Early laboratory spectra of molecular ions were generally recorded in emission, where ions are produced in highly excited states (for example by an electric discharge). Radiative relaxation from this state occurs and the emitted radiation is dispersed and collected to obtain spectra.
One high resolution spectrum recorded using this technique is that of the
transition of HeNe+ [6]. Dabrowski and Herzberg mixed high purity He and Ne in the ratio 10:1 inside a hollow cathode discharge, dispersed the resulting emission and recorded the spectrum using photographic plates.
Similarly, a rotationally resolved emission spectrum for the -
transition of CH+ was recorded at high resolution by Carrington and Ramsay [7] using a hollow cathode discharge lamp. The error in recording the positions of individual lines is considered to be
0.003 cm-1.
A recent example of a dispersed emission spectrum is the study of NeRg2+ (Rg=Ar,Kr and Xe), ArRg2+ (Rg=Kr,Xe) and KrXe2+ recorded by Tsuji et al. [8] using a rare gas flowing afterglow tube. The apparatus [9] involves the passage of a rare gas through a microwave discharge into a stainless steel reaction cell. A large (7000 s-1) vacuum pump draws the gas continuously away from the discharge region. The rare gas then passes over a gas inlet, where a relatively small amount of the sample gas flows into the chamber. Charge exchange between the rare gas ions and the neutral precursor gas occurs and emission spectra are recorded using a monochromator and photomultiplier (slightly downstream of the sample inlet). One problem of using such an apparatus is that the carrier species for any spectrum is unknown, and that emission bands for several species overlap with both each other and with the lines due to transitions in the rare gas atoms.
One commonly used method for recording either absorption or emission spectra is that of Laser Induced Fluorescence (LIF) where an upper electronic state is populated by driving transitions from a lower electronic state using a tunable laser. Two methods are commonly used to study the spectrum of the molecule:
LIF studies are commonly used in conjunction with a jet cooled source
which reduces the population of high lying vibrational and rotational states and hence the number of transitions' leading to spectral simplification. An example of a molecular ion recorded using LIF with jet cooling is the molecular complex
(where X is He, Ne, N2 or Ar) [10].
A novel technique used to study LIF spectra of HBr+ was that of Xie and Zare [11]. The A
-X
transition of HBr+ was recorded using 2+1 resonance-enhanced multi-photon ionisation (REMPI) to prepare HBr+ in the X state from the HBr precursor. This was followed by LIF, using a second (probe) laser to populate levels of the upper A
state. It proved possible to select the lower fine structure level of HBr+ by selecting different HBr Rydberg states in the REMPI process.
Laser induced fluorescence has been used in conjunction with fast ion beams to record high resolution spectra of molecular ions. Recently, a spectrum of the B
- X
transition of C2+ was recorded [12]. Transitions to the upper state were Doppler tuned into resonance (by varying the drift region potential) with the 501.7 nm line from an argon ion laser. The advantage of using fast ion beams in conjunction with laser induced fluorescence is that the line widths are considerably reduced in comparison to those found using standard Doppler-limited techniques. Similarly, this very sensitive technique has been used to record hyperfine structure in the (1,2) band of the B
transition in
and
[13]. Fast ion beam laser induced fluorescence has been applied recently to the
transition of SiO+ [14], where ions were produced by ionisation of heated SiO powder, extracted from the source and Doppler tuned into resonance with transitions from a Ti:sapphire ring dye laser.
Further techniques involving the LIF of fast ion beams use the principle of the in-flight Lamb-dip method [15]. An ion beam and laser beam are made collinear, but the laser frequency and ion beam velocity (acceleration voltage) are selected so that any transitions are off resonance. The ion beam then passes through a flight region where voltages are applied to the ion beam to tune a transition into resonance with the laser. Due to the velocity distribution in the ion beam, only ions with a selected velocity will be in resonance and a `hole' will be `burned' into the beam. Using radio-frequency (rf) magnetic dipole transitions, the `hole' is transferred from one lower state level to another level, without altering the translational velocity. The ions from which pumping occurred will fluoresce from the excited state, but only a few will return to their original state. In the second (downstream probe) flight region, the same process occurs and the spontaneous emission is detected by a photomultiplier. If the voltages in the two regions are the same, the laser beam will interact with the same velocity group of ions in both and a decrease in the detected fluorescence will be found. This `Lamb-dip' will been seen when the pump region is in resonance with an optical transition. In practice, the frequency of the rf voltage is fixed while the voltage applied to the pump region is scanned. This technique has been used to study the (0,1) band of the B
- X
transition for N2+ at extremely high resolution.
Emission spectra can also be recorded using Stimulated Emission Pumping (SEP) [16]. A fixed frequency laser induces a transition to an upper electronic state (pump), whereupon the frequency of a second laser is scanned in order to stimulate emission from that state back to the ground electronic state (dump). Resonances are detected by monitoring the decrease in spontaneous emission. This technique has been applied to the
transition in the diacetylene ion
[17].