The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. The experimental results are obtained from the fragmentation threshold yielding ClO+, which is observed at 11.52 +/- 0.025 eV. The O-O bond strength D(o) is derived from this value in comparison to the first ionization energy of ClO, yielding D(o)298 = 72.39 +/- 2.8 kJ mol(-1). The present work provides a new and independent method to examine the equilibrium constant K(eq) for chlorine peroxide formation via dimerization of ClO in the stratosphere. This yields an approximation for the equilibrium constant in the stratospheric temperature regime between 190 and 230 K of the form K(eq) = 1.92 x 10(-27) cm3 molecules(-1) x exp(8430 K/T). This value of K(eq) is lower than current reference data and agrees well with high altitude aircraft measurements within their scattering range. Considering the error limits of the present experimental results and the resulting equilibrium constant, there is agreement with previous works, but the upper limit of current reference values appears to be too high. This result is discussed along with possible atmospheric implications.

Abstract.The photon-ion merged-beams technique has been employed at the new PhotonIon spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe Absolute cross sections for 3d photoionization of Xe q+ ions (1 ≤ q ≤ 5) 2

The high-resolution He I photoelectron spectrum and the absolute vacuum-ultraviolet (vacuum-UV) absorption cross section (6-25 eV) as well as the ionic fragmentation of chlorine dioxide (OCIO) are reported. The photoelectron spectrum is interpreted in terms of exchange splitting effects of the various singlet and triplet cation states as well as by comparison to chemically related molecules. The vacuum-UV absorption spectrum shows different Rydberg series converging to the cation states. These Rydberg series and their vibrational progressions are assigned by term value arguments, dipole selection rules, and comparison with the photoelectron spectrum. Photoionization mass spectrometry is used for measurements of the ionization and fragmentation threshold of OCIO. The major fragment is C10+ which accurs above 13.4 eV. Thermochemical data such as heats of formation and bond dissociation energies are derived. No evidence for isomerization of OCIO+ is found, as observed for the electronically excited neutral molecule.

The C 1s -->pi* transition in molecular benzene and benzene clusters is investigated by photoion yields at high energy resolution. The vibrationally resolved band shows the same shape in clusters as in the bare molecule, but it is redshifted by 50 meV in small clusters, i.e. near the threshold of cluster formation. This redshift increases to 70 meV with increasing cluster size. The results are assigned in comparison with ab initio calculations on model structures of dimers, trimers, and tetramers. These indicate that different carbon sites in the molecular moieties give rise to distinct spectral shifts, where carbon sites that are pointing to the pi-system of another molecule show a larger redshift than the other ones. Such structural properties are found in solid benzene, so that the gas-to-solid shift of C 1s -->pi* excited benzene is derived to be a redshift which is of the order of 100-180 meV.

Photofragmentation of chlorine dioxide (OC10) and its aggregates is investigated in the UV regime [349-373 nm, A(2A2) -%(2B,) transition]. The products of UV photolysis are subsequently ionized with vacuum UV laser radiation followed by time-of-flight mass analysis. The isolated molecule decays predominantly by formation of vibrationally excited C10 (X(211)) as a result of predissociation. Highly vibrationally excited C10 is formed if even quanta of the asymmetric stretching vibration are excited. The results are discussed in relation to the UV absorption cross section and competitive fragmentation routes, such as formation of molecular oxygen. The UV photolysis of homogeneous chlorine dioxide aggregates [ (OClO),] yields fragments such as excited molecular oxygen, C~ZOS, and C1305. Evidence for evaporation of neutral molecules from excited aggregates is found as well. The results on aggregate photolysis are discussed in relation to experiments on chlorine dioxide in the gas and condensed phase. Ab initio calculations are performed in order to rationalize the experimental results. Properties, such as structures, stabilities, and vibrational frequencies, of different isomers of the OClO dimer and the photolysis product Cl2O3 are derived. Possible implications to stratospheric photochemistry are briefly discussed.