Werner Jakubetz

Theory of optimal laser pulses for selective transitions between molecular eigenstates

Abstract We present a new theoretical approach to optimal laser pulses designed for selective transitions between molecular eigenstates, within the semiclassical dipole approximation. Optimization proceeds iteratively, starting from a rather arbitrary, e.g. continuous wave, reference and yielding efficient modulations of the laser field intensities, frequencies, and phases. The method is verified by successful application to state-selective vibrational excitation of a Morse oscillator representing the OH bond in H 2 O by a subpicosecond laser pulse. The present theory is also compared with complementary approaches to optimal laser pulses inducing selective dissociations or excitations to bond-selective stretches in model polymers.

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Abstract Potential curves for proton transfer in [H 5 O 2 ] + and for the dissociation of one OH bond in [H 3 O] + were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H 5 O 2 ] + is very sensitive to electron correlation. At an OO distance of 2.74 A it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H 5 O 2 ] + and for the dissociation of one OH bond in [H 3 O] + . In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.

A simulation of ultrafast state-selective IR-laser-controlled isomerization of hydrogen cyanide based on global 3D ab initio potential and dipole surfaces

Abstract An ultrafast state-selective laser-controlled pump-dump scheme proceeding in the electronic ground state is simulated for HCN → HNC isomerization. The simulation is based on global 3D ab initio electronic ground state potential and dipole surfaces. The dipole surface obtained as part of the present work is a fit to 2010 single-reference AQCC data points. Isomerization dynamics including all three vibrational degrees of freedom is treated within the J = 0 vibrational manifold. Up to 550 J = 0 vibrational states previously reported by Bowman et al. [ J. Chem. Phys. 99 (1993) 308] are employed to obtain converged results. The laser polarization is fixed along the CN axis and molecular rotation is disregarded. Isomerization is initiated from HCN in its ( J = 0) vibrational ground sate, and control is exerted by a pulse sequence which splits the overall process into a sequence of state-specific sub-transitions. The intermediate states are chosen from a least-cost isomerization ladder obtained from an artificial intelligence algorithm, and include excited HCN bend states and a delocalized vibrational state above the isomerization barrier. We demonstrate that the molecule can be prepared in a specified HNC bend state with high overall selectivity (> 92%) and without concomitant ionization or dissociation, on a picosecond timescale using 4 or 5 sequential mid-infrared Gaussian pulses.

Studying vibrational wavepacket dynamics by measuring fluorescence interference fluctuations

The principle of coherence observation by interference noise [COIN, Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)] has been applied as a new approach to measuring wavepacket motion. In the COIN experiment pairs of phase-randomized femtosecond pulses with relative delay time τ prepare interference fluctuations in the excited state population, so the correlated noise of fluorescence intensity—the variance varF(τ)—directly mimics the dynamics of the propagating wavepacket. The scheme is demonstrated by measuring the vibrational coherence of wavepacket motion in the B-state of gaseous iodine. The COIN interferograms obtained recover propagation, recurrences and spreading as the typical signature of wavepackets. The COIN measurements were performed with precisely tuned excitation pulses which cover the bound part of the B-state surface up to the dissociative limit. In combination with preliminary numerical calculations, comparison has been made with results from previous phase-locked wavepacket interferometr...

Molecular quantum dynamics in a thermal system: Fractional wave packet revivals probed by random-phase fluorescence interferometry

The method of coherence observation by interference noise (COIN) [Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)] has been shown to be a useful tool for measurements of wave packet motion at the quantum-classical border. We present the first systematic interferometric study of fractional vibrational revivals in the B state of thermal iodine (I2) vapor. Experimental COIN interferograms ranging from 200 fs to 40 ps are presented for various excitation wavelengths. The complex temporal structure of the observed fluorescence includes rapid initial damping in the short-time regime and the appearance of quarter- and half-revivals on the quantum-mechanical long-time scale. These features arise from a delicate balance between rotational and vibrational molecular coherences. The clear observation of the wave packets on the long time scale is possible due to the long-time stability of the COIN interferometer. Lowest-order perturbative solutions nicely recover the experimental results, and closed-form analytical ex...

Uni- and bimodal product energy distributions for the reactions H + Cl2 (υ = 1) and D + Cl2 (υ = 1)

Abstract Vibrotational reaction probabilities for the reactions H + Cl 2 (υ = 1) and D + Cl 2 (υ = 1) at translational energies close to 0.1 eV are predicted to be bimodal, with the D reaction showing the more pronounced bimodality. The corresponding rotational and vibrational product distributions may be either uni- or bimodal. The results are from accurate quantum calculations for the collinear reactions, together with an information theoretic 1D → 3D transformation. An optimized, extended LEPS surface is used.

