Ion and velocity map imaging for surface dynamics and kinetics.

We describe a new instrument that uses ion imaging to study molecular beam-surface scattering and surface desorption kinetics, allowing independent determination of both residence times on the surface and scattering velocities of desorbing molecules. This instrument thus provides the capability to derive true kinetic traces, i.e., product flux versus residence time, and allows dramatically accelerated data acquisition compared to previous molecular beam kinetics methods. The experiment exploits non-resonant multiphoton ionization in the near-IR using a powerful 150-fs laser pulse, making detection more general than previous experiments using resonance enhanced multiphoton ionization. We demonstrate the capabilities of the new instrument by examining the desorption kinetics of CO on Pd(111) and Pt(111) and obtain both pre-exponential factors and activation energies of desorption. We also show that the new approach is compatible with velocity map imaging.

Single-field slice-imaging with a movable repeller: photodissociation of N2O from a hot nozzle.

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations--(0, v2, 0)--are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 ≤ 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O((1)D) + N2¹Σ(g)⁺ asymptote was determined to be 3.65 ± 0.02 eV.

Six-dimensional dynamics study of reactive and non reactive scattering of H(2) from Cu(111) using a chemically accurate potential energy surface.

We have studied the interaction of H(2) on Cu(111) using quasi-classical and quantum dynamics, and a chemically accurate six-dimensional potential energy surface (PES). The PES was computed using the specific reaction parameter (SRP) approach to density functional theory (DFT), in an implementation adapted to molecules interacting with metal surfaces. To perform this study we have applied the Born-Oppenheimer static surface (BOSS) approximation, i.e., we used both the Born-Oppenheimer (BO) and the static surface (SS) approximations. We show that our theoretical approach accurately describes experiments on dissociative adsorption, the effect of molecular vibrational and rotational motion on dissociative (associative) adsorption (desorption), and rotational excitation upon scattering. More specifically, dynamics calculations on reactive scattering of H(2) reproduce reaction probabilities measured in molecular beam experiments, effective barrier heights describing the dependence of reaction on the initial rovibrational state, and data on rotationally inelastic scattering with chemical accuracy (i.e., within 1 kcal mol(-1) approximately 4.2 kJ mol(-1)). These processes are not affected much by surface motion, either because they were measured using a low surface temperature, T(s), or because the computed observable is independent of T(s). However, we show that to account for the dependence of molecular orientation on a reaction the inclusion of surface motion is required. We have also found that vibrational excitation is poorly described within the BOSS approximation, suggesting a breakdown of this approximation.

Control of gradient-driven instabilities using shear Alfvén beat waves.

A new technique for manipulation and control of gradient-driven instabilities through nonlinear interaction with Alfvén waves in a laboratory plasma is presented. A narrow, field-aligned density depletion is created in the Large Plasma Device, resulting in coherent, unstable fluctuations on the periphery of the depletion. Two independent shear Alfvén waves are launched along the depletion at separate frequencies, creating a nonlinear beat-wave response at or near the frequency of the original instability. When the beat wave has sufficient amplitude, the original unstable mode is suppressed, leaving only the beat-wave response, generally at lower amplitude.

Chemically accurate simulation of a prototypical surface reaction: H2 dissociation on Cu(111).

Methods for accurately computing the interaction of molecules with metal surfaces are critical to understanding and thereby improving heterogeneous catalysis. We introduce an implementation of the specific reaction parameter (SRP) approach to density functional theory (DFT) that carries the method forward from a semiquantitative to a quantitative description of the molecule-surface interaction. Dynamics calculations on reactive scattering of hydrogen from the copper (111) surface using an SRP-DFT potential energy surface reproduce data on the dissociative adsorption probability as a function of incidence energy and reactant state and data on rotationally inelastic scattering with chemical accuracy (within approximately 4.2 kilojoules per mole).

Inverse velocity dependence of vibrationally promoted electron emission from a metal surface.

All previous experimental and theoretical studies of molecular interactions at metal surfaces show that electronically nonadiabatic influences increase with molecular velocity. We report the observation of a nonadiabatic electronic effect that follows the opposite trend: The probability of electron emission from a low-work function surface--Au(111) capped by half a monolayer of Cs--increases as the velocity of the incident NO molecule decreases during collisions with highly vibrationally excited NO(X(2)pi((1/2)), V = 18; V is the vibrational quantum number of NO), reaching 0.1 at the lowest velocity studied. We show that these results are consistent with a vibrational autodetachment mechanism, whereby electron emission is possible only beyond a certain critical distance from the surface. This outcome implies that important energy-dissipation pathways involving nonadiabatic electronic excitations and, furthermore, not captured by present theoretical methods may influence reaction rates at surfaces.

The work function of submonolayer cesium-covered gold: a photoelectron spectroscopy study.

Using visible and x-ray photoelectron spectroscopy, we measured the work function of a Au(111) surface at a well-defined submonolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61+/-0.08 eV, consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.

Observation of a change of vibrational excitation mechanism with surface temperature: HCl collisions with Au(111).

