CO Photofragment Research Articles

Wavelength-Dependent Dynamics of the O(1D2) Channel in the 1Δu State Photodissociation of CO2.

The wavelength-dependent dynamics of the O(1D2) channel, formed by photoexcitation of CO2 to the 1Δu state at 143.53-153.03 nm, is investigated by using the time-sliced velocity-mapped ion imaging method. The measured ionic peaks of the O(1D2) images are analyzed to determine the total kinetic energy release (TKER) spectra and image anisotropy parameters. The structures observed in the TKER spectra can be directly assigned to the ro-vibrational state distributions of the counter CO photofragments. Compared to those observed at 157 and 155 nm, the highly rotationally excited CO photofragments still obviously appear in v = 0 and 1, but the fraction of rotational excitations is significantly reduced. Conversely, the CO photofragments exhibit substantially higher vibrational excitations, implying that the nearly linear 21A' state also contributes to dissociation in addition to the bend configuration. The image anisotropy parameters display an extremely slow decreasing trend with an increase of the CO ro-vibrational state besides those for the highest ro-vibrationally excited CO photofragments. Nevertheless, the nonaxial recoil effect, suggested in previous photodissociation studies of CO2 and other triatomic molecular systems, is still appropriate to explain the observations of internal energy dependences of image anisotropy parameters.

Generation of Highly Vibrationally Excited CO in Sequential Photodissociation of Iron Carbonyl Complexes

Ultraviolet photochemistry of iron pentacarbonyl, Fe(CO)5, was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO photofragments, generated by ultraviolet irradiation of Fe(CO)5, showed the generation in the highly vibrationally excited states with v = 11-15. Analysis of the band intensities observed in the 213-235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe(CO)n=1-4, were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photodissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + hν → Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe(CO)4, and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe(CO)n intermediates.

Primary and Secondary Loss of CO and NO Ligands in the Ultraviolet Photodissociation of the Heteroleptic Co(CO)3NO Complex.

The photodissociation dynamics of the heteroleptic Co(CO)3NO complex were investigated in the metal-to-carbonyl (CO) ligand charge-transfer band to compare the reactivity of the CO and nitrosyl (NO) ligands. The final state distributions of both the CO and NO fragments were measured using resonance-enhanced multiphoton ionization (REMPI) spectroscopy and velocity-map ion-imaging. The primary CO photofragment was differentiated from the secondary fragments of the subsequent unimolecular decomposition of coordinatively unsaturated intermediates by comparing the momentum distributions. The internal energy of the Co(CO)2NO intermediate was sufficiently high (≥348 kJ/mol) to be generated in the electronic excited state, indicating the occurrence of the primary CO elimination on an excited state. The NO fragments exhibited two velocity components. The analysis of the final state distributions suggested that the higher- and lower-kinetic-energy components originated from the direct primary elimination and sequential elimination, respectively. The direct photoelimination through a transiently bent ligand conformation was illustrated on the basis of a two-dimensional REMPI approach and time-dependent density functional theory calculations. The present results of both ligands demonstrate the correlation between elimination mechanisms and possible ligand conformations in the electronic excited state.

Anomalous Intensities in the 2+1 REMPI Spectrum of the E 1Π-X 1Σ+ Transition of CO.

We report on one-color experiments near 214 nm involving the photodissociation of jet-cooled OCS to produce high rotational states (40 < J < 80) of CO (X 1Σ+, v = 0, 1) which were then ionized by 2+1 resonance-enhanced multiphoton ionization via the E 1Π state. The nominally forbidden Q-branch of the two-photon E 1Π-X 1Σ+ transition is observed with intensity comparable to the allowed R-branch. The bright character of the high- J Q-branch lines can be described quantitatively as intensity borrowing due to mixing of the E 1Π and C 1Σ+ states, using J-dependent mixing coefficients extrapolated from the observed Λ-doubling in the lower rotational levels of the E state. In addition to the significant enhancement of Q-branch intensities above the values predicted by conventional two-photon line strengths for a 1Π-1Σ+ transition, the high- J lines of the R- and P-branches appear to be suppressed in intensity by approximately a factor of 3 compared to the unperturbed low- J line strengths, most likely due to perturbations associated with a 1Σ- state. The E-state rotational term values for J < 80, v = 0 derived from the present spectra agree within our measurement and calibration uncertainties with the extrapolations based on the molecular constants previously derived from rotational levels with J < 50. The E-X transition is attractive for future application to photodissociation dynamics and rotational polarization measurements of CO photofragments, with convenient access to state-selective probing on multiple rotational branches, which exhibit different sensitivity to fragment alignment.

