Rf Ion Trap Research Articles

The vibrational predissociation spectroscopy of hydrogen cluster ions

The first infrared spectra of protonated hydrogen clusters in the gas phase have been observed. Predissociation spectra were taken with a tandem mass spectrometer: mass selected hydrogen cluster ions were irradiated inside a rf ion trap by a tunable infrared laser, and the fragment ions created by photodissociation of the clusters were mass selected and detected. Spectra for each product channel were measured by counting fragment ions as a function of laser frequency. Low resolution spectra (Δν=10 cm−1) in the region from 3800 to 4200 cm−1 were observed for the ions H+5, H+7, and H+9 at 3910, 3980, and 4020 cm−1, respectively. A band was also observed for H+5 at 3532 cm−1. No rotational structure was resolved. The frequencies of the band maxima agree well with the frequencies predicted by previous ab initio calculations for the highest modes.

Radiative lifetimes of trapped molecular ions: HCl+ and HBr+

Excitation spectra in the A 2Σ+−X 2Π system of HCl+(0,0) and HBr+(1,0) as well as radiative lifetimes of selectively excited rotational levels in the A 2Σ+ electronic states have been measured in a RF ion trap using laser induced fluorescence and time-resolved single-photon counting techniques. Coherent ultraviolet radiation was generated by frequency mixing of a Nd:YAG/dye laser system. For HBr+ the lifetimes of four rotational levels adjacent to the v′=1 predissociation limit at J′=25/2 were measured. No statistically significant variation with J was found. The average of these four values (3.89±0.15 μs) is therefore reported as the effective lifetime of the v′=1 level of A 2Σ+ HBr+. For HCl+ the lifetime measurements likewise indicate no strong variation with J. The average lifetime of the v′=0 level of the A 2Σ+ state of HCl+ is 3.4±0.4 μs. Measurements of radiative lifetimes of molecular ions by the RF ion trap technique eliminates the determinate error that results from drift of ions out of the detection volume in conventional experiments.

Transition probability of the Si III 189.2-nm intersystem line

Measurement of the lifetime of the metastable 3s3p(3)P(0)1 level of Si(2+) (Si III), which decays by photon emission at 189.2 nm to the 3s2(1)S0 state, is reported. The data were taken from spontaneous emission from metastable Si III stored in an RF ion trap. The Si III ions were produced through electron bombardment of SiH4 and SiF4 at pressures of 1/100,000,000-1/10,000,000 Torr. A photomultiplier was employed to count the photon emissions from the transitions. A total of 11 decay curves were generated for analysis, with Poisson statistics used to set the uncertainties at within 8 pct. Significant systematic effects were controlled, and the lifetime was found to be within 3.6 microsec of 59.9 microsec. The method used is concluded valid for determining the lifetimes of metastable levels of low-Z ions with low charge, and thereby the transition probabilities.

Investigation of the production of singly charged ions in a rf quadrupole ion trap

A three-dimensional rf quadrupole ion trap has been constructed to study ion production by electron-impact ionization. Gases and atoms in an atomic beam can be investigated. The method can be extended to the study of photon ionization and of the production of multiply charged ions by a single collision and/or by step-by-step ionization. Total electron-impact ionization cross sections of helium have been measured and are σe(350 eV) =0.26(6) ×10−16 cm2 and σe(500 eV) =0.21(4) ×10−16 cm2. Equations are derived to operate the rf ion trap in the symmetric storage mode.