The A 2Πu-X 2Σ+g band of the nitrogen molecular ion rotationally cooled to typically 50 K in a pulsed supersonic jet expansion has been observed by direct absorption spectroscopy with near-infrared diode laser source. Rotational lines belonging to the (2, 0) vibrational band were observed with the signal-to-noise ratio greater than 40 by the use of the discharge modulation technique combined with dual-gated integration.
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