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.

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.

Electron-induced dissociation dynamics studied using covariance-map imaging

Recently, covariance analysis has found significant use in the field of chemical reaction dynamics. When coupled with data from product time-of-flight mass spectrometry and/or multi-mass velocity-map imaging, it allows us to uncover correlations between two or more ions formed from the same parent molecule. While the approach has parallels with coincidence measurements, covariance analysis allows experiments to be performed at much higher count rates than traditional coincidence methods. We report results from electron-molecule crossed-beam experiments, in which covariance analysis is used to elucidate the dissociation dynamics of multiply-charged ions formed by electron ionisation over the energy range from 50 to 300 eV. The approach is able to isolate signal contributions from multiply charged ions even against a very large 'background' of signal arising from dissociation of singly-charged parent ions. Covariance between the product time-of-flight spectra identifies pairs of fragments arising from the same parent ions, while covariances between the velocity-map images ('recoil-frame covariances') reveal the relative velocity distributions of the ion pairs. We show that recoil-frame covariance analysis can be used to distinguish between multiple plausible dissociation mechanisms, including multi-step processes, and that the approach becomes particularly powerful when investigating the fragmentation dynamics of larger molecules with a higher number of possible fragmentation pathways.

The kinetic energy of PAH dication and trication dissociation determined by recoil-frame covariance map imaging

We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. In our experiments, ions are formed by interaction with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS2 multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision, ranging 1.94-2.60 eV and 2.95-5.29 eV for the dications and trications, respectively. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) simulations.

Imaging the Dynamics of the Electron Ionization of C2F6

The dissociation of C₂F₆ following electron ionization at 100 eV has been studied using multimass velocity-map ion imaging and covariance-map imaging analysis. Single ionization events form parent C₂F₆⁺ cations in an ensemble of electronic states, which follow a multiplex of relaxation pathways to eventually dissociate into ionic and neutral fragment products. We observe CF₃⁺, CF₂⁺, CF⁺, C⁺, F⁺, C₂F₅⁺, C₂F₄⁺, C₂F₂⁺, and C₂F⁺ ions, all of which can reasonably be formed from singly charged parent ions. Dissociation along the C–C bond typically forms slow-moving, internally excited products, whereas C–F bond cleavage is rapid and impulsive. Dissociation from the à state of the cation preferentially forms C₂F₅⁺ and neutral F along a purely repulsive surface. No other electronic state of the ion will form this product pair at the electron energies studied in this work, nor do we observe any crossing onto this surface from higher-lying states of the parent ion. Multiply charged dissociative pathways are also explored, and we note characteristic high kinetic energy release channels due to Coulombic repulsion between charged fragments. The most abundant ion pair we observe is (CF₂⁺, CF⁺), and we also observe ion pair signals in the covariance maps associated with almost all possible C–C bond cleavage products as well as between F⁺ and each of CF₃⁺, CF₂⁺, CF⁺, and C⁺. No covariance between F⁺ and C₂F₅⁺ is observed, implying that any C₂F₅⁺ formed with F⁺ is unstable and undergoes secondary fragmentation. Dissociation of multiply charged parent ions occurs via a number of mechanisms, details of which are revealed by recoil-frame covariance-map imaging.

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.

Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10-100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH^*, PAH^+* and PAH^2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH²⁺ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Partial and Contingent Recoil-Frame Covariance-Map Imaging

When applied to multimass velocity-map imaging data, covariance analysis reveals correlations between different fragment ions formed from the same parent molecule and can provide detailed insights into the fragmentation dynamics. Covariances between the time-of-flight signals for two different ions show that they are formed in the same event, while covariances between their velocity-map images, often referred to as "recoil-frame covariances", reveal details of the correlated motion of the two fragments. In many cases, covariance analysis is complicated by the fact that fluctuations in experimental parameters such as laser or molecular beam intensities can lead to apparent correlations between unrelated ions. In the context of time-of-flight covariance signals, this problem has been overcome by the introduction of partial covariance and contingent covariance approaches. Here, we apply these approaches to recoil-frame covariance-map images. We also demonstrate that in many cases the total signal within each experimental cycle can be used as a useful proxy for independent explicit measurements of the varying experimental parameter(s).

