Ion Production Environments

Figure: NH2+ and HCO+ Signal Strength. Signal strength of NH2+ (squares) and HCO+ (triangles) function of magnetic field.

The basis of our experimental work in this field was the development of a technique for ion spectroscopy that produces signals two orders of magnitude larger than previous techniques [1]. This is especially important because prior to this development, microwave ion spectroscopy was reputed to be extremely difficult and time consuming because of the long searches for weak lines. The basis of the method is the extension of the ion rich negative glow region of an anomalous glow discharge by means of an axial magnetic field. In addition to the very large gains in signal strength (~x100), the enhancement is also ion specific, thus providing a powerful discriminant.

Figure: NH2+ and HCO+ Signal Strength. Signal strength of NH2+ (squares) and HCO+ (triangles) function of magnetic field.

shows a particularly simple version of the magnetically lengthened negative glow cell [1].

In [1].it was pointed out that the magnetically lengthened negative glow cell is functionally equivalent to an electron gun injecting magnetically confined electrons into a electric field free region. Based on this equivalence a new cell has been developed.

Figure: Magnetic Enhancement. Cell for the study of molecular ions
based on the magnetic enhancement of the negative glow region of a
discharge.

Figure: Cell for Molecular Ion Study.

In the cell shown in Figure: Cell for Molecular Ion Study the electrons are produced by a thermonic cathode and guided through a metallic tube which can be cooled either by liquid helium or liquid nitrogen. Because the requirements for sustaining an anomolous glow discharge place severe constraints on pressure, gas mixture, voltage, and current, this new cell provides significantly greater flexibility for the optimization of the production of ions. Additionally, because the parameters of the systems are much better determined, it is possible to model the ion production. This makes possible more predictive ion production schemes as well as the quantitative measurement of the physical and chemical parameters involved.

Figure: Pulsed E-beam THz Probe. THz probe of a
pulsed e-beam induced ion-molecule reaction in a low
temperature H2 - CO mixture, showing (upper) the
initial formation of the HCO+ (the initial rise in probe
absorption), the rotational and translational cooling
of the average temperature after the end of the e-
beam pulse (the sharp rise at 1.8 msec), and the
eventual recombination of the HCO+ and the slow
electrons. The lower portion shows the translational
temperature calculated from the Doppler lineshape
information provided by the two probes shown in
the upper figure which probe different parts of the
Gaussian lineshape.

In addition to the spectroscopic study of energy levels, the THz has been used to probe both discharge [7]. Figure: Pulsed E-beam THz Probe shows a recent result which combines many of these experimental technique to establish an experimentally well characterized environment for the study of inelastic collision of molecular ions at very low temperatures.

More specifically, this figure shows an example of the results of a THz study of the chemistry and physics which results from a pulsed flux of electrons being focused through a mixture of H2 and CO at ~ 50 K to form HCO+. This experimental system is similar to that shown in

Figure: Magnetic Enhancement. Cell for the study of molecular ions
based on the magnetic enhancement of the negative glow region of a
discharge.

above, except that the electron flux is produced by an externally gated e-beam source rather than an abnormal glow discharge. The lower trace that shows the measured translational temperature is particularly interesting. Because the ions are formed with high translational temperature, early in the pulse the average over the ions in the cell is dominated by the hotter ions that have just been formed. Later an equilibrium average is reached. When the electron flux ends, the average translational temperature approaches that of the CO and H2. It is satisfying that this latter temperature is in good agreement with the results of standard thermometry applied to the cell walls.

References

  1. De Lucia, F. C., Herbst, E., Plummer, G. M. & Blake, G. A. The Production of Large Concentrations of Molecular Ions in the Lengthened Negative Glow Region of a Discharge J. Chem. Phys. 78, 2312-2316 (1983). Google Scholar
  2. Demuyuck, C. Millimeter-Wave Spectroscopy in Electric Discharges. Rare Molecules Show Themselves Only If You Look In The Other Direction J. Mol. Spectrosc. 168, 215-226 (1994). Google Scholar
  3. Warner, H. E., Conner, W. T., Petrmichl, R. H. & Woods, R. C. Laboratory Detection of the 110 - 111 Submillimeter Wave Transition of the H2D+ Ion J. Chem. Phys. 81, 2514 (1984). Google Scholar
  4. Bogey, M., Demuynck, C., Denis, M., Destombes, J. L. & Lemoine, B. Laboratory Measurement of the 110 - 111 Submillimeter Line of H2D+ Astron. Astrophys. 137, L15-L16 (1984). Google Scholar
  5. Blake, G. A., Laughlin, K. B., Cohen, R. C., Busarow, K. L. & Saykally, R. J. Laboratory Measurement of the Pure Rotational Spectrum of Vibrationally Excited HCO+ (v2 = 1) by Far-Infrared Laser Sideband Spectroscopy Astrophys. J. 316, L45-L48 (1987). Google Scholar
  6. Amano, T. & Maeda, A. Double-Modulation Submillimeter-Wave Spectroscopy of HOC+ in the nu2 Excited Vibrational State J. Mol. Spectrosc. 203, 140-144 (2000). Google Scholar
  7. Skatrud, D. D. & De Lucia, F. C. Dynamics of the HCN Discharge Laser Appl. Phys. Lett. 46, 631-633 (1985). Google Scholar
  8. McCormick, R. I., Everitt, H. O., De Lucia, F. C. & Skatrud, D. D. Collisional Energy Transfer in Optically Pumped Far-Infrared Lasers IEEE J. Quantum Electron. 23, 2069-2077 (1987). Google Scholar
  9. Everitt, H. O. & De Lucia, F. C. Rotational Energy Transfer in Small Polyatomic Molecules Adv. At., Mol., Opt. Phys. 35, 331-400 (1995). Google Scholar
  10. Oesterling, L. C., De Lucia, F. C. & Herbst, E. Millimeter-Wave Time-Resolved Studies of HCO+ - H2 Inelastic Collisions Spectrochim. Acta, Part A 57, 705-716 (2001). Google Scholar