Stark Deceleration Research Articles

Imaging the onset of the resonance regime in low-energy NO-He collisions.

At low energies, the quantum wave-like nature of molecular interactions results in distinctive scattering behavior, ranging from the universal Wigner laws near 0 kelvin to the occurrence of scattering resonances at higher energies. It has proven challenging to experimentally probe the individual waves underlying these phenomena. We report measurements of state-to-state integral and differential cross sections for inelastic NO-He collisions in the 0.2 to 8.5 centimeter-1 range with 0.02 centimeter-1 resolution. We studied the onset of the resonance regime by probing the lowest-lying resonance dominated by s and p waves only. The highly structured differential cross sections directly reflect the increasing number of contributing waves as the energy is increased. Only with CCSDT(Q) level of theory was it possible to reproduce our measurements.

Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide.

High Rydberg states of nitric oxide (NO) with principal quantum numbers between 40 and 100 and lifetimes in excess of 10 µs have been prepared by resonance enhanced two-color two-photon laser excitation from the X 2Π1/2 ground state through the A 2Σ+ intermediate state. Molecules in these long-lived Rydberg states were detected and characterized 126 µs after laser photoexcitation by state-selective pulsed electric field ionization. The laser excitation and electric field ionization data were combined to construct two-dimensional spectral maps. These maps were used to identify the rotational states of the NO+ ion core to which the observed series of long-lived hydrogenic Rydberg states converge. The results presented pave the way for Rydberg-Stark deceleration and electrostatic trapping experiments with NO, which are expected to shed further light on the decay dynamics of these long-lived excited states, and are of interest for studies of ion-molecule reactions at low temperatures.

High-resolution imaging of molecular collisions using a Zeeman decelerator.

We present the first crossed beam scattering experiment using a Zeeman decelerated molecular beam. The narrow velocity spreads of Zeeman decelerated NO (X2Π3/2, j = 3/2) radicals result in high-resolution scattering images, thereby fully resolving quantum diffraction oscillations in the angular scattering distribution for inelastic NO-Ne collisions and product-pair correlations in the radial scattering distribution for inelastic NO-O2 collisions. These measurements demonstrate similar resolution and sensitivity as in experiments using Stark decelerators, opening up possibilities for controlled and low-energy scattering experiments using chemically relevant species such as H and O atoms, O2 molecules, or NH radicals.

Generation of high-energy-resolved NH3 molecular beam by a Stark decelerator with 179 stages**Project supported by the National Natural Science Foundation of China (Grant Nos. 91536218, 11034002, 11274114, 11504112, and 11504318), the National Basic Research Program of China (Grant No. 2011CB921602), the Fundamental Research Funds for the Central Universities, China, Shanghai Pujiang Talents Plan, China (Grant No. 18PJ1403100), and Exploration Funds

We demonstrate the production of cold, slow NH3 molecules from a supersonic NH3 molecular beam using our electrostatic Stark decelerator consisting of 179 slowing stages. By using this long Stark decelerator, a supersonic NH3 molecular beam can be easily decelerated to trappable velocities. Here we present two modes for operating the Stark decelerator to slow the supersonic NH3 molecules. The first is the normal mode, where all 179 stages are used to decelerate molecules, and it allows decelerating the NH3 molecular beam from 333 m/s to 18 m/s, with a final temperature of 29.2 mK. The second is the deceleration-bunch mode, which allows us to decelerate the supersonic NH3 beam from 333 m/s to 24 m/s, with a final temperature of 2.9 mK. It is clear that the second mode promises to produce colder (high-energy-resolution) molecular samples than the normal mode. Three-dimensional Monte Carlo simulations are also performed for the experiments and they show a good agreement with the observed results. The deceleration-bunch operation mode presented here can find applications in the fields of cold collisions, high-resolution spectroscopy, and precision measurements.

Laser-assisted Stark deceleration of CaF in its rovibronic ground (high-field-seeking) state**Project supported by the National Natural Science Foundation of China (Grant No. 11604164).

A near-resonant, red-detuning laser-assisted Stark deceleration scheme is proposed to slow CaF in its high-field-seeking rovibronic ground state. The assisting Gaussian laser beam can confine CaF molecules transversely owing to the optical Stark effect. Simulations suggest that the present scheme is superior to previous Stark decelerators. Under typical experimental conditions, when the assisting laser frequency is red-detuned to the molecular transition (λ ∼606.3 nm) by 5.0 GHz and the laser power is about 5.6 W, the proposed decelerator can achieve a total number at the order of 104 CaF molecules with a number density at the order of 108 cm−3. The equivalent temperature of the obtained cold CaF molecules is 2.3 mK. Additionally, the desired assisting laser power can be as low as about 1.2 W if keeping the red-detuning value to be 1.0 GHz, which further suggests its experimental feasibility.