We have measured the vibrational excitation probability (Pv) of HCl incident on a Au(111) surface at kinetic energies (Ei) of 0.59 eV to 1.37 eV and surface temperatures (Ts) of 273 K to 1073 K. For all energies, the slope of the Pv as a function of Ts exhibits a sharp increase above Ts approximately 800 K. We show this change in slope and the threshold behavior of Pv to be consistent with a change in excitation mechanism from an electronically adiabatic mechanical mechanism to an electronically nonadiabatic mechanism involving excited electron-hole pairs.

Ionization-induced electron trapping in ultrarelativistic plasma wakes.

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.

Positron production by x rays emitted by betatron motion in a plasma wiggler.

Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

Observation of terahertz emission from a laser-plasma accelerated electron bunch crossing a plasma-vacuum boundary.

Coherent radiation in the 0.3-3 THz range has been generated from femtosecond electron bunches at a plasma-vacuum boundary via transition radiation. The bunches produced by a laser-plasma accelerator contained 1.5 nC of charge. The THz energy per pulse within a limited 30 mrad collection angle was 3-5 nJ and scaled quadratically with bunch charge, consistent with coherent emission. Modeling indicates that this broadband source produces about 0.3 microJ per pulse within a 100 mrad angle, and that increasing the transverse plasma size and electron beam energy could provide more than 100 microJ/pulse.

Associate degree graduates and the rapidly aging RN workforce.

This second segment of a four-part series examines the inter-relationship between the growth in associate degree nursing programs and the aging of the RN workforce. A growing proportion of new RNs have entered the workforce via associate degree programs, increasing from 40% in 1977 to 60% in 1996. New graduates, as well as working RNs, are approximately 5 years older in 1996 than 20 years earlier. Findings suggest that the rapid aging of the RN workforce can not be directly attributed to the rise in the number of older-aged graduates of associate degree programs. Rather, the declining propensity of those born after 1960 to enter nursing has resulted in fewer young RNs, and therefore: (1) an aging workforce, and (2) fewer new grads from baccalaureate programs (which have always attracted younger RNs) relative to grads from associate degree programs (which have always attracted older RNs).

Indications for a novel muscular dystrophy pathway. gamma-filamin, the muscle-specific filamin isoform, interacts with myotilin.

gamma-Filamin, also called ABP-L, is a filamin isoform that is specifically expressed in striated muscles, where it is predominantly localized in myofibrillar Z-discs. A minor fraction of the protein shows subsarcolemmal localization. Although gamma-filamin has the same overall structure as the two other known isoforms, it is the only isoform that carries a unique insertion in its immunoglobulin (Ig)-like domain 20. Sequencing of the genomic region encoding this part of the molecule shows that this insert is encoded by an extra exon. Transient transfections of the insert-bearing domain in skeletal muscle cells and cardiomyocytes show that this single domain is sufficient for targeting to developing and mature Z-discs. The yeast two-hybrid method was used to identify possible binding partners for the insert-bearing Ig-like domain 20 of gamma-filamin. The two Ig-like domains of the recently described alpha-actinin-binding Z-disc protein myotilin were found to interact directly with this filamin domain, indicating that the amino-terminal end of gamma-filamin may be indirectly anchored to alpha-actinin in the Z-disc via myotilin. Since defects in the myotilin gene were recently reported to cause a form of autosomal dominant limb-girdle muscular dystrophy, our findings provide a further contribution to the molecular understanding of this disease.

Why are shortages of hospital RNs concentrated in specialty care units?

This article is the first in a series examining the interplay between the aging of the nurse workforce and other factors driving the growing nursing shortage that are already affecting some specialty areas. Nearly 60% of the current RN workforce is over 40 years of age; and the percentage of RNs under age 30 has fallen by nearly 40% since 1980. The total number of FTE RNs is projected to shrink after 2010, likely resulting in shortages of RNs "when the large baby-boom generation of RNs starts to retire." Because ICUs have historically attracted younger RNs, the rapid decline in the number of RNs in the workforce under age 30 plays a large role in explaining the development of shortages in the ICU. The growing difficulties staffing operating rooms and other peri-operative services is seen as related to the aging work force as more diploma prepared nurses have been attracted to this specialty because they had educational exposure to this area.

Vibrational promotion of electron transfer.

By using laser methods to prepare specific quantum states of gas-phase nitric oxide molecules, we examined the role of vibrational motion in electron transfer to a molecule from a metal surface free from the complicating influence of solvation effects. The signature of the electron transfer process is a highly efficient multiquantum vibrational relaxation event, where the nitrogen oxide loses hundreds of kilojoules per mole of energy on a subpicosecond time scale. These results cannot be explained simply on the basis of Franck-Condon factors. The large-amplitude vibrational motion associated with molecules in high vibrational states strongly modulates the energetic driving force of the electron transfer reaction. These results show the importance of molecular vibration in promoting electron transfer reactions, a class of chemistry important to molecular electronics devices, solar energy conversion, and many biological processes.

Acute bilateral ophthalmoplegia in a young woman.

