Photodissociation Of Ketene Research Articles

Quasi-classical statistico-dynamical description of polyatomic photo-dissociations: state-resolved distributions

An alternative methodology to investigate indirect polyatomic processes with quasi-classical trajectories is proposed, which effectively avoids any binning or weighting procedure while provides rovibrational resolution. Initial classical states are started in terms of angle-action variables to closely match the quantum experimental conditions and later transformed into Cartesian coordinates, following an algorithm very recently published [J. Chem. Phys. 130, 114103 (2009)]. Trajectories are then propagated using the 'association' picture, i.e. an inverse dynamics simulation in the spirit of the exit-channel corrected phase space theory of Hamilton and Brumer [J. Chem. Phys. 82, 595 (1985)], which is shown to be particularly convenient. Finally, an approximate quasi-classical formula is provided which under general conditions can be used to add possible rotational structures into the vibrationally-resolved quasi-classical distributions. To introduce the method and illustrate its capabilities, correlated translational energy distributions from recent experiments in the photo-dissociation of ketene at 308 nm [J. Chem. Phys. 124, 014303 (2006)] are investigated. Quite generally, the overall theoretical algorithm reduces the total number of trajectories to integrate and allows for fully theoretical predictions of experiments on polyatomics.

State-resolved thermalization of singlet and mixed singlet-triplet states of CH2

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH(2) (~a (1)A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH(2) is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH(2) population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times.

Photodissociation dynamics of ketene at 157.6nm

Photodissociation dynamics of ketene at 157.6 nm has been investigated using the photofragment translational spectroscopic technique based on photoionization detection using vacuum-ultraviolet synchrotron radiation. Three dissociation channels have been observed: CH2+CO, CH+HCO, and HCCO+H. The product translational energy distributions and angular anisotropy parameters were measured for all three observed dissociation channels, and the relative branching ratios for different channels were also estimated. The experimental results show that the direct C-C bond cleavage (CH2+CO) is the dominant channel, while H migration and elimination channels are very minor. The results in this work show that direct dissociation on excited electronic state is much more significant than the indirect dissociation via the ground state in the ketene photodissociation at 157.6 nm.

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.

A velocity map ion-imaging study on ketene photodissociation at 208 and 213nm: Rotational dependence of product angular anisotropy

Photodissociation dynamics of ketene following excitation at 208.59 and 213.24 nm have been investigated using the velocity map ion-imaging method. Both the angular distribution and translational energy distribution of the CO products at different rotational and vibrational states have been obtained. No significant difference in the translational energy distributions for different CO rotational state products has been observed at both excitation wavelengths. The anisotropy parameter beta is, however, noticeably different for different CO rotational state products at both excitation wavelengths. For lower rotational states of the CO product, beta is smaller than zero, while beta is larger than zero for CO at higher rotational states. The observed rotational dependence of angular anisotropy is interpreted as the dynamical influence of a peculiar conical intersection between the (1)B(1) excited state and (1)A(2) state along the C(S)-I coordinate.

Predominance of Nonequilibrium Dynamics in the Photodissociation of Ketene in the Triplet State

The photodissociation of ketene is studied using direct surface-hopping classical trajectories where the energy and gradient are computed on the fly by means of state-averaged complete active space self-consistent field with a double-ζ polarized basis set. Three low-lying electronic states, singlets S0 and S1 and triplet T1, are involved in the process of photodissociation of triplet state ketene. We propagated a trajectory, starting at the Franck−Condon geometry on S1, and branched it out into many child trajectories every time the propagating potential energy surface (PES) crossed with another PES. The major photodissociation pathway to the triplet products was found to be S1 → S0 → T1 → CH2(X3B1) + CO(X1Σ+). It has been found that (1) the S0−T1 nonadiabatic transition creates the T1 species nonstatistically at restricted regions of phase space and (2) a large fraction of the T1 species thus created dissociates almost immediately, leaving no time for equilibration of internal degrees of freedom. Whether a specific T1 trajectory dissociates fast or not is determined by the amount of C−C stretch vibration at the S0−T1 branch point. In essence, the above observations suggest strongly that the T1 photodissociation process is highly nonstatistical, thus making equilibrium-based statistical theories inapplicable for computing the dissociation rate.

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.

