Hydrogen-atom production channels of acetaldehyde photodissociation: Direct DFT molecular dynamics study

Direct density-functional (DFT) molecular dynamics (MD) calculations have been carried out for the following two hydrogen-atom production channels in acetaldehyde photodissociation on the lowest triplet-state (T₁) potential energy surface (PES): CD₃CHO → CD₂CHO + D (1) and CD₃CHO → CD₃CO + H (2). The employed DFT method was B3LYP with the cc-pVDZ basis set. The average product hydrogen kinetic energies estimated from the results of the direct DFT MD calculations were 18.3 and 16.6 kcal mol⁻¹ for reactions 1 and 2, respectively, and these were half – two thirds of the previously measured values [T.Y. Kang, S.W. Kang, H.L. Kim, Chem. Phys. Lett. 434 (2007) 6]. This is because of the low reverse barrier heights predicted at the B3LYP/cc-pVDZ level. The present results for the product hydrogen kinetic energies, however, agree qualitatively with the experimental measurements and strongly supports the mechanisms taking place on the T₁ PES.     

Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism

The photodissociation of acetaldehyde in the molecular channel yielding CO and CH(4) at 248 nm has been studied, probing different rotational states of the CO(nu = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained. One of the mechanisms is consistent with the well-studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism. This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding to a high internal energy in the CH(4) counter-fragment.     

A classical trajectory study of the photodissociation of T1 acetaldehyde: the transition from impulsive to statistical dynamics

Previous experimental and theoretical studies of the radical dissociation channel of T(1) acetaldehyde show conflicting behavior in the HCO and CH(3) product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of CH(3)CHO into HCO and CH(3) fragments over the barrier on the T(1) surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) ab initio calculations. 20,000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and CH(3) products are vibrationally cold; there is a small amount of HCO rotational excitation and little CH(3) rotational excitation, and the reaction energy is partitioned dominantly (>90% at threshold) into relative translational motion. Close to threshold the HCO and CH(3) rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and CH(3) rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reaction dynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.     

Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ～308 nm. Towards the blue edge, at 230 nm, an upper limit of ～50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.     

The formation and decay mechanisms of HCO in the photodissociation of gas phase acetaldehyde

The kinetic mechanism for the formation and decay of HCO(0,0,0) following flashlamp excitation (10 μs pulse width) into the ¹A″ → ¹A′ absorption transition of gas phase acetaldehyde (0.2 Torr) was examined by time-resolved intracavity laser detection (TRMD) and by phosphorescence lifetime measurements. The HCO radical was found to appear primarily in the vibrationless level reaching a maximum concentration about 250 μs after the excitation of acetaldehyde. The formation rate of HCO(0,0,0) was observed to be insensitive to an order of magnitude change in the number of collisions of excited-state acetaldehyde with either argon, cyclohexane, or the cell wall. Contrastingly, the decay rate of HCO exhibited a strong dependence on the collisional environment. The rate constants for HCO(0,0,0) decay by collisions with acetaldehyde, argon, and cyclohexane and by reaction with O₂ were measured by TRILD. The rate constant for O₂, quenching of ³A″ phosphorescence was also obtained.The potential for HCO(0,0,0) being either a primary or secondary dissociation product is considered in the formulation of a kinetic mechanism describing both the formation and decay behavior observed. Evidence is presented in support of a mechanism in which (1) HCO(0,0,0) is formed by the thermal reaction between acetyl radicals. CH₃CO, and ground-state acetaldehyde after excited-state acetaldehyde undergoes primary dissociation to CH₃CO, and (2) HCO(0,0,0) decays principally by collisionally-induced dissociation at the cell wall.     

State-selected imaging of HCCO radical photodissociation dynamics

We present a dc sliced ion imaging study of HCCO radical photodissociation to CH and CO at 230 nm. The measurements were made using a two-color reduced Doppler probe strategy. The CO rotational distribution was consistent with a Boltzmann distribution at 3500 K. Using the dc slice ion imaging approach, we obtained CO images for various rotational levels of CO (v=0). The results are largely consistent with earlier work, albeit with a significant 0.9 eV peak seen previously in the translational energy distributions absent in our state-selected imaging study.     

Laser-induced fluorescence emission from HCO produced by 308 nm excimer laser photodissociation of acetaldehyde

The LIF emission spectrum of the HCO Ā ²A″-X̄ ²A′ transition has been analyzed, and ν₁″ = 2432 ± 20 cm⁻¹ was determined. The HCO(X̄) radicals generated by a photolysis pulse from the 5 Torr CH₃CHO sample disappear by second-order kinetics involving radical—radical reactions.     

