Thermal electron detachment rate constants for azulene − at atmospheric pressure by the photodetachment-modulated electron capture detector

Abstract

The direct measurement of rate constants k d for the thermal electron detachment of a polyatomic negative ion has been previously reported on only one occasion—for the azulene negative ion (Az −) by pulsed high-pressure mass spectrometry (PHPMS) at 4 torr total buffer gas pressure over the temperature range 140–200°C. We report here measurements of k d for Az − at a pressure of 2 atm over the temperature range 130–190°C, using a recently developed technique called the photodetachment-modulated electron capture detector (PDM-ECD). This method is based on the competition which exists in the PDM-ECD for destruction of Az − by three processes: thermal electron detachment, photodetachment and ion-ion recombination. The k d values for Az − reported here are found to be somewhat lower, about one-third as great at a given temperature, than those of the previous study. These differences in the two sets of k d measurements suggest the existence of a small but significant pressure dependence which causes k d to be slightly lowered by a 400-fold increase in buffer gas pressure. It is also recognized, however, that a systematic error in the treatment of PDM-ECD data may exist, owing to limitations of current knowledge concerning the rates of ion—ion recombination reactions at near-atmospheric pressures. The complete UV-vis photodetachment spectrum of Az − is also reported here and is shown to bear very strong resemblance to the condensed phase absorption spectrum of Az −.