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Investigations of the Potential Functions of Weakly Bound Diatomic Molecules and Laser-Assisted Excitive Penning Ionization

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Investigations of the Potential Functions of Weakly Bound Diatomic Molecules and Laser-Assisted Excitive Penning Ionization Mrnnw JULi am LBL-14278 Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Materials & Molecular Research Division INVESTIGATIONS OF THE POTENTIAL FUNCTIONS OF WEAKLY BOUND DIATOMIC MOLECULES AND LASER-ASSISTED EXCITIVE PENNING IONIZATION James H. Goble, Jr. (Ph.D. thesis) May 1982 wflfr ,a> *#ffi?" •'• rifciiiirff ^ ' £ a ii-» m,& (s nmm®-. Piv.posji5 .ortiie U.S. Department of Energy under Contr£ci;DE-ACO3-7ESP003&3 LBL-14278 1 INVESTIGATIONS OF THE POTENTIAL FUNCTIONS OF WEAKLY BOUND DIATOMIC MOLECULES AND LASER-ASSISTED EXCITIVE PENNING IONIZATION James H. Goble, Jr. Materials and Molecular Research Division Lawrence Berkeley Laboratory and LEL—14270 Department of Chemistry, University of California DEB2 017259 Berkeley, California 94720 ABSTRACT Three variations on the Dunham series expansion function of the potential of a diatomic molecule are compared. The differences among these expansions lie in the choice of the expansion variable, X. The functional form of these variables are X = 1-r /r for the Simon-Parr- s e Finlan version, A - l-(r /r)p for that of Thakkar, and X = l-exp(-p(r/r -1)) for that of Huffaker. A wide selection of molecular systems are examined. It is found that, for potentials in excess of thirty kcal/mole, the Huffaker expansion provides the best description of the three, extrapolating at large internuclear separation to a value within 10% of the true dis­ sociation energy. For potentials that result from the interaction of excited states, ail series expansions show poor behavior away from the equilibrium internuclear separation of the molecule. This property can be used as a qualitative diagnostic of interacting electronic states. The series representation of the potentials of weakly bound mole­ cules are examined in more detail. The ground states of BeAr , HeNe , NaAr, and Ar? and the excited states of HeNe , NaNe, and NaAr are best NOTICE PORTIONS Of THIS REPORT j>BE IUEGIBIE. It has been reproduced from the best available copy to permit the broadest possible availability. ms-wooTioii CF T>« 2 described by the Thakkar expansion. Presumably this is because of the r" attractive forces that are responsible for the binding of these systems. Close examination of the potentials of the alkaline earth dimers suggests that covalent forces may be of importance in the bonding of these metals. Finally, the observation of laser-assisted excitive Penning ioni­ zation in a flowing afterglow is reported. The reaction 3 + 2 Ar( P2) + Ca + hv -* Ar + Ca (5p P ) + e" occurs when the photon energy, hv, is approximately equal to the energy difference between the metastable argon and one of the fine structure levels of the ion's doublet. By monitoring the cascade fluorescence of the above reaction and comparing it to the fluorescence from the field-free process 3 + 2 Ar( P2) + Ca -»- Ar + Ca (4p P ) + e* , 3 °2 a surprisingly large cross section of 6.7x10 A is estimated. Mechanisms responsible for the resonant excitation and the large cross section are considered. i This is dedicated to the rest of the Gobies, and especially to my parents. It is as much their achievement as it is mine. ii Acknowledgement s Except for the abstract and this acknowledgement, I con­ sistently use the second person plural, "we", throughout this dissertation. This is not a pretense to royalty; it is ar. implicit acknowledgement of my research director, John Winn. For these many years he has been counselor, friend, and most important, teacher. He may have not found the burned out FET in the Baratron electronics, but he showed me how, and that I could. His support and encourage­ ment were crucial at those times when I tended towards despair. I have benefitted from his scientific insight and chemical intuition, especially as to what is publishable material. I want to express my gratitude to the "colleagues and co-workers" that have been as much as a family to me. Brian, Davf., Dennis, Henry, Susan, and Will provided the environment that made working there a joy. Especially appreciated are the legacy of Dennis Hartman and the assistance of' Will Hollingsworth that made Chapter IV possible. The intellectual discourses with Henry tuftman, sometimes accented with raised voices, helped maintain a stimulating, academic atmosphere. A virtual member of the family, George Webber, is responsible for a feeling I have of how things on paper go together when they are made of metal. Thanks to Howard Wood, for showing me how things are made out of metal. More important, the student shop provided a therapeutic means of working out my frustrations by making things instead of breaking them. Special thanks go to the Mahan group for all the equipment we iii borrowed