|MPQ: ||T. Schaetz, S. Kahra, G. Leschhorn, T. Dou, W. Fuss, W. Schmid (Area C2.2)
E. Botschafter, A. Schiffrin, R. Ernstorfer, R. Kienberger (Area A.1)
|LMU:||R. deVivie-Riedle, M. Kowalewski (Area C2.6)|
We present the preparation of one single molecular ion as a target for single-molecule imaging with atomic resolution. The methods of sympathetic laser cooling, Coulomb crystallization and state preparation allow for focusing anticipated MAP* x-ray pulses of high intensity to considerably better than 1Ám on a single molecule, providing very well defined conditions in a nearly background free environment. A launched pump-probe experiment with 4fs UV-pulses investigates the methodology and incorporates all building blocks for target preparation in future single molecule diffraction experiments.
The investigation of the dynamics within a molecule during a structural change on the relevant timescale (<10-15 fs) gets into reach due to the development of MAP*-light sources that will provide pulses of more than 1011 photons at wavelengths promising a resolution on atomic length scales. We should become capable of recording a diffraction pattern from a single molecule without the need of crystalline periodicity. The three-dimensional, time-resolved structure could be retrieved, if the destroyed molecule can be replaced by identically prepared copies, pounded one after the other.
Besides the work necessary to supply the photons at the required intensities, we have to provide reproducible and precise methods at sufficient repetition rates to prepare and handle the molecular target. Therefore, we try to transfer the unique operational fidelities for handling trapped ions in quantum information processing into the field of attosecond experiments with MAP-light sources and their possibilities of ultimate control of laser fields.
The laser sources of our MAP-collaborators A.1. recently achieved unique specifications (sub 4 fs UV-pulses of 1 ÁJ at 3 kHz). One could use these pulses, for example to investigate neutral molecules in gas phase. Large Coulomb crystals of molecular ions could provide spectroscopic data. Using the methods of quantum information processing based on trapped ions for the preparation and control of external and internal degrees of freedom[1,2,3] could allow merging the individual experimental advantages to provide single molecular ions as targets with unique operational fidelities.
We designed our system to enable diffraction pump-probe experiments, e.g. photo-triggered isomerization recorded by x-ray pulses, on single molecular ions to elucidate the structural changes during chemical reactions. We will handle undisturbed single molecular ions (m<104 u) by sympathetically laser cooling the external degrees of freedom in a Coulomb crystal confined in a Paul trap, cooling their ro-vibrational degrees of freedom by buffer-gas- or even direct laser cooling close to their ground states, separate single molecular ions into a diffraction zone by controlling electrical fields on electrodes and light pressure and 3D-orientate them in space by additional light pulses.
We will investigate charged molecules and plan to append charged atoms for trapping, not disturbing the dynamics of the molecule or to protonate molecules, simulating in some cases a natural surrounding. Our generic tool should enable the almost background-free investigation of dynamical effects by x-ray diffraction, driven by the identical but split laser pulse on a sub fs timescale. Originally, we planned to bridge the gap until we get access to the x-ray pulses with electron pulses to investigate structural changes in simple molecular ions in collaboration with group C.2.1. Much to our regret, it was not possible up to now to provide these electron pulses. Since we had reached our anticipated milestones, we searched and found theoretical (C.2.6) and experimental (A.1) support to provide the proof of principle experiment for our originally proposed method of pump-probe experiments on single molecules. In the future, we hope to just have to exchange the laser sources.
In close collaboration with the MAP-theory group (C.2.6.) we investigated the possibilities to perform the proof of method experiment depicted in Fig. 2. A pump pulse is expected to transfer the vibrational wave packet from the electronic ground (X) into the excited state (A) with a natural life time of ~4 ns. A probe pulse of variable time delay (0-2000 fs) should allow for the transfer from the excited state (A) to the dissociative channel (C). The dissociation of 24MgH+ into neutral 24Mg and H+ should lead to the loss of both particles the disappearance of the dark molecular ion providing the detection of the reaction.
We currently merge our Ultra-High-Vacuum (UHV) apparatus via a differential pumping section with the MAP* beam line. The unique coatings of our mirrors had been designed and will be fabricated by the MAP-service centre to match the spectral demands for the anticipated experiment.
In parallel: set up of Electro Spray Ionization (ESI) source
In a separate setup, we reanimated a ElectroSpray Ionisation source and achieved sufficient flux of (generic) single molecular ions. Currently we extend the source with a mass filter section and a rf-lens to allow merging the ESI with our ion guide setup.
We transferred (one floor, approx. 150m) our fully functional UHV apparatus including the optical- and laser setup to the MAP-beam line (AS4b at MPQ) to get access to 4 fs pulses at a wavelength of 290 nm.
We envision the investigation, whether the bandwidth (approximately 30 nm) of the 4 fs pulses allows to provide a coupling between the bound molecular states to realize scattering rates sufficient for direct detection of the fluorescence light of a single molecular ion. Theoretical predictions suggest shifting the spectrum sufficiently red to allow for a related direct laser cooling of the internal degrees of freedom of the molecule. In addition, we might be able to transfer the population of the ground state (X) directly via a two-photon transition to the dissociative state (C) and to apply the second (probe) pulse to transfer the population to another dissociative channel (B). The regained 24Mg+ might still be trapped therefore providing a new detection channel where molecular (dark) ions do not disappear put turn into bright ions again. For the farer future, this might allow to think of experiments, where the charged product of a reaction could be kept and further investigated.
As long-term goal with x-ray pulses of sufficient intensities, we want to adapt the proof of principle experiment to the specific needs. An important prerequisite is the envisioned field free orientation of the molecular ions via laser pulses.
Letzte ─nderung: 11.08.2009, 11.39