Investigation of 3-fragment photodissociation of O3 at 193.4 and 157.6 nm by coincident measurements.

Photodissociation of the ozone molecule at 193.4 nm (6.41 eV) and 157.6 nm (7.87 eV) is studied by fast-beam translational spectroscopy. Coincident detection of the dissociation products allows direct observation of the 3-fragment channel and determination of its kinematic parameters. The results indicate that at each wavelength, 3-fragment dissociation proceeds through synchronous concerted bond breaking, but the energy partitioning among the fragments is different. The branching fraction of the 3-fragment channel increases from 5.2(6)% at 193.4 nm to 26(4)% at 157.6 nm, in agreement with previous studies. It is shown that vibrational excitation of the symmetric stretch mode in O3 molecules created by photodetachment of O(3)(-) anion enhances the absorption efficiency, especially at 193.4 nm, but does not have a strong effect on the 3-fragment dissociation.

Spectroscopic and theoretical investigations of adenosine 5'-diphosphate and adenosine 5'-triphosphate dianions in the gas phase.

Doubly deprotonated adenosine 5'-diphosphate ([ADP-2H](2-)) and adenosine 5'-triphosphate ([ATP-2H](2-)) dianions were investigated using infrared multiple photon dissociation (IR-MPD) and photoelectron spectroscopy. Vibrational spectra acquired in the X-H stretch region (X = C, N, O) and augmented by isotope-labelling were compared to density functional theory (DFT) calculations at the B3LYP/TZVPP level. This suggests that in [ATP-2H](2-) the two phosphate groups adjacent to the ribose ring are preferentially deprotonated. Photoelectron spectra recorded at 4.66 and 6.42 eV photon energies revealed adiabatic detachment energies of 1.35 eV for [ADP-2H](2-) and 3.35 eV for [ATP-2H](2-). Repulsive Coulomb barriers were estimated at ~2.2 eV for [ADP-2H](2-) and ~1.9 eV for [ATP-2H](2-). Time-dependent DFT calculations have been used to simulate the photoelectron spectra. Photodetachment occurs primarily from lone pair orbitals on oxygen atoms within the phosphate chain.

Two- and three-body photodissociation dynamics of diiodobromide (I2Br-) anion.

The photodissociation of gas-phase I(2)Br(-) was investigated using fast beam photofragment translational spectroscopy. Anions were photodissociated from 300 to 270 nm (4.13-4.59 eV) and the recoiling photofragments were detected in coincidence by a time- and position-sensitive detector. Both two- and three-body channels were observed throughout the energy range probed. Analysis of the two-body dissociation showed evidence for four distinct channels: Br(-) + I(2), I(-) + IBr, Br+I(2) (-), and I + IBr(-). In three-body dissociation, Br((2)P(3∕2)) + I((2)P(3∕2)) + I(-) and Br(-) + I((2)P(3∕2)) + I((2)P(3∕2)) were produced primarily from a concerted decay mechanism. A sequential decay mechanism was also observed and attributed to Br(-)((1)S)+I(2)(B(3)Π(0u) (+)) followed by predissociation of I(2)(B).

Intramolecular competition in the photodissociation of C(3)D(3) radicals at 248 and 193 nm.

Motivated by recent experimental work, a theoretical study of the photodissociation of perdeuterated propargyl (D(2)CCD) and propynyl (D(3)CCC) radicals has been carried out, focusing on the C-C bond cleavage and D(2) loss channels. High-level ab initio calculations were carried out, and RRKM rate constants were calculated for isomerization and dissociation pathways. The resulting reaction barriers, microcanonical rate constants and product branching ratios are consistent with the experimental findings, supporting the overall mechanism of internal conversion followed by statistical dissociation on the ground state surface. We found loose transition states and very low exit barriers for two of the C-C bond cleavage channels and an additional CD(2) + CCD channel, which had not been reported previously. Our results probe the extent of propargyl and propynyl isomerization prior to dissociation at 248 and 193 nm and deliver a comprehensive picture of all ongoing molecular dynamics.

Photodissociation of the propargyl and propynyl (C(3)D(3)) radicals at 248 and 193 nm.

The photodissociation of perdeuterated propargyl (D(2)CCCD) and propynyl (D(3)CCC) radicals was investigated using fast beam photofragment translational spectroscopy. Radicals were produced from their respective anions by photodetachment at 540 and 450 nm (below and above the electron affinity of propynyl). The radicals were then photodissociated at 248 or 193 nm. The recoiling photofragments were detected in coincidence with a time- and position-sensitive detector. Three channels were observed: D(2) loss, CD+C(2)D(2), and CD(3)+C(2). Observation of the D loss channel was incompatible with this experiment and was not attempted. Our translational energy distributions for D(2) loss peaked at nonzero translational energy, consistent with ground state dissociation over small (<1 eV) exit barriers with respect to separated products. Translational energy distributions for the two heavy channels peaked near zero kinetic energy, indicating dissociation on the ground state in the absence of exit barriers.

D atom loss in the photodissociation of the DNCN radical: implications for prompt NO formation.

The photodissociation of DNCN following excitation of the C 2A"<--X 2A" electronic transition was studied using fast beam photofragment translational spectroscopy. Analysis of the time-of-flight distributions reveals a photodissociation channel leading to D+NCN competitive with the previously observed CD+N2 product channel. The translational energy distributions describing the D+NCN channel are peaked at low energy, consistent with internal conversion to the ground state followed by statistical decay and the absence of an exit barrier. The results suggest a relatively facile pathway for the reaction CH+N2-->H+NCN that proceeds through the HNCN intermediate and support a recently proposed mechanism for prompt NO production in flames.

Dissociative photodetachment studies of I2-.Ar: coincident imaging of two- and three-body product channels.

Van der Waals clusters serve as prototypical systems for studying processes of energy transfer. The I2.Ar system has attracted particular interest due to the wide array of decay processes occurring in competition with one another. Here, we present systematic dissociative photodetachment (DPD) studies of the I2- and I2-.Ar anions in the region 4.24-4.78 eV. The resulting neutral fragments are detected by time- and position-sensitive (TPS) coincident imaging. Photofragment mass distributions and translational energy distributions from the DPD of I2- are presented and facilitate understanding of the I2.Ar system. For the I2.Ar complex, channels resulting from two-body dissociation leading to I2+Ar photoproducts are observed at all photon energies employed. We also report the first direct observation of the previously inferred three-body dissociation channel leading to I+I+Ar photoproducts. The relative intensities of each decay channel are investigated in relation to the electronic state being accessed. Translational energy distributions of the I2.Ar complex lend further insight into the decay mechanism for each channel.