s to standstill, but also to obtain a cold molecular packet with a temperature of a few µK, and the corresponding fraction of cold molecules is 10⁻⁶-10⁻⁷, which strongly depends on the synchronous phase angle. And we also find that a pair of appropriate rising and falling times of laser pulses will lead to a better slowing effect than that produced by the top-hat temporal ones.

Scattering resonances in slow NH3–He collisions

We theoretically study slow collisions of NH(3) molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10(-4) cm(-1) to 130 cm(-1), using fully converged quantum close-coupling calculations. To describe the interaction between the NH(3) molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH(3) beam.

Generation of the three-dimensional array of optical trap by spatial light modulator

In this paper, a new scheme of generating a three-dimensional array of optical trap is proposed by using a composite phase grating that is fabricated by liquid crystal spatial light modulator. The composite phase grating is formed by combining the circular grating, which is generated by transforming a one-dimensional rectangular grating into a circular grating that can produce the longitudinal array of optical trap, with a two-dimensional rectangular grating. The grating that generates 5× 5× 5 array of optical trap is simulated according to the technical parameters of the spatial light modulator. The output intensity distribution is calculated by using the Gaussian light wave with ordinary power as input light and focusing the diffracting light with lens. The results show that three-dimensional array of optical trap with a very high peak value of intensity and an intensity gradient is obtained around the focus of the lens. The optical dipole potential of trapping cold atoms achieves the order of mK, and the interaction force between the atom and the optical field is much greater than the atom gravity. When the high power laser is used as input light, the generated array of optical trap can also be employed to trap the cold molecules produced by Stark deceleration.

Producing translationally cold, ground-state CO molecules

Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here, we outline that the required samples of translationally cold CO (X(1)Σ(+), v'' = 0, N'' = 0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a(3)Π(1)) molecules followed by optical transfer of the metastable species to the ground state via perturbed levels in the A(1)Π state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.

A high-energy, chirped laser system for optical Stark deceleration

We describe a high-energy, frequency chirped laser system designed for optical Stark deceleration of cold molecules. This system produces two, pulse amplified beams of up to 700 mJ with flat-top temporal profiles, whose frequency and intensity can be well controlled for durations from 20 ns–10 μs. The two beams are created by amplifying a single, rapidly tunable Nd:YVO4 microchip type laser at 1064 nm, which can be frequency chirped by up to 1 GHz over the duration of the pulse. Intensity modulation induced by relaxation oscillations in the microchip laser during the frequency chirp are virtually eliminated by injection locking a free running semiconductor diode laser before pulsed amplification.

Accumulation of Stark-decelerated NH molecules in a magnetic trap

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ− ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground-state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.

Scattering of Stark-decelerated OH radicals with rare-gas atoms

We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X2Π3/2, J = 3/2, f) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D2 as a function of the collision energy between ∼70 cm−1 and 400 cm−1. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by coupled channel scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006)], Ar [L. Scharfenberg, J. Kłos, P.J. Dagdigian, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D2 [M. Kirste, L. Scharfenberg, J. Kłos, F. Lique, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.

Stark deceleration of CaF molecules in strong- and weak-field seeking states

We report the Stark deceleration of CaF molecules in the strong-field seeking ground state and in a weak-field seeking component of a rotationally-excited state. We use two types of decelerator, a conventional Stark decelerator for the weak-field seekers and an alternating gradient decelerator for the strong-field seekers, and we compare their relative merits. We also consider the application of laser cooling to increase the phase-space density of decelerated molecules.

Cold heteromolecular dipolar collisions

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.