Siegel der Universität

Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

The Orotron Spectrometer

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A highly sensitive intracavity millimeter wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. An increase in sensitivity of a factor about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption lines of the CO rare isotopomers, the Ar-CO and Ne-CO van der Waals complexes. The high sensitivity, wide spectral range and simple tunability of the spectrometer make it a very efficient tool for searching of weakly absorbing species in a jet. The high potential of the new technique for observation of very weakly bound complexes with a small dipole moment, namely Ne-CO, CO-N2, CO-H2, CO-D2, He-CO, (CO)2 has been demonstrated.

The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality (Q = 104) Fabry-Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultra high vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10 - 15 kHz providing the capability of sub-Doppler spectral resolution without phase locking.

Key Features

  • very high sensitivity (gain 100 - 1000 compared to single pass spectrometer)
  • high spectral resolution (10 - 15 kHz without stabilization)
  • scanning range 107 - 178 GHz
  • easy tunability
  • internal electron detector
  • low rotational temperature (1-10 K)
  • low gas consumption (few ml/min)


Experimental Setup

  • source frequency modulation 25 kHz
  • pulse repetition rate 10 - 20 Hz
  • diameter of the valve 0.5 mm
  • diffusion pump 1000 l/s



The following spectra demonstrate the high sensitivity and resolution of the Orotron spectrometer.


Future Developments

  • It is planned to extend the frequency range of the Orotron to 300 GHz.
  • Currently we are developing a double resonance experiment, with the aid of a second microwave generator (20 - 50 GHz).