Microsolvatation & Microaggregation

Physical Chemistry II - Laser spectroscopy and Biophotonics

Microsolvatation & Microaggregation

We study microsolvation and microaggregation of molecular clusters in helium nanodroplets via high-resolution infrared spectroscopy using the Bochum He-nanodroplet apparatus in combination with a home-built cw-Optical Parametric Oscillator (OPO) with full frequency
coverage in the range from 2600-3400 cm-1, high output power (up to 2.7 W), and high resolution (0.0001 cm-1).

Helium droplets with an average size of several thousend He are formed in a supersonic expansion of precooled (16~K) gaseous helium through a 5 µm diameter nozzle. In a
partitioned second chamber, the suprafluid helium droplets at 0.37 K were doped with one molecule after the other. Each of the molecules is cooled to 0.37 K by evaporative cooling before aggregation.

After crossing the interaction zone with an IR laser, the nanodroplets are detected in a quadrupole mass spectrometer. Absorption of an infrared photon and subsequent cooling to the ground state results in the evaporation of several hundred helium atoms yielding a depletion of the mass spectrometer signal due to a reduced droplet ionization cross section.

Helium droplets provide an ideal environment to study ultracold chemistry. They act like a vacuum cleaner in that they pick up molecules from the vacuum chamber and cool them down to 0.37 K.The large droplet size of typically a few nm allows investigation of molecules with small vapor pressure (e.g. amina acids). The associated cooling times are shorter than the timescales for migration of the dopants within the droplet, as well as the timescale between single pickup processes.Thus, pre-cooled monomers aggregate sequentially which can yield non-equilibrium structures not found at higher temperatures. Helium nanodroplets are also found to be a soft matrix which due to their suprafluid nature induce only very small shifts (less than 1 cm-1) compared to the gas phase. At ultracold temperatures we can find entire new reaction and aggregation mechanisms.