The Role of Momentum Partitioning in Covariance Ion Imaging Analysis.

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data.

Two-Dimensional Projected-Momentum Covariance Mapping for Coulomb Explosion Imaging.

We introduce projected-momentum covariance mapping, an extension of recoil-frame covariance mapping for 2D ion imaging studies. By considering the two-dimensional projection of the ion momenta as recorded by the detector, one opens the door to a complex suite of analysis tools adapted from three-dimensional momentum imaging studies. This includes the use of different frames of reference to unravel the dynamics of fragmentation and the application of fragment momentum constraints to isolate specific fragmentation channels. The technique is demonstrated on data from a two-dimensional ion imaging study of the Coulomb explosion of the cis and trans isomers of 1,2-dichloroethene, following strong-field ionization by an intense near-infrared femtosecond laser pulse. Classical simulations are used to guide the interpretation of projected-momentum covariance maps. The results offer a detailed insight into the distinct Coulomb explosion dynamics for this pair of isomers and lay the groundwork for future time-resolved studies of photoisomerization dynamics in this molecular system.

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes via site-selective ionization.

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene (cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge.

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study.

The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis.

We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot-1). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.

Factors that influenced students to matriculate at a northeastern dental school: A comparative study.

PURPOSE/OBJECTIVES: There is a robust body of research on which factors dental schools consider when selecting students for acceptance, but there is considerably less research regarding what factors dental applicants consider when selecting a particular program. The aim of this investigation was to better understand which factors most influenced students to matriculate at a particular dental school and how the value of these factors has changed over time. METHODS: To answer this research question, an online survey was utilized to determine why students chose to attend Columbia University College of Dental Medicine (CUCDM). This survey was administered to students in the Class of 2023 and was compared to the responses of an identical survey that was administered to the Class of 2013. The response rate for the classes of 2013 and 2023 were 70.0% and 42.2%, respectively. Results from the Classes of 2013 and 2023 were analyzed using independent sample T-tests. RESULTS: The Class of 2013 ranked academic reputation, location, and admissions into specialty programs as the three most important factors in their selection of a dental school. The Class of 2023 ranked tuition cost, perceived patient pool, and financial aid as their three most important factors. CONCLUSION(S): For dental schools to continue to successfully attract and enroll students, they must understand what motivates a student to matriculate in a specific dental program. One way to accomplish this is through the use of a survey.

Observation of Rainbows in the Rotationally Inelastic Scattering of NO with CH4.

Using a combination of velocity-map imaging and resonance-enhanced multiphoton ionization detection with crossed molecular beam scattering, the dynamics of rotational energy transfer have been examined for NO in collisions with CH4 at a mean collision energy of 700 cm-1. The images of NO scattered into individual rotational (jNO') and spin-orbit (Ω) levels typically exhibit a single broad maximum that gradually shifts from the forward to the backward scattering direction with increasing rotational excitation (i.e., larger ΔjNO). The rotational rainbow angles calculated with a two-dimensional hard ellipse model show reasonable agreement with the observed angles corresponding to the maxima in the differential cross sections extracted from the images for higher ΔjNO transitions, but there are clear discrepancies for lower ΔjNO (in particular, final rotational levels with jNO' = 7.5 and 8.5). The sharply forward scattered angular distributions for these lower ΔjNO transitions better agree with the predictions of an L-type rainbow model. The more highly rotationally excited NO appears to coincide with low rotational excitation of the co-product CH4, indicating a degree of rotational product-pair anticorrelation in this bimolecular scattering.

Practice management curriculum in New York State dental schools.

The practice management curricula of the four dental schools in New York State were examined and were found to be limited. Although it was acknowledged that it is challenging to expand the hours devoted to teaching this discipline, steps should be taken to address the issue. The author suggests two initiatives that can help address this deficiency.

Satellite teaching hospitals and public-private collaborations in veterinary medical clinical education.