Theoretical study of the ArH+ photodissociation

The multireference Spin-Orbit (SO) Configuration Interaction (CI) method in its Lambda-S Contracted SO-CI (LSC-SO-CI) version is employed to calculate potential energy curves for the ground and low-lying excited states of the ArH(+) cation. For the first time, electric dipole moments are also computed in the approach, including SO coupling for transitions to the states responsible for the first absorption continuum (A-band) of ArH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that absorption in the A-band is dominated by the parallel A(1)Sigma(+)<--X(1)Sigma(+) transition. In the low-energy part of the band (<95 x 10(3) cm(-1)) the absorption is caused by the perpendicular B(1)Pi<--X(1)Sigma(+) excitation, but transitions to the b(3)Pi(0(+),1) states are also not negligible. The branching ratio Gamma for the final photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or= 10(5) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Ar(+)((2)P(1/2)) ions, and thus leads to the inverse population of the Ar(+)((2)P(1/2)-(2)P(3/2)) ion states.     

A photodissociation study of CH2BrCl in the A-band using the time-sliced ion velocity imaging method

Employing a high-resolution (velocity resolution deltanu/nu<1.5%) time-sliced ion velocity imaging apparatus, we have examined the photodissociation of CH2BrCl in the photon energy range of 448.6-618.5 kJ/mol (193.3-266.6 nm). Precise translational and angular distributions for the dominant Br(2P32) and Br(2P12) channels have been determined from the ion images observed for Br(2P32) and Br(2P12). In confirmation with the previous studies, the kinetic-energy distributions for the Br(2P12) channel are found to fit well with one Gaussian function, whereas the kinetic- energy distributions for the Br(2P32) channel exhibit bimodal structures and can be decomposed into a slow and a fast Gaussian component. The observed kinetic-energy distributions are consistent with the conclusion that the formation of the Br(2P32) and Br(2P12) channels takes place on a repulsive potential-energy surface, resulting in a significant fraction (0.40-0.47) of available energy to appear as translational energy for the photo fragments. On the basis of the detailed kinetic-energy distributions and anisotropy parameters obtained in the present study, together with the specific features and relative absorption cross sections of the excited 2A', 1A", 3A', 4A', and 2A" states estimated in previous studies, we have rationalized the dissociation pathways of CH2BrCl in the A-band, leading to the formation of the Br(2P32) and Br(2P12) channels. The analysis of the ion images observed at 235 nm for Cl(2P(32,12)) provides strong evidence that the formation of Cl mainly arises from the secondary photodissociation process CH2Cl + hnu --> CH2 + Cl.     

Ion imaging study of NO3 radical photodissociation dynamics: characterization of multiple reaction pathways

The photodissociation of NO(3) has been studied using velocity map ion imaging. Measurements of the NO(2) + O channel reveal statistical branching ratios of the O((3)P(J)) fine-structure states, isotropic angular distributions, and low product translational energy consistent with barrierless dissociation along the ground state potential surface. There is clear evidence for two distinct pathways to the formation of NO + O(2) products. The dominant pathway (>70% yield) is characterized by vibrationally excited O(2)((3)Σ(g)(-), v = 5-10) and rotationally cold NO((2)Π), while the second pathway is characterized by O(2)((3)Σ(g)(-), v = 0-4) and rotationally hotter NO((2)Π) fragments. We speculate the first pathway has many similarities to the "roaming" dynamics recently implicated in several systems. The rotational angular momentum of the molecular fragments is positively correlated for this channel, suggesting geometric constraints in the dissociation. The second pathway results in almost exclusive formation of NO((2)Π, v = 0). Although product state correlations support dissociation via an as yet unidentified three-center transition state, theoretical confirmation is needed.     

Slice imaging of quantum state-to-state photodissociation dynamics of OCS

Slice imaging experiments are reported for the quantum state-to-state photodissociation dynamics of OCS. Both one-laser and two-laser experiments are presented detecting CO(J) or S((1)D(2)) photofragments from the dissociation of hexapole state-selected OCS(v(2) = 0,1,2 / J = 1,2) molecules. We present data using our recently developed large frame CCD centroiding detector and have implemented a new high speed MCP high voltage pulser with an effective slice width of only 6 ns. Slice images are presented for quantum state-to-state photolysis, near 230 nm, of vibrationally excited OCS(v(2) = 0,1,2). Two-laser pump-probe experiments detecting CO(J = 63 or 64) show a dramatic change in the beta parameter for the same final state of CO(J) when the photolysis energy is reduced by about 1000 cm(-1). We attribute the observed change from large positive to large negative beta to a large increase of the molecular frame deflection angle at very slow recoil velocity, due to a breakdown of the axial recoil. Two-laser experiments on the S((1)D(2)) fragment reveal single well-separated rings in the slice images correlating with individual CO(J) states. Strong polarization effects of the probe laser are observed, both in the angular distribution of the intensity of single S((1)D(2)) rings and in the resolution of the radial velocity distribution. It is shown how the broadening of the velocity distribution can be reduced by a directed ejection of the electron in the ionization process perpendicular to the slice imaging plane. The dissociation energy of OCS(v(2) = 0, J = 0) --> CO(J = 0) + S((1)D(2)) is determined with high accuracy D(0) = (34 608 +/- 24) cm(-1).     

