Andrew J. Bell

Second harmonic generation by p-nitrophenol at water/air and water/heptane interfaces

Interfacial second harmonic generation by p-nitrophenol at air/water and heptane/water interfaces has been investigated. The preliminary results indicate that the orientation of the p-nitrophenol is slightly different in the two cases, but the influence of the resonant enhancement on the two important components of the molecular hyperpolarisability (βzzz and βzxx) differs significantly in the two systems. The difference is attributed to the change in the absorption spectrum of p-nitrophenol in heptane and water.

Photolysis of HOCl at 248 nm in a supersonic molecular beam : laser-induced fluorescence spectra of OH

The nascent OH rotational distribution has been observed following the 248 nm photolysis of HOCl in a supersonic molecular beam. A strong preference for the population of the OH 2Π3/2 spin state is found and a significant deviation from the statistical population of the OH lambda-doublet states is observed. The OH rotational distribution can be characterised by a temperature which is slightly different for the two spin–orbit states. These rotational temperatures decreases as the pressure of the Ar carrier gas used in the supersonic expansion is increased, but the spin–orbit population ratios and the lambda-doublet preference is essentially the same under all the expansion conditions studied. The experimental observations are consistent with dissociation from a 1A′ excited state and the rotational energy of the OH fragment originating partially from the zero-point bending motion of the HOCl and partially from the parent rotational motion.

Quenching of Ba(2P1) by CH4 molecules within argon clusters

Abstract A systematic study of the quenching of Ba(1P1) fluorescence by CH4 in Ar clusters has been performed for different argon cluster sizes and numbers of methane molecules in the clusters. The quenching rate as a function of the number of methane molecules in a given cluster follows a first-order rate law. Quenching rates for different cluster sizes have been deduced from the experimental data. The average time for Ba(1P1) to collide with CH4 and be quenched is 18±5 ns in an Ar1000 cluster. The quenching time varies with the cluster size as n 2 3 .

Energy dependence of the inelastic process Ba(6s6p1P1)+Ar,He→Ba(6s6p3P1,2)+Ar,He

The cross section of the inelastic transfer from Ba(6{ital s}6{ital p} {sup 1}{ital P}{sub 1}) to Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 1,2}) induced by collision with argon or helium is measured as a function of the collision energy. The existence of a 0.15 eV energy barrier in the formation of Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 1}) is observed and allows to explain why Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 2}) was the only product formed in a previous cell experiment.The cross section of the inelastic transfer from Ba(6s6p 1P1) to Ba(6s6p 3P1,2) induced by collision with argon or helium is measured as a function of the collision energy. The existence of a 0.15 eV energy barrier in the formation of Ba(6s6p 3P1) is observed and allows to explain why Ba(6s6p 3P2) was the only product formed in a previous cell experiment.

Raman-spectra of NOCl and Cl2 photodissociation dynamics

The resonance enhanced Raman spectra of NOCl and Cl2 were recorded at a range of laser wavelengths from 514nm to 355nm, in both static and flow cells. A progression of overtone transitions (up to ν′′ = 13) was observed in the Raman spectrum of Cl2 at 355nm. NO overtones were observed for the NOCl dissociation at any of the wavelengths investigated. This suggests that the NOCl photodissociation may not be as fast as was previously thought.

266 nm Photolysis of HOCl and DOCl: laser-excitation fluorescence detection of OH and OD

Fragment rotational population distributions for OH and OD were observed by laser-excitation fluorescence following the 266 nm photolysis in a free jet molecular beam of HOCl and DOCl, respectively. The photodissociation dynamics show a strong preference for the 2Π(A′) OH or OD lambda doublet component in both spin–orbit states. The 2Π3/2 state is more populated than the 2Π1/2 state of OH. The distributions are consistent with photolysis via a repulsive excited state and the measured rotational alignment (–0.28) is compatible with an excited-state symmetry of A′ for the dominant dissociation pathway. The high-J limiting value for the degree of electron alignment reaches the extreme value of –1 for the 2Π1/2 state for both OH and OD, but falls short of this for the 2Π3/2 state, reaching only –0.7.

Ab initio calculations on the ground and low-lying electronic states of the Cl3 radical : emission from Cl3

Abstract Broad band, low level emission is observed in the region 370–420 nm following the photolysis of Cl 2 in a flow system at a pressure of 400 Torr in the presence of nitrogen buffer gas. It is suggested that the decay of an excited electronic state of the Cl 3 radical is responsible for this emission. To support this claim a series of ab initio calculations on the Cl 3 radical were performed with 6-31G and 6-31G* basis sets at the SCF and CI levels. The calculated C 2v ground state agrees with previous calculations but a lower minimum with an asymmetric structure, probably a van der Waals state, was also found. Ab initio SCF calculations on the ground state of Cl − 3 were also performed.

The collision of electronically excited Ba (6s6p 1P1) with CO2 and H2O clusters

Abstract The reactions of Ba ( 1 P 1 ) with H 2 O and CO 2 monomers and large clusters are compared. By cooling the H 2 O or CO 2 beam with argon, one can eliminate all the monomers and produce a “pure” cluster beam. This allows one to compare the reactions of monomers and of clusters under clean conditions. While luminescence is observed from the reactive and inelastic collisions with the monomers, no emission is seen from the collisions with the clusters. The present observations support the idea that reaction with clusters results in the formation of a solvated product from which all visible luminescence is quenched.

Photolysis of HOBr and DOBr at 266 nm: OH and OD product-state distributions

The OH and OD rotational and vibrational product-state distributions have been recorded following the 266 nm photolysis of HOBr and DOBr, respectively, in a molecular beam. The photodissociation dynamics are similar to those observed for HOCl. The OH and OD distributions are rotationally cold and, for almost all of the states investigated, Gaussian in shape. Preferential population of the OH (or OD)2Π3/2 spin–orbit state was observed, together with a strong preference for the Π(A′)Λ-doublet levels. The alignment, β2o(02), tends towards the limiting value of –0.5 at the highest N levels that could be observed.In contrast to the photolysis of HOCl, about 10 % of the OH fragment was observed in the first excited vibrational state, with a similar rotational distribution to the OH (v= 0) products. The OD product distributions from DOBr were found to be sensitive to the molecular-beam expansion conditions with a bimodal distribution obtained for the colder expansions.The photodissociation is consistent with excitation of a non-bonding electron to a σ antibonding orbital which promotes rapid and direct bond fission via an upper state of A′ symmetry.

Molecular Collisions on Large Argon Clusters

For the experimental study of solution phase chemistry one would wish for a “perfect” system to be able to investigate. That is, a solution of known size, structure, and internal energy, with a knowledge of interaction energies of the solute and solvent. The need for such “ideal experiments” is made very acute by recent molecular dynamics calculations that open the route for modelling fundamental aspects of reaction dynamics in solution [1, 2].