Near-threshold photodissociation of cool OH+ to O + H+ and O+ + H.

We have measured the spectrum of laser photodissociation of OH+ molecular ions to O + H+ and O+ + H fragments for photon energies of 38 100-40 900 cm-1. The OH+ ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N'' = 0-11 of the vibrational ground state X3Σ-(v'' = 0), close to room temperature (≈300 K). The many resonances in the spectra reveal the energies, widths, and O/O+ branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v' = 9 and 10 of the A3Π electronic state. For the A3Π0,1 substates, isolated levels with v' = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v' = 0 and 1 states of the shallow 15Σ- electronic state, borrowing oscillator strength from nearby A3Π levels. Together, the near-threshold levels deliver a new value D0 = 40 253.8(1.1) cm-1 for the dissociation energy of OH+. Through a two-step photodissociation process, 72 levels from the lower bound states A3Π(v' = 7-8) appear as well and are rotationally analyzed. The level energies are used to construct improved A3Π and 15Σ- Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH+ photodissociation or of the related charge-exchange reaction O + H+ → O+ + H.

Autoresonance Cooling of Ions in an Electrostatic Ion Beam Trap.

Autoresonance (AR) cooling of a bunch of ions oscillating inside an electrostatic ion beam trap is demonstrated for the first time. The relatively wide initial longitudinal velocity distribution is reduced by at least an order of magnitude using AR acceleration and ramping forces. The hot ions escaping the bunch are not lost from the system but continue to oscillate in the trap outside of the bunch and may be further cooled by successive AR processes. Ion-ion collisions inside the bunch close to the turning points in the trap's mirrors contribute to the thermalization of the ions. This cooling method can be applied to any mass and any charge.

Structure and stability of the negative hydrogen molecular ion.

We present the results of a Coulomb explosion experiment that allows for the imaging of the rovibrational wave function of the metastable H2- ion. Our measurements confirm the predicted large internuclear separation of 6 a.u., and they show that the ion decays by autodetachment rather than by spontaneous dissociation. Imaging of the resulting H2 products reveals a large angular momentum of J = 25 ± 2, quantifying the rotation that leads to the metastability of this most fundamental molecular anion.

Hot water molecules from dissociative recombination of D3O+ with cold electrons.

Individual product channels in the dissociative recombination of deuterated hydronium ions and cold electrons are studied in an ion storage ring by velocity imaging using spatial and mass-sensitive detection of the neutral reaction fragments. Initial and final molecular excitation are analyzed, finding the outgoing water molecules to carry internal excitation of more than 3 eV in 90% of the recombination events. Initial rotation is found to be substantial and in three-body breakup strongly asymmetric energy repartition among the deuterium products is enhanced for hot parent ions.

Electron photodetachment cross sections of small carbon clusters: evidence for nonlinear isomers.

Absolute cross sections for photodetachment of negative carbon clusters are reported for Cn (n = 3, ..., 8). The results indicate that various neutral isomers exist, some with electron affinities as low as 1 electron volt. The method of production plays an important role in the characteristics of carbon clusters.

Ultraslow isomerization in photoexcited gas-phase carbon cluster [Formula: see text].

Isomerization and carbon chemistry in the gas phase are key processes in many scientific studies. Here we report on the isomerization process from linear [Formula: see text] to its monocyclic isomer. [Formula: see text] ions were trapped in an electrostatic ion beam trap and then excited with a laser pulse of precise energy. The neutral products formed upon photoexcitation were measured as a function of time after the laser pulse. It was found using a statistical model that, although the system is excited above its isomerization barrier energy, the actual isomerization from linear to monocyclic conformation takes place on a very long time scale of up to hundreds of microseconds. This finding may indicate a general phenomenon that can affect the interstellar medium chemistry of large molecule formation as well as other gas phase processes.

An experimental setup to study delayed electron emission upon photoexcitation of trapped polyatomic anions.

A Velocity Map Imaging (VMI) spectrometer has been designed and integrated with an electrostatic ion beam trap to study delayed electron emission from trapped polyatomic anions upon photodetachment. The VMI spectrometer is small in size and can record a wide range of photoelectron energies, with variable magnification. Delayed electron emission can be recorded in our experimental setup for any time duration after the photoexcitation of the polyatomic anions. Experiments were carried out with trapped O- and C5- ions to demonstrate the capability of the spectrometer. Delayed electron emissions from C5- as well as prompt photoelectrons from O- were detected by the VMI spectrometer upon photoexcitation. The design and performance of the spectrometer are presented in detail.

Observation of longitudinal quadrupole ion bunch oscillations in an electrostatic ion beam trap.

Ions in an ion bunch trapped inside an Electrostatic Ion Beam Trap (EIBT) exhibit collective oscillations within the bunch under the influence of an external driving force. These internal oscillations have been measured explicitly using a new method with a particle detector outside the EIBT. In this approach, the evolving ion bunch is monitored along the entire trap length, in contrast to the localized single point measurements that are often carried out in other techniques. In the present study, quadrupole oscillations have been measured for the first time in an EIBT along with the dipole oscillations that were measured previously. The frequency of the quadrupole oscillation is found to be about twice the dipole oscillation frequency. This is in agreement with the prediction of a theoretical model.

The cryogenic storage ring CSR.

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed, and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 ± 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion), and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm(-3) is derived, equivalent to a room-temperature pressure below 10(-14) mbar. Fast atomic, molecular, and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

Fourier transform time-of-flight mass spectrometry in an electrostatic ion beam trap

We report on the application of an electrostatic ion beam trap as a mass spectrometer. The instrument is analogous to an optical resonator; ions are trapped between focusing mirrors. The storage time is limited by the residual gas pressure and reaches up to several seconds, resulting in long ion flight paths. The oscillation of ion bunches between the mirrors is monitored by nondestructive image charge detection in a field-free region and mass spectra are obtained via Fourier transform. The principle of operation is demonstrated by measuring the mass spectrum of trapped Ar+ and Xe+ particles, produced by a standard electron impact ion source. Also, mass spectra of heavier PEGnNa+ and bradykinin ions from a pulsed MALDI ion source were obtained. The long ion flight path, combined with mass-independent charge detection, makes this system particularly interesting for the investigation of large molecules.

Absolute photo-destruction and photo-fragmentation cross section measurements using an electrostatic ion beam trap.

We describe a technique to measure absolute photo-induced cross sections for cluster anions stored in an electrostatic ion beam trap (EIBT) with a central deflector. The setup allows determination of total photo-destruction cross sections as well as partial cross sections for fragmentation and electron detachment. The unique properties of this special EIBT setup are investigated and illustrated using small Al(n)(-) clusters.

A cryogenic electrostatic trap for long-time storage of keV ion beams.

We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures <15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.