A cryogenic cylindrical ion trap velocity map imaging spectrometer.

A cryogenic cylindrical ion trap velocity map imaging spectrometer has been developed to study photodissociation spectroscopy and dynamics of gaseous molecular ions and ionic complexes. A cylindrical ion trap made of oxygen-free copper is cryogenically cooled down to ∼7 K by using a closed cycle helium refrigerator and is coupled to a velocity map imaging (VMI) spectrometer. The cold trap is used to cool down the internal temperature of mass selected ions and to reduce the velocity spread of ions after extraction from the trap. For CO ions, a rotational temperature of ∼12 K is estimated from the recorded [1 + 1] two-photon dissociation spectrum, and populations in spin-orbit excited XΠ and vibrationally excited states of CO are found to be non-detectable, indicating an efficient internal cooling of the trapped ions. Based on the time-of-flight peak profile and the image of N , the velocity spread of the ions extracted from the trap, both radially and axially, is interpreted as approximately ±25 m/s. An experimental image of fragmented Ar from 307 nm photodissociation of Ar shows that, benefitting from the well-confined velocity spread of the cold Ar ions, a VMI resolution of Δv/v ∼ 2.2% has been obtained. The current instrument resolution is mainly limited by the residual radial speed spread of the parent ions after extraction from the trap.