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Techniques based on picosecond resonance-enhanced multiphoton ionization and mass-selective ion photodissociation are described as a means for studying complex reaction mechanisms in molecular clusters. A set of experiments is described that can identify the cluster-size specific reactions that occur by the overlapping processes of neutral dissociation/fragment ionization, parent ionization/dissociation, and parent ion photoexcitation/dissociation. These molecular beam techniques are demonstrated for the case of neutral and ionic photon transfer and evaporation in clusters of phenol in (NH(3))(n), as well for radical chemistry and van der Waals dissociation in (CH(3)I) (n) clusters.

Optical excitation followed by ion mass detection in a supersonic molecular beam can offer unusually high levels of selectivity for detection of trace concentrations of material in the atmosphere. We describe recent work involving two-color excitation of aromatic molecules in a supersonic molecular beam, time-of-flight mass spectrometer. It is shown how double resonance excitation in cold molecules can provide two very selective stages of optical selectivity, further augmented by single mass detection. Three applications are described: (1) Two-color excitation to higher excited electronic states is illustrated for final states above and below the ionization potential. (2) Stimulated emission pumping is shown to allow selective identification using ion dip detection. (3) Resonance ion dissociation is demonstrated as a means for obtaining double optical and double mass selectivity.

An ultrasensitive detection method for atmospheric monitoring has been developed based on the technique of supersonic molecular beam, resonance enhanced multiphoton ionization, and time-of-flight mass spectroscopy (MB/REMPI/TOFMS). Several organophosphonate and organosulfide compounds, representing simulants to a class of toxic compounds, were studied. Detection levels as low as 300 ppt (dimethyl sulfide) were obtained. Single-vibronic-level REMPI of the cooled molecules in conjunction with TOFMS provided selectivity of ~10(4) against chemically similar compounds in humid air expansions. The fragment ions formed by REMPI excitation are shown for diisopropyl methylphosphonate to depend strongly on the resonant intermediate state of the neutral molecule.