All-Optical Three-Dimensional Electron Momentum Imaging

We report a new implementation of three-dimensional (3D) momentum imaging for electrons, employing a two-dimensional (2D) imaging detector and a silicon photomultiplier tube (siPMT). To achieve the necessary time resolution for 3D electron imaging, a poly(p-phenylene)-dye-based fast scintillator (Exalite 404) was used in the imaging detector instead of conventional phosphors. The system demonstrated an electron time-of-flight resolution comparable with that of electrical MCP pick-off (tens of picoseconds), while achieving an unprecedented dead time reduction (∼0.48 ns) when detecting two electrons.

C–I and C–F bond-breaking dynamics in the dissociative electron ionization of CF3I

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF₃I at energies ranging from 20 to 100 eV.

Photofragmentation dynamics of N,N-dimethylformamide following excitation at 193 nm

N,N-dimethylformamide, HCON(CH₃)₂, is a useful model compound for investigating the peptide bond photofragmentation dynamics. We report data from a comprehensive experimental and theoretical study into the photofragmentation dynamics of N,N-dimethylformamide in the gas phase at 193 nm. Through a combination of velocity-map imaging and hydrogen atom Rydberg tagging photofragment translational spectroscopy we have identified two primary fragmentation channels, namely, fission of the N-CO "peptide" bond and N-CH₃ bond fission leading to the loss of CH₃. The possible fragmentation channels leading to the observed products are rationalised with recourse to CASPT2 calculations of the ground and first few excited-state potential energy curves along the relevant dissociation coordinates, and the results are compared with the data from previous experimental and theoretical studies on the same system.

A meta-analysis of prevalence rates and moderating factors for cancer-related post-traumatic stress disorder

OBJECTIVE

Systematic reviews highlight a broad range of cancer-related post-traumatic stress disorder (CR-PTSD) prevalence estimates in cancer survivors. This meta-analysis was conducted to provide a prevalence estimate of significant CR-PTSD symptoms and full diagnoses to facilitate the psychological aftercare of cancer survivors.

METHODS

A systematic literature search was conducted for studies using samples of cancer survivors by using validated clinical interviews and questionnaires to assess the prevalence of CR-PTSD (k = 25, n = 4189). Prevalence estimates were calculated for each assessment method using random-effects meta-analysis. Mixed-effects meta-regression and categorical analyses were used to investigate study-level moderator effects.

RESULTS

Studies using the PTSD Checklist-Civilian Version yielded lower event rates using cut-off [7.3%, 95% confidence intervals (CI) = 4.5-11.7, k = 10] than symptom cluster (11.2%, 95% CI = 8.7-14.4, k = 9). Studies using the Structured Clinical Interview for Diagnostic and Statistical Manual, Fourth Edition (SCID), yielded low rates for lifetime (15.3%, 95% CI = 9.1-25, k = 5) and current CR-PTSD (5.1%, 95% CI = 2.8-8.9, k = 9). Between-study heterogeneity was substantial (I(2)  = 54-87%). Studies with advanced-stage samples yielded significantly higher rates with PTSD Checklist-Civilian Version cluster scoring (p = 0.05), and when assessing current CR-PTSD on the SCID (p = 0.05). The effect of mean age on current PTSD prevalence met significance on the SCID (p = 0.05). SCID lifetime prevalence rates decreased with time post-treatment (R(2)  = 0.56, p < 0.05).

DISCUSSION

The cancer experience is sufficiently traumatic to induce PTSD in a minority of cancer survivors. Post-hoc analyses suggest that those who are younger, are diagnosed with more advanced disease and recently completed treatment may be at greater risk of PTSD. More research is needed to investigate vulnerability factors for PTSD in cancer survivors. © 2014 The Authors. Psycho-Oncology published by John Wiley & Sons Ltd.