Ab initio study of nonadiabatic interactions in the photodissociation of ketene

Ab initio calculations have been carried out on potential-energy surfaces for the photodissociation of ketene. S0 and S1 state cross extensively around the Franck–Condon (F–C) region upon C–C–O bending, and the S1→S0 internal conversion is expected to be very efficient. S1 and T1 stay close in energy in the F–C region, but do not couple strongly due to the small spin–orbit coupling, and direct S1→T1 intersystem crossing is unlikely. The triplet state, which produces the ground-state products is likely to be formed via the process S1→S0→Tn. S0 crosses with the lowest triplet state (T1 or T2) at rather low energy near the triplet minimum. The S0/Tn crossing persists all along the C–C dissociation pathway. As C–C is stretched, the energy of the crossing increases and the crossing structure deviates substantially from the reaction path. These results suggest that, if intersystem crossing at higher potential energy is favored, the rate of reaction may reflect the dynamics of intersystem crossing and that on the triplet surface.

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.

K‐Conservation in the dissociation of singlet ketene

AbstractThe effect of K‐conservation on the photodissociation of ketene has been investigated. Rate constants and photo‐fragment excitation (PHOFEX) spectra have been calculated using phase space theory (PST) in a helicity basis. While the effect of K‐conservation on rate constants is relatively subtle, PHOFEX spectra with state‐selected reactant molecules appear to provide a feasible way to experimentally resolve the question of K‐conservation.

State-specific neutral time-of-flight of CO from ketene photodissociation at 351 nm: The internal energy distribution of CH2(X̃ 3B1)

Metastable time-of-flight (TOF) spectroscopy was used to measure the translational energy distribution of specific rotational states of CO formed from ketene photodissociation (CH2CO→CH2+CO) at 351 nm. This distribution could be directly related to the internal energy distribution of the other fragment (X̃ 3B1 CH2) formed in the reaction, thereby giving a correlated distribution of the internal states of the fragments. This technique overcomes the spectral complexity associated with detection the X̃ 3B1 state CH2. Previous measurements of the CO rotational distribution were simulated theoretically using the impulsive model and zero-point vibrational energy considerations. These models predicted that the rotational distributions of CO and CH2 should be uncorrelated, that ∼10% of the CH2 should be vibrationally excited with one quantum in the bending mode, and that the rotational energy distribution of CH2 should peak near zero. Measurements presented in this paper show a slight anticorrelation of CO and CH2 rotations, no vibrational excitation of CH2 and Gaussian-like rotational energy distributions of CH2 that peak at ∼1 kcal/mole and have a full width at half-maximum of ∼0.8 kcal/mol. Qualitative explanations for this behavior are presented.

The correlated product state distribution of ketene photodissociation at 308 nm

The correlated product state distribution for ketene photodissociation (CH2CO→CH2+CO) at 308 nm has been measured by using quantum-state-specific metastable time-of-flight (TOF) spectroscopy. This distribution is a matrix whose elements are the probability that if CO is produced in the dissociation with quantum-state ‖nCO〉, CH2 will be produced with quantum-state ‖nCH2〉. It was found that ketene photodissociation yields CH2 in three resolved states; the 1A1(000), and 1A1(010) states of CH2 are the major channels, while the 3B1 state is a minor channel. In addition to this scalar distribution, the vector correlations between the recoil velocity and the angular momentum of the CO fragment (v⋅j correlation), expressed by the β00(22) bipolar moment, have also been obtained as a function of the kinetic energy release of the photoreaction. The correlated product state distribution was found not to follow the predictions of phase space theory, suggesting that dynamic hindrances exist in the photoreaction that have not been previously observed. A phase space theory calculation with restricted impact parameter values was also performed and compared to experiment. The impact parameter restricted phase space theory more accurately reproduced all of the correlated product state information obtained in this work as well as previous uncorrelated product state distributions for CH2 and CO. Both the ranges and the values of the allowed impact parameters obtained from these restricted calculations increase as the rotational energy of CO increases. Also, the values of the allowed impact parameters for 1A1(010) CH2 are larger than for 1A1(000) CH2. This strongly suggests that C–C–O bending modes are hindered at the transition state and therefore play an important role in the photodissociation.

Photodissociation of ketene with a narrow-band tunable laser around 212.5 nm

The photodissociation of ketene with a narrow-band tunable laser has been studied at 218.0, 212.5 and 207.0 nm. A CH photofragment is formed in the C 2Σ + (ν′ = 0), B 2Σ − (ν′ = 0, 1) and A 2Δ (ν′ = 0, 1) states through a process involving the absorption of two laser photons. The visible emission observed above 580 nm is attributed to the CH 2(b 1B 1) photofragment formed in a single-photon process. The experimental results strongly suggest that electronically excited CH could be formed in a sequential mechanism involving the absorption of a second photon by the CH 2(b 1B 1) intermediate. Nevertheless, the simultaneous existence of several mechanisms must be considered to explain the formation of the CH fragment in the various observed electronic states.

