Speaker
Description
Intra- and intermolecular energy transfer is of fundamental importance in most chemical and physical processes. Yet, due to the complexity of the fundamental steps involved, its understanding is often largely phenomenological. To investigate energy transfer processes that involve defined quantum levels, various optical pump-probe schemes exist, which require the measurement of the probability of absorption of a probe photon, after the absorption of a pump photon. In most cases, these experiments are performed in the condensed phase and frequently suffer from sample heterogeneities and finite temperature as well as photon heating effects. Experiments on dilute gas-phase samples on the other hand are confronted with the challenges of detecting the correlated absorption of two photons while avoiding sample heating by the absorption of more photons.
Vibrational action spectroscopy of ions in ultracold helium nanodroplets will allow us to overcome those issues. Over the years, we have shown that helium nanodroplets can be used to obtain highly resolved infrared spectra of a large variety of molecular ions and ionic clusters. An important property of the helium droplet is that it provides for a dissipative environment which efficiently cools embedded species to its equilibrium temperature of 0.4 K. In the IR spectroscopic method developed and employed at the FHI, ions that are embedded in helium droplets absorb a large number of photons from the pulse train of the FHI-free-electron-laser (FHI-FEL). After each individual absorption event, a cascade of relaxation processes sets in, with first intramolecular vibrational redistribution (IVR) occurring on a picosecond (ps) timescale, followed by slower energy transfer to the helium droplet. By evaporation of helium atoms, the droplet then returns itself and the embedded ion to its equilibrium temperature. Each individual absorption event thus occurs from the ground state of the ultracold ion and narrow IR resonances are observed.
Signal in the experiment is observed after the helium droplet is completely evaporated. Experimentally, the droplet size can be adjusted and by this, the number of photons that need to be absorbed in order to obtain signal. This can be exploited for pump-probe experiments. The FHI-FEL in its two-color mode will provide two pulse trains with 500 MHz repetition rate. One of them delivers the pump, the other one the probe pulses. Each ion in a droplet will interact with a large number of pump-probe pulse pairs. Whether a probe photon will be absorbed depends (among other things) on its wavelength and the time delay between pump and probe, and by measuring this absorption probability, direct and unique insight into the vibrational dynamics and vibrational couplings are obtained.
Thus, IR-IR pump-probe measurement will become possible because a) each pump-probe pulse pair interacts with a 0.4 cold ion in its ground state and b) the helium droplet size can be varied, acting as a detector that signals the absorption of a well-defined and tunable number of photons. This will allow us to not only to measure the lifetime of excited vibrational modes, but also directly the energy flow in the molecule as well as the coupling between modes, giving us 2D‑IR information.
| Abstract Number (department-wise) | MP 09 |
|---|---|
| Department | MP (Meijer) |