The harmonization of cardiac troponin I measurement is independent of sample time collection but is dependent on the source of calibrator

We compared the agreement of cardiac troponin I (cTnI) values between seven cTnI assay systems in clinical samples collected from 1 h up to 11 days post acute coronary syndrome (ACS), before and after correction for calibration differences between systems. Values were referenced to two sources of common calibrator, namely, a manufactured cardiac troponin material containing the ternary ICT complex, and a serum from an ACS-positive subject. In 38 samples, cTnI varied up to sevenfold between systems, the among-systems coefficient of variation ranging from 49% to 127%. After correction for method calibration differences by reference to the cTnICT material, the inter-method variation was 43% to 99% compared with 13% to 68% for the serum-based material. Differences in cTnI isoform composition between the materials, or matrix interactions for the cTnICT calibrator, might contribute to the variation in harmonization effect. Despite the improved comparability of test values by the use of the serum-based calibrator, there was a residual variation in test values between systems that was not attributable to the time of collection of the sample post ACS. We concluded that due to the heterogeneity of clinical samples, use of a pooled human serum material might better serve as a secondary reference reagent for cTnI measurement.

Tricyclic drugs for depression in children and adolescents

BACKGROUND

There is a need to identify effective and safe treatments for depression in children and adolescents. While tricyclic drugs are effective in treating depression in adults, individual studies involving children and adolescents have been equivocal.

OBJECTIVES

To assess the effects of oral tricyclic antidepressants compared to placebo in the treatment of child and adolescent depression.

SEARCH STRATEGY

We searched MEDLINE (1966-1997), EMBASE, Excerpta Medica (June 1974-1997), the Cochrane Collaboration Depression, Anxiety and Neurosis Group trials register (most recent search 25/1/2000) and bibliographies of previously published reviews and papers describing original research were cross-checked. Current Contents was screened for recent publications. We contacted authors of relevant abstracts in conference proceedings of the American Academy of Child and Adolescent Psychiatry, and we hand searched the Journal of the American Academy of Child and Adolescent Psychiatry (1978-1999).

SELECTION CRITERIA

Randomised controlled trials comparing the efficacy of orally administered tricyclic medication with placebo in depressed people aged 6-18 years.

DATA COLLECTION AND ANALYSIS

Most studies reported multiple outcome measures including depression scales and clinical global impression scales. For each study the best available depression measure was taken as the index measure of depression outcome. Predetermined criteria were established to assist in the ranking of measures. Where authors reported categorical outcomes we calculated individual and pooled odds ratios for the odds of improvement in treated compared with control subjects. For continuous outcomes pooled effect sizes were calculated as the number of standard deviations by which the change in depression scores for the treatment group exceeded those for the control groups.

MAIN RESULTS

Thirteen trials (involving 506 participants) were included. No overall improvement with treatment compared to placebo was seen for children or adolescents (odds ratio = 0.84, 95% confidence interval 0.56 to 1.25). A statistically significant but small benefit of treatment over placebo was seen in reducing symptoms (effect size (standardised mean difference) = -0.31, 95% confidence interval -0.62 to -0.01). Subgroup analyses suggest a larger benefit among adolescents (effect size = -0.47, 95% confidence interval -0.92 to -0.02), and no benefit among children (effect size = 0.15, 95% confidence interval -0.34 to 0.64). Treatment with a tricyclic antidepressant caused more vertigo (odds ratio = 4.38, 95% confidence interval 2.33 to 8.25), orthostatic hypotension (odds ratio = 6.78, 95% confidence interval 2.06 to 22.26), tremor (odds ratio 6.29, 95% confidence interval 1.78 to 22.17) and dry mouth (odds ratio = 5.17, 95% confidence interval 2.68 to 29.99) than did placebo, but no statistically significant difference was found for other possible adverse effects.

REVIEWER'S CONCLUSIONS

Data suggest tricyclic antidepressants are not useful in treating depression in pre pubertal children. There is marginal evidence to support the use of tricyclic antidepressants in the treatment of depression in adolescents, although the magnitude of effect is likely to be moderate at best.