ChemInform Abstract: State Resolved Photofragmentation of Ni(CO)4 at 193, 248, and 308 nm: A Detailed Study of the Photodissociation Dynamics.

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser‐induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, wi...

Photodissociation of laboratory oriented molecules: Revealing molecular frame properties of nonaxial recoil

We report the photodissociation of laboratory oriented OCS molecules. A molecular beam of OCS molecules is hexapole state-selected and spatially oriented in the electric field of a velocity map imaging lens. The oriented OCS molecules are dissociated at 230 nm with the linear polarization set at 45 degrees to the orientation direction of the OCS molecules. The CO(nu=0,J) photofragments are quantum state-selectively ionized by the same 230 nm pulse and the angular distribution is measured using the velocity map imaging technique. The observed CO(nu=0,J) images are strongly asymmetric and the degree of asymmetry varies with the CO rotational state J. From the observed asymmetry in the laboratory frame we can directly extract the molecular frame angles between the final photofragment recoil velocity and the permanent dipole moment and the transition dipole moment. The data for CO fragments with high rotational excitation reveal that the dissociation dynamics is highly nonaxial, even though conventional wisdom suggests that the nearly limiting beta parameter results from fast axial recoil dynamics. From our data we can extract the relative contribution of parallel and perpendicular transitions at 230 nm excitation.

State-to-state photodissociation of OCS (ν2=0,1|JlM). I. The angular recoil distribution of CO (X 1Σ+;v=0|J)

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2=0,1|JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v=0|J) rotational distribution as well as the angular recoil anisotropy parameter β of the CO photofragment are reported for dissociation of single rovibrational (ν2=0,1|JlM) quantum states of OCS. A strong dependence of the anisotropy parameter β on the initial bending state, ν2=0 or 1, of OCS is observed. The effects of the initial bending state of OCS are rationalized in terms of the strong angular dependence of the transition dipole moment function of OCS for the 1 1Σ−(1 1A″) and 1 1Δ(2 1A′) excited state surfaces involved in the dissociation at 230 nm. The state-to-state imaging experiment provides a revised and improved determination of the binding energy of OCS (ν1,ν2,ν3=0,0,0|J=0)→CO (X 1Σ+;v=0|J=0)+S (1D2), D0=(4.284±0.009) eV.

State-resolved translation energy distributions for NCO photodissociation

The photodissociation dynamics of NCO have been examined using fast beam photofragment translational spectroscopy. Excitation of the 102, 301, and 102302 transitions of the B̃ 2Π←X̃ 2Π band produces N(4S)+CO photofragments exclusively, while excitation of the 103303 transition yields primarily N(2D)+CO photoproducts. The translational energy [P(ET)] distributions yield D0(N–CO)=2.34±0.03 eV, and ΔHf,00(NCO)=1.36±0.03 eV. The P(ET) distributions exhibit vibrationally resolved structure reflecting the vibrational and rotational distributions of the CO product. The N(2D)+CO distribution can be fit by phase space theory (PST), while the higher degree of CO rotational excitation for N(4S)+CO products implies that NCO passes through a bent geometry upon dissociation. The P(ET) distributions suggest that when the B̃ 2Π←X̃ 2Π band is excited, NCO undergoes internal conversion to its ground electronic state prior to dissociation. Excitation of NCO at 193 nm clearly leads to the production of N(2D)+CO fragments. While conclusive evidence for the higher energy O(3P)+CN(X 2Σ+) channel was not observed, the presence of this dissociation pathway could not be excluded.

