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The dynamics of cyclopentadiene (CP) following optical excitation at 243 nm was investigated by time-resolved pump–probe X-ray scattering using 16.2 keV X-rays at the Linac Coherent Light Source (LCLS). We present the first ultrafast structural evidence that the reaction leads directly to the formation of bicyclo[2.1.0]pentene (BP), a strained molecule with three- and four-membered rings. The bicyclic compound decays viaa thermal backreaction to the vibrationally hot CP with a time constant of 21 ± 3 ps. A minor channel leads to ring-opened structures on a subpicosecond time scale.

The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.

The ability to communicate scientific concepts to expert and nonexpert audiences is an essential skill for chemistry and STEM students. Current chemistry curricula mainly focus on developing skills to communicate technical information to expert audiences, while relaying the same information to general audiences receives much less attention. Although numerous initiatives work to close this gap, many have logistical or financial barriers that make them difficult to integrate in a large classroom setting. Herein, we present an assignment focused on improving advanced students’ (i.e., graduate and advanced undergraduate students) ability to communicate current organometallic chemistry research (i.e., technical information) to a general audience using infographics. Our assignment features a unique, iterative feedback model incorporating instructor, peer, and near-peer (general audience) groups to provide students with multiple opportunities to refine their communication skills. Anonymous student self-assessments of advanced undergraduate/graduate students (infographic creators and peer reviewers) and first year, non-major undergraduate students (near-peer reviewers and general audience) indicate that the assignment led to (i) increased confidence in communication skills (advanced undergraduate/graduate students), (ii) a broadened understanding of advanced chemistry in everyday life (both), and (iii) increased recognition for the importance of scientific communication to different audiences (both). Reflections on student outcomes as well as recommendations and considerations for instructors are discussed.

We present a method to calculate total X-ray scattering cross sections directly from ab initioelectronic wave functions in atoms and molecules. The approach can be used in conjunction with multiconfigurational wave functions and exploits analytical integrals of Gaussian-type functions over the scattering operator, which leads to accurate and efficient calculations. The results are validated by comparison to experimental results and previous theory for the molecules H2and CO2. Importantly, we find that the inelastic component of the total scattering varies strongly with molecular geometry. The method is appropriate for use in conjunction with quantum molecular dynamics simulations for the analysis of new ultrafast X-ray scattering experiments and to interpret accurate gas-phase scattering experiments.

A technique for the spatially resolved and molecule-specific detection of chemical vapors is presented. The chemical specificity arises from a transient absorption spectrum where an ultraviolet (UV) pulse excites the molecule to a Rydberg state, and a near-infrared (NIR) or visible probe pulse records a transient absorption spectrum. By recording the NIR pulse reflected off a random, distant object and measuring the elapsed time between the emission of the UV pulse and the absorption of a counter-propagating NIR pulse, the distance to the absorber is obtained. The feasibility of the approach is demonstrated by detecting acetone plumes with millimeter scale spatial resolution.

To study transition structures that are formed in real time during a chemical reaction process, it is necessary to use ultrafast methods to follow the structural dynamics of molecular systems. For many decades, optical methods have been used to study the electronic states and time course giving rise to structural intermediates of chemical reactions with an ever higher time resolution. Although in some cases optical methods show many details, ultimately the results of these observations give only indirect information about the structure of the chemical reaction intermediates. Experimental observation of the behaviour of matter in the space-time continuum on ultrashort time scale is the necessary first step to explain and, subsequently, to control the nonequilibrium processes and functionality of the systems under study, to trace the relationship between the elements of the triad 'Structure - Dynamics - Function' to fully understand material properties. The results of these studies provide the necessary new information for testing theoretical approaches to the description of nonequilibrium chemical dynamics of molecular systems. The study of the time sequence of ultrafast processes occurring during the evolution of intermediate structures in the course of chemical reactions requires the integration of information that can be obtained by using complementary diffraction and spectroscopic methods based on various physical phenomena. Integration of data from ultrafast diffraction and spectroscopy makes it possible to investigate timescales prior to the onset of dissipation in which the coherent dynamics of matter can be observed. The use of quantum chemical calculations has reached a level of sophistication that makes it possible to explain the results of these experimental studies, the features of nonadiabatic behaviour of intermediate structures, and transition states of chemical reactions of molecular systems. This review analyzes the achievements of this rapidly developing field of modern chemistry, femtochemistry, to observe the primary events directing chemistry.The bibliography includes 580 references.