Ab initio study of the KrH+ photodissociation

The multireference spin-orbit configuration interaction method is employed to calculate potential energy curves for the ground and low-lying excited states of the KrH(+) cation. For the first time, the spin-orbit interaction is taken into account and electric dipole moments are computed for transitions to the states responsible for the first absorption continuum (A band) of KrH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that the A-band absorption is dominated by the parallel A (1)Sigma(+)<--X (1)Sigma(+) transition. In the low-energy part of the band (<83x10(3) cm(-1)) the absorption is mainly caused by the spin-forbidden b (3)Pi(0(+) )<--X (1)Sigma(+) excitation, while perpendicular transitions to the B (1)Pi and b (3)Pi(1) states are significantly weaker. The branching ratio Gamma for the photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or=90x10(3) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Kr(+)((2)P(12)) ions, which may be used to obtain laser generation on the Kr(+)((2)P(12)-(2)P(32)) transition.     

A slice imaging and multisurface wave packet study of the photodissociation of CH3I at 304 nm

The role of the conical intersection between the (1)Q(1) and (3)Q(0) excited states in the photodissociation of CH(3)I at 304 nm is investigated drawing a comparison between the adiabatic--through direct absorption to the (3)Q(1) state--and non-adiabatic--via the (1)Q(1)→(3)Q(0) conical intersection--production of I atoms in the ground (2)P(3/2) state. The versatility of the slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of I((2)P(3/2)) atoms allow distinct measurements of the competing processes. The I((2)P(3/2)) atom kinetic energy distributions (KEDs) obtained in both cases reflect inverted vibrational progressions of the ν(2) umbrella mode of the CH(3) co-product. The experimental results show a satisfactory agreement with multisurface wave packet calculations using a reduced dimensionality (pseudotriatomic) model carried out on the available ab initio potential energy surfaces.     

The photodissociation of H2S: A study of indirect photodissociation

We have performed calculations for the photodissociation of H₂S using surfaces constructed to test a model proposed by van Veen et al., in which the dissociation occurs via predissociated levels of the bound ¹B₁ excited state. Total Franck-Condon factors for the photodissociation and partial Franck-Condon factors for the product vibrational distributions are presented.     

High-resolution slice imaging of quantum state-to-state photodissociation of methyl bromide

The photodissociation of rotationally state-selected methyl bromide is studied in the wavelength region between 213 and 235 nm using slice imaging. A hexapole state selector is used to focus a single (JK=11) rotational quantum state of the parent molecule, and a high speed slice imaging detector measures directly the three-dimensional recoil distribution of the methyl fragment. Experiments were performed on both normal (CH(3)Br) and deuterated (CD(3)Br) parent molecules. The velocity distribution of the methyl fragment shows a rich structure, especially for the CD(3) photofragment, assigned to the formation of vibrationally excited methyl fragments in the nu(1) and nu(4) vibrational modes. The CH(3) fragment formed with ground state Br((2)P(3/2)) is observed to be rotationally more excited, by some 230-340 cm(-1), compared to the methyl fragment formed with spin-orbit excited Br((2)P(1/2)). Branching ratios and angular distributions are obtained for various methyl product states and they are observed to vary with photodissociation energy. The nonadiabatic transition probability for the (3)Q(0+)-->(1)Q(1) transition is calculated from the images and differences between the isotopes are observed. Comparison with previous non-state-selected experiments indicates an enhanced nonadiabatic transition probability for state-selected K=1 methyl bromide parent molecules. From the state-to-state photodissociation experiments the dissociationenergy for both isotopes was determined, D(0)(CH(3)Br)=23 400+/-133 cm(-1) and D(0)(CD(3)Br)=23 827+/-94 cm(-1).     

Computational study toward understanding the photodissociation mechanism of sarin

The potential energy surface for sarin (C₄H₁₀FO₂P) dissociation into (CH₃)₂CHO+PO(F)(CH₃), CH₃+(CH₃)₂CHOPO(F), and F+(CH₃)₂CHOPO(CH₃) in the T₁ and S₁ states were investigated at the complete‐active‐space self‐consistent field (CASSCF) with the 6‐31G** and aug‐cc‐PVDZ basis sets. The different reaction pathways are characterized on the basis of the computed potential energy surface and surface crossing point, the time‐dependent density functional theory (TD‐DFT) was used to calculate the vertical energies based on the CAS(8,7)/6‐31G** optimized excited structures, which may provide some new insights into the mechanism of the ultraviolet photo‐degradation of sarin molecules. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Time-resolved electron detachment imaging of the I- channel in I2Br- photodissociation

The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment's electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.     

乙烯酮光解途径的从头算研究

本文分别考察了C~2~α方式以及C~3-面内弯曲和C~3 一面外弯曲方式下乙烯酮光解脱氢反应(CH~2CO→H~2+C~2O)的状态相关图. 对由此给出的对称性允许的光解脱氢途径进行了从头算研究.发现最具可能的途径是在C~3-面内弯曲方式中^1A″态势能面上进行的绝热光反应,其能垒高度为263.83KJ.mol^-^1.     

Velocity map imaging of HBr photodissociation in large rare gas clusters

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.