|
We constructed a helium droplet source as a tool to investigate secondary structures of charged biomolecules in a low-temperature environment. Helium nanodroplet spectroscopy is a well-established method to obtain high-resolution spectra of embedded foreign species, which are cooled down to the liquid helium temperature of 0.37 K. By means of a pulsed valve with a 0.8 mm orifice, which is kept at a temperature of 5-20 K, superfluid 4He nanodroplets are formed via an adiabatic expansion of gaseous helium at high pressure (30-50 bar) into vacuum. Under these initial stagnation conditions, helium undergoes a phase transition, following an isentrope into the superfluid phase regime of LHeII. The He droplet beam passes through an ion trap, in which the ions are stored by an electric potential of a few electron volts; there the ions can be picked up by helium clusters. Because of their great mass and high kinetic energy, the loaded He droplets are supposed to not being influenced by the trapping potential. Therefore they can pass the trap straightforward and consequently drag along ions out of the trap. Subsequently, IR spectroscopy of cold ions can be performed and, finally, detection takes place in a time of flight mass spectrometer. |
|