Laser photodissociation of ketene at 230 nm

Ketene in a molecular beam is excited at the onset of its X ̃ 1 A 1 → B ̃ 1 B 1 absorption band by laser light at 230 nm. The rovibrational population distribution in the CO photofragment, formed in the dissociation, is probed by REMPI. The phase space theory (PST) is applied to the dissociation channels leading respectively to CO+CH 2( a ̃ 1 A 1 ) and CO+CH 2( b ̃ 1 B 1 ). For the latter, the PST calculations lead to an average rotational energy in CO which is not compatible with the measured value, thus suggesting that this channel does not represent an important contribution to the dissociation process.

How a Transition State Tightens: The Singlet Photodissociation of Ketene as a Test Case

The tightening of the transition state as energy increases above the dissociation threshold is studied for the dissociation of singlet ketene. Rate constants and quantum yields have been determined for the photodissociation of ketene to produce CH2(ã1A1)(0,0,0) + CO(X̃ 1Σ+)(v=1). At 57, 110, 200, 357, and 490 cm-1 above this threshold, vibrational branching ratios for the singlet products were measured and compared to theory. Above 100 cm-1, the experimental values are consistent with the separate statistical ensembles (SSE) and variational RRKM models. CO(v=1,J) photofragment excitation (PHOFEX) spectra were observed up to 300 cm-1 over the threshold for production of CO(v=1) and used to calculate the total yield of the state probed. The J dependence of these yields is statistical, consistent with the observed 1CH2 rotational distributions. Thus, the total CO(v=1) singlet yield and rate constant are determined as a continuous function of energy up to 300 cm-1. Rate constants are given accurately by phase space theory (PST) up to 35 ± 5 cm-1. The ab initio rate constants of Klippenstein, East, and Allen match the experimental rate constants from 10 to 6000 cm-1 well within experimental uncertainty without any adjustment of parameters for the unified statistical model of Miller. Thus the rate appears to be controlled by an inner transition state near 3 Å and the outer PST transition state acting in series with the outer transition state dominating for energies below 50 cm-1 and the inner for energies above a few hundred cm-1. From the measured rate constants, an experimental density of states is calculated to be 0.94 times the anharmonic ab initio density of states. This allows the degeneracy gt of the coupled triplet channels to be determined from the rates at the triplet threshold, gt = 1.0 ± 0.1. The vibrational branching ratios and product yields for the vibrationally excited triplet products were also estimated and found to be nearly constant over this energy region. These values are about 17% of those predicted for vibrationally adiabatic dynamics in the exit valley after passage through the triplet transition state.

Evidence for a second molecular channel in the fragmentation of formaldehyde

This paper reports CO(v,J) distributions from photolysis on the 2141, 45, 2161, 2143, and 2341 bands of H2CO and the 2243 band of D2CO. A significant fraction of the CO(v=0) photofragment is found in low rotational states (JCO&amp;lt;15) for excitation above the threshold of the H+HCO dissociation channel. Photolysis on the 2141 band, which lies below this radical threshold, shows no measurable population in low-JCO states. The fraction of the total population in low-JCO states increases with increasing photolysis energy. In contrast, the CO(v=1, J) distributions do not broaden significantly with increasing excitation energy. Similar results are found for D2CO. Two alternative models addressing the dynamics of this dissociation are proposed. First, anharmonic motion at the transition state may lead the molecule to dissociate from configurations with smaller impact parameters and thus produce a broadened rotational distribution. More likely, a second fragmentation path, related to the exit channel of the H+HCO→H2+CO abstraction reaction and accessed through the radical channel, may open.