Structured emission induced by ArF laser excitation of ketene in a molecular beam

Structured emission at wavelengths 540–690 nm is observed following ArF laser excitation of a molecular beam of ketene. The emission depends linearly on laser energy. The fluorescence shows a fast risetime and biexponential decay. Under the lowest collision regime studied the fast and slow components of the decay have time constants of 1.9 ± 0.7 μs and 7.2 ± 0.8 μs respectively. The observed emission is tentatively assigned to fluorescence from vibrationally excited CH 2( 1B 1); formation of the latter in the photodissociation of ketene has been predicted theoretically but no previously experimental observation has been reported.

Determination of the singlet/triplet branching ratio in the photodissociation of ketene

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm−1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm−1 above the singlet threshold (30 116.2 cm−1 ), are measured in a supersonic free jet of ketene. The CO(v″=0) rotational distributions at 56, 110, 200, 325, and 425 cm−1 are bimodal. The peaks at low J′s, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v″=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm−1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm−1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm−1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm−1 excess energy, the CO(v″=1) rotational distribution is obtained, and the ratio of CO(v″=1) to CO(v″=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.

Rate constants for removal of ( 1B 1) prepared in the (0,13,0), (0,14,0) and (0,18,0) overtones by helium and several hydrocarbons.

Methylene 1B 1 was prepared by a two-step process: photodissociation of ketene at 308 nm (XeCl excimer laser) gave “cold” methylene 1A 1 and CO. CH 2CO → 308 nm CH 2 ( 1A 1) + CO Subsequent excitation of the 1A 1 electronic state using a dye laser produced the 1B 1 state CH 2 ( 1A 1) → hv CH 2 ( 1B 1) Bending overtones v 2=13, 14 and 18 could be selected and their time resolved fluorescence studied in the presence of several gases. Very little dependence of overtone radiative lifetimes are found. Quenching rate constants are in all cases very fast. they range from 5*10 −11 cm 3 molec −1 s −1 for quenching of overtone v 2=14 by helium to 1.1*10 −9 cm 3 molec −1 s −1 for quenching of the same overtone by trans-2-butene. Chemical forces seem to be involved in the quenching by hydrocarbons.

Dissociation on ground-state potential-energy surfaces

Two new techniques allowing for the resolved spectral study of dissociation on ground-state potential-energy surfaces are presented. First, Stark level-crossing spectra of highly vibrationally excited ground-state formaldehyde (S*0) characterize the initial rovibrational wavefunctions of dissociative formaldehyde states. Individual S*0 D2CO states are resolved near the barrier to dissociation but broaden and overlap at higher energies. Neighbouring resolvable states possess enough individual vibrational character to vary by over one order of magnitude in lifetime. A potential barrier height of 78.0–81.1 kcal mol–1 has been determined for H2CO → H2+ CO dissociation. Second, photofragment excitation (PHOFEX) spectra probe the production of correlated fragment states from ketene photodissociation. Molecular-beam PHOFEX spectra show that ketene possesses no observable rotational structure and only weak vibrational structure. The PHOFEX spectrum of an individual rotational state of the lowest vibronic state of singlet methylene gives the energetic threshold for formation of that state along with ground-state CO. The threshold for the combination with each successive CO(v″= 0, J″) channel is clearly resolved. A statistical calculation reproduces the main features of the PHOFEX spectra. Analysis of the PHOFEX spectra gives a H2CCO →1CH2+ CO dissociation threshold of 30 102 ± 15 cm–1. Finally, the effect of a potential barrier in the dissociation coordinate on statistical vs. dynamical control of dissociation is discussed.

Rotational excitation in the carbon monoxide product of ketene photodissociation

Rotational excitation in the carbon monoxide product of ketene photodissociation

ChemInform Abstract: METHYLENE SINGLET‐TRIPLET ENERGY SPLITTING BY MOLECULAR BEAM PHOTODISSOCIATION OF KETENE

The singlet–triplet splitting in methylene has been determined from the measurements of fragment velocities from ketene photodissociation at 351 and 308 nm in a molecular beam. The splitting is found to be 8.5±0.8 kcal/mol. This agrees with many experimental results, but not with the value of 19.5 kcal/mol derived from recent photodetachment experiments on CH−2.

Methylene singlet–triplet energy splitting by molecular beam photodissociation of ketene

The singlet–triplet splitting in methylene has been determined from the measurements of fragment velocities from ketene photodissociation at 351 and 308 nm in a molecular beam. The splitting is found to be 8.5±0.8 kcal/mol. This agrees with many experimental results, but not with the value of 19.5 kcal/mol derived from recent photodetachment experiments on CH−2.