Studies of oriented molecules using coincidence time-of-flight mass spectroscopy

Abstract

A beam of symmetric top (CX3Y) molecules is state-selected using an inhomogeneous hexapole electric field and then oriented by homogeneous electric fields. A fast neutral atomic potassium beam produced via charge-exchange crosses the molecular beam at a right angle. Continuous electric fields extract ion pair products as they are formed into dual time-of-flight (TOF) mass spectrometers, where the arrival time difference is characteristic of the mass difference of the particles. The positive product (K+) is known for these reactions, allowing identification of the negative products based on the time difference. Construction of the dual time-of-flight (TOF) mass spectrometers is described, with an explanation of how coincidence detection is used to identify the negative ion products of reactions between fast neutral K atoms and small molecules (≤7 atoms). Examination of the results of initial characterization studies including reactions with CH3Cl, CH3Br, CH3I, CF 3Br, and SF6 indicate that coincidence TOF should be a powerful tool for investigations into the dynamics of these reactions. Preliminary results suggest that orientation data will be obtainable to address the question of whether different approach vectors result in different product channels. The importance of a well-calibrated alkali source, well-defined electric fields, and a good vacuum is indicated. An appendix discusses radio-frequency (RF) spectroscopy experiments performed to explore characteristics of the rotational distribution in the molecular beam. A possible application of RF spectroscopy to "tag" single quantum states is discussed.