Laser cooling and trapping of polar molecules

Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.

A better method for Stark deceleration uses a new high-voltage amplifier

A new high-voltage amplifier design provides the capabilities and flexibility needed for traveling-wave Stark deceleration.

A high-voltage amplifier for traveling-wave Stark deceleration.

Controlling high voltage is a critical aspect of Stark deceleration, a technique that uses electric fields to produce slow molecules. Traditionally, Stark deceleration required only commercial high-voltage switches to operate. However, a new continuous version of Stark deceleration, which promises significantly improved performance, requires chirped sinusoidal voltages. The complexity of the electronics needed to run this new decelerator has restricted the implementation for all but a few groups. The challenge is to create high-voltage amplifiers that have a frequency range of tens of kilohertz down to DC and that can source and sink enough current (∼1 A) to drive the capacitive load of the decelerator. We present a new high-voltage linear amplifier for driving in-vacuum electrodes for traveling-wave Stark deceleration. The amplifier has a gain of 12 000, output voltages up to ±10 kV, instantaneous currents up to 1.5 A, and a frequency range from 30 kHz down to DC. This makes the amplifier suitable for traveling-wave Stark deceleration of a supersonic molecular beam down to rest.

Pulsed operation of a ring Stark decelerator

Stark deceleration is a technique that uses time-varying inhomogeneous electric fields to decelerate polar molecules for various molecular beam and trapping experiments. New ring-geometry Stark decelerators with continuously varying voltages offer a method to produce a more intense source of molecules than in the more typically used crossed-pin geometry Stark decelerators. However, this new technique, traveling-wave Stark deceleration, is more experimentally challenging due to the electronic requirements. A third method, pulsed-ring Stark deceleration, uses ring-geometry electrodes with high-voltage pulses to decelerate molecules. The discrete voltages make it easier to implement, and it is more efficient in certain situations due to an increased longitudinal potential gradient. Here, we present an experimental realization of a ring-geometry Stark decelerator using both continuously varying and discrete voltages. Using the pulsed-ring method, we demonstrate a newly accessible low-velocity regime for moderate peak voltages on ring electrodes. This opens up the opportunity to efficiently decelerate various molecular species using a medium length decelerator. A comparison of experimental and simulated results between traveling-wave and pulsed-ring Stark deceleration is presented along with a simple model for determining when each mode is more efficient.

Optical Stark deceleration of neutral molecules from supersonic expansion with a rotating laser beam**Project supported by the National Natural Science Foundation of China (Grant Nos. 11504112, 91536218, and 11604100).

Cold molecules have great scientific significance in high-resolution spectroscopy, precision measurement of physical constants, cold collision, and cold chemistry. Supersonic expansion is a conventional and versatile method to produce cold molecules with high kinetic energies. We theoretically show here that fast-moving molecules from supersonic expansion can be effectively decelerated to any desired velocity with a rotating laser beam. The orbiting focus spot of the red-detuned laser serves as a two-dimensional potential well for the molecules. We analyze the dynamics of the molecules inside the decelerating potential well and investigate the dependence of their phase acceptance by the potential well on the tilting angle of the laser beam. ND3 molecules are used in the test of the scheme and their trajectories under the impact of the decelerating potential well are numerically simulated using the Monte Carlo method. For instance, with a laser beam of 20 kW in power focused into a pot of in waist radius, ND3 molecules of 250 m/s can be brought to a standstill by the decelerating potential well within a time interval of about 0.73 ms. The total angle covered by the rotating laser beam is about 5.24° with the distance travelled by the potential well being about 9.13 cm. In fact, the molecules can be decelerated to any desired velocity depending on the parameters adopted. This scheme is simple in structure and easy to be realized in experiment. In addition, it is applicable to decelerating both molecules and atoms.

