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17 december 2018

Closed FOM programme

Number 59.
Title Molecular dynamics studies with intense IR radiation (MDIR) 
Executive organisational unit Rijnhuizen
Programme management Dr. J. Oomens and Prof.dr. G.J.M. Meijer
Duration 2003-2008
Cost estimate M€ 1.5


In this research programme various studies will be undertaken that explore and exploit the unique capabilities of the IR user facility FELIX in molecular physics, under the common theme 'Molecular dynamics studies with intense IR radiation'. In particular, by constructing two different experimental set-ups inside the optical FEL cavity, optical studies on molecules, clusters and nano-particles will be undertaken throughout the IR spectral region with an unprecedented photon flux. Infrared multi photon excitation, leading to dissociation, ionisa­tion or to reactions, will be used to determine the vibrational properties and to unravel the geometrical structure of molecules and particles that are, for instance, of relevance in astro­physics in this intra-cavity set-up. Manipulation, i.e. slowing and trapping, of neutral mole­cules in intense pulsed IR light fields will be studied. Using feedback control to produce opti­mally shaped FEL micro-pulses, coherent multi-photon vibrational excitation of molecules will be pursued, and the possibilities of the Free Electron Laser for Intra-Cavity Experiments, FELICE, set-up for coherent control of molecular dynamics will be investigated.

Background, relevance and implementation

The Free Electron Laser for Infrared eXperiments, FELIX, at Rijnhuizen has been designed and constructed with the aim to supply the wide scientific community with tunable IR radiation. During the last years, this radiation has been successfully used for a variety of multi-photon excitation and ionisation experiments on atoms, molecules, clusters, and nanocrystals. These IR multi-photon excitation (MPE) and ionisation experiments have demonstrated that FELIX is particularly well suited to resonantly pump large amounts of vibrational energy into isolated (bio)molecules, clusters and nano-particles. With the output energy per micro-second of FELIX it is possible to explore and exploit the potential that IR-MPE experiments already promised to have in the late seventies. The energy deposited in an isolated system by IR-MPE can be used to trigger reactions, emission of photons (fluorescence), ejection of fragments (disso­ciation) or ejection of electrons (ionisation). By monitoring these IR laser induced processes rather than the direct IR absorption process, extremely sensitive detection schemes can be employed, via which accurate information on the structure and dynamics of the species under investigation is obtained.

The attainable signal levels in IR-MPE experiments scale highly nonlinearly with FEL power and fluence (energy per unit area), and will greatly benefit from an increased IR laser intensity, such as is available inside the optical FEL cavity. There, the power and fluence are at least two orders of magnitude above the values outside the cavity. Gas-phase studies are fully compa­tible with such an intra-cavity set-up; the number of photons absorbed by the low-density gas-phase species is negligible compared to the total number of photons emitted by the FEL, and the laser properties of the FEL will therefore not be affected. At those increased IR intensities many more, and significantly larger, molecules and cluster materials become amenable for optical investigations via IR-MPE over a much wider wavelength range. In addition, we will be able to enter a new regime with the IR-MPE experiments and efficient coherent multi-photon vibrational excitation in a single pico-second pulse will become feasible.

We will construct an FEL optical cavity with two intra-cavity user-stations for experiments in molecular physics. The available pulse energies in this FELICE set-up will be around 1-3 mJ of tunable IR radiation per (picosecond duration) micro-pulse, or some 10 Joules of pulsed IR radiation in a (several microsecond duration) macro-pulse. The two experimental set-ups that will be constructed inside the FEL-cavity are a state-of-the-art ion trap and a pulsed molecular beam machine.