.“ .... ,. LBNL-42877 .. ”--’ .-, , . ~ >. -,. .! .: .C,. ; ,- fimm ~lRNEST ~RLANDEl bAWRENCE .. ~ERIKEILEY ~ATUC21NAL- kAIH~RATaRY *:.-,..:.*..ish_.? . ..–. ) -k ;:.. 4.., ..’ ..... Femtosecond Photoelectron ~-. Spectroscopy A NewTool for . ,. the Studyof AnionDynamics Benjamin J. Greenblatt Chemical SciencesDivision February 1999 Ph.D. Thesis .. .. ~./.- — .-..-.— —----- - -.,-- ... ;>;., c.‘>;:::.‘..,” $+’.,. ‘.’.:,. ‘A-,,{. DISCLAIMER Thisdocumentwaspreparedm m accountofworksponsoredbythe UnitedStatesGovernment.Whilethisdocumentisbelievedto contain correctinformation,neitherthe UnitedStatesGovernmentnor any agencythereof,norTheRegentsof theUniversityof California,norany of theiremployees,makesanywarranty,expressor implied,orassumes anylegalresponsibilityfortheaccuracy,completeness,or usefulnessof trny information,apparatus,product, or process dkclosed, or representsthat its use wouldnot infringeprivatelyownedrights. 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(Haverford College) 1993 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Chemistry in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Daniel M. Neumark, Chair Professor Ronald C. Cohen Professor Jeffrey Bokor Spring 1999 This work was supported in part by the Director, Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SFOO098, by the National Science Foundation under Grant No. CHE-97 10243, and by the Defense University Research Instrumentation Program under Grant No. F49620-95- 1-0078. ... .. ~<~.--,,..,,.. -.,,.. -. ., ,.- . .. ~,, - -- -..--.——.- ,., Femtosecond Photoelectron Spectroscopy: A New Tool for the Study of Anion Dynamics Copyright @ 1999 by Benjamin Jefferys Greenblatt The U.S. Department of Energy has the right to use this document for any purposewhatsoeverincludingthe right to reproduce all or anypart thereof ..-. --- .,-- 1 Abstract Femtosecond Photoelectron Spectroscopy: A New Tool for the Study of Anion Dynamics by Benjamin Jefferys Greenblatt Doctor of Philosophy in Chemistry University of California, Berkeley Professor Daniel M. Neumark, Chair A new experimental technique for the time-resolved study of anion reactions is presented. Using femtosecond laser pulses, which provide extremely fast (- 100 fs) time resolution, in conjunction with photoelectron spectroscopy, which reveals differences between anion and neutral potential energy surfaces, a complex anion reaction can be followed from its inception through the formation of asymptotic products. Experimental data can be modeled quantitatively using established theoretical approaches, allowing for the refinement of potential energy surfaces as well as dynamical models. After a brief overview, a detailed account of the construction of the experimental apparatus is presented. Documentation of the data acquisition program is contained in the Appendix. The first experimental demonstration of the technique is then presented for Iz- photodissociation, modeled using a simulation program which is also detailed in the Ap- pendix. The investigation of Iz- photodissociation in several size-selected Iz-(Ar). (n = 6- 20) and 12-(C02). (YZ= 4- 16) clusters forms the heart of the dissertation. In a series of chapters, the numerous effects of solvation on this fundamental bond-breaking reaction .,..= -- ,,,-- - ,-,. ,: ../ ,- - -- .7.--.:-.-,,... 2 Abstract are explored, the most notable of which is the recombination of 12-on the ground ~(2X~) state in sufficiently large clusters. Recombination and trapping of 12-on the ex- cited ~(211~,2,~) state is also observed in both types of clusters. The studies have re- vealed electronic state transitions, the first step in recombination, on a -500 fs to -10 ps timescale. Accompanying the changes in electronic state is solvent reorganization, which occurs on a similar timescale. Over longer periods (-1 ps to >200 ps), energy is trans- ferred from vibrationally excited 12-to modes of the solvent, which in turn leads to sol- vent evaporation. These effects become more important as cluster size increases. In addi- tion, differences in timescale and mechanism are observed between clusters of Ar, which binds to r and 12-rather weakly, and C02, whose large quadruple moment allows sub- stantially stronger binding to these anions. ... 111 Dedication This dissertation is dedicated to my father Raymond Benjamin Greenblatt His unwavering love for me throughout my life has allowed me to soar ;able of contents 1 Abstract.-. ..............---------------------------------------------------------------------------------------------------------------------... Dedication .....................-----------------------------------------------------------------------------------------------------------111 Table of contents .......................................................................................................................iv.. Preface .........----------------------------------------------------------------------------------------------------------------...........Vlll Acknowledgments .........................................................------.......------......................................xii Chapter 1. Introduction ---------------------------........-......................................................................... References .........--------------------------------------------------------------------------------------------------...............1. Chapter 2. Experimental AppuatUs ...................................................................................... 14 1. Vacuum system --.............................................................-----------------------------------------------14 1.1. Pumps...- ..............-.-....................-.........-........-..--........-..-........-................................l6 1.2. Interlock ----------------------------.------.......-------...................................-------....................... 18 1.3. Source and zeroeth differential regions ------------------.............................................. 21 1.3.1. Pulsed valve ------------------.....................................--------.................................. 21 1.3.2. Electron ~n ....................................................................................... 23 1.3.3. Zeroeth differential region .......................................................................... 25 1.3.4. Extraction and acceleration ..................---------............................................. 27 1.4. First and second differential regions ..................................................................... 28 1.4.1. Ion deflectors ..-----..........................................-------.......................................28 1.4.2. Einzel lenses ......---------------...........................................................................29 1.5. Detector region ...............-------------.------..............--------------........................................ 30 1.5.1. Laser windows and baffles ..............................--------------------------------------------34 1.5.2. Retractable ion detector ...............................------------------------------------------------ 1.5.3. Magnetic bottle ,------..-------....................------....................................___..:~ 1.5.4. Electron detector ..............-------.................................................................... 38 1.5.5. Mass gate ......-.--..........................................................................-----------------40 1.5.6. Pulsed ion decelerator .................................................................................41 1.5.7. Reflection ........----------.........---------......................................................----------41 1.6. Timing. -----.................-..................-..-........-----.........................-......-........................43 1.7. Ion time of flight ---------------------...............................................................................44 1.8. Electron time of flight ............................................................................................47 1.8.1. Resolution --------....-.-------...............................................................----------------48 1.8.2. Pulsed ion deceleration .............................................---------......................... 50 1.9. Reflection ion time of flight ...------.....................----------------------------------------------------53 2. Laser system ....................................................................................................................55