Related Literature

Molecular beams are applied in a wide variety of chemical and physical problems ranging from studies on the smallest diatomic molecule to those on large organic molecules. For reviews see [110,111,112]. The number of studies using cooled molecular ions are considerably fewer than those for the corresponding neutral molecules, due to the low beam strength attainable for molecular ions. This study uses the technique of ionisation directly after expansion to create molecular ions. In the design of the apparatus and the experimental techniques used to create molecular ions influences have been taken from from several earlier studies, outlined below.

Larsson et al. [113] have used a supersonic jet source in conjunction with a high frequency deflection technique [114] to make lifetime measurements on various molecular ions and neutral molecules. A molecular beam was crossed by an electron beam, skimmed and emission spectra observed for N₂⁺, CO⁺ and N₂. Temperatures of around 15 K were achieved, although this was less than the predicted achievable temperature. This was attributable to ambient heating of the beam during the ionisation process. The jet cooled N₂⁺ ${\rm B^{2}\Sigma_{u}^{+}~-~X^{2}\Sigma}_{g}^{+}$ (0-0) band was shown to be significantly less congested than the corresponding room temperature spectrum. A redistribution of state population occurred, where low lying states became more highly populated (and hence their transitions had a greater intensity).

Carrington, Shaw and Taylor [115] recorded microwave spectra of Ar₂⁺ and Ne₂⁺. A neutral free jet from a liquid nitrogen cooled 20 $\mu$m nozzle was crossed with an electron beam. Stagnation pressures of up to 3.5 bar were used, giving pressures of 5 x 10^-5 mbar in the source region. In the apparatus, ions were accelerated out of the source using potentials of up to 10 kV in a direction perpendicular to the molecular beam, then mass selected, and enter a waveguide whereupon transitions are induced using microwave radiation. Daughter ions were separated from the parent beam using an electrostatic analyser and detected at a Faraday cup or an electron multiplier.

Bae et al. [116] demonstrated a cluster ion source capable of producing collimated, intense beams of positively or negatively charged clusters. Cluster ions were created using an electron gun focussed directly on the aperture of the pulsed nozzle. The ions were then skimmed and ion optics used to focus the ions, followed by mass selection. Laser radiation from a Nd-YAG system placed anti-parallel to the beam direction created photofragments which were selected using an electrostatic analyser and detected on a multichannel plate. Through the study of the mass spectra it was shown to be possible to create the nitrogen dimer cluster ions (N $_{2}^{+})_{\rm n}$ with an n value of up to 14.

Coe et al. [117] used a new technique for measuring molecular ions with sub-Doppler resolution. They used direct absorption techniques coupled with fast ion beams, to record a spectrum of HF⁺. 3.5 torr of HF was expanded from a water cooled 0.5 mm diameter nozzle, and directed towards an emitting filament. A current of 1$\times10^{-5}$ A was attained at 3 kV. This is (at least) an order of magnitude larger than normal ion beam experiments. The absorption path length of the laser system was increased using two 98% reflecting mirrors. Transitions were Doppler tuned into resonance with an Infrared laser by varying the acceleration voltage. Decreases in laser power transmitted through the ion beam occurred where a transition was located. A minimum linewidth of 40 MHz was found for the source and the resolution proved to be capable of resolving hyperfine splittings in the X$^{2}\Pi$  state of HF⁺.

Okumura, Yeh and Lee recorded vibrational spectra for ${\rm H_{3}^{+}(H_{2})_{n}}$. Molecular ions were created using a corona discharge source in which the stagnated gas was ionised before expansion using a corona discharge tip (a nickel plated sewing needle). Care was taken to ensure the purity of the hydrogen gas, and that all water vapour was eliminated. The 75 $\mu$m platinum nozzle was cooled by liquid nitrogen or Freon, decreasing the temperature of gases leaving the nozzle, thus improving clustering conditions and decreasing the number of collisions. The ion beam was skimmed 7 mm downstream from the nozzle, and ions focussed and accelerated after skimming. Cluster ions of up to H₁₅⁺ were found to be created in the source.

Bieske and Maier have recorded spectra of a wide range of molecular ions using free jet sources. An example of their work can be found in the study of the B-X transition of ${\rm N_2^{+}..He}$ [118]. A water cooled pulsed nozzle with a stagnation pressure of 3 bar (1:100 Nitrogen/Neon) was used as a free jet source. Ionisation takes place inside a shielding cage using a double filament arrangement (constructed from spark plugs). Ions were skimmed and ion optics focussed the beam into a quadrupole mass spectrometer. A dye laser was scanned and daughter ions were collected using a channeltron detector. The entire assembly of the ion source (skimmer, filament and shielding cage) was sprayed with graphite in order to reduce the accumulation of surface charges and stray electric fields which accelerate and warm the ions. To reduce the warming, which had a visible effect on the spectrum, a re-coating was required every few days. The photofragment spectra corresponded to transitions in the N₂⁺ chromophore suggesting that the molecule behaves as a free internal rotor. This same apparatus has been used successfully for a number of jet cooled molecular ions including N ${\rm _{2}^{+}-Ne_{n}}$ ( $1\le{\rm n}\le8$) [119][120], N ${\rm _{2}^{+}-He_{n}}$ ( $1\le{\rm n}\le 3$) [121] and ${\rm H_{2}-HN_{2}^{+}}$ [122]