Ab initio calculations of the ground and excited states of I2− and ICl−

Relativistic four-component MRCISD+Q calculations of the six lowest valence states of molecular I 2 − anion including breit interactions

CONTEXT AND RESULTS

This study aimed to obtain potential energy curves within a multireference 4-component relativistic method and to present spectroscopic constants (Re ,ω e ,ω e xe ,ω e ye , De , D0 , Be ,α e ,β e ,γ e ), accurate extended Rydberg analytical form, and rovibrational levels for the 6 low-lying states of the I2 − anion. For these states, some spectroscopic constants, rovibrational levels, and an accurate analytical form are presented for the first time in literature, and they are of interest for femtosecond and dynamics experiments of I2 − as well as for electron attachment of I2 . This study suggests that the inclusion of relativistic and correlation effects treated at the MRCISD+Q level is needed to obtain reliable results, specially for De .

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The potential energy curves of the ground and the excited states of the molecular iodine anion (I2 − ) were investigated at multireference configuration interaction (MRCISD) with Davidson size-extensivity correction (denoted as +Q) within a fully relativistic four-component relativistic framework including Breit interaction.

Energy Transfer in the 2 u (1 D 2) Ion-Pair State of I2 by Inelastic Collisions with Noble Gas Atoms

We investigated the energy transfer in the 2 u (1 D 2) ion-pair state of I2 by collision with noble gas atoms, Ar, Kr, and Xe, using an optical-optical double resonance/fluorescence detection technique. By analyzing the temporal profiles of the emission from the laser-excited 2 u (1 D 2) state at various noble gas pressures, the quenching rate constants were determined to be (4.55 ± 0.42) × 10-10, (4.23 ± 0.11) × 10-10, and (6.83 ± 0.16) × 10-10 cm3 molecule-1 s-1 for quenching by Ar, Kr, and Xe, respectively. The 2 g (1 D 2) ion-pair state, lying in the vicinity of the 2 u (1 D 2) state, was identified as a destination state by collision with Ar and Kr. Collision with Xe provided a new reactive pathway forming the excimer XeI(B). The rate constants were determined to be = (9.61 ± 0.63) × 10-11 cm3 molecule-1 s-1 and = (4.87 ± 0.34) × 10-11 cm3 molecule-1 s-1 for the formation of the 2 g (1 D 2) state by collision with Ar and Kr, respectively, and = (6.55 ± 0.19) × 10-11 cm3 molecule-1 s-1 for the formation of XeI(B). The collisional cross sections calculated from the quenching rate constants were considerably larger than the molecular size, owing to the harpoon mechanism.

Collision induced state-to-state energy transfer dynamics between the 2u ((1)D2) and 2g ((1)D2) ion-pair states of I2

We report the first observation of collision induced state-to-state energy transfer from the 2u ((1)D2) (v2u = 3-7) ion-pair state of I2 using a perturbation facilitated optical-optical double resonance technique through the c (1)Πg∼ B (3)Π(0) hyperfine mixed double-faced valence state as the intermediate state. The excitation of the 2u ((1)D2) state yielded the weak UV fluorescence from the wide range of vibrational levels in the nearby 2g ((1)D2) state. The vibrational distribution in the 2g ((1)D2) state derived by the Franck-Condon simulation of the UV fluorescence showed that the population in the 2u ((1)D2) state transfers mostly to the 2g ((1)D2) vibronic levels which are located energetically above the laser-prepared level. The radiative lifetimes and the self-quenching rate constants were determined to be 21.3 ± 0.1 and 44.6 ± 0.8 ns, and (1.30 ± 0.01) × 10(-9) and (2.26 ± 0.17) × 10(-9) cm(3) molecule(-1) s(-1) for the 2u ((1)D2) (v2u = 3) and 2g ((1)D2) (v2g = 5) states, respectively. The rate constant for the 2u ((1)D2) - 2g ((1)D2) collision induced state-to-state energy transfer was also evaluated to be (1.89 ± 0.01), (3.07 ± 0.07), and (3.77 ± 0.05) × 10(-10) cm(3) molecule(-1) s(-1) for the v2u = 3, 5, and 7 levels, respectively. The very large self-quenching cross sections for the ion-pair states of I2 could be explained by the harpoon mechanism.

Identification of photofragmentation patterns in trihalide anions by global analysis of vibrational wavepacket dynamics in broadband transient absorption data

Photodissociation pathways of a trihalide series are systematically investigated by globally fitting vibrational wavepacket signals in broadband transient absorption spectra.

Structure and photoelectron spectral properties of I(-)·nCO2 clusters: an ab initio study

Structures and photoelectron spectral properties of I(-)·nCO(2) (n=1-7) clusters are presented at the level of second-order Møller-Plesset perturbation theory with relativistic corrections. Triple split-valence 6-311++G(d,p) basis set functions are employed herein. It is observed that the CO(2) molecules approach the I(-) anion from one side in all the clusters and that I(-)·nCO(2) clusters prefer the surface structure. The calculated vertical detachment energy of these clusters is in excellent agreement with the reported experimentally measured values (within 4%). Efforts are also made to extract vertical detachment energy of large size of clusters, including the bulk. The extracted vertical detachment energy values for larger clusters (n=8-13) by employing the microscopic theory-based expression are also close (within 4%) to that of the experimentally measured values.

