**Photodissociation dynamics of small van der Waals clusters**

** a) Direct photodissociation processes**

When the HX (X=halogen atom) chromophore is excited from the ground electronic state ^{1}Sigma^{+} to the A ^{1}Pi state (whose potential surface is repulsive) direct photodissociation of the molecule takes place. The HX molecules can interact weakly with rare gas atoms, forming complexes of the type Rg-HX binded by weak hydrogen bonds. It is interesting to explore how the direct photodissociation process may be affected by the presence of the Rg rare gas atom, in order to understand solvation effects in photodissociation dynamics. In this sense, the photodissociation of the Rg-HX clusters has been studied for different members of the family with Rg=Ar, Kr and X=F, Cl, Br, and I. By replacing Rg by a HX molecule leads to another interesting family of complexes: The (HX)_{2} dimers. Within this family, the photodissociation of the (HI)_{2 }dimer has been investigated. In this case a nonadiabatic photodissociation process takes place in addition to the adiabatic direct one, which makes the photolysis process richer in complexity and interest.

J.C. Juanes-Marcos and A. García-Vela, *An energy-resolved study of of the partial fragmentation dynamics of Ar-HCl into H + Ar-Cl after ultraviolet photodissociation*, J. Chem. Phys. **112**, 4983 (2000); S. López-López, R. Prosmiti, and A. García-Vela, *Nonadiabatic photodissociation dynamics in (HI) _{2 }induced by intracluster collisions*, J. Chem. Phys.

**126**, 161102 (2007).

** b) Indirect photodissociation: Vibrational predissociation
**

Indirect photodissociation mediated by resonances are fundamental processes in order to understand photodissociation dynamics. The van der Waals (vdW) complexes X

_{2}-Rgn (X=halogen atom, Rg=rare gas atom, n>=1)are prototypical systems that support resonance states and undergo indirect photodissociation. The study of the vibrational predissociation processes that take place in these clusters upon vibronic excitation of the X

_{2}chromophore allow one to understand the underlying mechanisms of intramolecular vibrational redistribution (IVR) that take place. Such processes have been investigated for several triatomic complexes X

_{2}-Rg with X=Cl, Br, and I, and Rg=He, Ne using exact time-dependent quantum mechanical (wave packet) methods. In addition, the vibrational predissociation of the tetraatomic Cl

_{2}-He

_{2}complex has been studied considering the full dimensionality of the system, by developing an exact wave packet treatment. In the tetraatomic clusters X

_{2}-Rg

_{2}the predissociation dynamics involves the sequential dissociation of the two vdW bonds, which makes the theoretical description of this indirect photodissociation process remarkably more complex than in the case of the triatomic X

_{2}-Rg systems.

A. García-Vela, *A full-dimensional quantum dynamical approach to the vibrational predissociation of Cl _{2}-He_{2}*, J. Chem. Phys.

**122**, 014312 (2005); A. García-Vela and K.C. Janda,

*Quantum dynamics of Ne-Br*, J. Chem. Phys.

_{2}vibrational predissociation: The role of continuum resonances as doorway states**124**, 034305 (2006).

**Quantum coherent control of the lifetime of excited resonance states by means of laser pulses**

The goal of this line is to investigate the quantum coherent control of the lifetime of a system in an excited resonance state by creating superpositions of overlapping resonances. The control strategy is based on the quantum interference mechanism occurring between overlapping resonances, which can be controlled by varying the population of the different resonances in the superposition created. Two types of control schemes have been applied to a realistic model of the Br_{2}-Ne predissociation decay dynamics described by a three-dimensional wave packet method. In one of the control schemes the population of the overlapping resonances in the superposition prepared is varied by changing the width of the excitation pulse used. In the second, more flexible control scheme, two excitation laser pulses are used, each of them exciting a single resonance overlapping with the other one. In this scheme control is exerted by varying two typically experimental parameters like the delay time and the ratio of intensities between the two pulses, which allows one to exert selective control of the population excited in the two overlapping resonances, and therefore of the interference between them. The two control schemes applied provide an extensive and selective degree of control on the lifetime of the overlapping resonances, particularly in the case of the scheme using two pulses.

A. García-Vela, *Active control of the lifetime of excited resonance states by means of laser pulses*, J. Chem. Phys. **136**, 134304 (2012); A. García-Vela, *Strong Enhancement of the lifetime of a resonance state by using a combination of two laser pulses*, J. Phys. Chem. Lett. **3**, 1941 (2012).

