High resolution infrared direct absorption spectroscopy of ionic complexes

A sensitive and generally applicable technique for high resolution infrared spectroscopy of weakly bound ionic complexes is presented. The method is based on the direct absorption of tunable diode laser radiation in an expansion cooled planar plasma. The plasma is generated by electron impact ionization of gas that is expanded supersonically through a long and narrow slit. This technique allows a fast and effective production modulation. Online monitoring by a quadrupole mass spectrometer yields direct information on the plasma characteristics. Rotational temperatures as low as 15 K and densities of the order of 10(9)-10(10) complexes/cm3 are routinely obtained. The technique is discussed on the example of N-4(+) and demonstrated with results on the proton bound complexes N-2-H+-N-2, Ar-HN2+, and Ar-HCO+. (C) 2000 American Institute of Physics. [S0034-6748(00)00804-2].