We present a technique for standoff trace chemical sensing that is based on the dependence of excited electronic state lifetimes on the amount of internal vibrational energy. The feasibility of the technique is demonstrated using N,N-dimethylisopropylamine (DMIPA). Time-resolved measurements show that the lifetime of the S_1 state in DMIPA exponentially decreases with the amount of vibrational energy. This property is employed to acquire molecular spectral signatures. Two laser pulses are used: one ionizes the molecule through the S_1 state; the other alters the S_1 state lifetime by depositing energy into vibrations. Reduction of the S_1 state lifetime decreases ionization efficiency that is observed by probing the laser-induced plasma with microwave radiation.

The effects of spectral shape on two photon fluorescence excitation are investigated experimentally using an acousto-optic pulse shaper to modify femtosecond pulses from a Ti:sapphire laser. By using different spectral window shapes, we find that the measured two photon efficiency can vary by a factor of 2 for differently shaped spectra with the same full width at half maximum. We find that these effects are described well by a simple model assuming transform-limited pulses. The fact that even small changes in the spectral wings can significantly affect the efficiency of nonlinear processes has implications for biological multiphoton imaging, where it may be desirable to minimize sample exposure to radiation and maximize fluorescence or harmonic efficiency. © 1999 Society of Photo-Optical Instrumentation Engineers.

Theoretical studies of the anisotropic distribution of atoms in the liquid/vapour interface have led to two conclusions. First, the density along the normal to the interface (the longitudinal one particle distribution function) has different forms in a liquid dielectric and a liquid metal. In a dielectric1, the density decreases monotonically as one passes from the liquid to the vapour, while in a metal2the liquid/vapour interface is stratified for about three atomic diameters into the bulk liquid, Fig. 1 shows examples. Second, the transverse structure factor at the surface of a liquid (the Fourier transform of the in-plane distribution of pairs of atoms) is very similar to the bulk liquid structure factor except near the origin, the difference arising from long-range correlations in the surface1. The very few experiments3,4that can be interpreted in terms of the properties of the density along the normal to the liquid surface are in agreement with the first set of predictions. There have been no published reports on the transverse structure factor in the liquid/vapour interface. Here we describe the transverse structure factor of the room temperature liquid/vapour interface of mercury, determined using the method of grazing incidence X-ray diffraction5. Our experimental results are consistent with the above predictions.

To study transition structures that are formed in real time during a chemical reaction process, it is necessary to use ultrafast methods to follow the structural dynamics of molecular systems. For many decades, optical methods have been used to study the electronic states and time course giving rise to structural intermediates of chemical reactions with an ever higher time resolution. Although in some cases optical methods show many details, ultimately the results of these observations give only indirect information about the structure of the chemical reaction intermediates. Experimental observation of the behaviour of matter in the space-time continuum on ultrashort time scale is the necessary first step to explain and, subsequently, to control the nonequilibrium processes and functionality of the systems under study, to trace the relationship between the elements of the triad ‘Structure – Dynamics – Function’ to fully understand material properties. The results of these studies provide the necessary new information for testing theoretical approaches to the description of nonequilibrium chemical dynamics of molecular systems. The study of the time sequence of ultrafast processes occurring during the evolution of intermediate structures in the course of chemical reactions requires the integration of information that can be obtained by using complementary diffraction and spectroscopic methods based on various physical phenomena. Integration of data from ultrafast diffraction and spectroscopy makes it possible to investigate timescales prior to the onset of dissipation in which the coherent dynamics of matter can be observed. The use of quantum chemical calculations has reached a level of sophistication that makes it possible to explain the results of these experimental studies, the features of nonadiabatic behaviour of intermediate structures, and transition states of chemical reactions of molecular systems. This review analyzes the achievements of this rapidly developing field of modern chemistry, femtochemistry, to observe the primary events directing chemistry.

Review: 74 refs.

For Abstract see ChemInform Abstract in Full Text.