Tunable diode laser measurements of carbon monoxide energy distributions from acrolein photodissociation at 193 nm

Rovibrational energy distributions and average translational energies for CO photofragments produced by the 193-nm photolysis of gas-phase acrolein have been measured. A tunable infrared diode laser system was used to probe the CO fragment. Energy release to the CO translational [P 1,0 (16)], rotational (=0), and vibrational degrees of freedom can be described by the effective temperatures 1380, 2750, and 2230 K, respectively. A simple phase space model, consistent with a two-step mechanism where radical pair formation occurs prior to the formation of free CO, accurately predicts rotational and vibrational energy distributions

Photodissociation dynamics of Mo(CO)6 at 266 and 355 nm: CO photofragment kinetic-energy and internal-state distributions

The internal-state and kinetic-energy distributions of the CO photofragments from the 266 and 355 nm photolysis of Mo(CO)6 have been measured under collision-free conditions using vacuum-ultraviolet laser-induced fluorescence. The rotational-state distributions for CO(v″=0) and (v″=1) are well represented by Boltzmann distributions with effective rotational ‘‘temperatures’’ of Tr(v″=0)=950±70 K and Tr(v″=1)=935±85 K for 266 nm and Tr(v″=0)=750±70 K and Tr(v″=1)=1150±250 K for 355 nm photolysis. The CO(v″=1/v″=0) vibrational-state ratios for 266 and 355 nm photolysis are 0.19±0.03 and 0.09±0.02, respectively. The Doppler-broadened CO photofragment line shapes indicate that the translational energy distributions are isotropic and Maxwellian. There is no photolysis-laser wavelength or internal-state dependence to the extracted translational ‘‘temperatures.’’ The observed energy partitioning and kinetic-energy distributions are inconsistent with an impulsive ejection of a single CO ligand. CO photofragment line shapes for 266 nm photolysis are not consistent with a mechanism involving the repulsive ejection of the first CO ligand, followed by the statistical decomposition of the Mo(CO)5 fragment. While phase-space theories do not predict quantitatively the energy disposal, the photodissociation mechanism appears to be dominated by statistical considerations. The results also suggest that the photodissociation of Mo(CO)6 at 266 and 355 nm involves a common initial ‘‘state’’ and that similar exit channel effects are operative.

State resolved photofragmentation of Ni(CO)4 at 193, 248, and 308 nm: A detailed study of the photodissociation dynamics

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser-induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, with about 2.8 eV of electronic excitation. The Ni(CO)n products from 248 nm photolysis are formed in both the ground and excited states, with a 3:1 branching ratio. The data taken at 308 nm also indicate the presence of both channels, with the excited state channel still important. These results are consistent with predictions of earlier ab initio work, although the detailed mechanism is somewhat different. This earlier work is discussed in light of the present results.

Photodissociation dynamics of carbonyltrihydridoboron at 193 nm

The photodissociation dynamics of the reaction BH{sub 3}CO {yields} BH{sub 3} + CO at 193 nm have been investigated by using time-resolved infrared absorption spectroscopy and ultraviolet/visible emission spectroscopy. Partitioning of the available energy into the translational and vibrational degrees of freedom has been measured for the nascent CO photofragment. Time-resolved carbon monoxide laser absorption spectroscopy has been used to observe the CO photofragment in vibrationals levels {nu}{double prime} = 0-4. The nascent CO photofragment can be characterized by a vibrational temperature, T{sub vib} = 2,760 {plus minus} 160 K. Time-resolved tunable diode laser absorption spectroscopy has also been employed to observe internal excitation in the nascent BH{sub 3} photofragment, as well as measure the quantum yield for its production from BH{sub 3}CO photolysis.

ChemInform Abstract: State‐Resolved Photofragmentation Dynamics of Fe(CO)5 at 193, 248, 266, and 351 nm.

