The photodissociation dynamics of tetrachloroethylene.

We present a direct current slice imaging study of tetrachloroethylene (C(2)Cl(4)) photodissociation, probing the resulting ground state Cl ((2)P(3/2)) and spin-orbit excited state Cl* ((2)P(1/2)) products. We report photofragment images, total translational energy distributions and the product branching ratio of Cl*/Cl following dissociation at 235 and 202 nm, obtained using a two-color reduced-Doppler dissociation/probe. Near 235 nm, the Cl translational energy distribution shows a peak at the limit of the available energy, indicating a direct dissociation through a σ*(C-Cl) ← π (C=C) transition, which is superimposed on a broader underlying distribution. The ground state Cl image and associated translational energy distribution at 202 nm is broad and peaked at lower energy, suggesting either internal conversion to the ground state or a lower excited state prior to dissociation. The Cl* images are similarly broad at both wavelengths. The branching ratio is presented as a function of recoil energy, but after integration shows a near-statistical average of Cl:Cl* as 70:30 at both wavelengths. All the images are largely isotropic, with anisotropy parameters (ß) of 0.05 ± 0.03.

Slice imaging of nitric acid photodissociation: The O(1D) + HONO channel.

We report an imaging study of nitric acid (HNO(3)) photodissociation near 204 nm with detection of O((1)D), one of the major decomposition products in this region. The images show structure reflecting the vibrational distribution of the HONO coproduct and significant angular anisotropy that varies with recoil speed. The images also show substantial alignment of the O((1)D) orbital, which is analyzed using an approximate treatment that reveals that the polarization is dominated by incoherent, high order contributions. The results offer additional insight into the dynamics of the dissociation of nitric acid through the S(3) (2 (1)A(')) excited state, resolving an inconsistency in previously reported angular distributions, and pointing the way to future studies of the angular momentum polarization.

Masked velocity map imaging: a one-laser-beam Doppler-free spectroscopic technique.

A novel spectroscopic technique has been developed which makes it possible to record Doppler-free resonance-enhanced multiphoton ionization (REMPI) spectra with just one laser beam. The approach simply involves masking the outer side of the phosphor screen under velocity map imaging conditions so that only those species having no velocity component parallel to the laser beam propagation direction are detected. The benefits of this method are demonstrated in spectroscopic characterization of highly translationally and rotationally excited CO fragments resulted from the 230 nm photolysis of OCS and acetone, yielding substantially improved values of the rotational constants for the B state (v'' = 0) of the CO molecule. The resolving power and the state distribution analysis of reaction products are also demonstrated for room-temperature H atoms generated by dissociation of background hydrogen molecules and oxygen atom detected from the 225.6 nm photolysis of ozone.

Roaming dynamics in acetone dissociation.

DC slice imaging has been employed to study the photodissociation dynamics of acetone at 230 nm, with detection of the CO photoproduct via the B (v' = 0) (1)Sigma(+) <-- X (v'' = 0) (1)Sigma(+) transition. A bimodal translational energy distribution observed in the CO fragments points to two distinct dissociation pathways in the 230 nm photolysis of acetone. One pathway results in substantial translational energy release (E(ave) approximately 0.3 eV) along with rather high rotational excitation (up to J'' = 50) of CO, and is attributed to the thoroughly investigated stepwise mechanism of bond cleavage in acetone. The other dissociation pathway leads to rotationally cold CO (J'' = 0-20) with very little energy partitioned into translation (E(ave) approximately 0.04 eV) and in this way it is dynamically similar to the recently reported roaming mechanism found in formaldehyde and acetaldehyde dissociation. We ascribe the second dissociation pathway to an analogous roaming dissociation mechanism taking place on the ground electronic state following internal conversion. For acetone, this would imply highly vibrationally excited ethane as a coproduct of rotationally cold CO, with the ethane formed above the threshold for secondary decomposition. We estimate that about 15% of the total CO fragments are produced through the roaming pathway. Rotational populations were obtained using a new Doppler-free method that simply relies on externally masking the phosphor screen under velocity map conditions in such a way that only the products with no velocity component along the laser propagation direction are detected.

Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices.

The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC71BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor-acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC71BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances.

Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells.

Structural characteristics of the active layers in organic photovoltaic (OPV) devices play a critical role in charge generation, separation and transport. Here we report on morphology and structural control of p-DTS(FBTTh2)2:PC71BM films by means of thermal annealing and 1,8-diiodooctane (DIO) solvent additive processing, and correlate it to the device performance. By combining surface imaging with nanoscale depth-sensitive neutron reflectometry (NR) and X-ray diffraction, three-dimensional morphologies of the films are reconstituted with information extending length scales from nanometers to microns. DIO promotes the formation of a well-mixed donor-acceptor vertical phase morphology with a large population of small p-DTS(FBTTh2)2 nanocrystals arranged in an elongated domain network of the film, thereby enhancing the device performance. In contrast, films without DIO exhibit three-sublayer vertical phase morphology with phase separation in agglomerated domains. Our findings are supported by thermodynamic description based on the Flory-Huggins theory with quantitative evaluation of pairwise interaction parameters that explain the morphological changes resulting from thermal and solvent treatments. Our study reveals that vertical phase morphology of small-molecule based OPVs is significantly different from polymer-based systems. The significant enhancement of morphology and information obtained from theoretical modeling may aid in developing an optimized morphology to enhance device performance for OPVs.

Roaming-mediated isomerization in the photodissociation of nitrobenzene.

Roaming reactions comprise a new class of reaction in which a molecule undergoes frustrated dissociation to radicals, followed by an intramolecular abstraction reaction. Nitro compounds have long been known to dissociate to give NO as a major product. However, rates based upon isomerization via calculated tight transition states are implausibly slow, so the key dissociation pathway for this important class of molecules remains obscure. Here, we present an imaging study of the photodissociation of nitrobenzene with state-specific detection of the resulting NO products. We observe a bimodal translational energy distribution in which the slow products are formed with low NO rotational excitation, and the fast component is associated with high rotational excitation. High-level ab initio calculations identified a 'roaming-type' saddle point on the ground state. Branching ratio calculations then show that thermal dissociation of nitrobenzene is dominated by 'roaming-mediated isomerization' to phenyl nitrite, which subsequently decomposes to give C(6)H(5)O + NO.