Aqueous bromine atoms were produced by laser flash photolysis of 1,2-dibromoethane at 248 nm in solutions containing bromide ions. Forward and reverse rate constants of the title reaction were determined as functions of temperature. An analysis of potential sources of systematic errors shows that the measured forward and reverse rate constants have relative uncertainties ((σ k /k) of ∼10 and ∼25%, respectively, over the temperature range from 10.5 to 50°C. The Arrhenius parameters are (k f ( 10%) ) 5.1 × 10 12 exp (-1812/T) M -1 s -1 and (k r ( 25%) ) 2.5 × 10 10 exp (-4068/T) s -1 . The equilibrium constant is found from the ratio of k f /k r : (K eq ( 30%) ) 2.0 × 10 2 exp (2256/T) M -1 or (3.9 ( 1.2) × 10 5 M -1 at 298 K. The reaction entropy and enthalpy are ∆S R°) 44 ( 6 J mol -1 K -1 and ∆H R°) -19 ( 2 kJ mol -1 , respectively. The corresponding reaction reduction potential is ∆E°) 0.33 ( 0.01 V, in very good agreement with that calculated from half-cell potentials. In addition, preliminary rate constants for Br 2 -• + Br 2 -• f Br 3 -+ Br -and the hydrogen abstraction reaction (Br • + BrCH 2 CH 2 Br f • CBrH-CH 2 -Br + H + + Br -) are reported.
Cavity ring-down spectroscopy (CRDS) is used in two quite different contexts which demonstrate the power and versatility of the method for answering spectroscopic and dynamical questions about isolated molecules and their molecular clusters. In the first example, CRDS is used to record the direct absorption spectrum of the spinforbidden 3 Δ U <-1 Σg + transition in diacetylene (C 4 H 2 ). The upper state of this transition is thought to be responsible for the efficient photochemistry of diacetylene. An estimate of the integrated absorption cross section for the transition leads to a prediction for the intrinsic phosphorescence lifetime of the upper state of about 1 sec. In the second example, CRDS is combined with laser-induced fluorescence spectroscopy to determine relative single-vibronic level fluorescence quantum yields of jet-cooled tropolone and tropolone-(H 2 O) n clusters with n=l-4 in its S 1 state. The complexation of a single water molecule to tropolone is seen to increase the fluorescence quantum yield of tropolone by a factor of 8.0 ± 1.0.Many interesting problems in electronic spectroscopy are those which seek to answer a question of dynamical importance involving such processes as intramolecular vibrational redistribution (IVR), intersystem crossing, internal conversion, photoisomerization, photo-dissociation, and excited state photochemical reactivity.This paper presents an overview of two areas in which cavity ringdown methods have been used to address questions of dynamical interest regarding: (i) the photo-chemically reactive states of diacetylene, and (ii) solvent effects on the 'Current address:
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