The photofragmentation dynamics of Fe(CO)5 in a supersonic molecular beam have been studied for photolysis wavelengths of 193, 248, 266, and 351 nm. The CO photofragments formed under these collision‐free conditions were detected by vacuum ultraviolet laser‐induced fluorescence (VUV LIF). This allowed for the determination of the rotational and vibrational distributions for the CO products and, by using Doppler spectroscopy, the translational energy distributions were also determined. These data are presented along with the details of a statistical model calculation which reproduces the experimental data very precisely. The statistical model for the photodissociation dynamics, which works at all photolysis wavelengths studied, is based on sequential elimination of CO ligands, with complete energy randomization in the intermediate ground state Fe(CO)n fragments between elimination steps. The detailed mechanism for the ultraviolet photochemistry of Fe(CO)5 is discussed in light of these results and previous...

State-resolved photofragmentation dynamics of Fe(CO)5 at 193, 248, 266, and 351 nm

The photofragmentation dynamics of Fe(CO)5 in a supersonic molecular beam have been studied for photolysis wavelengths of 193, 248, 266, and 351 nm. The CO photofragments formed under these collision-free conditions were detected by vacuum ultraviolet laser-induced fluorescence (VUV LIF). This allowed for the determination of the rotational and vibrational distributions for the CO products and, by using Doppler spectroscopy, the translational energy distributions were also determined. These data are presented along with the details of a statistical model calculation which reproduces the experimental data very precisely. The statistical model for the photodissociation dynamics, which works at all photolysis wavelengths studied, is based on sequential elimination of CO ligands, with complete energy randomization in the intermediate ground state Fe(CO)n fragments between elimination steps. The detailed mechanism for the ultraviolet photochemistry of Fe(CO)5 is discussed in light of these results and previous experimental and theoretical work.

State-resolved photodissociation of OCS monomers and clusters

Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four-wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one characterized by β=0 near J=54, (3) that the CO velocity vector is aligned nearly perpendicular to its angular momentum vector, and (4) that the CO angular momentum vector is also aligned parallel to that component of the transition dipole which lies perpendicular to the recoil velocity. These results are interpreted in terms of a model for the dissociation in which excitation takes place to two surfaces of A′ and A″ symmetry derived from a bent 1Δ configuration. Dissociation of OCS clusters was also investigated and was found to produce a photochemistry completely different from that of the monomers. Rotationally cold CO as well as S2 in both the X3Σ−g and a1Δg states was observed.

Photofragment spectroscopy of metal carbonyls: a molecular beam study of iron pentacarbonyl photolysis at 193 nm

The detailed product energy distribution has been determined for the 193-nm photofragmentation of Fe(CO)/sub 5/. In these experiments, done under molecular beam conditions, the CO photofragment was detected by vacuum ultraviolet laser-induced fluorescence. The vibrational and rotational distributions were determined from the spectra, and the translational energy distribution by Doppler spectroscopy. The measured energy release is well described by a completely statistical dissociation in which energy randomization is complete before each step in the series of three sequential CO eliminations.

Rotational state distribution of CO photofragments from triplet ketene

The nascent rotational state distribution of CO(v″=0,J″) following excimer laser photolysis of ketene at 351 nm has been determined under collisionless conditions in a flow cell. At this low excitation energy dissociation can only take place on the triplet potential surface leading to CH2(X̃ 3B1) and CO(X̃ 1Σ+). The available energy permits only the vibrational ground state of CO to be populated. The observed rotational distribution of CO(v″=0,J″) deviates drastically from a phase space theory statistical distribution as well as from a thermal one. A Boltzmann plot of this distribution exhibits a population inversion for J″&amp;lt;13. The nonstatistical behavior is attributed to a barrier along the dissociation path. The fragments are repelled too rapidly for energy to be randomized between them. Thus the photofragmentation dynamics of triplet ketene contrasts markedly with dissociation on the singlet surface which has no barrier and gives a statistical CO rotational state distribution. An impulsive model calculation for the ab initio transition state geometry is in surprisingly good agreement with the experimental energy partitioning among the fragment degrees of freedom. This suggests that the CCO bond angle is strongly bent at the top of the barrier and that the barrier height is a substantial fraction of available energy.