Vibrationally stimulated ionization of carbon monoxide in optical pumping experiments

Abstract

Ionization in vibrationally excited CO is investigated. The carbon monoxide is excited by resonance absorption of CO laser radiation and subsequent vibration-vibration (V-V) pumping in an optical cell. Ionization involving CO molecules at high vibrational states occurs in the laser beam region. Thus, a non-self-sustained electric discharge (Thomson discharge), supported by the laser beam, is ignited between two plane electrodes placed in the cell. The applied voltage does not exceed the breakdown voltage for the cell gases. The measured discharge current is several microamperes. The discharge voltage-current characteristic satisfactory corresponds to the Thomson discharge theory. This allows measuring the ionization rate in the plasma with an accuracy of a factor of 2. The vibrational distribution function (VDF) of CO in the cell is measured by infrared emission spectroscopy. It is shown that the CO vibrational levels are strongly overpopulated up to level v=40. By adding helium to the CO-Ar mixture it is demonstrated that definite correlation exists between the discharge current and the high vibrational level populations. It is concluded that the ionization occurs in collisions of two vibrationally excited CO molecules. An ionization rate constant for this process of k i=(8±5)×10 −15 cm 3/s is inferred from the VDF measurements. Since the Thomson discharge environment allows regulation of the electron concentration by changing the applied voltage, it becomes possible to investigate the influence of electrons on the VDF. The effect of vibration-to-electron (V-e) coupling is measured in the experiment for the first time. This property of the Thomson discharge makes it very promising for a thorough investigation of the interaction between electrons and highly vibrationally excited molecules.