The crossed beam technique is for the first time used in a mass spectrometric investigation of photoionization products. The photoionization threshold of atomic hydrogen is 911 Å and its photoionization cross section is exactly calculable. This allows the amount of ionization to be calculated, if the intensity of the radiation and the particle density are known. On the other hand, an unknown photon flux can be determined from the measured proton current. With the new technique the absolute photoelectric yield and/or the photoionization cross section of gases can be measured.The experimental device consists essentially of a UV monochromator, a hydrogen atomic beam, and a mass spectrometer. By the atomic beam technique it is also possible to measure the photoionization cross section of unstable particles such as atomic oxygen and atomic nitrogen.
The ionic species N
Photoionization has been used to produce ions of N2 , 02 . and CO in definite excited states. Deactivating collisions of these ions with molecular gases were described in paper I, where the cross sections of ions with thermal velocity are given for various electronic and vibrational states.By application of an electric field the charged particles are accelerated yielding information on the influence of the kinetic energy of the collision partners on these deactivating collisions. The cross section is found to follow an exponential law o ~ E~a , with a varying between 0.34 and 0 41 for the different excited stales of the molecular ions. The observations were carried out for a range of kinetic energies from the thermal energy up to 6 eV maximum. This simple exponential law is followed for kinetic energies up to at least 2 eV. At higher energies slight deviations were found to occur. IntroductionOne spectroscopic means for analysis of a gaseous sample is to measure its emission spectrum.Luminescence spectra give detailed information about the state of the molecules in question. The measurement of luminescence allows conclusions to be drawn about other competing processes, e. g. collisional deactivation, if the remaining experimental parameters such as time and gas pressure are considered. This procedure has been described in paper 1 1 for excited molecular ions. In this earlier publication molecular ions at room temperature having a thermal velocity distribution were treated.The measured cross sections for collisional deactivation are thus valid for this mean energy only. In general the method provides information about a large number of molecular states. This makes it possible to measure the influence of the excitation energy of a molecule on the ongoing collisional processes. Also the influence of the translational energy in these collisions can be measured. This is comparatively easy in the case of ionic collision partners. Only a relatively simple additional experimental setup as compared to that described in paper I is required. A variable electric field of defined geometry is introduced inside the collision chamber so that the ions can be accelerated after their formation. The ions will not have a uniform energy * In the past many experiments were undertaken to measure the energy dependence of ion-neutral collisions 2 ' 3 . Charge transfer and ion-molecule reactions were emphasized. For both processes theoretical models were developed which are, however, restricted to atoms in the case of charge transfer.GIOUMOUSIS and STEVENSON 4 applying Langevin's calculations to describe ion-molecule reactions predict that the product of cross section and particle velocity for such a reaction is constant. The quantum state of the ion is not taken into account; only the polarisability of the neutral collision partner is considered. Many experimental attempts were made to check this theory.The way to determine the energy dependence of such cross sections, however, differs in a luminescence experiment from other experimental...
An experiment to determine the absolute value of the photoionization cross section of atomic oxygen is described. The atoms are produced in an electrical discharge in oxygen gas with 1% hydrogen added. In order to prevent recombination a crossed beam technique is employed. The ions formed are detected by a time-of-flight mass spectrometer. The concentration of oxygen atoms in the beam is 57%.The measured photoionization cross section of atomic oxygen is compared with theoretical data. The results show the participation of autoionization processes in ionization. The cross section at the autoionizing levels detected is considerably higher than the absorption due to the unperturbed continuum. Except for wavelengths where autoionization occurs the measured ionization cross section is in fair agreement with theory. This holds up to 550 Å whereas for shorter wavelengths the theoretical values are much higher.
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