Hgl + 1 or I* 2 The process above, studied by Zewail shows the potential of fs measurements (40) A beam of H g l molecules was dissociated by a fs pulse of 311 nm light. The precursor and products were subsequently ionized by a 622 nm fs pulse and then mass analyzed in a time-of-flight (TOF) mass spectrometer. The latter wavelength was chosen because it is in resonance with the Β*Σ<--Χ Σ transition of Hgl. This is the initial, controlling absorption on the way to ionization. The Ε vector of the dissociating light E was fixed parallel to the axis of the TOF tube.
The resulting ions are then accelerated into a pair of microchannel plates. Roughly 10 electrons come out the back side of these plates for each ion that produces a pulse, about 5080% of the ions that strike the front of the plate. The electrons are accelerated into a fluorescent screen. Finally, a digital camera takes a picture of the screen and sends it to a computer. The experiment is repeated at 10 H z . The experiment may be performed i n the configuration shown or with the detection apparatus rotated 9 0 ° and pointed toward the pulsed nozzle.