Ponente
Dr.
Klaus von Haeften
(K-nano)
Descripción
The interaction of molecules determines chemical reactions and biological processes. Very fine details of such interactions can be unravelled using rotational spectroscopy thanks to its great resolving power. However, rotational spectroscopy is usually restricted to the gas phase. In the condensed phases, interactions are usually so strong that rotational features are overshadowed. An exception is liquid helium where interactions are exceptionally weak. Furthermore, its properties are strongly affected by quantum effects. Also, it is an attractive model substance for theory and for experiment: (i) helium atoms, having only two electrons, greatly facilitate high level ab initio calculations of clusters. (ii) At the temperatures where helium becomes liquid all other substances freeze. Liquid helium is therefore one of the purest, if not the purest of all condensed substances. This exceptional purity has recently been exploited for the investigation of nucleation, growth and solidification of nanoparticles [1].
This presentation will highlight two recent experiments where rotational spectroscopy has been pushed to new limits. The full rotational spectrum of a molecular complex was derived using femtosecond wave packet spectroscopy. In another experiment in liquid helium, molecules were
identified in their lowest rotational quantum state in thermal equilibrium.
Impulsive alignment of clusters in a beam and fluorescence spectroscopy in bulk helium
We have excited a supersonic beam of small C2H2-Hen clusters non-resonantly with intense femtosecond laser pulses - a technique called impulsive alignment - thereby creating wave packets composed of rotational eigenstates. The clusters were then probed with a second laser pulse after a set time delay which led to Coulomb explosion. Using the fragment velocity distribution of the C2H2 molecules the state of alignment was determined and the propagation of rotational wave packets was measured in the time domain. A Fourier-transform of the time-spectrum yielded the pure complete rotational spectrum of C2H2-He in excellent agreement with theory [2]. The spectrum showed strong delocalisation of the complex indicating liquid-like character. The excited complex nevertheless rotated coherently over the entire duration of the experiment of 600 ps and showed no signs of dephasing [3].
These results demonstrate that impulsive alignment is well suited to derive structural and dynamical information from clusters, including weakly bound complexes. Production of these complexes requires strong cooling with the consequence that normally only the lowest rotational quantum states are populated. Unlike traditional frequency domain spectroscopy, where selection rules limit the quantum number of states to ΔJ=1, impulsive alignment provides the control that is necessary to excite and probe all J levels, up to the dissociation threshold.
In another experiment bulk helium was electronically excited using a corona discharge, creating a rich fluorescence spectrum which was measured as a function of temperature and pressure. Intense fluorescence in the visible region showed the rotationally resolved d u+ b3g transition of He2*. With increasing pressure, the rotational lines merged into single features. The observed pressure dependence of line width, shapes and line shifts established that within liquid helium excimers are either solvated, and cold, or ‘boiling’ within rotationally hot gas pockets. Increase of hydrostatic pressure was found to rotationally cool the excimers at a rate of at least 1010 to 1011 K/s in collisions with the liquids until they occupied the lowest available quantum state [4].
These findings are important with regard to the quest of achieving greatest possible control over molecules, including cooling their degrees of freedom. Also, they suggest that it should be possible to investigate liquid and superfluid helium at the nanoscale over a large pressure and temperature range using molecules as rotational probes. Previous experiments used helium droplets and were therefore restricted to fixed pressures and temperatures. They suggest that by additional control of pressure, temperature and thermodynamic phase unprecedented insight into the structure of solvation layers and interfaces can be achieved.
Funding is acknowledged from the Royal Society, The Leverhulme Trust, Erasmus, COST action MOLIM, CONACYT, the Iraq government and the University Joseph Fourier for a visiting professorship for KvH.
References
[1] H. Gharbi Tarchouna, N. Bonifaci, F. Aitken, L. G. Mendoza-Luna, and K. von Haeften, J. Phys. Chem. Lett. 6 (2015) 3036
[2] G. Galinis, L. G. Mendoza-Luna, M. J. Watkins, C. Cacho, R. T. Chapman, A. M. Ellis, M. Lewerenz, L. G. Mendoza Luna, R. S. Minns, M. Mladenovic, E. Springate, I. C. E. Turcu, M. J. Watkins, L. Kazak, S. Gode, R. Irsig, S. Skruszewicz, J. Tiggesbaumker, K-H. Meiwes-Broer, A. Rouzee, J. G. Underwood, M. Siano and K. von Haeften, Faraday Discuss. 171 (2014) 195
[3] G. Galinis, L. G. Mendoza-Luna, M. J. Watkins, C. Cacho, R. T. Chapman, A. M. Ellis, M. Lewerenz, L. G. Mendoza Luna, R. S. Minns, M. Mladenovic, A. Rouzee, E. Springate, I. C. E. Turcu, M. J. Watkins, and K. von Haeften, Phys Rev. Lett. 113 (2014) 043004
[4] L. G. Mendoza-Luna, N. M. K. Shiltagh, M. J. Watkins, N. Bonifaci, F. Aitken, and K. von Haeften, J. Phys. Chem. Lett. 7 (2016) 4666
Autor primario
Dr.
Klaus von Haeften
(K-nano)