Experimental progress in positronium laser physics

The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.Graphical abstract

Theoritical research on optical Stark deceleration and trapping of neutral molecular beams based on modulated optical lattices

According to the optical Stark deceleration theory of using a stationary quasi-cw red-detuned optical lattice to slow and trap an arbitrary pulsed molecular beam, we propose a novel idea of using a modulated optical lattice instead of a stationary one to realize a multistage optical Stark deceleration. We analyze the motion of the decelerated molecules inside the optical decelerator, and study the dependence of the velocity of the decelerated molecular packet on the synchronous phase angle and the number of the deceleration stages (i.e. half the number of the optical-lattice cells) by using the Monte-Carlo method. The simulation results show that it takes longer time for the molecules to reach the detector as the number of the deceleration stages increases. The decelerated molecular wave packets are gradually separated from the large wave packets of the original molecular velocity distribution. And the higher the number of the deceleration stages, the lower the decelerated molecular speed is. In addition, we also study the influence of the initial phase angle of synchronous molecules under the same conditions. It is demonstrated that the higher the initial phase angle of synchronous molecules, the lower the decelerated molecular speed is and the smaller the number of molecules in the deceleration wave packet, so the phase space is compressed. The result also shows that the modulated optical Stark decelerator does not have the process of molecular free flight, and thus improving the efficiency of deceleration for molecules. The ultra-cold molecules can be trapped in the optical lattice by rapidly turning off the modulation signal of the lattice. Comparing with the previous scheme, the doubled number of the deceleration stages is reached in the same optical lattice length since a modulated optical lattice is used. For a length of optical lattice of 3.71 mm, theoretical simulation results demonstrate that the speed of methane molecules is decelerated from 280 m/s to 172 m/s. Comparing with the previous results from 280 m/s to 232 m/s, the deceleration effect is improved by 26%. Our scheme can not only obtain an ultra-colder molecular packet under the same molecular-beam parameters and deceleration conditions, but also be directly used to trap the slowed cold molecules after the deceleration without needing to use other techniques for molecular trapping.

State-to-State Differential Cross Sections for Inelastic Collisions of NO Radicals with para-H2 and ortho-D2.

We present state-to-state differential cross sections for collisions of NO molecules (X2Π1/2, j = 1/2f) with para-H2 and ortho-D2 molecules, at a collision energy of 510 and 450 cm–1, respectively. The angular scattering distributions for various final states of the NO radical are measured with high resolution using a crossed molecular beam apparatus that employs the combination of Stark deceleration and velocity map imaging. Rotational rainbows as well as diffraction oscillations are fully resolved in the scattering images. The observed angular scattering distributions are in excellent agreement with the cross sections obtained from quantum close-coupling scattering calculations based on recently computed NO–H2 potential energy surfaces, except for excitation of NO into the j = 7/2f channel. For this particular inelastic channel, a significant discrepancy with theory is observed, despite various additional measurements and calculations, at present, not understood.

Laser-Assisted Stark Deceleration of Polar Molecules HC2n+1N (n=2, 3, 4) in High-Field-Seeking State

Laser-assisted Stark deceleration scheme was proposed to decelerate the high-field-seeking molecule ICl in its rovibronic ground state. However, the laser intensity of 1.0×1010 W/cm2 is hard to realize in experiment. The time-of-flight signals of HC2n+1N (n=2, 3 and 4) by three-dimensional Monte-Carlo simulation suggest that deceleration of such molecules is more feasible experimentally as only one-tenth laser intensity is needed.

Imaging diffraction oscillations for inelastic collisions of NO radicals with He and D2.

We present state-to-state differential cross sections for collisions of NO molecules (X2Π1/2,j=1/2,f) with He atoms and ortho-D2 (j = 0) molecules as a function of collision energy. A high angular resolution obtained using the combination of Stark deceleration and velocity map imaging allows for the observation of diffraction oscillations in the angular scattering distributions. Differences in the differential cross sections and, in particular, differences in the angular spacing between individual diffraction peaks are observed. Since the masses of D2 and He are almost equal and since D2(j = 0) may be considered as a pseudo-atom, these differences directly reflect the larger size of D2 as compared to He. The observations are in excellent agreement with the cross sections obtained from quantum close-coupling scattering calculations based on accurate ab initio NO-He and NO-D2 potential energy surfaces. For the latter, we calculated a new NO-D2 potential energy surface.

