QUALITY
NIST 3
(cos 0)/(cos Θ) instead of an angle (Θ). However, SHG measurements of the molecular orientation at several wave lengths can remove many of these am biguities and assumptions, and the ori entation calculation methodology used in the analysis of the surface SHG ex periments is constantly being refined (18,19). In addition to electrochemical interfaces, SHG is currently being ap plied to studies of molecular orienta tion in Langmuir-Blodgett films, at sil ica surfaces, and at liquid interfaces (1-4). •
UNITED STATES DEPARTMENT OF COMMERCE NATIONAL NSTITUTE OF STANDARDS AND TECHNOLOGY
The National Institute of Standards and Technology has developed a series of SRM's to serve as calibrants, test mixtures, and standardization materials for Quality Control of analytical instrumentation and methodology. MEASUREMENTS and STANDARDS are important to everyone who needs quality. NIST has over 1,000 Standard Reference Materials that can help you calibrate instruments and check on measurement accuracy. For more information phone or write for a free catalog.
Nonresonant SHG studies of chemisorption
In the absence of any molecular transi tions at the fundamental or second har monic wavelengths, the second har monic response from platinum elec trodes is dominated by the nonlinear susceptibility of the surface metal at oms. On a variety of surfaces the changes in this nonresonant SHG (Δχι in Equation 2) have been used to indi rectly monitor the chemisorption of species (5-7, 9, 11-13). For platinum surfaces the SHG signal was shown to provide a quantitative measure of the relative surface coverage of iodide, chloride, and bisulfate ions and other chemisorbed species (13). The surface sensitivity and selectiv ity of the SHG signal enables one to measure surface chemisorption in the presence of a large amount of solution species and during solution reactions. However, in contrast to resonant SHG studies of molecules at electrode sur faces, use of the nonresonant SHG sig nal is a relatively nonspecific method that is only capable of monitoring the absorption of species that perturb the metal surface. One chemisorbed species for which large changes are observed upon che misorption is the monatomic hydrogen that exists on the platinum electrode prior to H 2 evolution. In Figure 7a the SHG signal at 302 nm from a polycrystalline platinum electrode in 0.1 M per chloric acid solution is plotted as a function of potential along with the cy clic voltammogram. Very large changes are observed in the hydrogen adsorp tion region; these changes can be quan titatively related to the amount of ad sorbed hydrogen on the surface (13a). An increase in the SHG signal from a P t ( l l l ) surface upon the chemisorp tion of hydrogen has also been ob served in ultrahigh vacuum (7). The presence of adsorbed hydrogen can also be monitored on single crystal platinum electrodes by changes in the nonresonant SHG signal from the sur face. For P t ( l l l ) electrodes, there has been much controversy concerning the
MEASUREMENTS = STANDARD REFERENCE MATERIALS
Telephone (301) 975-OSRM (6776) FAX (301) 948-3730
SRM
STANDARD REFERENCE MATERIAL PROGRAM Building 2 0 2 , Room 204 National Institute of Standards and Technology Gaithersburg, MD. 2 0 8 9 9
CIRCLE 94 ON READER SERVICE CARD
Organic Chemist When virtually every phase of the communications, data management, industrial controls, defense systems and automobile industries began converting their electro mechanical technology to integral circuit technology, AVX developed the ceramic multilayer capacitors essential to such applications. That kind of innovation and responsiveness to market needs characterizes AVX; it's what has made us the nation's largest manufacturer of monolithic capacitors for the electronics industry. To maintain that number one position, we must constantly develop new technologies for the design and applications of capacitors. With this goal in mind, our Corporate Research and Development Laboratory is currently looking for an engineer with a background in organic/analytical chemistry, plus hands-on experience with FTIR, Raman, GC, DCP, HPLC, and similar characterization methods. Additionally, a background in characterization of resins, solvents, organic additives and elemental chemical analysis of inorganic and organic materials will be helpful. You must have a M.S., with one (1) year industrial experience or B.S. with three to four (3-4) years experience. Degree should be in chemistry or related field. In addition to the opportunity for growth in a highly professional environment, we can offer you an excellent salary and generous company benefits, including medical, life and dental insurance, various stock bonus programs, profit sharing, 401K and cash bonus plans, and a full relocation package. Not to mention the recreational advantages of living in one of the nation's loveliest resort communities — Myrtle Beach, South Carolina. To apply, send your confidential resume to: John Taylor, Human Resources Manager, AVX Corporation, P.O. Box 867, Myrtle Beach, SC 29577. Our growth today can be your success tomorrow
AVX
CORPORATION An equal opportunity employer, m/f/h/v. ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991 • 289 A
