Study by Mass Spectrometry of the Decomposition of Ammonia by Ionizing Radiation in a Wide-Range Radiolysis Source
Authors: Melton Charles E. 
Published: January 2005

A wide-range radiolysis source which can be used to approximate an ideal experimental study of a chemical reaction induced by ionization has been designed and constructed for a study of the decomposition of ammonia. The source consists of three separate compartments in series, each with a separate electron beam which can be independently controlled over a wide range of intensity and energy. The pressure in the source can be varied over a range of 10−9 to 10 torr and decreases progressively from compartment to compartment. Reactions can be followed in time starting from about 10−14 sec after irradiation up to the time for the formation of final stable products. Each compartment has a distinct function and is used to observe reactions which occur at different times after irradiation. Compartment 3 is a high-sensitivity ion source which ionizes reaction products produced in a preceding compartment for mass identification by a sensitive mass spectrometer to which the apparatus is attached. Also, cross sections for ionization of both positive and negative ions by electrons are measured in this compartment. Reactions which occur from ∼10−14 to ∼10−3 sec after irradiation are studied in Compartment 3. Compartment 2, the medium-pressure compartment (10−8−1 torr), is used to measure (a) rate constants for ion—molecule reactions, (b) cross sections for the production of free radicals by electrons, and (c) energy levels of excited states. In Compartment 2, reactions are studied which require times of the order of 10−3 to 1 sec. Compartment 1 (pressure to 10 torr) is used for the initial radiation of the sample to produce final products and to measure (a) the threshold energy, (b) the percentage of each due to ion—molecule reactions, and (c) G values.
Irradiation of NH3 with 100-eV electrons at a pressure of 1 torr in the wide-range radiolysis source produced 74.9% H2, 24.9% N2, and 0.2% N2H4 as reaction products with G values of 8.8, 2.9, and 0.03, respectively. Positive-ion—molecule reactions produced 54% of the H2, 63% of the N2, and 20% of the N2H4. All of the products appeared at a threshold energy of about 4 eV corresponding to the energy necessary to produce H− and NH2− from NH3 by electrons. The abundance of the products increased sharply at about 10 eV corresponding to the ionization potential of NH3.
The primary products of irradiation were 58.8% positive ions, 40.8% free radicals and neutral species, and 0.4% negative ions. Cross sections for the production of each of the species by 100-eV electrons and appearance potentials of the negative ions are reported. The total cross sections observed were: positive ions, 2×10−16; negative ions, 1.4×10−18; and free radicals, 1.4×10−16 cm2/molecule. In addition to the free radicals produced by dissociation of NH3, the secondary radicals NH4, N2H2, N2H3, and N2H5 were observed. Appearance potentials were measured and in some cases calculated (by an energy-calibrated molecular-orbital scheme) for the free radicals and the following values in electron volts were obtained: experimental, H (13.8), N (14.6), NH (12.8), NH2 (11.7), N2H2 (9.9), N2H3 (7.6), and N2H4 (8.8); theoretical: NH2 (10.5), N2H2 (10.1), and N2H3 (7.8). The most abundant positive and negative ions observed at 1 torr pressure were NH4+, 83% and NH3−, 51%. Rate constants and cross sections are given for the positive-ion—molecule reactions; the negative-ion spectrum is reported for a pressure of 1 torr.
By generalizing from the data collected, an attempt is made to specify the elementary reactions leading to the reaction products. It is shown that NH2, H, and NH4+ are the most important transient species in the reaction mechanism and that essentially all of the primary ions formed react with NH3 to produce NH4+ and either H or NH2. In certain respects the conclusions from this study agree with those obtained by electric-discharge techniques.