An ultrasensitive and mass selective detection of electronic spectra with vibrational and (depending on the spectral width of the laser) rotational resolution can be achieved by two color resonant two photon ionization (R2PI) of the cluster. The wavelength of the first laser is scanned over resonances of the intermediate electronic state of interest while the temporally and spatially overlapping second laser pulse is fixed to excite just above the vertical ionization threshold of the cluster. In this way fragmentation due to ion excess energy which could lead to an assignment of an electronic spectrum to a cluster of lower size is avoided or reduced. One color R2PI is often limited by this problem, if the intermediate electronic state does not lie below halfway the adiabatic ionization limit plus dissociation energy of the cluster ion. The cluster ions are accelerated in an electric field and pass a field-free drift chamber with the lighter cluster ions arriving earlier at the micro channel plate (MCP) ion detector. The resulting time of flight (TOF) spectrum is digitized by typically a 500 MHz oscilloscope for each laser wavelength increment and the intensity of selected masses transferred to a computer. By that the vibronic spectra of clusters of all different sizes present in the jet can be obtained from a single wavelength scan. In praxis, however, best quality spectra are obtained by tuning the jet expansion conditions to a cluster of specific size and recording the spectrum just for this cluster. A mass resolution of Dm/m > 500 could be achieved in this linear TOF mass spectrometer.

Careful optimization of the time of flight mass spectrometer is prerequisite for a sensitive detection of low cluster concentrations within a broad mass (cluster size) range. Not all extracted ions are from the molecular beam. The laser ionizes residual gas molecules evaporating from the chamber walls as well, which may cause a strong background signal. These background ions are strongly suppressed by applying a linear voltage ramp to the acceleration plates. Both the starting time and the slope of the voltage rise are chosen so that only ions, travelling with the velocity of the jet, are focussed on the detector. A small aperture between ionization and drift chamber increases the spatial resolution.

Further improvement of detection sensitivity is achieved by optimization of the field geometry which allows a comparably large detectable ionization volume of 50 mm³ in our apparatus. Thus loss of mass resolution which could arise from different accelerations at different positions in the extraction field is avoided. Furthermore, the sensitivity of detection especially of larger cluster ions is increased by post acceleration of the signal ions before they reach the detector. This increase in kinetic energy of the heavy ions greatly enhances the yield of secondary electrons in the MCP detector. In this way it was possible to detect the vibronic spectra of phenol water clusters up to 12 water molecules.