Nr. 66 - Ultrafast molecular dynamics

Approved FOM programme

Objectives

The programme 'ultrafast molecular dynamics' is devoted to the probing and control of the dynamics of molecules on ultrashort time scales. The proposed research will focus on low-frequency degrees of freedom like hydrogen-bonds, conformational motions, and molecular rotations. To this purpose, advanced femtosecond pulse shaping and imaging detection tech­niques will be developed and applied.

Background, relevance and implementation

In the last decades, our understanding of molecular and electronic dynamics of atoms and small molecules has strongly increased as a result of the use of ultrafast laser systems and multi-particle detectors. In the earlier years, these techniques were mostly applied to the study of high-frequency, electronic degrees of freedom. These studies have led to a wealth of information on ionisation dynamics, properties of electronic excited states, and relaxation behaviour. More recently however, the attention has been shifted more and more to the study of low-energy degrees of freedom, such as hydrogen bonds and conformational vibrations, since these degrees of freedom, i.e. the relative motions of molecules and molecular groups, play a major role in reaction dynamics and biomolecular functions.

The structure of biomolecules in aqueous solutions is to a large extent determined by solvation (hydrogen-bond) interactions. In the past, these interactions have been studied by exciting and probing a dissolved probe molecule with ultrashort visible laser pulses. However, the information obtained with this technique turned out to be very limited, because the measured dynamics in most cases do not represent the spontaneous, ground state dynamics of the liquid. Here aqueous solvation interactions will be studied via a direct real-time monitoring of the motion of water molecules. In these experiments, the vibrational excitation of a water molecule is used as a label to follow the thermal fluctuations of the liquid on a (sub)picosecond time scale. These experiments require the use of far- and mid-infrared non-linear spectroscopic techniques. These tech­niques will also be used to study thermal conformational fluctuations in small biomolecules in aqueous solution, i.e. small peptides and DNA molecules.

Present-day laser technology is not only capable of probing low-energy internal and external molecular degrees of freedom, it also allows the generation of light pulses at such high intensities that low-frequency motions can be manipulated, leading for example to light-induced molecular alignment. In earlier days, control methods focussed on the manipulation of the phase of high-frequency optical excitations. However, high-frequency excitations often show an extremely rapid dephasing within tens of femtoseconds. In contrast, low-frequency molecular degrees of freedom often show a much longer coherence lifetime, and thus form far more effective tools to control reactions and energy-transfer processes. The dynamics of low-energy molecular degrees of freedom will be controlled with trains of phase-modulated pulses that are obtained via so-called adaptive pulse-shaping techniques. These techniques will be used to manipulate photodissociation reactions and molecular orientations in the gas phase, and the energy flow in biomolecules and proton-transfer reactions in the liquid phase. In the latter case, control will be achieved by launching collective motions of solvating water molecules with phase-modulated femtosecond mid-infrared pulses.

The research programme UMD consists of three central, related themes that will be carried out by four closely collaborating research groups:

1. Control of the interaction with ultra-intense light fields
   Manipulation of the interaction between atomic and diatomic systems and highly intense light to produce attosecond pulses, and to study bond softening/hardening effects.
2. Control of reactions and motions in (bio)molecular systems
   Control of molecular orientation, energy transfer, and proton-transfer reactions with adaptive femto-second pulse-shaping techniques.
3. Fluctuations in biomolecules and water
   Study of aqueous solvation interactions and ultrafast conformational fluctuations of biomolecules with femtosecond vibrational spectroscopy and numerical simulation techniques.

Remarks

None.