Product State Distributions Research Articles

Reactions of oxygen atoms with van der Waals complexes: The effect of complex formation on the internal energy distribution in the products

Reactions of atomic oxygen with complexes containing HCl are investigated and the OH product state distributions are compared to those observed for the corresponding reactions of HCl monomers. In previous studies of reactions of O(3P) with HCl and hydrocarbon complexes, rotationally colder OH product state distributions were observed, when compared to the corresponding reactions of monomers. In contrast, we find that reactions of O(1D) with HCl clusters yield OH rotational distributions that are unaffected by the incorporation of HCl into a van der Waals complex. Quasiclassical trajectories are run on collisions of oxygen with HCl and Ar⋯HCl at 1 eV collision energies to investigate the differences in the dynamics of the O(1D) and O(3P) reactions. It is found that when the van der Waals complex is longer lived than the collision complex, rotational and vibrational cooling are observed. In contrast, when the dissociation of the van der Waals complex is prompt, compared to the collision complex lifetime, the effects of complex formation on the internal energy of the OH product become negligible.

Quantum-state resolved products via vacuum ultraviolet photostimulated desorption from geologic calcite

We report the results of a photostimulated desorption (PSD) study of neutral CO products from room temperature geologic calcite utilizing fluences<300 μJ/cm 2 of 157-nm excimer laser radiation. At this wavelength, we are coincident with an intense absorption peak (absorption coefficient=4.39×10 5 cm −1) such that most excitation occurs near or at the crystal surface. Detailed product state distributions are obtained using (2+1) resonance enhanced multiphoton ionization. The CO translational energy can be characterized by a temperature, T=105±10 K, significantly lower than that of the substrate. The CO rotational and vibration state distributions appears thermalized to the surface. This maybe contrasted with our previous study of CO PSD from room temperature calcite done using 193-nm and 213-nm excitation. Bulk UV absorption decreases by two orders of magnitude at these wavelengths and excitation occurs throughout numerous bulk crystal layers. In those studies, CO translational energy could be characterized by a temperature of T=110 K with product internal energy distributions which showed both thermal and non-thermal components. This seemed to indicate a rapid, scattered release from the surface. Broadly similar PSD results over a range of excitation wavelengths and crystal absorptivities confirms a final surface state decay mechanism in the production of product CO. Subtle variations may reflect state differences in desorption for excitations occurring near or within the band gap and those occurring more deeply within the `charge-transfer' band.

HCO (N,Ka,Kc,J) distributions from near-threshold photolysis of H2CO (J,Ka,Kc)

The dynamics of the reaction H2CO+hν(λ≈330 nm)→H+HCO have been studied following excitation of formaldehyde into the Ã(1A2) state, just above the dissociation threshold of the X̃(1A1) state. Formaldehyde was excited via specific J, Ka, Kc rotational states and the ensuing rotational distribution of HCO measured by fully resolving N, Ka, Kc, and J=N±S of the fragment. When only the N and Ka quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are generally modeled well by phase space theory (PST). Within ≈10 cm−1 of the threshold, however, the PST predictions consistently exceed the experimental populations. This was accounted for by the inclusion of a centrifugal barrier in the PST model. The attractive part of the effective centrifugal potential was modeled by a dipole-induced dipole plus dispersion interaction. The barrier is weak and long range (&amp;gt;5 Å). Resolution of Kc in the reaction, in both parent and product, gave large deviations from the PST model. The HCO population distributions separate according to whether Kc was the upper- or lower-energy state. Additionally, the upper/lower preference was sensitive to the choice of Kc in the parent. Insufficient data are currently available to quantify this observation. The product state distribution was also found to be independent of the spin–rotation state of HCO.

The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (100), (020), and (0220) bath vibrational modes

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data.

Nonstatistical unimolecular dissociation over a barrier

A general formulation is presented to model photodissociation processes in which internal conversion is followed by unimolecular dissociation over an exit barrier; this classification of dissociation mechanism results in a nonstatistical product state distribution. The energy available to products is divided into independent statistical and impulsive energy reservoirs. The statistical reservoir considers direct projections of a vibrational microcanonical ensemble at the transition state (TS) onto product quantum states, conserving vibrational adiabaticity and angular momentum. The impulsive reservoir represents the energy released in passing from the TS to products; this reservoir is treated assuming sudden dissociation of the zero-point TS wave function using a combination of Franck-Condon and impulsive models. We derive the statistical adiabatic impulsive model, which convolutes these two energy reservoirs, to predict the product translational energy distribution for nonstatistical dissociation over a barrier. Two test cases are modeled and compared with experimental data: unimolecular dissociation of acetyl radicals and photodissociation of vinoxy radicals via the B̃ 2A″-X̃ 2A″ band.

