Kenneth P. Lawley

EINSTEIN A-COEFFICIENTS AND TRANSITION DIPOLE-MOMENTS FOR SOME ION-PAIR TO VALENCE TRANSITIONS IN I2

The relative fluorescence intensities of all the strong electronic transitions from five of the first cluster of ion-pair (IP) states of I2 and from three states in the second cluster have been measured. The fluorescence, in most cases from ν′ = 0, was dispersed and comprised both discrete and continuous (bound → free) spectra. These were fitted and each simulated electronic spectrum integrated separately to give relative values of the Einstein A-coefficients (summed over final vibrational states) for each transition. Using previously reported lifetimes, transition dipoles for each of the parallel transitions from nine IP states are deduced, together with some of the stronger perpendicular transition dipole moments. At R e of the ion-pair states, μ‖ (five transitions observed) is almost constant at 0·2–0·3 D, but μ‖ ranges between 0·7 and 3·7 D. There is a marked difference in the parallel transition moments associated with some g/u pairs in the same cluster of IP states which can be accounted for by the ...

The resonance enhanced (2+1) multiphoton ionization spectrum of I2

Abstract The resonance enhance (2+1) multiphoton ionization spectrum of I 2 has been recorded in the region 48000–75300 cm −1 using both linearly and circularly polarized light. Rydberg states ( ns ) up to n =11 based on the [ 3 2 ]g ion core and up to n = 8 on the [ 1 2 ]g core have been observed, together with three 5d states based on each core. Of these, twelve states have been observed for the first time and six other states have been reassigned. Assignments have been made by comparing the observed molecular quantum defects with those of atomic iodine, by using band contour analysis and by measurement of polarization ratios. Polarization ratios which are widely different from those expected have been observed for some systems.

Dispersion and polarization forces associated with the ion-pair states of diatomic molecules

Abstract Heteronuclear ion-pair states have large dipole moments that are absent in the corresponding homonuclear states. It is shown that the large intermolecular polarization forces that arise in the heteronuclear case are precisely matched by anomalously large dispersion forces in the homonuclear case that come from a giant g ↔ u electronic transition. In the halogen and interhalogen IP states orbiting cross sections at thermal energies are typically > 150 A2, making most inelastic and reactive processes of IP states orbiting controlled. Collision induced g ↔ u transitions may have asmall (⪅20 A2) direct contribution from b values greater than the critical value for orbiting.

Structural isomerism in the I2Ar Van der Waals complex studied by ZEKE-PFI spectroscopy: evidence for both linear and T-shaped isomers

Evidence for the existence of two structural isomers of the I2Ar Van der Waals complex has been obtained from REMPI spectroscopy and ZEKE-PFI (zero kinetic energy-pulsed field ionisation) spectroscopy. The mass-resolved REMPI spectrum of the complex in the [2∏32]c 7s; 2g Rydberg state shows two overlapping vibrational progressions which have been assigned to excitation of the T-shaped and linear isomers. The two-colour ZEKE-PFI spectrum of the ground electronic state of the ion recorded via a number of vibrational levels in the [2∏32]c 7s; 2g Rydberg state show two well separated vibrational progressions. The measured difference of 43 cm−1 in the band origins for the two progressions provides an indication of the relative stabilities of the two isomers in the ion, with the linear isomer being the more weakly bound.

Spectroscopy and electronic structure of ion-pair states

Ion-pair states are a distinctive sub-set of the valence states of molecules. In a few cases they constitute the ground electronic state, but more usually their term values lie close to the lowest Rydberg states. Their unusual spectroscopic properties and the techniques used to access them are discussed with particular reference to the diatomic halogens. Their electronic structure is best approached from two complementary points of view, as a single molecular orbital configuration or the product of separated ion configurations, according to the ionic separation. The consequences of this dual character for absorption and fluorescence spectroscopy are outlined. Finally, the state-specific coupling of Rydberg and ion-pair states is surveyed.

Vacuum ultraviolet absorption and fluorescence excitation spectra of Br2

Abstract Absorption and fluorescence excitation spectra of Br 2 in the region 125–170 nm have been recorded using tunable synchrotron radiation. Computer simulations of the absorption and dispersed fluorescence spectra have allowed identification of the upper electronic state responsible for the main fluorescence excitation system (150–167 nm), as the D(0 + u ) ion-pair state. A potential function for this state is presented which accounts for vibrational levels up to ν′ ≈ 170 and a pronounced inflection on the inner wall of this potential, due to an avoided crossing, is identified at T e + 15000 cm −1 . The mean radiative lifetime of the D(0 + u ) state has been determined as ≈ 9 ns. An analysis of the 320–360 nm structured continuum fluorescence, from the D(0 + u ) state to a lower repulsive state, is also given.

Re-analysis of the ultraviolet absorption spectrum of ozone

A re-analysis of the Huggins and Hartley bands in the ultraviolet absorption spectrum of O3 is presented in which the structure in both bands is assigned to vibrational progressions in the symmetric stretching mode ν1 and the bending mode ν2 but not the asymmetric stretching mode ν3. The present improved fit of a larger range of bands results in relatively large anharmonicity terms, whereas previous assignments have relied on the motion being largely harmonic in the upper state. From a consideration of the shape of single surface required to support both sets of vibrational data, it is concluded that the Huggins bands almost certainly terminate on the 2 1A1 state rather than the 1 1B2 state.

Elastic scattering of alkali atoms from iodine atoms and molecules at thermal energies

High-resolution, small-angle measurements of the elastic differential cross sections have been made for the systems K/I and K/I2. In all cases the structure at small angles is found to arise predominantly from collisions occurring along the adiabatic potential. The off-diagonal matrix element of the electronic hamiltonian between the covalent and ionic states in the K/I system is found to be ∼ 6×10-15 ergs.

Photodissociation of ozone between 335 and 352 nm to give O2(b)+O()

Abstract The previously unobserved O 2 (b 1 Σ + g )+O( 3 P J ) spin-forbidden photodissociation channel of ozone is identified following absorption in the Huggins band (335–352 nm). The nascent O 2 (b 1 Σ + g ) photofragments are detected by two-colour resonance enhanced multiphoton ionisation (REMPI) via the d3sσ g 1 Π g Rydberg state.

Resonance enhanced (2+1) multiphoton ionization spectrum of Br2: The lower Rydberg states

Abstract The resonance enhanced (2+1) multiphoton ionization spectrum of Br 2 has been recorded in the region 61000–85000 cm −1 and gerade Rydberg states up to n =11 have been identified. Two series are seen, based on the Ω= 3 2 g and 1 2 g core states. All of the higher states are members of the n s series but four components of the [Ω] 4d multiplet close in energy to [Ω] 6s have been identified for the first time. The quantum defects are very similar to those of the Br atom and to those of the lower members of the d series in CH 3 Br. The s series is at least an order of magnitude more intense than the d series, but intensities within each [Ω c ] n s←-X and [Ω c ]4d←X multiplet present some unexpected features.