The double resonance technique has been applied to a molecular ion. We have observed microwavehyphen;infrared double resonance signals of HN+2produced in a hollow cathode discharge at low pressures (20ndash;200 mTorr). A color center laser with 2ndash;20 mWatts of power provided the infrared signal radiation, and a millimeter wave klystron with sim;400 mWatts of power provided the microwave pump radiation. The frequency of the microwave radiation was swept and the double resonance signal was detected through variation of the infrared power. Using theR(1) andP(1)infrared transitions we have observed theJ=1larr;0 rotational transition in both the ground state and the ngr;1state. We have observed not only the direct threehyphen;level double resonance signals, but also collisionhyphen;induced fourhyphen;level double resonance signals indicating the existence of some selection rules for the Langevin potential dominated ionndash;neutral interaction. The major results of this paper are: (1) The double resonance method is now applicable to molecular ions and (2) ionndash;molecule collisions have selection rules.
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