Zeeman modulation technique for Rapid Scanning FTR

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Theory Theoretical analysis has made for z Zeeman modulated interferogram of rapid scanning FTR. It is believed that a modulation frequency(fM) must be at least one or two magnitude higher than a frequencies of interferogram(fF)to eliminate modulation signals from interferogram by electronic low pass filter. This leads a practical modulation frequency of 〜20 kHz. For the Zeeman modulation, the modulation frequency of 20 kHz is sometimes difficult to achieve, so some modification which avoids a limitation of fM ≫fF. In this work low pass filter in a lock in amplifier was removed and demodulated interferogram was accumulated in the memory of the computer. Accumulation of the interferogram was showed to eliminate a modulation signal. This method allow us to use a Zeeman modulation with 20 Hz for rapid scanning FTIR.Measurements Absorption cell equipped a solenoid coil which generated an ac magnetic field of 500 G at 20 Hz. Absorption spectrum of nitric oxide was measured at a resolution of 0.005 cmィイD1-1ィエD1. Nitric oxide has unpaired electron, leading a Zeeman effect. The vibrational spectrum of nitric oxide was selectively measured from IR back ground which is not affected by external magnetic field.