Stark deceleration

The development of the experimental techniques for the production of slow molecular beams described in Chapter 14 offers the unique possibility of studying state and angle-resolved differential scattering of molecules in the presence of external electromagnetic fields. Molecular beam experiments with Stark decelerated and guided molecular beams can be designed to probe fully state-resolved differential cross-sections (DCSs), which contain detailed information about the collision process. This... 

STARK DECELERATION OF NEUTRAL POLAR MOLECULES 14.2.1 The Stark Decelerator... 

The Stark decelerator (or accelerator) for neutral polar molecules is the equivalent of a linear decelerator (or accelerator) for charged particles. The Stark decelerator exploits the quantum-state specific force that a polar molecule is subjected to in an electric field. This force is rather weak, typically some eight to ten orders of magnitude weaker than the force that the molecule, when singly ionized, would experience in an equivalent electric field. Nevertheless, this force suffices to exert a complete control over the motion of polar molecules using principles akin to those developed to manipulate charged particles. 

Central to the understanding of the operation principle of a Stark decelerator are the concepts of a synchronous molecule and of phase stability. Returning to Figure 14.6, we call the position z of a molecule at the time when the fields are switched the... 

In the phase-stability diagrams of Figure 14.7, contours of constant energy are shown that result from a numerical integration of Equation 14.4 for OH radicals in the 7 = 3/2, Af = - 9/4 state. The computations were carried out with the parameters of a Stark decelerator operated in our laboratory, at the values of the synchronous... 

FIGURE 14.8 Longitudinal acceptance of a Stark decelerator for OH radicals for different values of the synchronous phase angle Here is the velocity of the nonsynchronous molecule along the longitudinal coordinate z and is its phase. The value = 0 corresponds to the velocity of the synchronous molecule. 

A more extensive description of phase stability in a Stark decelerator revealed that additional phase-stable regions exist these have indeed been observed in an experiment [30]. The higher-order phase-stable regions can be understood as resulting from higher partial waves in the Fourier expansion of the time-dependent inhomogeneous electric field and from their interferences [31]. 

A schematic of the Stark deceleration and trapping machine that has been used to decelerate and trap OH radicals in our laboratory is shown in Figure 14.9. 

FIGURE 14.9 Schematic of the experimental setup. A pulsed beam of OH radicals is produced via ArF-laser photodissociation of HNO3 seeded in a heavy carrier gas. The molecular beam passes through a skimmer, hexapole, and Stark decelerator into the detection region. State-selective laser-induced fluorescence (LIF) detection is used to measure the arrival time distribution of the OH(7 = 3/2) radicals in the detection zone. (From van de Meerakker, S.Y.T. et al., Annu. Rev. Chem., 57, 159-190, 2006. Copyright (2006) Annual Reviews www.annualreviews.org. With permission.)... 

Longitudinal Focusing of a Stark Decelerated Molecular Beam... 

Inspired in part by the manipulation of polar molecules by electric fields, a magnetic analog of the Stark decelerator has recendy been developed. Deceleration based on the magnetic interaction allows the manipulation of a wide range of atoms and molecules to which the Stark deceleration technique cannot be applied. The requisite rapid switching of the magnetic fields posed a considerable experimental challenge. 

ND3 molecules were loaded into the ac trap by Stark-decelerating them to a standstill using their LFS state, and subsequently pumping about 20% of the molecules, with a microwave pulse, to a HFS state. Figure 14.18 shows the density of ND3 molecules at the center of the ac trap as a function of the switching frequency for molecules in either the HFS or LFS component of the ground state of para-ammonia [68]. 

The inversion spectrum of the J,K) = 1,1) level of ND3 consists of 72 hyperfine transitions in a frequency interval of about 300 kHz. Due to this spectral congestion, the hyperfine structure could not be resolved in an earlier molecular beam experiment [71]. We have therefore carried out this experiment with a Stark-decelerated molecular beam, using the setup shown schematically in Figure 14.19a. A beam of ammonia molecules is decelerated from 280 m/sec to either 100 or 50 m/sec, and focused into a microwave zone. The microwave zone provides a nearly rectangular... 

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