and the coraradery. of elbow-bending at the Cheshire Cat. I am even beginning to miss the flavor of Anchor Steam on tap (a longing I did not think I would experience). X would also like to thank Yuan T. Lee for the loan of his argon ion laser, dye laser, and the facilities for performing a series of photoacoustic spectroscopy experiments. Although this work is not reported here, the confidence- and character-building I experienced there has proven invaluable. I also thank the members of his research group for their assistance. The "can-do" attitude they enjoy continues to be an inspiration. Thanks go to Steve Winkle, Pat Pakes, and Ken Smith for sharing, at one time or the other, their friendship and the cost of an abode. Thanks, "Wink", for inspiring me to learn how to cook. I thank Dennis and Paula Trevor for their warmth, hospitality, and the "little chickens" served on several Thanksgivings. I also thank Kate and the rest of the Lawheads for Christmas, 1980. Thanks, Ninon, for the coffee cup with the frog inside. I want to express my appreciation to Sheila Walsh for her work on the second virial coefficients of the Ar^ potentials. I also thank Yuan Lee and Chris Becker for the Ar scattering data and scattering program. 1 also thank John Winn for the "refined Thakkar" of Ar« and the two Be2 Thakkar potentials fitted to the ab initio points. I also thank Bowen Liu of IBM for his unpublished results on Mg2- Thanks go to Ann Weightman and Vera Spanos for turning my chicken scratchings into a readable dissertation and to Nancy Monroe and Andy Poynter for their artistic interpretation Df my figures. iv I also thank James Kinsey and his research group for their patience with my divided attentions while I have been writing the last three-quarters of this dissertation. This work has been supported by the National Science Foundation, the National Resource for Computation in Chemistry, and the Division of Chemical Sciences, Office of Basic Energy Sciences, U. S. Depart­ ment of Energy under Contract No; DE-AC03-76SFQ0098. The laser discussed in Chapter IV was borrowed from the San Francisco Laser Center, supported by NSF under Grant #CHE79-16250, awarded to the University of California at Berkeley in collaboration with Stanford University. I gratefully acknowledge the support from a University Regents' Intern Fellowship. Finally, with all respects and reverences, I thank the Good Lord that this part of my life is completed. V TABLE OF CONTENTS DEDICATION 1 ACKNOWLEDGEMENTS ii TABLE OF CONTENTS V BIOGRAPHICAL NOTE vii CHAPTER I. Introduction and Overview 1 References 6 CHAPTER II. Diatomic Potential Determination and the Revival of Series Potential A. Introduction 8 B. JWKB Solution 10 C. The Dunham Solution 14 D. RKR Method 18 E. The Empirical Series Strikes Back 22 F. Further Convergence Considerations 30 G. The Program 32 H. The Roads-Not-Taken 35 I. General Observations 39 J. The Question of Higher a 's 49 K. Conclusions 53 References 55 Tables 61 Figures 69 CHAPTER III. Series Potentials Applied to Weakly Bound Molecules A. Introduction 72 B. BeAr+ 76 vi C. Other Molecular Ions 86 D. NaNe and NaAr 92 E. Ar2 103 F. Alkaline Earth Dimers 113 G. Direct Fit 122 H. Conclusion 127 References 129 Tables 136 Figures 159 CHAPTER IV. Laser Assisted Penning Ionization A. Introduction 176 B. Experimental 181 C. Signal Treatment and Results 185 D. Discussion 188 E. Theoretical Considerations 192 F. Future Considerations 198 G. Conclusion 200 References 201 Figures 205 Appendix I 209 Appendix II 256 vii James Henry Goble, Jr. was born December 16, 1952 in Longview, Texas and was raised in Fort Worth and Houston, Texas. He holds a Bachelor of Science degree in chemistry from the University of Texas, awarded in 1975. He is a member of Phi Beta Kappa, the American Chemical Society, the American Physical Society, and is a University of California Regents1 Intern Fellow. He is, at this writing, a Postdoctoral Assoc Late at the Massachusetts Institute of Technology. 1 Chapter I Introduction and Overview The main purpose of this work is to examine the nature of the bonding of weakly bound diatomic molecules. By "weakly bound", we mean binding energies of 30 kcal per mole or less. This energy regime is responsible for phenomena such as condensation, physical adsorption on surfaces, the adhesion in polycrystalline materials like cerauics, the binding in molecular crystals, the rates of ion-mclecule reactlous, and ever, the mechanism responsible fcr the high selectivity of enzymatic catalysis. He hope to shed some light on these kinds of forces by examining the attraction of the simplest of these systems; the weakly bound diatomic molecule. The quantity that best describes this interaction is the inter- molecular potential. Although there have been significant advances in the ab initio calculation of this potential from first principles , they rely on the cancellation of electron correlation errors between the molecule and those of the separaced atoms.
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