Further aspects on the control of photodissociation in light-induced potentials

In this work we show how to control the photodissociation of a diatomic molecule in the frame of light-induced potentials for different shapes of the transition dipole moments. A sequence of a half-cycle or control pulse and a delayed pump pulse is used for achieving state-selective photodissociation with high yields. The effect of the control is to shift the photodissociation bands to higher frequencies. It is also possible to dissociate the molecule in a superposition of electronic states of the fragments, even when the photodissociation bands corresponding to the different electronic states of the products are largely separated. In this case one needs to engineer the sequence delaying the half-cycle pulse after the pump pulse and additionally turning off rapidly the control pulse. Depending on the shape of the dipole functions the duration of the pulses in the sequence must be constrained to shorter times as well. Finally we show that the control scheme affects the velocity of the fragments. Although broad kinetic energy distributions are always obtained when the half-cycle pulse is short, if the Stark effect implies a blueshifting in the energy of the electronic states, the distribution of the relative speed of the fragments will be redshifted.

Dynamic molecular interferometer: Probe of inversion symmetry in I2− photodissociation

Time-resolved photoelectron imaging of negative ions is employed to examine 780-nm dissociation dynamics of I2(-), emphasizing the effects of interference in time-resolved photoelectron angular distributions obtained with 390-nm probe. No energetic changes are observed after about 700 fs, but the evolution of the photoelectron anisotropy persists for up to 2.5 ps, indicating that the electronic wave function of the dissociating anion continues to evolve long after the asymptotic energetic limit of the reaction has been effectively reached. The time scale of the anisotropy variation corresponds to a fragment separation of the same order of magnitude as the de Broglie wavelength of the emitted electrons (lambda=35 A). These findings are interpreted by considering the effect of I2(-) inversion symmetry and viewing the dissociating anion as a dynamic molecular-scale "interferometer," with the electron waves emitted from two separating centers. The predictions of the model are in agreement with the present experiment and shed new light on previously published results [A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys. 118, 999 (2003)].

Time-resolved imaging of the reaction coordinate

Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr(-). The results are discussed in a side-by-side comparison with the dissociation of I(2) (-), examined under similar experimental conditions. The I(2) (-) anion, extensively studied in the past, is used as a reference system for interpreting the IBr(-) results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I(2) (-) and IBr(-) photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr(-) dissociation potential as a function of the reaction coordinate.

Photodissociation dynamics of IBr−(CO2)n, n<15

We report the ionic photoproducts produced following photoexcitation of mass selected IBr(-)(CO(2))(n), n=0-14, cluster ions at 790 and 355 nm. These wavelengths provide single state excitation to two dissociative states, corresponding to the A(') (2)Pi(1/2) and B 2 (2)Sigma(1/2) (+) states of the IBr(-) chromophore. Excitation of these states in IBr(-) leads to production of I(-)+Br and Br(-)+I( *), respectively. Potential energy curves for the six lowest electronic states of IBr(-) are calculated, together with structures for IBr(-)(CO(2))(n), n=1-14. Translational energy release measurements on photodissociated IBr(-) determine the I-Br(-) bond strength to be 1.10+/-0.04 eV; related measurements characterize the A(') (2)Pi(1/2)<--X (2)Sigma(1/2) (+) absorption band. Photodissociation product distributions are measured as a function of cluster size following excitation to the A(') (2)Pi(1/2) and B 2 (2)Sigma(1/2) (+) states. The solvent is shown to drive processes such as spin-orbit relaxation, charge transfer, recombination, and vibrational relaxation on the ground electronic state. Following excitation to the A(') (2)Pi(1/2) electronic state, IBr(-)(CO(2))(n) exhibits size-dependent cage fractions remarkably similar to those observed for I(2) (-)(CO(2))(n). In contrast, excitation to the B 2 (2)Sigma(1/2) (+) state shows extensive trapping in excited states that dominates the recombination behavior for all cluster sizes we investigated. Finally, a pump-probe experiment on IBr(-)(CO(2))(8) determines the time required for recombination on the ground state following excitation to the A(') state. While the photofragmentation experiments establish 100% recombination in the ground electronic state for this and larger IBr(-) cluster ions, the time required for recombination is found to be approximately 5 ns, some three orders of magnitude longer than observed for the analogous I(2) (-) cluster ion. Comparisons are made with similar experiments carried out on I(2) (-)(CO(2))(n) and ICl(-)(CO(2))(n) cluster ions.

Ultrafast nonadiabatic dynamics: Quasiclassical calculation of the transient photoelectron spectrum of I2−⋅(CO2)8

In this paper we investigate the transient photoelectron spectrum of I2(-) in CO2 clusters recently measured by Neumark and co-workers. This work reveals a rich excited state dynamics with various competing electronic output channels. We find good agreement with experiments and we are able to relate the transient signal to different dynamical events that occur during the evolution of the cluster and its fragmentation products.