No abstract

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, we have derived the residence time (τnozzle) of the gas pulse in the nozzle to be around 30 μs.

We report the photochemistry of (OCS) n Ϫ cluster ions following 395 nm (nϭ2-28) and 790 nm (nϭ2-4) excitation. In marked contrast to (CO 2) n Ϫ , extensive bond breaking and rearrangement is observed. Three types of ionic products are identified: S 2 Ϫ ͑OCS͒ k , S Ϫ ͑OCS͒ k /OCS 2 Ϫ ͑OCS͒ kϪ1 , and (OCS) k Ϫ. For nϽ16, 395 nm dissociation is dominated by S 2 Ϫ-based fragments, supporting the theoretical prediction of a cluster core with a C 2v (OCS) 2 Ϫ dimer structure and covalent CC and S-S bonds. A shift in the branching ratio in favor of S Ϫ-based products is observed near nϭ16, consistent with an opening of the photodissociation pathway of OCS Ϫ core-based clusters. These monomer-based cluster ions may coexist with the dimer-based clusters over a range of n, but electron detachment completely dominates photodissociation as long as their vertical electron detachment energy, increasing with addition of each solvent molecule, is less then the photon energy. An (OCS) 2 Ϫ conformer of C 2 symmetry with a covalent CC bond is believed to be responsible for 790 nm dissociation of (OCS) 2 Ϫ , yielding primarily OCS Ϫ products. The yield of OCS Ϫ , and thus the importance of the C 2 form of (OCS) 2 Ϫ cluster core, decreases with increasing n, perhaps due to more favorable solvation of the C 2v form of (OCS) 2 Ϫ and/or a solvent-induced increase in the rate of interconversion of conformers. The (OCS) k Ϫ products observed in 395 nm photodissociation of the larger (nу7) clusters are attributed to photofragment caging. Formation and dissociation mechanisms of clusters with different core types are discussed. The photochemical properties of (OCS) n Ϫ are compared to those of the isovalent (CO 2) n Ϫ and (CS 2) n Ϫ species.

We report time-resolved photodissociation and geminate recombination dynamics of I2− in size-selected I2− Arn and I2−(CO2)n cluster ions by using ultrafast pump–probe techniques at 790 nm in conjunction with a tandem time-of-flight mass spectrometer. The absorption recovery, which reflects the time scale for photodissociation followed by recombination and vibrational relaxation of I2− inside the cluster, shows a strong dependence on the composition of the surrounding cluster solvent. The absorption recovery time for I2−(CO2)16 is ∼1 ps, whereas for I2−Ar20 it is ∼130 ps. This difference is discussed in terms of electrostatic and hard sphere interactions. We also observe the time dependence of the destruction of the Ar solvent cage for I2−Ar16. Finally, absorption recovery data for I2−(CO2)n cluster ions taken with 790 nm pump–probe wavelengths are compared with the greater energy release 720 nm data.