The lack of standardization of cardiac troponin I assay systems

The lack of standardization of cardiac troponin I (cTnI) assay systems has meant an inability to compare absolute values between methods, and non-traceability of cTnI measurement in clinical laboratories. The present study tested whether a common calibrator for cTnI would harmonize values between different methods. Twenty fresh-frozen plasma samples and a common calibrator were analyzed in duplicate using the Dade Behring Stratus II, Chiron ACS:180, Abbott AxSYM, and Beckman Coulter Access immunoassay systems. Uncorrected cTnI values varied up to 60-fold between systems, and when correlated to the Stratus, linear regression slopes were 0.97 (ACS:180), 4.86 (AxSYM), and 0.03 (Access), verifying the differences in calibration. After correction for calibration differences by reference to the common calibrator, among-assay CV ranged from 2.7% to 55%, and was >25% for eight samples. However, the exclusion of Access results reduced the CV to 32% with only four outliers. The results show that comparable cTnI values between methods are possible by the use of a common calibrator. The lack of method harmonization for some samples may be due to non-equal antibody immunoreactivity of different plasma cTnI forms. The complete standardization of cTnI measurement requires both a secondary reference material and standardization of manufacturers' cTnI antibodies.

Gravitropic responses of the Avena coleoptile in space and on clinostats. II. Is reciprocity valid?

Experiments were undertaken to determine if the reciprocity rule is valid for gravitropic responses of oat coleoptiles in the acceleration region below 1 g. The rule predicts that the gravitropic response should be proportional to the product of the applied acceleration and the stimulation time. Seedlings were cultivated on 1 g centrifuges and transferred to test centrifuges to apply a transverse g-stimulation. Since responses occurred in microgravity, the uncertainties about the validity of clinostat simulation of weightlessness was avoided. Plants at two stages of coleoptile development were tested. Plant responses were obtained using time-lapse video recordings that were analyzed after the flight. Stimulus intensities and durations were varied and ranged from 0.1 to 1.0 g and from 2 to 130 min, respectively. For threshold g-doses the reciprocity rule was obeyed. The threshold dose was of the order of 55 g s and 120 g s, respectively, for two groups of plants investigated. Reciprocity was studied also at bending responses which are from just above the detectable level to about 10 degrees. The validity of the rule could not be confirmed for higher g-doses, chiefly because the data were more variable. It was investigated whether the uniformity of the overall response data increased when the gravitropic dose was defined as (gm x t) with m-values different from unity. This was not the case and the reciprocity concept is, therefore, valid also in the hypogravity region. The concept of gravitropic dose, the product of the transverse acceleration and the stimulation time, is also well-defined in the acceleration region studied. With the same hardware, tests were done on earth where responses occurred on clinostats. The results did not contradict the reciprocity rule but scatter in the data was large.

Efficacy of tricyclic drugs in treating child and adolescent depression: a meta-analysis

OBJECTIVE

To examine whether tricyclic antidepressants are superior to placebo in the treatment of child and adolescent depression.

DESIGN

Meta-analysis of 12 randomised controlled trials comparing the efficacy of tricyclic antidepressants with placebo in depressed subjects aged 6-18 years.

MAIN OUTCOME MEASURES

Most studies employed several depression rating scales. For each study the "best available" measure was chosen by using objective criteria, and individual and pooled effect sizes were calculated as the number of standard deviations by which the change scores for the treatment groups exceeded those for the control groups. Where authors had reported numbers "responding" to treatment we calculated individual and pooled ratios for the odds of improvement in treated compared with control subjects.

RESULTS

From the six studies presenting data which enabled an estimation of effect size the pooled effect size was 0.35 standard deviations (95% confidence interval of -0.16 to 0.86) indicating no significant benefit of treatment. From the five studies presenting data on the number of "responders" in each group, the ratio of the odds of a response in the treated compared with the control subjects was calculated and the pooled odds ratio was 1.08 (95% confidence interval of 0.53 to 2.17); again indicating no significant benefit of treatment. The pooled sample had more than an 80% chance of detecting a treatment effect of 0.5 standard deviations or greater. There was an inverse relation between study quality and estimated treatment effect.

CONCLUSIONS

Tricyclic antidepressants appear to be no more effective than placebo in the treatment of depression in children and adolescents.