MOLECULAR BEAM KINETICS. Item Type text; Thesis-Reproduction (electronic) Authors Sentman, Judith Barlow. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 03/10/2021 03:47:31 Link to Item http://hdl.handle.net/10150/274898 INFORMATION TO USERS This reproduction was made from a copy of a document sent to us for microfilming. While the most advanced technology has been used to photograph and reproduce this document, the quality of the reproduction is heavily dependent upon the quality of the material submitted. 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Zeeb Road, Ann Arbor. MI 48106 MOLECULAR BEAM KINETICS by Judith Barlow Sentman A Thesis Submitted to the Faculty of the DEPARTMENT OF CHEMISTRY In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 19 8 3 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfill­ ment of requirements for an advanced degree at The Univer­ sity of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowl­ edgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the inter­ ests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: 3 / Walter B. Miller, III Date Lecturer in Chemistry To my husband, Bob, for his love, patience and understanding ACKNOWLEDGMENTS The author wishes to express her sincere appreciation to her thesis director, Dr. Walter B. Miller, for his encouragement and guidance. The author also wishes to thank her colleague, Barbara Lancaster, who designed and built the supersonic nozzle source used in these studies. iv TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS vi LIST OF TABLES vii ABSTRACT viii 1. INTRODUCTION 1 2. SEEDED MOLECULAR BEAMS 3 Prototype Experiments 3 General Principles of Nozzle Beam Operation . 9 Comparison of Effusive and Nozzle Sources . 15 Velocity Distributions 16 Beam Energies . 17 3. APPARATUS AND PROCEDURE 18 Vacuum System 18 Ovens and Nozzle 19 Surface Ionization Detector 2 3 Velocity Selector 25 Electronics 28 Planned Experimental Procedure 30 4. CROSSED BEAM ANALYSIS TECHNIQUE AS APPLIED TO REACTION OF INTEREST 34 Ca + KC1 K + CaCl 34 Center of Mass to Laboratory Conversion .... 34 Kinematics 37 The Conservation of Energy 37 The Conservation of Momentum 38 The Conservation of Intensity 39 Computational Factors 40 5. DISCUSSION 44 LIST OF REFERENCES 47 v LIST OF ILLUSTRATIONS Figure Page 1. Comparison of effusive- and nozzle-source systems with nozzle and effusive source chamber at similar locations 10 2. Comparison of effusive- and nozzle-source systems with skimmer and effusive source at similar locations 12 3. Geometrical arrangement of the ovens and detector 21 4. General Newton diagram for A + BC -*• product for y = 90° 36 vi LIST OF TABLES Table Page 1. Velocity selector dimensions 26 vii ABSTRACT The crossed molecular beam reaction of calcium, from a nozzle source, with potassium chloride, from an effusive source was to have been studied experimentally. The product alkali atoms in the reaction were to have been detected with a Pt-W filament, surface ionization detector. We expect that this reaction proceeds through a long-lived intermediate complex. Calculation of the angular distribution of product in the reaction of calcium with potassium chloride can be done on the basis of the statistical theory. viii CHAPTER 1 INTRODUCTION The crossed molecular beam technique offers the most direct means of studying the molecular mechanics of chemical reactions. Ever since Kantrowitz and Grey (1951) first proposed the supersonic jet expansion of gases as a high intensity source, a large number of research groups have used nozzle beams to carry out reactive and nonreactive scattering experiments to study the structural properties of the nozzle beam molecules through molecular beam reso­ nance techniques. The majority of the reactions studied involve either an alkali atom or an alkali halide molecule as one of the reactants because suitable detectors are readily available for these chemical species. The main advantage of the molecular beam technique is that it allows one to look at the reaction product after a single encounter of the reactants, thus providing information about the elementary processes of classical kinetics. Chapter 2 of this thesis is a statement of the history and general principles of nozzle beam operation. This review is not an exhaustive list of seeded nozzle molecular beam experiments but rather a brief overview of some prototype studies. 1 2 Chapter 3 is a description of the apparatus, including the nozzle source and detection system and the planned experimental procedure. Chapter 4 includes the conversion of data from the center of mass coordinate system and the kinematic analysis as applied to the reaction of interest. A discussion of expected results is found in the final chapter. CHAPTER 2 SEEDED MOLECULAR BEAMS Supersonic nozzle sources are frequently used to produce beams of atoms or molecules for collision and spec­ troscopic experiments. The direct aim of seeded nozzle beams is to obtain molecular beams with higher intensities, narrower velocity distributions, lower rotational and vibrational energies, and higher translational energies. In the "seeding" technique the beam gas of interest is mixed with a large excess of a usually light diluent gas. In a seeded beam the beam species and the diluent are charac­ terized by velocity distributions which have the same peak velocity. For very dilute mixtures (a few mole percent seed gas), this peak velocity is nearly equal to that in a pure beam of diluent gas. By varying the mass of the diluent gas, the seed species can be accelerated or decelerated. It effectively allows gas temperatures much higher than the nozzle wall temperatures. Prototype Experiments This technique was first suggested by Kantrowitz and Grey (1951). They proposed the use of continuum source molecular beams and presented a design for a system utiliz­ ing an axisymmetric converging-diverging nozzle together 3 4 with a cone-shaped skimmer. Kantrowitz and Grey's nozzle converted most of the random translational and internal energy of the oven gas into directed mass motion. They showed that the intensities of molecular beams formed by the method might be several orders of magnitude higher than the intensities of beams from conventional effusive sources. They also anticipated narrow velocity distributions result­ ing from the cooling of a gas during expansion and the consequent reduction of random thermal molecular velocities. Kistiakowsky and Slichter (1951) reported the first attempts to implement the Kantrowitz-Grey design in a companion paper. Although their experiments showed that higher intensities could be achieved than with conventional sources, lack of sufficient pumping capacity prevented an adequate demonstration of the effectiveness of the method. Within a few years of the Kistiakowsky-Slichter experiments, Becker and Bier (1954) assembled a system of substantially higher pumping capacity (500 £/sec) and obtained beam intensities approaching those predicted for the Kantrowitz-Grey design. They found that eliminating the diverging section of the nozzle had little effect on the performance and thereafter used a simple free jet source. They also observed a substantial increase in beam intensity when a mixture of hydrogen and argon gas was substituted for pure hydrogen in the source. These and subsequent experi­ ments led to the realization that heavy molecules could be 5 accelerated by a light gas in a nozzle flow to produce molecular beams with energies approaching 10 eV or higher.