Optical Stark decelerator for molecules with a traveling potential well

We propose a versatile scheme to slow supersonically cooled molecules using a decelerating potential well, obtained by steering a focusing laser beam onto a pair of spinning reflective mirrors under a high-speed brake. The longitudinal motion of molecules in the moving red-detuned light field is analyzed and their corresponding phase-space stability is investigated. Trajectories of $\mathrm{C}{\mathrm{H}}_{4}$ molecules under the influence of the potential well are simulated using the Monte Carlo method. For instance, with a laser beam of power 20 kW focused onto a spot of waist radius 40--100 \ensuremath{\mu}m, corresponding to a peak laser intensity on the order of $\ensuremath{\sim}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$, a $\mathrm{C}{\mathrm{H}}_{4}$ molecule of $\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$ can be decelerated to $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$ over a distance of a few centimeters on a time scale of hundreds of microseconds.

Deceleration of a Supersonic Beam of SrF Molecules to 120 m s-1.

A beam of SrF molecules is decelerated from 290 to 120 m s-1 . Following supersonic expansion, the molecules in the X2 Σ (ν=0, N=1) low-field-seeking state are trapped by the moving potential wells of a traveling-wave Stark decelerator. With a deceleration strength of 9.6 km s-2 the removal of 85 % of the initial kinetic energy in a 4 m-long modular decelerator is demonstrated. The absolute amount of kinetic energy removed is a factor of 1.5 higher compared to previous Stark-deceleration experiments. The demonstrated decelerator provides a novel tool for the creation of highly collimated and slow beams of heavy diatomic molecules, which serve as a good starting point for high-precision tests of fundamental physics.

Decelerating and Trapping Large Polar Molecules

Manipulating the motion of large polyatomic molecules, such as benzonitrile (C6 H5 CN), presents significant difficulties compared to the manipulation of diatomic molecules. Although recent impressive results have demonstrated manipulation, trapping, and cooling of molecules as large as CH3 F, no general technique for trapping such molecules has been demonstrated, and cold neutral molecules larger than 5 atoms have not been trapped (M. Zeppenfeld, B. G. U. Englert, R. Glöckner, A. Prehn, M. Mielenz, C. Sommer, L. D. van Buuren, M. Motsch, G. Rempe, Nature 2012, 491, 570-573). In particular, extending Stark deceleration and electrostatic trapping to such species remains challenging. Here, we propose to combine a novel "asymmetric doublet state" Stark decelerator with recently demonstrated slow, cold, buffer-gas-cooled beams of closed-shell volatile molecules to realize a general system for decelerating and trapping samples of a broad range of volatile neutral polar prolate asymmetric top molecules. The technique is applicable to most stable volatile molecules in the 100-500 AMU range, and would be capable of producing trapped samples in a single rotational state and at a motional temperature of hundreds of mK. Such samples would immediately allow for spectroscopy of unprecedented resolution, and extensions would allow for further cooling and direct observation of slow intramolecular processes such as vibrational relaxation and Hertz-level tunneling dynamics.

Laser-assisted Stark deceleration of polar diatomic molecules in the Χ1Σ state

The traditional Stark deceleration method is difficult to apply in chemically stable polar diatomic molecules in their ground (Χ1Σ) state because the Χ1Σ state normally experiences little Stark shift and the rovibronic ground level is mostly high-field-seeking. To solve this problem, we propose a laser-assisted Stark deceleration scheme to decelerate such molecules in the present paper. Our results show that, owing to the transverse bunching effect of the applied red-detuning laser beam, the molecules of the high-field-seeking level |J = 0, M = 0〉 in the Χ1Σ state can be effectively decelerated. Furthermore, the present scheme is more effective because the interaction between the molecules and the combined fields can produce the pseudo-first-order Stark effect, and thus increase the depth of the effective potential. Compared to those molecules in the low-field-seeking state |J = 1, MΩ = −1〉 in the usual electrostatic Stark deceleration, a higher molecular density and lower velocity can be achieved under an equivalent initial phase angle.

Dependences of Q-branch integrated intensity of linear-molecule pendular spectra on electric-field strength and rotational temperature and its potential applications.

We calculate the pendular-state spectra of cold linear molecules, and investigated the dependences of “Q-branch” integrated intensity of pendular spectra on both electric-field strength and molecular rotation-temperature. A new multi-peak structure in the “Q-branch” spectrum is appearing when the Stark interaction strength ω = μE/B equal to or larger than the critical value. Our study shows that the above results can be used not only to measure the electric-field vector and its spatial distribution in some electrostatic devices, such as the Stark decelerator, Stark velocity filter and electrostatic trap and so on, but also to survey the orientation degree of cold linear molecules in a strong electrostatic field.