Ignacio R. Sola

SHARC: ab Initio Molecular Dynamics with Surface Hopping in the Adiabatic Representation Including Arbitrary Couplings.

We present a semiclassical surface-hopping method which is able to treat arbitrary couplings in molecular systems including all degrees of freedom. A reformulation of the standard surface-hopping scheme in terms of a unitary transformation matrix allows for the description of interactions like spin-orbit coupling or transitions induced by laser fields. The accuracy of our method is demonstrated in two systems. The first one, consisting of two model electronic states, validates the semiclassical approach in the presence of an electric field. In the second one, the dynamics in the IBr molecule in the presence of spin-orbit coupling after laser excitation is investigated. Due to an avoided crossing that originates from spin-orbit coupling, IBr dissociates into two channels: I + Br((2)P3/2) and I + Br*((2)P1/2). In both systems, the obtained results are in very good agreement with those calculated from exact quantum dynamical simulations.

Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine.

Ab initio molecular dynamics including nonadiabatic and spin-orbit couplings on equal footing is used to unravel the deactivation of cytosine after UV light absorption. Intersystem crossing (ISC) is found to compete directly with internal conversion in tens of femtoseconds, thus making cytosine the organic compound with the fastest triplet population calculated so far. It is found that close degeneracy between singlet and triplet states can more than compensate for very small spin-orbit couplings, leading to efficient ISC. The femtosecond nature of the ISC process highlights its importance in photochemistry and challenges the conventional view that large singlet-triplet couplings are required for an efficient population flow into triplet states. These findings are important to understand DNA photostability and the photochemistry and dynamics of organic molecules in general.

Control of ultrafast molecular photodissociation by laser-field-induced potentials

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. Here we explore the transition from the weak- to the strong-field regimes of laser control for the dissociation of a polyatomic molecule, methyl iodide. The control over the yield of the photodissociation reaction proceeds through the creation of a light-induced conical intersection. The control of the velocity of the product fragments requires external fields with both high intensities and short durations. This is because the mechanism by which control is exerted involves modulating the potentials around the light-induced conical intersection, that is, creating light-induced potentials.

Separation of enantiomers by ultraviolet laser pulses in H2POSH: π pulses versus adiabatic transitions

Different strategies to separate enantiomers from a racemate using analytical laser pulses in the ultraviolet frequency domain are proposed for the prototype model system H2POSH. Wave-packet propagations on ab initio ground- and electronic-excited state potentials show that it is possible to produce 100% of enantiomeric excess in a sub-picosecond time scale using a sequence of π and half-π pulses. Alternatively, the previous transitions can be substituted by adiabatic counterparts, using chirped laser pulses and a half-STIRAP (stimulated Raman adiabatic passage) method which only transfers half of the population between appropriate levels. Such an overall adiabatic mechanism gains stability concerning the pulse areas and frequencies at the expense of introducing new control variables, like the chirp and time delay.

Transferring vibrational population between electronic states of diatomic molecules via light-induced-potential shaping

We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials (LIPs). Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses (intuitive or counter-intuitive) and on the sign of the detuning (below or above the first transition) four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of ...

Nonadiabatic ab initio molecular dynamics including spin–orbit coupling and laser fields

Nonadiabatic ab initio molecular dynamics (MD) including spin-orbit coupling (SOC) and laser fields is investigated as a general tool for studies of excited-state processes. Up to now, SOCs are not included in standard ab initio MD packages. Therefore, transitions to triplet states cannot be treated in a straightforward way. Nevertheless, triplet states play an important role in a large variety of systems and can now be treated within the given framework. The laser interaction is treated on a non-perturbative level that allows nonlinear effects like strong Stark shifts to be considered. As MD allows for the handling of many atoms, the interplay between triplet and singlet states of large molecular systems will be accessible. In order to test the method, IBr is taken as a model system, where SOC plays a crucial role for the shape of the potential curves and thus the dynamics. Moreover, the influence of the nonresonant dynamic Stark effect is considered. The latter is capable of controlling reaction barriers by electric fields in time-reversible conditions, and thus a control laser using this effect acts like a photonic catalyst. In the IBr molecule, the branching ratio at an avoided crossing, which arises from SOC, can be influenced.

Selective excitation of diatomic molecules by chirped laser pulses

A new method for the selective excitation of diatomic molecules in single vibrational states on excited electronic potentials by two-photon absorption is proposed. The method implies the use of two chirped strong pulse lasers detuned from the optical transition to an intermediate electronic state. We show under what scenarios the method is successful on the time–energy scale in which the pulses operate. They involved a long-time (nanosecond) weak-field regime and a short-time (picosecond) strong-field regime. The adiabatic representation in terms of energy levels or in terms of light-induced potentials is used to interpret the physical mechanism of the excitation. The efficiency and robustness of the scheme are demonstrated by the excitation of the ground vibrational state of the 1Σg(4s) electronic potential of the Na2 molecule.

Application of trajectory surface hopping to vibrational predissociation

Abstract The recent MDQT (molecular dynamics with quantum transitions) trajectory surface hopping technique developped by Tully is applied to vibrational predissociation of the Van der Waals complex Ar⋯I 2 . The vibration of I 2 is treated quantum mechanically while the relative motion of the Ar atom with respect to I 2 is treated classically. The results show that even for this extreme case where no actual crossings between quantum potential energy surfaces exist, the method provides satisfactorily results for the vibrational predissociation rates.

Mixed Quantum-Classical Dynamics in the Adiabatic Representation To Simulate Molecules Driven by Strong Laser Pulses

The dynamics of molecules under strong laser pulses is characterized by large Stark effects that modify and reshape the electronic potentials, known as laser-induced potentials (LIPs). If the time scale of the interaction is slow enough that the nuclear positions can adapt to these externally driven changes, the dynamics proceeds by adiabatic following, where the nuclei gain very little kinetic energy during the process. In this regime we show that the molecular dynamics can be simulated quite accurately by a semiclassical surface-hopping scheme formulated in the adiabatic representation. The nuclear motion is then influenced by the gradients of the laser-modified potentials, and nonadiabatic couplings are seen as transitions between the LIPs. As an example, we simulate the process of adiabatic passage by light induced potentials in Na(2) using the surface-hopping technique both in the diabatic representation based on molecular potentials and in the adiabatic representation based on LIPs, showing how the choice of the representation is crucial in reproducing the results obtained by exact quantum dynamical calculations.

Further aspects on the control of photodissociation in light-induced potentials

In this work we show how to control the photodissociation of a diatomic molecule in the frame of light-induced potentials for different shapes of the transition dipole moments. A sequence of a half-cycle or control pulse and a delayed pump pulse is used for achieving state-selective photodissociation with high yields. The effect of the control is to shift the photodissociation bands to higher frequencies. It is also possible to dissociate the molecule in a superposition of electronic states of the fragments, even when the photodissociation bands corresponding to the different electronic states of the products are largely separated. In this case one needs to engineer the sequence delaying the half-cycle pulse after the pump pulse and additionally turning off rapidly the control pulse. Depending on the shape of the dipole functions the duration of the pulses in the sequence must be constrained to shorter times as well. Finally we show that the control scheme affects the velocity of the fragments. Although broad kinetic energy distributions are always obtained when the half-cycle pulse is short, if the Stark effect implies a blueshifting in the energy of the electronic states, the distribution of the relative speed of the fragments will be redshifted.