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Examines the complex between nitric oxide and nitrogen gas. Discussion on the appearance of the spectrum; Results showing high-level of ab initio calculations; Use of Rydberg-function-augmented basis sets for the study.

High-level ab initio calculations are used to calculate the thermodynamics and kinetics of a number of ligand-exchange reactions involving Na+·X complexes (X=O,O2,N2,CO2 and H2O). The exchange reaction Na+·N2+O⇌Na+·O+N2 is examined in detail using RRKM theory, where the microcanonical rate coefficients are determined from inverse Laplace transformation of the Langevin collision frequencies. This shows that ligand-switching dominates over recombination (to form O·Na+·N2) up to pressures above 1000 Torr. A model of the ion–molecule chemistry of sodium in the mesosphere/lower thermosphere region is then constructed by calculating the rate coefficients of these ligand-switching reactions using Langevin theory, constrained by detailed balancing. The model predicts vertical profiles of Na+, Na+·CO2 and Na+·H2O in very good agreement with the limited set of rocket-borne mass spectrometer measurements. The lifetime of Na+ against neutralisation to Na is shown to decrease exponentially from about 1 day at 110 km to about 1 min at 90 km, which supports the theory that sporadic sodium layers form from Na+ in descending sporadic E layers. [Copyright &y& Elsevier]

The A˜ [sup 2]Σ[sup +] state of Ar·NO is studied using (1+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Higher quality spectra than obtained in other studies allow the identification of a number of previously unreported features. The spectrum is analyzed using two models: a rigid van der Waals complex in which NO is weakly bonded to Ar; and a complex in which the free internal rotation of NO is hindered by the anisotropy caused by the presence of the Ar atom. It is concluded that as the intermolecular stretch is excited, then the anisotropy decreases, and the angular motion of the complex becomes more and more like that of a free rotor. Near the origin, the complex has an average geometry approaching linear, whereas when the intermolecular stretch is excited, an average geometry closer to T-shaped occurs; however, when the anisotropy is small, the concept of geometry becomes ill-defined. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman (267 nm pump and 400 nm probe excitation wavelengths) investigation of the initial formation and vibrational cooling of the iso-CH[sub 2]I-I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH[sub 2]I-I photoproduct species is predominantly responsible for the ∼385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Similar results were obtained in both nonpolar solution (cyclohexane solvent) and polar solution (acetonitrile) solvent. The picosecond resonance Raman spectra confirm that the iso-CH[sub 2]I-I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently undergoes vibrational cooling on the 4-50 ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transient absorption study. We discuss a possible qualitative scenario for the formation of the iso-CH[sub 2]I-I species that is in agreement with the available gas phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH[sub 2]I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase spectra of the iso-CH[sub 2]I-I photoproduct as well as previously reported femtosecond transient absorption bands that are broad and weak in the 300-500 nm region over the 0.3-3 ps time scale. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]