A time‐of‐flight mass filter for ion and cluster ion photodissociation studies

A sensitive method is described for detecting photodissociation products from a specific ion or cluster ion in the absence of the initially formed fragment ions that would otherwise interfere with the signals of interest. By using a simple pulse sequence, the conventional three‐grid time‐of‐flight (TOF) assembly can be operated as a low‐mass rejection filter capable of eliminating all ions below an adjustable threshold mass value. The method has been applied using different ionization sources [e.g., picosecond and nanosecond photoionization, and electron impact (EI) ionization]. Molecular‐beam applications are demonstrated for (1) high‐resolution resonance ion dissociation spectroscopy of rovibrationally cold ions, and (2) stepwise solvation of neat and seeded cluster ions formed by either photoionization or EI ionization. The low‐mass filter (LMF) is especially powerful for cluster ion photodissociation studies. For example, the elimination of successively larger cluster ions leads to photofragment mass spectra that can uniquely establish the photodissociation and metastable decay fragments from specific cluster ions.