Guy Ashkenazi

Classical/quantal method for multistate dynamics: A computational study

We discuss a classically‐motivated method for modeling ultrashort laser pulse optical excitation. The very same method can be used to treat the breakdown of the Born–Oppenheimer approximation. The results are compared to numerically‐exact quantum mechanics for a model problem representing excitation from the X (ground) state to the B (excited) state of molecular iodine. Expectation values and final B state populations are predicted quantitatively. The method provides a new way to simulate pump–probe experiments in particular and multistate dynamics in general. The method appears extendible to multidimensional problems. We argue that the increase of effort with dimensionality will be similar to that encountered in classical mechanical simulations as opposed to the exponential scaling of numerically‐exact quantum mechanical propagation techniques.

Newtonian propagation methods applied to the photodissociation dynamics of I3

A uniformly convergent propagation scheme designed for non‐hermitian Hamiltonian operators is presented. The method is based on a Newtonian interpolation polynomial which is created by a recursive application of the Hamiltonian operator on an initial wavefunction. The interpolation points used to construct the Newtonian polynomial are located in the complex eigenvalue space of the Hamiltonian. A new algorithm is developed to construct the interpolation points. Both time dependent and time independent quantities can be obtained using the same polynomial expansion. The method is particularly useful when negative imaginary potentials are used. The photodissociation dynamics of I3− is studied as an example of the utility of the scheme to gain insight on a dynamical encounter. The bond cleavage is followed in time simultaneously with the calculation of the Raman spectra. The study addresses the role of vibrational excitation of the reactant I3− on the nascent I2− spectral modulations and Raman spectra.

Connecting Levels of Representation: Emergent versus submergent perspective

Chemical phenomena can be described using three representation modes: macro, submicro, and symbolic. The way students use and connect these modes when solving conceptual problems was studied, using a think‐aloud interview protocol. The protocol was validated through interviews with six faculty members, and then applied to four graduate and six undergraduate chemistry students. We used a ‘levels of complexity’ framework to analyse responses: the macro and symbolic modes were considered system‐level representations, and the submicro mode a component‐level representation. We found that faculty members thought of system‐level properties as emerging from mechanistic interactions between particles on the component level—an emergent perspective. In many cases, the students either failed to connect the system and component levels, or thought of system‐level properties as guiding the behaviour of particles on the component level—a ‘submergent’ perspective. Some students used their familiarity with a symbolic equation describing the behaviour of a substance as the starting point of a thought process that leads them to impose mechanistically unwarrantable behaviour upon its particles. We concluded that a submergent perspective inhibits students from confronting their misconceptions regarding particle behaviour, and explains why students are often able to correctly solve algorithmic problems while failing to solve conceptual ones. It is suggested that the directionality of connecting particle behaviour to system‐level properties should be emphasized in teaching.

Interactive lecture demonstrations: a tool for exploring and enhancing conceptual change

Interactive Lecture Demonstrations (ILD) are a student centered teaching method, in which students are asked to predict the outcome of an experiment, observe the outcome, and discuss it with respect to their former expectations. The demonstrations are designed to contradict students’ known misconceptions, generate cognitive conflict and dissatisfaction with the existing conception, and promote a process of conceptual change. An ILD based course was used to explore the effect of cognitive conflict on the conceptual change process, and the role of student interactivity in this process. Three major levels of conceptual change were identified: high – students who remember the outcome of the demonstration, and explain it using the consensus model; medium – students who can recall the outcome, are dissatisfied with their alternative model, but do not switch to the consensus model; and low – no meaningful recollection of the outcome, and no change in the alternative model. A multiple-choice test based on the lecture demonstration was given to two groups, one of which only observed the demonstrations, without predicting and discussing. We found a significant difference between the groups, with an obvious drop in students’ ability to recall the outcome of the demonstrations in the non-interactive group. [Chem. Educ. Res. Pract., 2007, 8 (2), 197-211]

Using lecture demonstrations to promote the refinement of concepts: the case of teaching solvent miscibility

Novices often lack the descriptive knowledge of phenomena that is the basis for an expert’s interpretation of scientific concepts. Such lack of knowledge may lead to poor conceptual understanding, and misinterpretation of these concepts. Lecture demonstrations can provide essential experiences that serve as a context for discussion of over-generalized or over-simplified concepts. The design of such demonstrations starts from surveying the limited knowledge base of the student, followed by exploration of the richness of relevant contexts of the expert, and identifying key instances that can serve as meaningful discussion topics. An example of the design of a demonstration set for teaching solvent miscibility and its relation to intermolecular interactions is given, followed by results of its application in two different presentation modes: confrontation (aims at generating a conflict with existing conceptions) and refinement (aims at promoting differentiation and contextualization of scientific concepts). The students’ involvement in peer discussion, associated with these demonstrations, is evaluated by considering the distribution of students’ predictions. [Chem. Educ. Res. Pract., 2007, 8 (2), 186-196]

Photodissociation Dynamics of I− 3: Comparison of Ultrafast Pump Probe Experiments to Raman Scattering

When a UV light pulse is applied to I− 3 in solution, it either absorbs the light leading to fragmentation to I− 2 +I, or scatters the light usually as a Raman process. Since both the fragmentation and the Raman processes are governed by the same ground and excited potential energy surfaces, crossing the information gained from the two independent measurements leads to enhanced insight on the photodissociation event. A unified quantum computational scheme addresses simultaneously the dynamics of photodissociation and the absorption and Raman cross sections.