Clusters containing a phenylethylamine (PEA) or amphetamine (AMP) molecule and a solvent species such as Ar, CH 4 , CF 3 H, CO 2 , H 2 O, and other small molecules are formed in a supersonic jet expansion. Spectral studies of the solvation and related chemistry of PEA and AMP are pursued by using both fluorescence and mass spectroscopy techniques. To help analyze the experimental results, ab initio and atom-atom LennardJones-Coulomb (LJC) potential calculations are employed to calculate cluster geometries and binding energies. The LJC potential parameters for the 10-12 hydrogen-bonding potentials have been reevaluated on the basis of new ab initio partial atomic charge values and new experimental binding energies and geometries. The observed dependence of the relative spectral intensities of PEA and AMP conformers and their clusters on the cooling conditions (backing pressure and coolants employed) suggests that these species undergo population redistribution in the cooling and clustering process. The amount of excess energy (binding energy) available to the forming cluster plays a major role in the conformational conversion of PEA and AMP during cluster formation. If strong interactions (hydrogen bonding) exist between the solute and the solvent, such conversion/ redistribution processes occur among all conformers and their clusters. The conversion/redistribution process is restricted within the anti or gauche conformer sets and their clusters for weakly interacting solute/solvent pairs. All PEA and AMP clusters studied experience complete fragmentation upon ionization. The observed gradual dependence of photo ion intensity on the ionization laser energy suggests a significant change in geometry for both PEA and AMP, as well as their clusters, upon ionization. Consequently the high vertical ionization energy leads to an excess energy in the vibrational modes of the ions, causing fragmentation of the clusters. The clusters can fragment along two different general paths: (1) simple loss of the solvent molecules and (2) breaking the R-carbon bond of PEA or AMP, with additional loss of solvent molecules in some cases. Those clusters with weaker solute/solvent binding tend to fragment through solvent loss, while those forming hydrogen bonds tend to favor the R-carbon bond cleavage. Reactions are observed for PEA and AMP with NO. NO can completely quench PEA and AMP monomer spectra.
The lowest excited singlet state of biphenyl (BP) and a number of its isotopically and chemically substituted analogs have been studied by supersonic jet laser spectroscopy. The symmetry species of this excited state in BP can be classified as B 2-:; in the G I6 extended molecular symmetry group G I6 (EM). The symmetry-allowed origin of the biphenyl-h SI +-So electronic transition occurs at 35 268 cm-I. The frequency ofthe torsional moti~n in S is determined to be-65 cm-I. The potential parameters for this motion in SI are V 2 = 1195 1 cm-I, V 4 =-190 cm-I, and V6 = 18 cm-I. The torsional motion for the ground state (-50 cm-I) can be described by V 2 = 50 cm-1 and V 4 =-148 cm-I. The change in the dihedral angle between the two benzene rings in BP upon So to SI excitation is determined to be _44• based on a Franck-Condon factor calculation. Several fundamentals of the molecular vibrations are assigned in the SI state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (> 10 3 cm-1).
Decomposition studies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX-C 3 H 6 N 6 O 6 , see Fig. 1͒ isolated in the gas phase and cooled in a supersonic expansion are reported for the excited electronic state near 225 nm. The RDX is handled safely and effectively through matrix-assisted laser desorption ͑MALD͒ of a thin film of RDX/R6G laser dye ͑1:1͒ adsorbed on an aluminum oxide coating on an aluminum drum. The aluminum oxide coating is generated by plasma electrolytic oxidation of aluminum. The combination of MALD and supersonic molecular beam techniques generates intact and cold RDX molecules isolated in the gas phase. Two basic conclusions are reached in this study. ͑1͒ Photodissociation of RDX at Ϸ225 nm generates NO as an initial product. ͑2͒ Nascent NO thus generated is vibrationally hot (T vib ϳ1800 K͒ and rotationally cold (T rot ϳ20 K͒.
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