T. P. Softley

Rotational line intensities in zero kinetic energy photoelectron spectroscopy (ZEKE-PES)

Abstract Recent advances in the understanding of the factors which determine line intensities in zero kinetic energy photoelectron spectroscopy (ZEKE-PES) are reviewed. The relative importance of direct ionization and autoionization is assessed. Explicit consideration of the channel interactions which take place in the vicinity of molecular ionization thresholds leads to a general discussion of rotational line intensities in ZEKE-PES. A series of limiting cases is proposed to assist in the interpretation of experimental results. Finally a new dynamical interpretation of ZEKE experiments based on the trapping of the ZEKE electrons in non-penetrating high-l Rydberg orbitals by weak electric fields appears to give a satisfactory explanation of most experimental results obtained to date.

Ultracold molecules and ultracold chemistry

The recent development of a range of new methods for producing samples of gas-phase molecules that are translationally cold ( K) or ultracold ( mK) is driving efforts to study reactive and inelastic collisional processes in these temperature regimes. In this review article the new methods for cold/ultracold molecule production are reviewed in the context of their potential or current use in collisional studies and progress in the application of these methods is highlighted. In these sub-Kelvin temperature ranges, where the de Broglie wavelength is long compared with molecular dimensions, quantum effects may play a crucial role in the collision dynamics. Reactions with no potential energy barrier are of greatest importance, and this review article summarizes some of the principal theoretical approaches to understanding quantum effects in these barrierless processes.

Electric field effects on zero-kinetic-energy photoelectron spectra: An explanation of observed trends

Abstract An explanation is offered for the propensity for large negative changes of rotational angular momentum in ZEKE photoelectron spectra, which is increasingly observed as the rotational constant of the photoion decreases; Stark mixing becomes progressively more important, facilitating multistep rotational autoionization processes. The Inglis—Teller limit serves as a useful benchmark for assessing the importance of electric field effects. A discussion is presented of the relative importance of l mixing and J mixing.

New experimental method for studying rotationally state‐selected ion‐molecule reactions

A new apparatus is described in which a beam of molecular ions in a selected vibration–rotation state is prepared by field ionization of high Rydberg states, in an adaptation of the zero‐kinetic‐energy photoelectron technique. The state‐selected ions undergo low energy reactive collisions within a molecular beam and the ionic products are detected in a quadrupole mass filter. The Rydberg states are populated by two‐color stepwise multiphoton excitation, and by tuning to the pseudocontinuum of high‐Rydberg states associated with different vibration–rotation states of the ion core, different states of the ion are selected and the effect on reactivity determined. Some preliminary results for the H2+H+2→H+3+H reaction are reported.

Photoionisation and ZEKE photoelectron spectroscopy of Ar, H2 and CO2 using a coherent XUV laser source

Abstract Tunable extreme ultraviolet (XUV) radiation of narrow bandwidth (0.6 cm −1 ) is generated in the region of 130000 cm −1 . This radiation is successfully applied to the study of autoionising Rydberg states of argon and hydrogen, and to record one-photon zero kinetic energy (ZEKE) photoelectron spectra of argon and carbon dioxide.

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.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.

Applications of molecular Rydberg states in chemical dynamics and spectroscopy

Molecules in high Rydberg states, in which one electron has been excited into a hydrogenic orbital of large mean radius, have many unusual properties compared to ground state molecules. These properties, which are reviewed in this article, make them suitable for a diverse and growing number of applications in chemical dynamics. The most recent methods for studying molecular Rydberg states using high-resolution spectroscopy and theory, including effects of electric fields, are described here. An important feature is the high susceptibility of Rydberg states to external field perturbation which not only has a profound effect on the observable energy levels, spectroscopic intensities and lifetimes, but is also useful for state-selective detection through field ionization. The large dipole moment that can be created in a field is also useful for controlling the motion of molecules in Rydberg states. The applications reviewed here include: ZEKE (zero kinetic energy), MATI (mass-analyzed threshold ionization) and PIRI (photo-induced Rydberg ionization) spectroscopy; pulsed-field recombination of ions and electrons; the state selection and reaction of molecular ions; collisions of Rydberg states with neutrals, ions and metallic surfaces; Rydberg tagging and imaging of products of photodissociation; and the control of translational motion and orientation via the use of inhomogeneous fields.

High-resolution threshold-ionization spectroscopy of NH3

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X+(2A2″) ground state of NH3+ is dominated by the emission of even l photoelectron partial waves, and a s...

Rotational autoionization dynamics in high Rydberg states of nitrogen

The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+=0,1,2,3) of the ground vibronic X 2Σ+g (v+=0) state of the N+2 cation have been investigated by delayed pulsed field ionization (PFI) following two‐photon enhanced (2+1′) three‐photon excitation via the a″ 1Σ+g (v′=0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200–2000 ions/mm3. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N+=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into ne...