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Ion Beam Techniques

The first fast ion beam experiment was performed by von Busch and Dunn [34] who crossed photons from an arc lamp with a beam of H2+ ions in order to measure the total cross section for the photo-production of H+, and the population distribution of the vibrational states. Photofragment spectroscopy techniques flourished with the introduction of lasers into the field. Ozenne et al [35] pioneered a technique where an ion beam was crossed with a ruby pulsed laser in order to measure the energy releases of H+ from H2+. Shortly after this study, van Asselt et al., [36] probed the kinetic energy distributions of the photofragments using a laser beam inside an argon ion laser cavity. The photofragmentation of H2+ was probed as a function of laser wavelength and the relative population distributions in the ground ${\rm 1s\sigma_{g}}$ state determined.

Wing et al. [37] first applied `Doppler tuning' to fast ion beams, by coaxially irradiating a fast ion beam of HD+ with a CO2 laser. Scanning was achieved by varying the potential across a section of the flight region, thus varying the velocity of the ions and hence a single frequency source was able to `scan' rovibrational transitions of the molecule. The first four vibrational levels of the ground state of HD+ were observed, between 1,600 and 1,900 cm-1, by passing the irradiated ion beam through a target gas and detecting the resulting current. As transitions between rovibrational states change the population of states, the survival time for the ions in the gas target is changed and therefore the current arriving at the detector will be dependant on the transition. The laser beam was chopped to improve the sensitivity of the technique through lock-in detection methods.

The next logical step was to investigate the absorption spectra using predissociated upper states. To obtain photofragments, laser radiation was used to drive a transition to a quasibound upper state, whereupon dissociation occurred. The fragment ion current could then be obtained as a function of the wavelength of laser radiation and an absorption spectrum of transitions between bound and quasibound electronic states recorded. This was first achieved by Moseley et al., who studied the O+ fragments arising from the predissociation of an upper state of O2+ [38] using the technique of Threshold Photofragment Spectroscopy (TPFS). A fast ion beam was crossed by the output from a pulsed tunable dye laser operating between 578 and 586 nm and the relative photofragment current recorded as a function of the laser frequency. A peak in the photoproduct O+ current was observed. This was later correctly identified as the ${\rm b^{4}\Sigma_{g}^{-} - a^{4}\Pi_{u}}$ transition by Carrington, Roberts and Sarre [39]. Carrington and co-workers devised an experiemnt which involved accelerating O2+ ions into a tandem mass spectrometer to high voltages, whereupon parent and daughter species could be selected using magnetic sectors and an electron multiplier used to detect the O+ fragment ions. Transitions of O2+ were `Doppler tuned' into resonance with the 496 nm line of an argon ion laser by applying of a small subsidiary voltage to the flight region between the two magnetic sectors. This assignment was confirmed shortly afterwards when a detailed recording of the 17,060-17,300 cm-1  region was instigated using a fast ion beam in conjunction with a tunable dye laser [40].

These early studies formed the basis of laser photofragment spectroscopy studies as they are now known. Reviews of fast ion beam spectroscopy techniques include [4,41,42].

Carrington and co-workers went on to develop this technique which offered several advantages including: absolute selection of the spectral carrier, the possibility of selecting the fragment of interest, the high resolution probing of energy level structure, investigation of quasibound levels, the measurement of excess translational energy of the fragment ion and orientation of the dipole transition moment from the spatial distribution of the fragment ions. This technique has since been used in studies of CH+ [43,44], HeH+ [45] and HD+ [46,47]

Laser photofragment spectroscopy techniques have proved particularly adept at investigating the `re-coupling region' between `molecular' and `atomic' limits. Several different dissociation mechanisms have been identified in laser photofragment spectra. For example, the $^{1}\Pi $- $^{1}\Sigma ^{+}$transition of the astrophysically important molecule CH+ [48,49,50] was found to be dominated by transitions involving shape resonances. Predissociation experiments are the ideal investigative tool to probe the astrophysically important radiative association reaction due to the half-collision aspects of the reaction:


$\displaystyle {\rm C^{+}(^{2}P_{\frac{1}{2}}) + H(^{2}S) \rightarrow CH^{+}(^{1}\Pi)}$     (1.1)

The rate of Equation 1.1 is expected to be enhanced due to the shape resonances which exist for the high molecular rotation in the $^{1}\Pi $  state. This cannot, however, account for the unexpectedly large abundance of CH+ found in interstellar clouds [51].

SiH+ [52] has a photodissociation spectrum which is best described in terms of Feshbach resonances (where a bound electronic state is coupled to the continuum of state[s] correlating to a lower dissociation asymptote than the bound state). Similarly, Carrington and Softley have shown that HeNe+ [53] dissociates along an asymptote lower than that to which the upper electronic state correlates.

Over the last twenty years, many molecular ions have been studied using fast ion beams and both fixed frequency rare gas ion lasers with Doppler tuning and tunable dye lasers, interesting examples include PH+ [54], SH+ [55], NH+ [56], OH+ [57,58], NO+ [59], N22+ [60] and HF+ [61]

A novel technique has recently been used by Hechtfischer et al. to record the near-threshold photodissociation spectra of CH+ [62]. A CH+ beam is created using a Van-der-Graaf accelerator to electron strip CH-. The CH+ molecules are injected into an ion storage `ring' of 55.4 m circumference. Inside the `ring', photodissociation is induced using a pulse laser in the frequency range 300-330 nm which is made collinear with the ion beam in a straight section of the `ring'. Shortly after the interaction region the fragment ions are separated from the parent beam and their arrival is detected on a micro-channel plate. Due to the long storage time of ions inside the molecular beam (up to 30 s), it proved possible to examine different rovibronic transitions as the CH+  cooled. Transitions from the lowest vibrational state of CH+  to near-threshold states were probed using UV laser radiation. The majority of transitions in the region between 32,000 and 33,000 cm-1 are thought to be $^{1}\Pi $- $^{1}\Sigma ^{+}$  transitions [63] and are associated with storage times of between 15 and 30 seconds. The b $^{3}\Sigma^{-}$-a$^{3}\Pi$ transitions (between 31,500-35,000cm-1) are associated with storage times of between 0.4 and 1 s.

Another interesting possibility for future fast ion beam studies involves an `electrostatic bottle', developed by Zajfman and co-workers [64]. This apparatus is based around the idea of trapping a moving ion beam between two sets of cylindrical electrodes in much the same way as an optical resonator traps photons between two mirrors. A conventional fast ion beam is created in an external ion source, accelerated up to energies of approximately 4 keV and mass selected. The ion beam is collimated, focussed and directed into the trap. After passing through the (initially earthed) entrance electrodes, the beam reaches the exit electrodes, where it is stopped, focussed and reflected back by voltages of up to 10 kV. The entrance electrodes then have a similar field applied as the exit electrodes and the ion beam is trapped between the two sets of electrodes. This apparatus offers several advantages over conventional ion storage techniques due to the long storage times available (up to a few seconds); cold molecular ions (due to radiative cooling in the trap) could be easily be studied, long path lengths are possible and hence sensitive absorption techniques such as CRDS can be used. Initial experiments [65], have shown that the photodissociation of HD+ and D2+ is possible using a YAG laser aligned perpendicular to the direction of the beam. Neutral fragments could be detected using a multichannel plate.


next up previous contents
Next: Experimental Up: Introduction Previous: Photodissociation of molecular ions   Contents
Tim Gibbon
1999-09-06