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.

In this work we analyze clusters between the methoxy radical (CH3O, an open-shell molecule) and the nonpolar solvents Ar, N2, CH4, and CF4. CH3O is formed through the photolysis of CH3OH vapor in a supersonic expansion of CH3OH and a solvent gas (Ar, N2, CH4, CF4) seeded in a carrier gas of He. The radical and solvent molecules are cooled to ∼15–20 K and form clusters. These clusters are probed using laser induced fluorescence (LIF) of the CH3O radical. An extensive set of calculations, including ab initio and atom–atom potential calculations and rotational contour simulations are performed for each cluster in order to elucidate the cluster structure and the nature and relative importance of the limiting types of interactions that are responsible for cluster binding. A final minimum energy structure is presented for each cluster, together with the analysis of the limiting type of interactions that generate the van der Waals binding of the cluster.

The B←X and Ã←X transitions of the NCO radical and its clusters with nonpolar solvents are studied in a supersonic jet expansion by employing laser-induced fluorescence techniques. Fluorescence excitation ͑FE͒ and hole burning spectra are recorded for the NCO radical and compared to previous work. NCO is clustered with Ar, N 2 , CH 4 , and CF 4 nonpolar molecules to elucidate the effect of solvation on the radical energy levels and dynamics. FE spectra are detected for NCO 1:1 clusters showing blue shifts in their spectra with respect to that of the isolated NCO radical, while their 1:n counterparts show either red or blue shifts. Potential energy surface calculations are performed to evaluate the binding energies and geometries of 1:1 clusters in the X, Ã, and B electronic states. The relatively long decay lifetime and red shifted fluorescence wavelength range observed for B state clusters suggests that they decay first through internal conversion ͑IC͒ to à vibronic levels, and then experience rapid intracluster vibrational redistribution ͑IVR͒ and vibrational predissociation ͑VP͒, yielding ground state solvent molecules and NCO radicals at lower à vibronic levels. These à state NCO radicals subsequently emit, generating the Ã→X band.

2 + 1 ) Resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

Cyclopentadienyl (cpd), methylcpd (mcpd), fluorocpd (Fcpd), and cyanocpd (CNcpd) are generated photolytically, cooled in a supersonic expansion, and clustered with nonpolar solvents. The solvents employed are Ar, N2, CH4, CF4, and C2F6. These radicals and their clusters are studied by a number of laser spectroscopic techniques: Fluorescence excitation (FE), hole burning (HB), and mass resolved excitation (MRE) spectroscopies, and excited state lifetime studies. The radical D1←D0 transition is observed for these systems: The radical to cluster spectroscopic shifts for the clusters are quite large, typically 4 to 5 times those found for stable aromatic species and other radicals. Calculations of cluster structure are carried out for these systems using parameterized potential energy functions. Cluster geometries are similar for all clusters with the solvent placed over the cpd ring and the center-of-mass of the solvent displaced toward the substituent. The calculated cluster spectroscopic shifts are in reasonable agreement with the observed ones for N2 and CF4 with all radicals, but not for C2F6 with the radicals. The Xcpd/Ar data are sacrificed to generate excited state potential parameters for these systems. CH4 is suggested to react with all but the CNcpd radical and may begin to react even with CNcpd. van der Waals vibrations are calculated for these clusters in the harmonic approximation for both D1 and D0 electronic states; calculated van der Waals vibrational energies are employed to assign major cluster vibronic features in the observed spectra.