A 34-year-old woman presented with bilateral ophthalmoplegia, ptosis, and mild gait ataxia. Tensilon test, magnetic resonance imaging, and cerebrospinal fluid analysis were normal. She initially denied any alcohol intake but later admitted to significant alcohol and multisubstance abuse. The patient improved dramatically after treatment with thiamine. Wernicke's encephalopathy is discussed, highlighting that it may be present with normal mentation. It should be considered in the differential diagnosis of bilateral ophthalmoplegia even in the absence of altered mental status.

Implications of an aging registered nurse workforce.

CONTEXT: The average age of registered nurses (RNs), the largest group of health care professionals in the United States, increased substantially from 1983 to 1998. No empirically based analysis of the causes and implications of this aging workforce exists. OBJECTIVES: To identify and assess key sources of changes in the age distribution and total supply of RNs and to project the future age distribution and total RN workforce up to the year 2020. DESIGN AND SETTING: Retrospective cohort analysis of employment trends of recent RN cohorts over their lifetimes based on US Bureau of the Census Current Population Surveys between 1973 and 1998. Recent workforce trends were used to forecast long-term age and employment of RNs. PARTICIPANTS: Employed RNs aged 23 to 64 years (N = 60,386). MAIN OUTCOME MEASURES: Annual full-time equivalent employment of RNs in total and by single year of age. RESULTS: The average age of working RNs increased by 4.5 years between 1983 and 1998. The number of full-time equivalent RNs observed in recent cohorts has been approximately 35% lower than that observed at similar ages for cohorts that entered the labor market 20 years earlier. Over the next 2 decades, this trend will lead to a further aging of the RN workforce because the largest cohorts of RNs will be between age 50 and 69 years. Within the next 10 years, the average age of RNs is forecast to be 45.4 years, an increase of 3.5 years over the current age, with more than 40% of the RN workforce expected to be older than 50 years. The total number of full-time equivalent RNs per capita is forecast to peak around the year 2007 and decline steadily thereafter as the largest cohorts of RNs retire. By the year 2020, the RN workforce is forecast to be roughly the same size as it is today, declining nearly 20% below projected RN workforce requirements. CONCLUSIONS: The primary factor that has led to the aging of the RN workforce appears to be the decline in younger women choosing nursing as a career during the last 2 decades. Unless this trend is reversed, the RN workforce will continue to age, and eventually shrink, and will not meet projected long-term workforce requirements. JAMA. 2000.

A novel marker for vertebrate embryonic heart, the EH-myomesin isoform.

Myomesin is a structural component of the M-band that is expressed in all types of striated muscle. Its primary function may be the maintenance of the thick filament lattice and its anchoring to the elastic filament system composed of titin. Different myomesin isoforms have been described in chicken and mice, but no particular function has been assigned to them. Here we investigate the spatio-temporal expression pattern of myomesin isoforms by means of reverse transcriptase-polymerase chain reaction and isoform-specific antibodies. We find that two alternative splicing events give rise to four myomesin isoforms in chicken contrary to only one splicing event with two possible isoforms in mice. A splicing event at the C terminus results in two splice variants termed H-myomesin and S-myomesin, which represent the major myomesin species in heart and skeletal muscle of avian species, respectively. In contrast, in mammalian heart and skeletal muscle only S-myomesin is expressed. In embryonic heart of birds and mammals, alternative splicing in the central part of the molecule gives rise to the isoform that we termed EH-myomesin. It represents the major myomesin isoform at early embryonic stages of heart but is rapidly down-regulated around birth. Thus, the strict developmental regulation of the EH-myomesin makes it an ideally suited marker for embryonic heart.

Slow growth in the United States of the number of minorities in the RN workforce.

PURPOSE: To assess the extent to which the number of minority RNs has grown during the past 20 years, and to identify and compare key trends in personal and professional characteristics among minority groups and between minority and majority populations of RNs. Nursing education programs, employers, philanthropic organizations, and governments have expended considerable effort and resources to increase the number of minorities in nursing. DESIGN: Longitudinal analysis of trends in the number, education, employment, and earnings of minority RNs from 1977 to 1997. METHODS: Descriptive analysis of data from the U.S. National Sample Surveys of the Population of Registered Nurses, 1977-1996; and data from the U.S. Bureau of the Census Current Population Survey (CPS) Outgoing Rotation Group Annual Merged Files, 1977-1997. FINDINGS: In the past 20 years, the number of minority RNs has grown from 87,386 (or 6.3% of the total supply of RNs) in 1977 to 246,364 RNs (9.7%) in 1996. The number of Black (nonHispanic), Asian Pacific/Islanders, and American Indian/Alaskan Native nurses nearly tripled in this period while the number of Hispancis doubled. Although these rates of growth are impressive, the percentage of minorities in nursing lags considerably behind the percentage (18.3%) who are teachers, and the percentage (28.2%) in the U.S. population. CONCLUSIONS: Studies are needed to determine the barriers that exist in nursing education programs, health care organizations, and society in general that deter minorities from a nursing career. Without this understanding, efforts to design and implement ideas to attract, educate, and retain minorities in nursing education and later in the workplace are hampered.