The isomer of the I2–Ar complex which yields discrete bands in the B←X spectrum is shown, as expected, to be T shaped on the basis of rotational structure observed in the vibronic bands. Precise fluorescence quantum yields for I2–Ar relative to I2 were measured via simultaneous acquisition of absorption and fluorescence excitation spectra in a slit nozzle expansion. These fluorescence quantum yields provide vibrational predissociation efficiencies for B state I2–Ar as a function of vibrational state from v′ of 15 to 26. This is an oscillating function with local maxima at v′ of 16, 22, and 26. For v′=22 and 26, 73%±3% of the complexes undergo vibrational, rather than electronic predissociation. Fluorescence intensities of combination bands with excitation in the van der Waals modes were also found to have oscillating v′ dependencies with patterns nearly identical to that for the bands without van der Waals mode excitations. Thus, these oscillations must arise from the electronic predissociation channel, rather than the vibrational channel. Deconvolution of the lifetime of B state I2–Ar into vibrational and electronic lifetimes indicates that the similar overall lifetimes at v′ of 18 and 21 result from a twofold increase in the electronic lifetime at v′=21, which compensates for a decrease in the vibrational lifetime. Assumption of a smooth v′ dependence for the vibrational lifetime leads to oscillatory predicted overall lifetimes of 35, 77, 82, 51, and 30 ps over the v′ range of 20–24, respectively. Based on symmetry arguments, as well as the observed vibrational predissociation efficiencies, the electronic predissociation of I2–Ar must arise from coupling of the B state to the Πg state. This coupling may also be the dominant channel for collisional quenching of B state I2.
Optical spectra recorded with Ar and I2 in a He expansion exhibit fluorescence from an excitation continuum through a broad region of the discrete B←X transitions of I2 and I2–Ar. This fluorescence emanates from B-state I2 and arises from excitations of a bimolecular I2–Ar van der Waals complex. These results were obtained in order to test a proposed mechanism for the one-atom cage effect in I2–Ar, whereby continuum excitation to the repulsive Πu state precedes coupling onto the B state, dissociation of the complex, and fluorescence from B-state I2. The variation of the relative intensity of the observed fluorescence with excitation wavelength can be adequately reproduced with this model, but the Πu←X transition is much too weak to explain the observed absolute intensities. We consider the possible existence of a linear I2–Ar isomer in the expansion along with the well-documented T-shaped isomer. Large geometry changes for the linear isomer upon B←X excitation would result in highly dispersed Franck–Condon factors and thus split this stronger transition over a continuum. Both absolute intensities and wavelength dependences observed for fluorescence from continuum excitation fit well to the linear isomer model. Linear isomers could also be responsible for the one-atom cage effect observed at higher excitation energies.
Chemiluminescence spectra have been recorded for low-pressure, highly dilute, fuel-lean atomic oxygen flames of C 2 H 2 and C 3 O 2 ; under typical flame conditions the total pressure was 0.83 Torr with 1.1 mTorr of O atoms, 0.44 mTorr of O 2 , 0.20 mTorr of fuel, and the balance Ar. Spectral coverage was complete from 183 to 1500 nm for C 2 H 2 , and from 183 to 885 nm for C 3 O 2 . Strong CO a f X Cameron band emission was found to be the dominant feature of the chemiluminescence signature from 190 to 260 nm for C 2 H 2 oxidation under these conditions, with distinct but relatively minor CO A f X fourth positive emission between 185 and 195 nm. There is strong emission from higher triplet states of CO throughout the visible and near-IR, with CO e f a, d f a, and a′ f a emission features observed. Nearly identical CO emission spectra were obtained for both C 2 H 2 and C 3 O 2 fuels, which indicates that for both fuels the source of electronically excited CO is the C 2 O + O reaction. Quantification of emission yields for the four triplet electronic states of CO observed here indicates that little or no CO(a) is formed directly, rather its population results from radiative emission from higher triplet states. Approximately 70% of the triplet CO formed directly from the C 2 O + O reaction is in the a′ state, which had not been previously identified in this system.
Sulfur dioxide was adsorbed near 120 K on Pd( 100); dampasition occurred above 240 K to yield adsorbed SO which dissociated to form adsorbed sulfur and oxygen atoms at higher temperatures. For high SOz coverages on clean Pd( 100) or for SO2 adsorbed on the surface with a low oxygen precoverage, an T~-SO, species was formed above 300 K which decomposed at 450 K to yield SO,,,,. In the presence of p(2X2)O islands (local Bo = 0.25) SO2 reacted to form q3-S04. The v3 species also decomposed to yield it was approximately 30 kJ mol-' more stable than the v2 species. Several sparsely populated desorption states were also observed; they are attributed to destabilization of adsorbed SO4 by atomic adsorbates and by A three-parameter equation of state for strong electrolytes has been used to analyze osmotic coefficient data of chloride electrolytes in water at 25 "C up to concentrations in excess of 6 M. The analysis indicates cluster formation of neutral species at concentrations beyond 3 M. For the alkali metal halides, NaCl to CsCI, the data may be interpreted with dimer formation and a predicted mass action constant of -0.2 M-'. The other electrolyte (except SrC12) systems can be rationalized with formation of clusters larger than dimers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.