Magnetic field modulated infrared laser spectroscopy of molecular ions: The ν2 band of HCO+

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Abstract
The magnetic field effect on dc discharge production of molecular positive ions was investigated using infrared diode laser spectroscopy. When a magnetic field of about 100 G was applied, the ion signal strength in a system where the ions were generated by a hollow cathode discharge was found to decrease by an order of magnitude. This effect was used to discriminate absorption lines of ions from those of neutral species; an ac magnetic field was superimposed on the dc field and the signal was detected with a lock‐in amplifier. Using this magnetic field modulation technique, the ν2 band of HCO+ was observed for the first time and the following molecular parameters were determined: ν0=828.2305(9), B’=1.490 274(14), q=0.007 051 0(44), and D’=2.822(45)×10−6 in cm−1 units with three standard errors in parentheses.