Chris H. Greene

Determination of product population and alignment using laser‐induced fluorescence

For collision systems having axial symmetry, the anisotropy of the laser‐induced fluorescence is given explicitly in terms of the zeroth, second, and fourth rank moments of the angular momentum distribution, which, respectively, are proportional to the population, the quadrupole alignment, and hexadecapole alignment of the product internal state under study. Expressions are presented for determining these three quantities from the dependence of the fluorescence intensity on the polarizations of the absorbed and detected photons. Results are presented for an arbitrary excitation‐detection geometry which is then specialized to the commonly occurring cases where the direction of fluorescence detection is at right angles to the axis of cylindrical symmetry and the direction of the incoming light beam is either along the axis of cylindrical symmetry or at right angles to it and to the fluorescence detection direction. The approach of these expressions to the high‐J limit is considered. The effect of nuclear sp...

Lorentz meets Fano in spectral line shapes: a universal phase and its laser control.

A Phase for Fano In spectroscopy, samples placed between a steady light source and a detector are characterized based on the relative intensities of light absorbed at different frequencies. Temporal behavior—the relaxation of a photoexcited state—can be indirectly inferred from the absorption band shapes. The advent of ultrafast laser technology has enabled increasingly sophisticated measurements directly in the time domain. Ott et al. (p. 716; see the Perspective by Lin and Chu) present an analytical framework to account for asymmetric band shapes, termed Fano profiles, on the basis of a phase shift in the temporal dipole response. An analytical framework bolstered by attosecond spectroscopy conveys a clear understanding of asymmetric spectral line shapes. [Also see Perspective by Lin and Chu] Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase ϕ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.

Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio

Context: The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H2 ratio through the H3+ fractionation. Aims: We want to constrain the ortho/para H2 ratio across the L183 prestellar core. This is required to correctly describe the deuteration amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H2D+ abundance to the sole ortho-H2D+ column density measurement. Methods: To constrain this ortho/para H2 ratio and derive its profile, we make use of the N2D^+/N2H+ ratio and of the ortho-H2D+ observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models. Results: We estimate the H2D+ ortho/para ratio in the L183 cloud, and constrain the H2 ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (≈0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 × 105 years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained. Conclusions: A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H2 ratio should also place useful constraints on the dynamical evolution of prestellar cores. Appendices A and B are only available in electronic form at http://www.aanda.org

Mechanism for the destruction of H3+ ions by electron impact

The rate at which the simplest triatomic ion (H3+) dissociates following recombination with a low-energy electron has been measured in numerous experiments. This process is particularly important for understanding observations of H3+ in diffuse interstellar clouds. But, despite extensive efforts, no theoretical treatment has yet proved capable of predicting the measured dissociative recombination rates at low energy, even to within an order of magnitude. Here we show that the Jahn–Teller symmetry-distortion effect—almost universally neglected in the theoretical description of electron–molecule collisions—generates recombination at a much faster rate than any other known mechanism. Our estimated rate constant overlaps the range of values spanned by experiments. We treat the low-energy collision process as a curve-crossing problem, which was previously thought inapplicable to low-energy recombination in H3+. Our calculation reproduces the measured propensity for three-body versus two-body breakup of the neutral fragments, as well as the vibrational distribution of the H2 product molecules.

Low-energy electron scattering from DNA and RNA bases: Shape resonances and radiation damage

Calculations are carried out to determine elastic-scattering cross sections and resonance energies for low-energy electron impact on uracil and on each of the DNA bases (thymine, cytosine, adenine, and guanine), for isolated molecules in their equilibrium geometry. Our calculations are compared with the available theory and experiment. We also attempt to correlate this information with experimental dissociation patterns through an analysis of the temporary anion structures that are formed by electron capture in shape resonances.

Dissociative recombination of H3+ in the ground and excited vibrational states

The article presents calculated dissociative recombination (DR) rate coefficients for H3+. The previous theoretical work on H3+ was performed using the adiabatic hyperspherical approximation to calculate the target ion vibrational states and it considered just a limited number of ionic rotational states. In this study, we use accurate vibrational wave functions and a larger number of possible rotational states of the H3+ ground vibrational level. The DR rate coefficient obtained is found to agree better with the experimental data from storage ring experiments than the previous theoretical calculation. We present evidence that excited rotational states could be playing an important role in those experiments for collision energies above 10meV. The DR rate coefficients calculated separately for ortho- and para-H3+ are predicted to differ significantly at low energy, a result consistent with a recent experiment. We also present DR rate coefficients for vibrationally excited initial states of H3+, which are fo...

Monte Carlo hyperspherical description of helium cluster excited states

The J=0 many-body Schrodinger equation for 4HeN clusters with N=3–10 is solved numerically by combining Monte Carlo methods with the adiabatic hyperspherical approximation. We find ground state and excited state energies for these systems with an adiabatic separation scheme that reduces the problem to motion in a one-dimensional effective potential curve as a function of the hyperspherical radius R. We predict the number of J=0 bound states for these clusters, and also the He+HeN−1 elastic scattering lengths up to N=10. For N=5–10, these are the first such calculations reported.

Depolarization of optically prepared molecules by two randomly oriented spins

A general expression is developed for the time development of the alignment of an ensemble of isolated molecules having two random internal spins when the ensemble is optically pumped by absorption of a linearly polarized light pulse. This derivation assumes that the internal spin structure (fine structure/hyperfine structure) is coherently excited and is not resolved. As an example the pulsed infrared excitation of hydrogen fluoride is considered, in particular the vibration-rotation transition HF(ν″ = 0, N″) → HF(ν = 1, N). The presence of the nuclear spins I H = 1/2 and I F = 1/2 is found to cause a significant reduction in the degree of alignment of the low rotational levels but the extent of depolarization diminishes rapidly with increasing N.

Nature of spinor Bose-Einstein condensates in rubidium

We perform detailed close-coupling calculations for the rubidium isotopes

Xenon Clusters in Intense VUV Laser Fields

{}^{85}\mathrm{Rb}