S. R. Procter

Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields

Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.

Controlling the motion of hydrogen molecules

The deflection and deceleration of neutral hydrogen molecules in the presence of an inhomogeneous electric field is demonstrated. The molecules are laser-excited in a molecular beam to selected Rydberg–Stark states in the field of an electric dipole for a fixed period, and their trajectories are monitored using ion imaging and time-of-flight measurement. The Rydberg states (n � 17) show remarkably long lifetimes (>100 ls) and these results point to a novel method for trapping and cooling non-polar, non-magnetic molecules. 2003 Elsevier Science B.V. All rights reserved.

Ionization of H2 Rydberg molecules at a metal surface

The ionization of a beam of H2 Rydberg molecules in collision with a metal surface (evaporated Au or Al) is studied. The Rydberg states are excited in an ultraviolet-vacuum ultraviolet double-resonant process and are state selected with a core rotational quantum number N+=0 or 2 and principal quantum numbers n=17-22 (N+=2) or n=41-45 (N+=0). It is found that the N+=0 states behave in a very similar manner to previous studies with atomic xenon Rydberg states, the distance of ionization from the surface scaling with n2. The N+=2 states, however, undergo a process of surface-induced rotational autoionization in which the core rotational energy transfers to the Rydberg electron. In this case the ionization distance scales approximately with nu0(2), the effective principal quantum number with respect to the adiabatic threshold. This process illustrates the close similarity between field ionization in the gas phase and the surface ionization process which is induced by the field due to image charges in the metal surface. The surface ionization rate is enhanced at certain specific values of the field, which is applied in the time interval between excitation and surface interaction. It is proposed here that these fields correspond to level crossings between the N+=0 and N+=2 Stark manifolds. The population of individual states of the N+=2, n=18 Stark manifold in the presence of a field shows that the surface-induced rotational autoionization is more facile for the blueshifted states, whose wave function is oriented away from the surface, than for the redshifted states. The observed processes appear to show little dependence on the chemical nature of the metallic surface, but a significant change occurs when the surface roughness becomes comparable to the Rydberg orbit dimensions.

The Stark effect in the predissociating Rydberg states of NO

A spectroscopic study of the Stark effect in the predissociating v + = 0 Rydberg states of nitric oxide, (n = 13–16) is reported. The states are excited by two-colour excitation via the A2Σ+, v = 0, N = 0 state in the presence of a field in the range 0–1125 V cm−1, and the excitation is observed by using (2 + 1) REMPI detection of nitrogen (2 D) atoms formed by predissociation. The spectra recorded over a range of fields are compared with, and show excellent agreement with, multichannel quantum defect theory (MQDT) simulations of the absorption spectra applied in the bound state formalism of Sakimoto [J. Phys. B. 22, 2727 (1989)]. The spectroscopy demonstrates the possibility of producing N(2 D) or O(3 P) atoms with very narrow centre-of-mass frame speed distributions and arbitrarily selected speed.

The dynamics of high Rydberg states in the presence of time-dependent inhomogeneous fields

This paper presents calculations of the evolution of an optically prepared Rydberg wavepacket in the presence of time-dependent inhomogeneous electric fields and the results have relevance to the stabilization of Rydberg states as appropriate to ZEKE spectroscopy. The field is considered to arise from the combination of an applied field, which may be ramped in time, and the presence of microscopic charges, e.g., a pseudo-random distribution of ions, whose positions may also change with time. The results of the calculations lead to a clearer definition of the conditions under which Rydberg stabilization is achieved, such as in field switching experiments (Baranov et al., Chem. Phys. Lett., 1998, 291, 311), and also confirm the mechanisms by which the randomization of population between blue-shifted and red-shifted Stark states occurs in the presence of micro-fields due to ions (Palm et al., Philos. Trans. R. Soc. London, Ser. A, 1997, 355, 1551). The motion of the ions is found to have a significant m-locking effect in the calculations, providing a possible mechanism for the commonly observed long-lifetime tail in the population decay of high-n Rydberg states.

Cooling effects in the Stark deceleration of Rydberg atoms/molecules with time-dependent electric fields

This paper presents calculations for realizing the deceleration of H2 Rydberg molecules with n = 16, where n is the principal quantum number. A double-dipole decelerator (Softley T P, Procter S R, Yamakita Y, Maguire G and Merkt F 2005 J. Elec. Spectrosc. Relat. Phenom. 144–147 113) operated with an optimum time-dependent electric field allows, in principle, complete deceleration of the Rydberg molecules to zero mean velocity. A bunch of molecules in a supersonic beam is decelerated from the initial velocity centered at ~900 ms−1 and translational temperature 1 K to the final velocity 0 ms−1 and temperature 13 mK. The calculations are performed using the 4th-order symplectic integrator based on representations in phase space {q, p}, and show that an ensemble with narrow δq0 and broad δp0 distribution is converted to one at standstill with broad δq and narrow δp. Cooling effects are reinforced by field ionization in which the fast components that move to regions of high electric field are effectively filtered out.