530 research outputs found
Femtosecond wave packet spectroscopy: Coherences, the potential, and structural determination
Recently, we presented a formalism for extracting highly resolved spectral information and the potential of bound isolated systems from coherent ultrafast laser experiments, using I2 as a model system [Gruebele et al., Chem. Phys. Lett. 166, 459 (1990)]. The key to this approach is the formation of coherent wave packets on the potential energy curve (or surface) of interest, and the measurement of their scalar and vector properties. Here we give a full account of the method by analyzing the coherences of the wave packet in the temporal transients of molecules excited by ultrashort laser pulses, either at room temperature, or in a molecular beam. From this, some general considerations for properly treating temporal data can be derived. We also present a direct inversion to the potential and quantum and classical calculations for comparison with the experiments
Intramolecular vibrational energy redistribution as state space diffusion: Classical-quantum correspondence
We study the intramolecular vibrational energy redistribution (IVR) dynamics
of an effective spectroscopic Hamiltonian describing the four coupled high
frequency modes of CDBrClF. The IVR dynamics ensuing from nearly isoenergetic
zeroth-order states, an edge (overtone) and an interior (combination) state, is
studied from a state space diffusion perspective. A wavelet based
time-frequency analysis reveals an inhomogeneous phase space due to the
trapping of classical trajectories. Consequently the interior state has a
smaller effective IVR dimension as compared to the edge state.Comment: 5 pages, 3 figure
On Readout of Vibrational Qubits Using Quantum Beats
Readout of the final states of qubits is a crucial step towards implementing quantum computation in experiment. Although not scalable to large numbers of qubits per molecule, computational studies show that molecular vibrations could provide a significant (factor 2â5 in the literature) increase in the number of qubits compared to two-level systems. In this theoretical work, we explore the process of readout from vibrational qubits in thiophosgene molecule, SCCl2, using quantum beat oscillations. The quantum beats are measured by first exciting the superposition of the qubit-encoding vibrational states to the electronically excited readout state with variable time-delay pulses. The resulting oscillation of population of the readout state is then detected as a function of time delay. In principle, fitting the quantum beat signal by an analytical expression should allow extracting the values of probability amplitudes and the relative phases of the vibrational qubit states. However, we found that if this procedure is implemented using the standard analytic expression for quantum beats, a non-negligible phase error is obtained. We discuss the origin and properties of this phase error, and propose a new analytical expression to correct the phase error. The corrected expression fits the quantum beat signal very accurately, which may permit reading out the final state of vibrational qubits in experiments by combining the analytic fitting expression with numerical modelling of the readout process. The new expression is also useful as a simple model for fitting any quantum beat experiments where more accurate phase information is desired
Femtosecond probing of bimolecular reactions: The collision complex
Progress has been made in probing the femtosecond
dynamics of transition states of chemical reactions.(1) The
"half-collision" case of unimolecular reactions has been
experimentally investigated for a number of systems and
much theoretical work has already been developed.(2) For
bimolecular reactions, the case of full collision, the zero of
time is a problem which makes the femtosecond temporal
resolution of the dynamics a difficult task
Femtosecond real-time probing of reactions. VIII. The bimolecular reaction Br+I2
In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atomâmolecule Br+I_2 reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radicalâhalogen molecule exchange reactions
Ultrafast Reaction Dynamics
A decade ago this magazine devoted a special issue to laser chemistry (see PHYSICS TODAY, November 1980). One of the articles emphasized the importance of time scales in chemical reactions and the possible use of ultrashort lasser pulses to induce chemistry. Over the past 10 years new laser techniques, and gasâphase and molecularâbeam experiments, have revealed much about the fundamental steps of elementary chemical reactions. These approaches and the tremendous detail they have exposed about the dynamics of chemical reactions are the subject of the present article.
With new laser techniques and with gas phase and molecular beam experiments, it is now possible to determine the ultrafast motion in isolated chemical reactionsâchemistry on the 10^(â13)âsecond time scale
Ultrafast Reaction Dynamics
A decade ago this magazine devoted a special issue to laser chemistry (see PHYSICS TODAY, November 1980). One of the articles emphasized the importance of time scales in chemical reactions and the possible use of ultrashort lasser pulses to induce chemistry. Over the past 10 years new laser techniques, and gasâphase and molecularâbeam experiments, have revealed much about the fundamental steps of elementary chemical reactions. These approaches and the tremendous detail they have exposed about the dynamics of chemical reactions are the subject of the present article.
With new laser techniques and with gas phase and molecular beam experiments, it is now possible to determine the ultrafast motion in isolated chemical reactionsâchemistry on the 10^(â13)âsecond time scale
Understanding highly excited states via parametric variations
Highly excited vibrational states of an isolated molecule encode the
vibrational energy flow pathways in the molecule. Recent studies have had
spectacular success in understanding the nature of the excited states mainly
due to the extensive studies of the classical phase space structures and their
bifurcations. Such detailed classical-quantum correspondence studies are
presently limited to two or quasi two dimensional systems. One of the main
reasons for such a constraint has to do with the problem of visualization of
relevant objects like surface of sections and Wigner or Husimi distributions
associated with an eigenstate. This neccesiates various alternative techniques
which are more algebraic than geometric in nature. In this work we introduce
one such method based on parametric variation of the eigenvalues of a
Hamiltonian. It is shown that the level velocities are correlated with the
phase space nature of the corresponding eigenstates. A semiclassical expression
for the level velocities of a single resonance Hamiltonian is derived which
provides theoretical support for the correlation. We use the level velocities
to dynamically assign the highly excited states of a model spectroscopic
Hamiltonian in the mixed phase space regime. The effect of bifurcations on the
level velocities is briefly discussed using a recently proposed spectroscopic
Hamiltonian for the HCP molecule.Comment: 12 pages, 9 figures, submitted to J. Chem. Phy
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