On the information theoretic synthesis of three dimensional vibrotational reaction probabilities from collinear results

Abstract A modified version of the Bernstein—Levine collinear to three dimensional (1D→3D) transformation is described in detail and tested. As input data, the procedure requires 1D reaction probabilities and a preassigned value of the average fraction of rotational product energy. We obtain these quantities from accurate quantum and classical trajectory calculations respectively and compare the 1D→3D vibrotational reaction probabilities with those from the trajectory calculations. We consider first the H + F 2 → HF + F reaction, which previous work suggests is a favourable case. The procedure is then applied to the F + H 2 → HF + H reaction at various fixed energies. It is found that the 1D→3D transformation is useful at low energies but becomes unreliable at higher energies. This may be because bent configurations play a more important role as the energy increases. The significance of almost linear surprisal plots is also discussed.

Molecular isomerization induced by ultrashort infrared pulses. I. Few-cycle to sub-one-cycle Gaussian pulses and the role of the carrier-envelope phase

Using 550 previously calculated vibrational energy levels and dipole moments we performed simulations of the HCN-->HNC isomerization dynamics induced by sub-one-cycle and few-cycle IR pulses, which we represent as Gaussian pulses with 0.25-2 optical cycles in the pulse width. Starting from vibrationally pre-excited states, isomerization probabilities of up to 50% are obtained for optimized pulses. With decreasing number of optical cycles a strong dependence on the carrier-envelope phase (CEP) emerges. Although the optimized pulse parameters change significantly with the number of optical cycles, the distortion by the Gaussian envelope produces nearly equal fields, with a positive lobe followed by a negative one. The positions and areas of the lobes are also almost unchanged, irrespective of the number of cycles in the half-width. Isomerization proceeds via a pump-dumplike mechanism induced by the sequential lobes. The first lobe prepares a wave packet incorporating many delocalized states above the barrier. It is the motion of this wave packet across the barrier, which determines the timing of the pump and dump lobes. The role of the pulse parameters, and in particular of the CEP, is to produce the correct lobe sequence, size and timing within a continuous pulse.

The Reaction X + Cl2→XCl + Cl (X = Mu, H, D). II. Comparison of experimental data with theoretical results derived from a new potential energy surface

We consider experimental implications for the Mu + Cl2, H + Cl2, and D + Cl2 reactions of the extended London—Eyring—Polanyi—Sato (LEPS) potential energy surface derived from experimental data in paper I. In the present calculations, it is necessary to make additional implicit and explicit assumptions concerning the three-dimensional (3D) nature of the potential surface, since the inversion procedure of paper I yields information only on the collinear (1D) part of the surface. We have performed accurate 1D quantum calculations of reaction probabilities, which are then transformed into 3D by an information theoretic 1D → 3D transformation incorporating a constraint to allow for angular momentum transfer effects in light+heavy—heavy atom reactions. This procedure implicitly accounts for the 3D nature of the potential surface. The calculated vibrational and vibrotational product distributions are in good agreement with those determined in thermal chemiluminescence experiments. The Sato parameters for the 1D surface also define a full 3D surface. This is used as an approximation to the true surface, and its properties are explored in 3D quasiclassical trajectory calculations. Comparison is made for the H and D reactions with available chemiluminescence, molecular beam and kinetic experimental data for differential and total reaction cross sections, energy disposal, rate coefficients and Arrhenius parameters. Some kinetic isotope effects in the Mu, H, and D reactions are discussed using vibrationally adiabatic theory. Comparison is also made with results from other calculations in the literature for the H + Cl2 and D + Cl2 reactions.

Comparison of quasiclassical and quantum dynamics for resonance scattering in the Cl+HCl→ClH+Cl reaction

We present the results of quasiclassical trajectory (QCT) and quantum centrifugal sudden hyperspherical (CSH) scattering calculations for the Cl+HCl→ClH+Cl reaction using a semiempirical potential energy surface. In particular, we report state‐to‐state integral and differential cross sections in the vicinity of a transition state resonance that occurs at a total energy E of 0.642 eV. This resonance, which is labeled by the transition state quantum numbers (0,0,2), strongly perturbs the cross sections for the initial rovibrational state HCl(v=1, j=5), which was therefore considered in all our calculations. For E≥0.680 eV, which is well removed from the resonance energy, the QCT and CSH results are in good agreement, but for E near the resonance energy, important quantum effects are found in the integral cross sections, product state distributions, and differential cross sections. The CSH integral cross sections show smooth steplike increases for E≊0.642 eV, which are not seen in the QCT results. Associated...