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Oka and co-workers were the first group to report a direct infrared absorption spectrum of a molecular ion. They passed coherent infrared radiation through a multiple reflection, liquid nitrogen cooled, discharge cell and recorded a vibration-rotation spectrum of H3+; the simplest triatomic molecular ion. The laser radiation source was constructed from the output of a dye and argon laser mixed in a LiNbO3 crystal. The ratio of ions to neutral species in such a cell is approximately 1:106, and so an inherent disadvantage of such a technique is that the molecular ion absorption spectra can become obscured beneath (even weak) neutral species absorptions. Three modulation techniques have been developed to overcome these disadvantages and to enable the direct infrared absorption spectra of molecular ions to be recorded:
- Velocity modulation: Charged species in a gas mixture consisting of neutrals and ions experience a Coulombic force as a result of the potential gradient of the discharge. The ions therefore have a net velocity in one direction superimposed onto their random thermal motion and will `observe' the laser at a Doppler shifted frequency. If the polarity is reversed, the ions will travel in the opposite direction, and the Doppler shift will be reversed. By alternating the polarity, absorptions of the charged species are seen to be in phase. Lock-in detection methods can then be used to detect the absorptions due only to ions. This technique was developed by Saykally [18] and is particularly suitable for use with semiconductor diode lasers: absorption spectra for the fundamental band of SiH+ [19], the (2,0) band of the
transition of N2+ [20,21] and rotationally quasibound levels in v = 0,1 and 2 of the
state of HeH+ [22] have recently been recorded using velocity modulated spectroscopy.
- Discharge modulation: The precursor gas is contained inside a large conducting cathode and a discharge current is rapidly switched on and off. As the cathode allows current of one polarity to pass through, the current flows in one direction only. Ions are therefore only present during one phase of the alternating current. The method of lock-in detection is again used to detect absorptions. This technique has been applied to the
band of the astrophysically important HCO+ molecule [23]. A recent variation on discharge modulation has seen a technique in which an electron ionisation source was modulated [24]. The modulated electrons cross an expansion slit, where supersonic gas is ionised. This experiment allowed the first observation of the
(1
0) transition of N4+ in the gas phase and shows it to be a linear molecule.
- Magnetic modulation: In this case, high frequency longitudinal AC field in conjunction with a DC offset magnetic field is applied to a hollow cathode discharge tube using a solenoid. A strong magnetic field gives the ions a helical trajectory reducing collisions with the wall and hence ion concentrations are increased. Such apparatus has been used to study the
band of HCO+ [25]
Microwave spectroscopy generally involves transitions between rotational states in the ground vibrational state. It is an extremely high resolution technique which allows a very precise determination of many terms in the Hamiltonian.
Laboratory studies of microwave spectra for molecular ions were first performed in a glow discharge spectrometer on CO+ by Dixon and Woods [26].
Microwave spectroscopy has also recently been used in conjunction with Fourier Transform techniques to study the rotational spectra of a number of polyatomic ions using a Pulsed Discharge Nozzle (PDN) coupled with a Fourier-transform microwave spectrometer. Shortly after leaving the nozzle the ions are subject to a Lorentz force due to a static magnetic field passing through a coil. This focusses ions into a region in the centre of the microwave cavity, where spectra can be recorded. Spectra of HOCO+, HOCS+, D3+-Ar and D
-Ar have been recorded in this manner [27].
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Up: Molecular ions
Previous: Emission Spectroscopy
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Tim Gibbon
1999-09-06