Matthew Van Duzor

Direct and Indirect Detachment in the Iodide-Pyrrole Cluster Anion: The Role of Dipole Bound and Neutral Cluster States

Photoelectron imaging probes both molecular electronic structure and electron molecule interactions. In the current work images were recorded for detachment from the I(-) x C(4)H(5)N (I(-) x pyrrole) cluster anion at wavelengths between 360 and 260 nm. The direct detachment spectra show strong similarities to those of I(-), although a strong solvent shift, broadening and some structure is observed. A nondirect, dissociative or autodetachment feature is also observed over a range of wavelengths. Ab initio calculations identify several local minima associated with neutral and anion isomers. Energy and Franck-Condon arguments are used to assess the role of these in the detachment process. The cluster anion structure is essentially an I(-) atomic anion in the presence of a neutral pyrrole molecule. The spectral structure arises due to interactions in the open shell neutral cluster residue resulting from detachment. The indirect detachment feature arises through the formation of an intermediate dipole bound cluster anion state which subsequently dissociates. The energy dependence of this channel (observed over a 0.6 eV range of photon energies) is discussed in terms of the wide amplitude motions associated with the van der Waals modes of the cluster anions.

Threshold effects in I−⋅CH3CN and I−⋅H2O cluster anion detachment: The angular distribution as an indicator of electronic autodetachment

I(-) x H(2)O and I(-) x CH(3)CN cluster anion photodetachment properties (photoelectron spectra and angular distributions) are recorded via velocity mapped photoelectron imaging for wavelengths between 270 and 340 nm, in small energy increments. These are compared with free I(-) detachment results and reveal the presence of a sharp change in the angular distribution for the (2)P(3/2) spin orbit channel in the vicinity of the (2)P(1/2) threshold. The effect is seen at this threshold in the I(-) x H(2)O cluster anion and just below threshold for I(-) x CH(3)CN. The effect is attributed to an electronic autodetachment process, which is dependent on electronic energy transfer mediated by the electron-neutral complex produced in the excitation process. These results highlight the potential of cluster anion detachment as a probe of electron-molecule interactions and in particular the sensitivity of the angular distributions to intracluster electron transfer processes.

The effect of intracluster photoelectron interactions on the angular distribution in I−⋅CH3I photodetachment

I(-) and I(-)*CH(3)I velocity mapped photodetachment images are recorded over wavelengths between 270 and 370 nm. Spectral similarities, in conjunction with ab initio calculations show that the cluster anion comprises an atomic iodide anion moiety solvated by a relatively unperturbed CH(3)I molecule. Between 340-370 nm and at 280 nm, free I(-) is produced via a process analogous to dissociative electron attachment within the cluster anion. More strikingly, the photoelectron angular distribution for each species at a given electron kinetic energy is very different, despite detachment occurring from the iodide 5p orbital in each case. These observations reveal the effect of interaction of the photoelectron with the neutral cluster residue and are discussed in terms of resonances associated primarily with the CH(3)I molecule.

I−⋅CH3X(X=Cl,Br,I) photodetachment: The effect of electron-molecule interactions in cluster anion photodetachment spectra and angular distributions

The electron kinetic energy dependence of the photoelectron spectra and angular distributions of I(-)⋅CH(3)X (X=Cl,Br,I) cluster anions are measured via velocity mapped imaging at wavelengths between 350 and 270 nm. Processes analogous to those encountered in free CH(3)X-electron interactions are revealed. In particular, the presence and energies of resonances associated with a low lying σ(∗) state have a marked effect on the results of I(-)⋅CH(3)X photoexcitation. These effects (vibrational excitation, product anion production, and alteration of the photoelectron angular distribution) are far more prominent for I(-)⋅CH(3)I. However, in the vicinity of the (2)P(1/2) threshold there is a sharp deviation in the (2)P(3/2) channel angular distribution and an enhancement of the (2)P(3/2) channel vibrational structure of all three cluster anions. These latter effects are specific to the cluster anion environment through the relaxation of the partner excited I atom and subsequent electronic autodetachment.

Intra-cluster photoelectron interactions: Scattering and dissociative attachment in halide – methyl halide cluster anions

I- and I-CH3I photodetachment over a range of wavelengths generally shows similar spectral features but strikingly different photoelectron angular distributions. The difference in angular distribution is due to intracluster electron scattering. Additionally, within narrow energy ranges, the cluster anion detachment yields new spectral transitions. These correspond to excitation of a vibrational Feshbach resonance and subsequent fragmentation, analogous to a dissociative electron attachment process. Fs time resolved studies of dissociative electron attachment are discussed.

I−·(CH3I)2 photoexcitation: The influence of dipole bound states on detachment and fragmentation

We present the results of a photoelectron imaging study of the I(-)·(CH(3)I)(2) cluster anion over excitation wavelengths 355-260 nm. The resulting spectra and photoelectron angular distributions (PADs) suggest extensive electron-molecule interaction following photoexcitation. Fragmentation channels are observed subsequent to excitation between 355 and 330 nm. The origin of these features, which begin 200 meV and peak 70 meV below the X band direct detachment threshold, is described in terms of a predissociative dipole bound state. The nature of the fragments detected and the energetics of the channel opening argue strongly in favor of an asymmetric, head to tail cluster anion geometry posited by Dessent et al. [Acc. Chem. Res. 31, 527 (1998)]. Above the direct detachment threshold, PADs display evidence of phenomena akin to electron-molecule scattering. The fragment anions disappear above the X band threshold but reappear some distance below the second (A) direct detachment band. At these energies there is also rapid variation of the X band PAD, an observation attributed to autodetachment via spin-orbit relaxation of the iodine core of the cluster.

Near threshold Cl−·CH3I photodetachment: Apparent 2P1/2 channel suppression and enhanced 2P3/2 channel vibrational excitation

Cl(-)·CH(3)I cluster anion photoelectron images are recorded over a range of detachment wavelengths in the immediate post threshold region. The photoelectron spectral features fall into two categories. A number of weak, photon energy dependent transitions are observed and attributed to atomic anion fragmentation products. Several more intense, higher electron binding energy transitions result from single photon cluster anion detachment. Comparison with I(-)·CH(3)I suggests that the detachment process is more complicated for Cl(-)·CH(3)I. The single photon transition spacing is consistent with CH(3)I ν(3) mode excitation, but the two distinct vibronic bands of I(-)·CH(3)I detachment are not easily distinguished in the Cl(-)·CH(3)I spectra. Similarly, while the spectral intensities for both cluster anions show non-Franck Condon behavior, the level of vibrational excitation appears greater for Cl(-)·CH(3)I detachment. These observations are discussed in terms of low lying electronic states of CH(3)I along the C-I coordinate, and the influence of the CH(3)I moiety on the neutral halogen atom states.

Photon-initiated charge transfer in iodide-pyrrole clusters: Direct detachment and dipole bound states

Photoelectron imaging studies of I??Pyn cluster anions, (Py = pyrrole, n = 1, 2) reveal two distinct photodetachment signatures. Transitions in which the binding energy is independent of the detachment wavelength are ascribed to direct detachment. Additional, low energy features showing a binding energy dependence on the detachment wavelength are the result of dissociative detachment via an excited, dipole bound state of the cluster anion.