M. W. Gealy

New molecular‐dissociation furnace for H and O atom sources

A new type of molecular‐dissociation furnace has been developed for producing thermal beams of H and O atoms to be used as targets for fast atomic projectiles in various scattering experiments. Target‐particle densities in the beam from the furnace of about 1011 cm−3 have been achieved with molecular‐dissociation fractions in excess of 0.7 for each species. The techniques used are described and compared with more traditional furnace designs.

Cross sections for electron capture and loss. II. H impact on H and H/sub 2/

The electron-capture and electron-loss reactions for H impact on H and H/sub 2/ have been examined for projectile energies between 2.0 and about 0.1 keV. Relative cross sections for these reactions were measured directly, and the data for H targets were placed on an absolute scale by normalizing to the available results for H/sub 2/ targets. For the electron-capture (ion-pair formation) reaction, some new data are also reported here for H/sub 2/ targets. The crossed-beam techniques used to accomplish the measurements are described, and the results are compared with other experimental and theoretical data where possible.

Two-dimensional electronic spectroscopy using incoherent light: theoretical analysis.

Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.

Halogen Bonding in Iodo-perfluoroalkane/Pyridine Mixtures

Mole fraction and temperature studies of halogen bonding between 1-iodo-perfluorobutane, 1-iodo-perfluorohexane, or 2-iodo-perfluoropropane and pyridine were performed using noisy light-based coherent anti-Stokes Raman scattering (I((2)) CARS) spectroscopy. The ring breathing mode of pyridine both is highly sensitive to halogen bonding and provides a strong I((2)) CARS signal. As the lone pair electrons from the pyridinyl nitrogen interact with the sigma-hole on the iodine from the iodo-perfluoroalkane, the ring breathing mode of pyridine blue-shifts proportionately with the strength of the interaction. The measured blue shift for halogen bonding of pyridine and all three iodo-perfluoroalkanes is comparable to that for hydrogen bonding between pyridine and water. 2-Iodo-perfluoropropane displays thermodynamic behavior that is different from that of the 1-iodo-perfluoroalkanes, which suggests a fundamental difference at the molecular level. A potential explanation of this difference is offered and discussed.

Ion-Pair Interaction in Pyridinium Carboxylate Solutions

Ion-pair interactions between pyridinium cations and various carboxylate anions are explored using noisy light based coherent anti-Stokes Raman scattering (I(2)CARS). Binary mixtures of pyridine and various carboxylic acids (including halo-acetic acids, straight-chain carboxylic acids, and pivalic acid) are prepared. A Brønsted type acid-base reaction occurs in these mixtures to create pyridinium and carboxylate ions. Both pyridine, itself, and pyridinium have strong I(2)CARS signals originating from their ring breathing modes. The vibrational frequency of the ring breathing mode for pyridine is blue-shifted by hydrogen bonding, and that same mode for pyridinium is red-shifted by ion-pair interaction. Frequency shift data for the ring breathing mode of pyridine and pyridinium are presented. These data are discussed in terms of a simplistic model for the electronic behavior of these compounds.

Complete cancellation of noise by means of color-locking in nearly degenerate, four-wave mixing of noisy light

A nearly degenerate, four-wave mixing (ND4WM) setup with noisy light called I^(2)ND4WM(-) is investigated both experimentally and theoretically through factorized-time-correlation (FTC) diagram analysis. This third order nonlinear signal is of interest because the phenomenon of color-locking appears very directly in the measured signal. Color locking is a fundamental aspect of noisy-light spectroscopies that often plays a critical but subtle role in production of the signal. For I^(2)ND4WM(-), color-locking results in complete cancellation of the spectrally broad noise carried by the noisy light used to create it. When combined with FTC diagram analysis the qualitative features of the I^(2)ND4WM(-) can be understood. In addition to the striking noise cancellation, the qualitative features addressed in this work include the time profile, dynamic range, and polarization dependence of the signal. For comparison the spectrally broad I^(2)ND4WM(+) signal is presented.

Ejected-electron spectra in proton-atomic hydrogen collisions

The fundamental process of ionization in the collisions of protons with atomic hydrogen is considered at an impact energy of 70 keV by comparing experimental and theoretical doubly differential cross sections for electron ejection. This collision system provides a critical test of intermediate energy theories since effects such as electron-electron interaction (correlation) are not present and the roles of the ionization mechanism and subsequent evolution of the ejected electron in the two-centre Coulomb field are isolated. Results of the classical-trajectory Monte Carlo technique, the continuum distorted wave-eikonal initial state method and the plane wave Born approximation are compared with the present experimental data which are the first differential measurements that have been performed for the ionization of atomic hydrogen by proton impact.

Use of an Accelerator in Undergraduate Modern Physics Courses at Concordia College

In the physics department at Concordia College, Moorhead, MN there are three standard experiments in the undergraduate curriculum for which a 350kV Cockroft‐Walton ion accelerator is used. These include 1) Rutherford scattering of protons by thin metallic foils, 2) the nuclear reaction of protons with 12C to produce 13N + γ and 3) reaction of protons with 7Li to produce two α particles detected in coincidence. This paper will present typical data from these experiments and describe the fundamental principles and techniques demonstrated in each experiment.

Zero-degree auger electron spectroscopy of quasi-free electrons scattered by highly charged ions

Ion-atom collisions can be used to determine singly differential electron scattering cross-sections for the direct and resonant, elastic and inelastic electron-ion collisions. A new high-resolution tandem parallel-plate electron spectrometer system capable of sub eV resolution has been designed and constructed. This spectrometer has been used to analyze electrons up to 12 keV in energy. An estimated analysis limit of 21 keV has been determined, which will allow the measurement of the 2p2 resonances up to Z=22. We will report the results of direct and resonant elastic scattering studies for hydrogen-like F, Mg, and Si ions.