Veterinary teaching hospitals (VTHs) are facing more and greater challenges than at any time in the past. Changes in demand, expanding information, improving technology, an evolving workforce, declining state support, and an increasingly diverse consumer base have combined to render many traditional VTH modes of operation obsolete. In pursuit of continued success in achieving their academic mission, VTHs are exploring new business models, including innovative collaborations with the private sector. This report provides details on existing models for public-private collaboration at several colleges and schools of veterinary medicine, including those at Kansas State University, Purdue University, the University of Florida, and Tufts University. Although each of these institutions' models is unique, several commonalities exist, related to expansion of the case load available for teaching, the potential positive impact on recruitment and retention of clinical faculty, and the potential for easing financial pressures on the associated VTH. These new models represent innovative approaches that work to meet many of the key emerging challenges facing VTHs today.

Models for delivering school-based dental care.

School-based health centers (SBHCs) often are located in high-need schools and communities. Dental service is frequently an addition to existing comprehensive services, functioning in a variety of models, configurations, and locations. SBHCs are indicated when parents have limited financial resources or inadequate health insurance, limiting options for primary care and preventive services, or within low-access areas such as dental health professional shortage areas. Poor health and concomitantly poor oral health can lead to attendance problems. Oral health services in school-based setting are often the only access to services a child may have. Children who attend schools with SBHCs have immediate access to services that are coordinated with the student'sfamily and school personnel or administrators. Comprehensive services can be collaborative, with support or administration provided by more than 1 organization. For example, the Children's Aid Society (CAS), Columbia University School of Dental and Oral Surgery (CUSDOS), and Columbia University Mailman School of Public Health developed, implemented, and currently operate SBHCs in 2 communities in the northern Manhattan section of New York City (Central Harlem and Washington-Heights/Inwood). The clinics operate in or are affiliated with public schools in New York City. All CAS and Columbia University sites include dental components, using a variety of delivery models. Determining which dental delivery system to use for a particular community or population is a complex decision. The models, reasons for selection, and sustainability of each system are described.

Root fracture. Report of case supporting decreased splint times.

This is a case presentation of a root fracture that was treated and followed intermittently for 23 years. The treatment outcome supports the newly evolving protocol that root fractures are most successfully treated using a flexible splint over a relatively short duration.

Dental caries among disadvantaged 3- to 4-year-old children in northern Manhattan.

PURPOSE: The study was conducted to determine the prevalence of early childhood caries (ECC), untreated caries, and the ratio of posterior to anterior caries in a disadvantaged predominantly Hispanic or African-American urban population. Data are compared to NHANES III to assess the caries burden in our cohort. Comparisons are made to the aggregate and to minorities within the national database. METHODS: A retrospective chart review was conducted for children enrolled in a Head Start or day care program in the communities of Washington-Heights and Central and East Harlem and seen on the community organization's mobile dental van between 1995 and 1997. The study included only children 3 to 4 years of age at the initial examination (n=1,605). A single examiner provided all the examinations. The mean number of decayed and filled surfaces (dfs), decayed surfaces (ds) and filled surfaces (fs), the percentage of decayed of total decayed and filled surfaces (%d/dfs), decayed and filled teeth (dft), decayed teeth (dt) and filled teeth (ft), and the percentage of decayed of total decayed and filled teeth (%d/dft) were calculated. Posterior vs anterior d, f, dft, dfs and d-anterior/total d, and d-posterior/total d were tabulated. Northern Manhattan data was tabulated and compared to NHANES III (1988-1994) in the aggregate and for subpopulations categorized by gender and ethnicity. All results are also reported for children with at least one decayed or filled tooth. T-tests were used to assess for significant differences. RESULTS: There was even representation of males (50%) and females (50%). Mean dft was 1.08 overall, and 3.14 for children with dft>0. The level of untreated decay, %d/ dft, was 91%, significantly higher than the US national population which is 76% overall, and 76% for African Americans and Mexican Americans within the US national population. CONCLUSIONS: The children in this population have higher caries prevalence and a higher level of untreated caries than the national means as reported in NHANES III. The high level of untreated decay found in this particularly disadvantaged community suggests that enhanced dental services targeting the very young are needed in these communities.