RESONANCE-ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY IN AN ION TRAP DETECTOR

The potential benefits of performing laser photoionization experiments in an ion trap detector (ITD) are investigated. The 2 + 2 resonance-enhanced multiphoton ionization (REMPI) of nitric oxide NO through the A2-SIGMA+ (nu = 1) <-- X2-PI-1/2 (nu = 0) transition is studied. Mass spectra which reveal various isotopic combinations of NO are recorded simultaneously as the laser is scanned. With 6 ppm of NO in the He buffer gas the P11 + O-12 bandheads of the three most abundant NO isotopomers can be identified. The corresponding concentration of the (NO)-N-14-O-18 isotopomer is 13 +/- 3 ppb which is considered to be the limit of detection. The trap volume contains almost-equal-to 10(6) molecules at the detection limit and almost-equal-to 50 of these molecules are accessible to the laser at the bandhead. The concentration and molecular-number detection limits are factors of 30 and 150 better, respectively, than those obtained by other workers with a supersonic beam time-of-flight apparatus using a 2 + 1 REMPI scheme. This photoionization scheme is also at least 10 times more sensitive than electron impact ionization in the ITD.