The laser-induced fluorescence and action spectra of I2 in a helium supersonic expansion have been recorded in the I2 B-X, 20-0 region. Two features are identified within the spectra. The lower-energy feature arises from transitions between states that are localized in a T-shaped conformation on both the X- and B-state potentials. The higher-energy feature reflects transitions from states that are localized in a linear conformation on the X state to states that have energies that are larger than the barrier for free rotation of the rare gas atom about the I2 molecule on the B-state potential. Ground-state binding energies of 16.6(6) and 16.3(6) cm-1 were determined for the T-shaped and linear conformers, respectively. These spectra are compared to those calculated using the experimentally determined rotational temperatures. Based on the agreement between the experimental and calculated spectra, the binding energies of the J'=0 states with 0 and 2-6 quanta of excitation in the He...I2 bending mode on the B state were determined. Several models for the B-state potential were used to investigate the origins of the shape of the contour of the higher-energy feature in the spectra for He...I2 and He...Br2. The shape of the contours was found to be relatively insensitive to the choice of potential. This leads us to believe that the spectra of these systems are relatively insensitive to the parameters of the B-state potential energy surface and are more sensitive to properties of the halogen molecule.

The general mechanism for photo-cross-linking of preformed polymers is studied by comparing the efficacy of monofunctional benzophenone (BP) and bifunctional bis-benzophenone (BP-BP) photoinitiators for inducing radical chain branching reactions in glassy polystyrene (PS) and rubbery poly(n-butyl acrylate) (PnBA). Upon UV irradiation, macroradicals form and initiate a variety of cross-linking and scission reactions. The kinetics and mechanisms of these macroradical reactions were monitored by gel permeation chromatography (GPC) measurements of changes in the polymer molecular weight distributions. Molecular weight increases are associated with chain branching while decreases in molecular weight are indicative of chain scission. We study the early stages of radical recombination where branching is manifest as the formation of three-and four-arm star polymers that are soluble and can be detected/differentiated by GPC. Branching is observed even in glassy PS; however, the reactions are much faster in rubbery PnBA, consistent with the expected influence of main chain mobility. At equal chromophore equivalents, BP-BP was found to be more efficient than BP for producing macroradicals, primarily due to a lower degree of self-quenching in BP-BP. When added to glassy PS, the higher efficiency of BP-BP did not translate into more chain branching, except at high additive concentration where the probability of BP-BP chain bridging reactions becomes significant, but instead led to a higher degree of main chain scission. The latter result was attributed to the larger distance between chromophores for BP-BP than for BP at equal benzophenone equivalents. In marked contrast, almost no main chain scission was found for either additive in PnBA, and BP-BP proved more effective for promoting chain branching. The susceptibility to main chain scission is found to be dependent upon the location of radical formation by hydrogen abstraction from the polymer. In PS, radicals can form on the chain backbone, and radical scission reactions lead to fragmentation of the polymer chain. In PnBA, radicals form preferentially on the pendant side chains, and radical scission reactions do not lead to main chain breakage. A simple probability-based model was found to capture the salient features of radiation-induced branching in preformed polymers.

Supporting Information. PS (M n :152K, M w /M n =1.05) was purchased from Pressure Chemical. The PS-b-PtBA (M n :16K-7.5K, M w /M n =1.08 and M n :6.4K-7K M w /M n =1.33) block copolymers were purchased from Polymer Source Inc. The photoacid generator (PAG), (tertbutoxycarbonylmethoxyphenyl)diphenylsulfonium triflate, was purchased from Aldrich.Nickel sulfate (98%, crystalline; source of Nickel II ions) was purchased from Alfa Aesar. Sodium citrate (complexant) was purchased from Fisher. Lactic acid (complexant and buffer) was purchased from Fluka. Dimethylamine-borane complex (97%; reductant) was purchased from Sigma-Aldrich. Ammonium hydroxide (5.08N solution in water; pH adjustor) was purchased from Sigma-Aldrich. Tetraammine palladium(II) chloride monohydrate (99.99+%; positive Pd catalyst) was purchased from Aldrich. Toluene (extra dry) was purchased from Acros Organics. Ethanol (ACS/USP grade) was purchased from Pharmco-AAPER. Deionized water (>18.2 MΩ cm, Millipore, Milli-Q) was used in aqueous solutions and rinsing procedures. Acetone (HPLC-UV grade-Reagent grade), purchased from Pharmco-AAPER, was used in rinsing procedures. All chemicals were used without further purification.

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