**Nonadiabatic photodissociation processes in polyatomic molecules**

Nonadiabatic effects play an important role in numerous photodissociation processes in polyatomic molecules, and therefore their study and understanding is of great interest. One of the most interesting molecules in order to investigate the influence of nonadiabatic processes in the fragmentation dynamics is CH_{3}I, which has been extensively studied both experimentally and theoretically. Most of the experiments, however, were energy-resolved ones. Time resolution, on the other hand, makes possible the study of the nonadiabatic photodissociation dynamics of CH_{3}I from a different perspective which provides additional and more detailed information, and requires to develop new theoretical models. Thus the goal of this line is the theoretical study of the CH_{3}I photodissociation dynamics from the time-resolved point of view. The work of this line is carried out in collaboration with the experimental group of Prof. Luis Bañares (U. Complutense de Madrid), where time-resolved pump-probe experiments on CH_{3}I photodissociation have been carried out using femtosecond laser pulses. One of the main motivations of the theoretical work developed is to understand and rationalize the data obtained in these time-resolved experiments.

R. de Nalda, J. Durá, A. García-Vela, J.G. Izquierdo, J. González-Vázquez, and L. Bañares, *A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH _{3}I,* J. Chem. Phys.

**128**, 244309 (2008); L. Rubio-Lago, A. García-Vela, A. Arregui, G.A. Amaral, and L. Bañares,

*The photodissociation of CH*, J. Chem. Phys.

_{3}I in the red edge of the A band: Comparison between slice imaging experiments and multisurface wave packet calculations**131**, 174309 (2009); L. Rubio-Lago, J.D. Rodríguez, A. García-Vela, M.G. González, G.A. Amaral, and L. Bañares,

*A slice imaging and multisurface wave packet study of the photodissociation of CH*, Phys. Chem. Chem. Phys.

_{3}I at 304 nm**13**, 8186 (2011)

**Characterization of potential energy surfaces of van der Waals clusters by empirical and ab initio methods**

_{2}(B)-He, I

_{2}(B)-Ne, Cl

_{2}(B)-He, and Br

_{2}(B)-He have been obtained so far in the excited electronic state B of the diatomic moiety. Empirical methods have been used, consisting of fitting an analytical function for the potential surface such that the available spectroscopic (blue shifts) and dynamical (resonance lifetimes and predissociation product vibrational and rotational distributions) experimental data are reproduced. Three-body effects have been included in the model functions used to represent the potential surface of some of the systems.

Ab initio methods have been also aplied to obtain the potential energy surface in the ground electronic state of other van der Waals clusters like Ar-HI, involving a weak hydrogen bond. In this case the CCSD(T) level of theory (coupled cluster with single and double (triple) configurations) was used. The spectroscopic magnitudes calculated with this potential surface agree very well with the available experimental ones, assessing the high quality of the potential.

A. García-Vela, *An empirical potential energy surface for He-Cl _{2}(B) based on a multiproperty fit*, J. Chem. Phys.

**119**, 5583 (2003); R. Prosmiti, S. López-López, A. García-Vela,

*Potential Energy Surface and Rovibrational States of the Ground State Ar-HI Complex*, J. Chem. Phys.

**120**, 6471 (2004)

**Approximate quantum and semiclassical dynamics methods for photodissociation**

Exact quantum mechanical methodologies become very expensive with increasing dimensionality of

the system studied. Thus, the development of approximate methods computationally less expensive,

but still reasonably accurate, becomes a need. Approximate quantal and semiclassical models for

photodissociation have been suggested.

In the case of the approximate quantum mechanical methods, the models include the full dimensionality of the system, but it applies a decoupling scheme of some of the degrees of freedom. More specifically, some of the modes are fully coupled between them, but the coupling to the other modes is only approximate, in the framework of the Time-Dependent Self-Consistent-Field (TDSCF) approach. This approximate model has been applied to the photodissociation of triatomic

systems like I_{2}-Ne, Cl_{2}-Ne, and Ar-HCl (including three degrees of freedom in all cases), and to

the vibrational predissociation of the tetraatomic complex Cl_{2}-He_{2}, considering six degrees of freedom. Comparison with experimental data or with exact quantum calculations showed a reasonable accuracy of the models.

The development of semiclassical models has also been pursued. Among them, a hybrid classical/quantal method has been proposed to describe the sequential photodissociation in complexes of the type X_{2}-Rg_{2} (X=halogen, Rg=rare gas atom) including the full dimensionality of the system. The strategy is to describe classically one of the steps of the sequential dissociation process, and quantum mechanically the other step. It was applied to the Cl_{2}-He_{2} complex. Quasiclassical methods have also been investigated where quantum-like phase space distributions have been proposed to weight the initial conditions of the system for classical trajectory dynamics.

A. García-Vela, *A full-dimensional quantum approach to the vibrational predissociation of tetraatomic complexes based on the partially-separable Time-Dependent Self-Consistent-Field approximation*, J. Chem. Phys. **116**, 6595 (2002); M.I. Hernández, A. García-Vela, C. García-Rizo, N. Halberstadt, P. Villarreal, G. Delgado-Barrio, *A hybrid classical/quantum approach to cluster fragmentation dynamics: Application to the vibrational predissociation of He _{2}-Cl_{2}*, J. Chem. Phys.

**108**, 1989 (1998); A. García-Vela,

*On the importance of an accurate representation of the initial state of the system in classical dynamics simulations*, J. Chem. Phys.

**112**, 8302 (2000)