Thomas A. Holme

Vibrational energy flow into a reactive coordinate: A theoretical prototype for a chemical system

A realistic model is developed to illustrate vibrational energy flow into a reactive coordinate. The isomerization of C≡N–H to H–C≡N is enhanced nonstatistically through overtone excitations of the H–N bond. The mechanism determined for this theoretical model is dominated by resonant energy transfer from the anharmonically suppressed H–N mode into a combination of the C≡N plus the bending reaction coordinate. Implications of this classical prototype to mode specific chemistry, in particular the prediction of possible quantum tunnelling enhancement, are discussed in terms of experimentally observable systems.

Theoretical and computational studies of highly vibrationally excited acetylene

A consistent interpretation of the intramolecular vibrational dynamics of highly excited acetylene on the ground potential energy surface is proposed. Classical trajectory computations in the time domain (and their Fourier transform) and quantum-algebraic computations of the spectrum provide a picture which is in accord with the results of an analysis of the experimental stimulated emission pumping. A separation of time scales suggested by the algebraic Hamiltonian is verified in the classical simulations. There are four distinct, nested relaxation stages following the initial state preparation, each corresponding to energy exchange amongst a bigger subset of states. Initially (<1 ps) only the two CH stretch motions are effectively coupled. It is suggested that the two CH quanta states (which are primarily local mode in character) are responsible for the broad features in the spectra at ≈26000 cm−1. Next the CH stretches and the (Franck-Condon) excited CC stretch share energy. This stage is yet to be identified in the spectrum. By ≈ 10 ps, the CH trans bend excitation is also fully coupled, resulting in the “clumps” in the experimental spectrum. Finally, by about 15 ps there is complete relaxation corresponding to a concerted orbiting motion of the H atoms about the CC bond with a wide frequency band centered at ≈6 cm−1.

Enhancing the role of assessment in curriculum reform in chemistry

The role of assessment in the chemistry classroom is ultimately tied to the nature of the assessments available for use. Because they provide data that can inform decisions about curricular changes, or new pedagogies, the incorporation of new assessment strategies can play an important role in how educational and curriculum reform is carried out. Several recent developments in assessment have been tied together to investigate the benefits of using multiple assessment strategies in decision making about teaching innovation. These new tools include measures of student problem solving, metacognition, cognitive development within the chemistry content at the college level and evaluation of students in affective aspects of learning. Summaries of how these new tools may be combined and what measures arise from such combinations are presented.

Dynamics of overtone excitation processes for a polyatomic model

The excitation dynamics of a polyatomic system are presented in terms of both classical and quantum dynamics. These studies show that classical mechanics is qualitatively inaccurate in its description of the overtone excitation process. The quantum excitation is therefore an example of dynamic tunneling, which like most tunneling processes is slow. In a polyatomic molecule, intramolecular vibrational relaxation occurs much faster than does excitation, by several orders of magnitude. The disparity of time scales results in the excitation being very eigenstate specific in the small to medium molecule limit. We discuss the importance of these observations to photochemical experiments (possible preparation of long‐lived localized excitations) and to theoretical interpretations of molecular wave functions (possible interference effects in the molecular state function).

Energy flow pathways and their spectral signatures in vibrationally excited acetylene

Algebraic methods and classical trajectory studies for a model Hamiltonian which includes all in‐plane vibrational modes are used to elucidate the origin of spectral structure in the stimulated emission spectra of acetylene. The molecular oscillators responsible for previously established gateway states are determined as well as a plausible description of the overall IVR mechanism.

A theory for long-lived high-energy excitation of a local mode using two lasers

Abstract Quantum-mechanical calculations on a realistic system show that excitation of a local mode may be accomplished by using two lasers. More importantly, this local mode does not decay into the bath modes of the molecule even at very long times. This excitation is accomplished using two lasers tuned to two eigenstates whose wavefunctions interfere to give the local excitation. Most significantly, this observation for a model system implies that mode specific control of chemical reactions may be accomplished using two lasers to excite two overtone transitions.

A theoretical application of coherent multicolor laser spectroscopy to selective control of singlet and triplet excitations in carbon monosulfide

A theoretical proposal for preparation of chemically interesting coherent superpositions of molecular eigenstates using more than one laser was recently presented [T. A. Holme and J. S. Hutchinson, Chem. Phys. Lett. 124, 181 (1986)]. In this paper, the proposed coherent two‐color excitation process is applied to the separation and selective control of excitations of singlet and triplet manifolds. The method is applied to diatomic CS, using experimentally derived vibronic states and spin‐orbit couplings, and realistic laser intensities. We show that frequency tuning of two lasers can yield control of branching ratios between different excited triplet manifolds. We also generalize the previous two‐laser proposal to multicolor excitations, and illustrate the extension with a three‐laser highly selective excitation of a singlet vibronic state.

Biochemistry Instructors' Views toward Developing and Assessing Visual Literacy in Their Courses

Biochemistry instructors are inundated with various representations from which to choose to depict biochemical phenomena. Because of the immense amount of visual know-how needed to be an expert biochemist in the 21st century, there have been calls for instructors to develop biochemistry students’ visual literacy. However, visual literacy has multiple aspects, and determining which area to develop can be quite daunting. Therefore, the goals of this study were to determine what visual literacy skills biochemistry instructors deem to be most important and how instructors develop and assess visual literacy skills in their biochemistry courses. In order to address these goals, a needs assessment was administered to a national sample of biochemistry faculty at four-year colleges and universities. Based on the results of the survey, a cluster analysis was conducted to group instructors into categories based on how they intended to develop visual literacy in their courses. A misalignment was found between the vis...

Short-time vibrational dynamics of acetylene versus its isotopic variants

Abstract The short-time intramolecular dynamics of highly vibrationally excited HCCD and DCCD, as determined by classical trajectories, have qualitative features distinct from HCCH and from one another. The possible differences are also considered from the point of view of the symmetries of the normal modes. The short-time evolution will be reflected in the coarse-grained frequency spectrum and could be detectable via stimulation emission pumping.

A test of density functional theory for dative bonding systems

Abstract Density functional calculations are carried out for the H3B—NH3 system and compared with experimental and ab initio theoretical data. Calculations that use non-local exchange-correlation potentials are capable of providing excellent geometries, dative bond energies and dipole moments, as well as a adequate estimates of vibrational frequencies and barriers to internal rotations. The local density approximation can provide reasonable structure information, but significantly overestimates the dative bond energy, without perturbative non-local corrections.