VIBRATIONALLY STIMULATED IONIZATION OF CARBON-MONOXIDE IN OPTICAL-PUMPING EXPERIMENTS

Ionization in vibrationally excited CO is investigated. The carbon mon oxide is excited by resonance absorption of CO laser radiation and sub sequent vibration-vibration (V-V) pumping in an optical cell. Ionizati on involving CO molecules at high vibrational states occurs in the las er 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 th e 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 fac tor of 2. The vibrational distribution function (VDF) of CO in the cel l is measured by infrared emission spectroscopy. It is shown that the CO vibrational levels are strongly overpopulated up to level v = 40. B y adding helium to the CO-Ar mixture it is demonstrated that definite correlation exists between the discharge current and the high vibratio nal level populations. It is concluded that the ionization occurs in c ollisions of two vibrationally excited CO molecules. An ionization rat e constant for this process of k(i) = (8 +/- 5) x 10(-15) cm3/s is inf erred from the VDF measurements. Since the Thomson discharge environme nt allows regulation of the electron concentration by changing the app lied voltage, it becomes possible to investigate the. influence of ele ctrons 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 investigatio n of the interaction between electrons and highly vibrationally excite d molecules.