A MOLECULAR-BEAM TIME-OF-FLIGHT MASS-SPECTROMETER USING LOW-ENERGY-ELECTRON IMPACT IONIZATION

We have constructed a versatile apparatus to study photoinitiated processes in molecular beams using a variety of generally applicable techniques. The instrument contains a pulsed, low-energy electron gun that delivers space-charge-limited electron beams into the ionization region of a time-of-flight mass spectrometer. The electron energy is tunable from 8 to 200 eV, and the electron energy distribution is relatively narrow (FWHM ∼0.3 eV), which allows us to ionize laser-excited species and their decomposition products selectively. We have used low-energy electron impact ionization and mass spectrometry to characterize molecular beams, to detect vibrationally excited molecules prepared by vibrational overtone excitation, and to detect primary photodissociation products in the presence of precursor molecules.