Haywood, Hoppes, Hudson, and Wu. In addition, 

 work by Maxwell at NBS demonstrated the iso- 

 tope effect in superconductivity simultaneously 

 with workers at Rutgers, an experiment which 

 laid the ultimate basis for the Bardeen, Cooper, 

 Schrieffer(BCS) theory. 



Physical Measurements 



Measurement science as practiced at NBS is 

 synonymous with the development and use of the 

 tools and instruments of experimental science and 

 technology for more precise and powerful meas- 

 urements of the physical world. Although every 

 experimentalist is therefore also a practitioner of 

 measurement science, NBS has an institutional 

 responsibility to foster it. Thus, as mentioned ear- 

 lier, measurement science colors the entire re- 

 search effort at NBS and receives continuing high 

 priority. One indication of the marked progress in 

 measurement science can be seen by comparing 

 the best values of the physical constants listed of 

 1959 with those of 1975. In general, the precision 

 has increased by factors of thousands. 



As a specific example, the frequency spectrum 

 standards system has been expanded in overlap- 

 ping steps from 4 x IQiOHz to about lO-OHz (mi- 

 crowave frequencies to 7-rays). Not only has the 

 level of precision been improved markedly, but 

 the long-standing dichotomy between the optical 

 and x-ray regimes has been removed. 



Of all the other examples of areas where meas- 

 urement science is pursued at NBS, we mention 

 analytical chemistry. This area is also of interest 

 because of the important national problems which 

 are addressed. For example, pollution abatement, 

 industrial quality control, and clinical chemistry 

 are all analytical chemistry intensive. A major 

 challenge for the future is in the development of 

 sensitive, accurate techniques for the determina- 

 tion of organic compounds in diverse media. Here 

 the ability to separate and characterize becomes 

 paramount since the samples may have hundreds 

 or even thousands of potentially interfering con- 

 stituents. 



In the future as in the past, measurement sci- 

 ence will be the principal driving force for basic 

 research at NBS and to a large extent will deter- 

 mine the style of NBS research. The needs in mea- 

 surement science will be for more precise values 

 of the basic physical constants, for more sophisti- 

 cated chemical analysis, for more accurate charac- 

 terization of materials, and for more accurate and 

 economic techniques for making all sorts of physi- 

 cal measurements of temperature, sound, electro- 

 magnetic fields, etc. In pursuit of this challenge, 

 NBS expects to continue to operate at the fore- 

 front of the physical science disciplines and inte- 



grate research in the disciplines to provide the 

 measurement services needed in areas of major 

 national concern. Examples are nuclear safe- 

 guards, energy, government regulation requiring a 

 technical basis, health, safety, and technology 

 underlying better consumer information. 



Atoms and Molecules 



This category of research encompasses new and 

 promising research opportunities covering much of 

 physics and chemistry. In it, NBS conducts experi- 

 ments and develops theory for describing the basic 

 properties, such as configurations, interactions, 

 and transitions of atomic, molecular, and ionic 

 species. The data and theories developed form the 

 basis for modeling and understanding the equilibri- 

 um and nonequilibrium states of such important 

 systems as fusion plasmas, lasers, and the earth's 

 ozone layer. They also form the basis for predic- 

 tion and control of chemical reaction rates in all 

 their multifarious applications. 



NBS is presently analyzing the very hot gases of 

 interest to the national thermonuclear program. 

 An extensive series of measurements is being 

 made of radiation spectra, energy levels, transition 

 probabilities, and electron impact excitation. Of 

 particular scientific interest is the first definitive 

 beam measurement of the cross section for elec- 

 tron impact of a multiply-charged ion (C+^). 



By pioneering in the use of synchrotron radia- 

 tion and the development of monoenergetic elec- 

 tron sources, NBS played a leading role over the 

 past decade in discovering and understanding 

 atomic and molecular resonances due to doubly 

 excited states. A beam of electrons derived from 

 laser-induced photo-ionization of metastable bar- 

 ium was used to resolve doubly excited states to 

 about 1.5 meV, or more than 10 times better than 

 previous techniques. This new precision will allow 

 much more thorough tests of the theory, much of 

 which was developed at NBS. 



A new spectroscopy has been developed at NBS 

 and is becoming widely adopted elsewhere, where- 

 in molecular continuum radiation is quantitatively 

 measured and analyzed to determine molecular 

 potentials of colliding gases, when one of the colli- 

 sion partners is excited. The increasing application 

 of lasers to atomic and molecular spectroscopy has 

 brought with it the additional complexities of non- 

 linear or multiphoton processes. The physics of 

 such processes is being explored theoretically and 

 through experimental studies of multiphoton ioni- 

 zation in very diffuse alkali vapor as well as in 

 dense alkali plasmas. 



Further laser research at NBS has led to their 

 use for selectively exciting molecular transitions 

 which allow very sensitive analytical detection of 



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