Reactant–product decoupling approach to state-to-state dynamics calculation for bimolecular reaction and unimolecular fragmentation

The main purpose of the study is to explore a number of computational methods to be used in the RPD approach to state-to-state quantum dynamics calculation of polyatomic reactions. Specifically, a mixed time-dependent (TD) and energy-dependent (ED) approach to solve the RPD equations is investigated. In the mixed TD and ED approach, the reactant wavefunction is computed by the method of wavepacket propagation while the product wavefunction is calculated by energy-dependent methods. As a result, the reactant–product coordinate transformation only needs to be carried out for the number of energies at which the state-to-state S-matrix elements are needed, which is advantageous if state-to-state information at only a few energies are needed. Similar implementation of the mixed RPD approach is given for calculation of product state distribution in molecular photofragmentation dynamics.

Unimolecular Dissociation of HOCl Near Threshold: Quantum State and Time-Resolved Studies

The unimolecular dissociation of HOCl has been examined by accessing several rotational resonances of the 6νOH vibrational level using overtone−overtone double resonance. An examination of the energetic thresholds for opening various OH fragment rotational states allows us to determine the dissociation energy for the Cl−O bond to be D0 = 19 288.8 ± 0.6 cm-1 as well as confirm the absence of any appreciable barrier (≤2 cm-1) along the reaction coordinate. Time-resolved measurements of the unimolecular dissociation rates arising from excitation to selected |J,Ka〉 quantum states of the 6νOH vibrational level are found to be substantially slower than that expected on the basis of RRKM theory, and as a function of energy, these rates exhibit recurrent maxima that approximately correlate with the rotational energy level spacings of the OH(2Π3/2) manifold. The OH fragment product state distributions resulting from near-threshold unimolecular dissociation are found to be hotter than those predicted by phase space...

Photodissociation dynamics of jet-cooled H2O and D2O in the non-Franck–Condon regime: Relative absorption cross sections and product state distributions at 193 nm

Quantum state distributions for nascent OH and OD fragments generated by Franck–Condon “forbidden” 193 nm photodissociation of H2O and D2O are reported, with the two isotopomers initially prepared in their zero-point vibrational and lowest ortho/para nuclear spin allowed rotational states (i.e., JKaKc=101 and 000 in a 3:1 ratio for H2O and 1:2 ratio for D2O) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence (LIF) with cylindrical mirror collection optics optimized for the slit expansion geometry, which makes photodissociation studies feasible with cross sections as low as ≈10−26 cm2. The OH and OD fragments are formed exclusively in v=0, but with highly structured quantum state distributions in rotational, Λ-doublet, and fine structure levels (2Π3/2+, 2Π1/2+, and 2Π3/2−) that exhibit qualitatively different trends than observed in previous jet photolysis studies at 157 nm in the Franck–Condon “allowed” regime. The relative OH/OD fragment yields at 193 nm indicate a 64±10 times greater propensity for OH vs OD bond cleavage in H2O than D2O, which is more than three-fold smaller than predicted from full three-dimensional quantum scattering calculations on ground (X̃ 1A1) and first excited state (Ã 1B1) potential surfaces. One-dimensional semiclassical calculations of the Franck–Condon overlap matrix elements confirm these discrepancies to be considerably outside uncertainties associated with the ground and excited state potential surfaces. These results indicate that the photodissociation dynamics for this benchmark system are not yet fully understood and suggest either non-Born–Oppenheimer effects or contributions from other electronic surfaces may be important in the extreme non-Franck–Condon photolysis regime.

Theoretical study of the dissociation reaction HFCO→HF+CO: New ab initio potential function and classical trajectory analysis

A total of 3261 ab initio energies calculated at the RHF/MP2 level were used to obtain an analytical representation of the potential energy surface (PES) for the title reaction considering all the vibrational degrees of freedom. The analytical potential is constructed by switching three local representations of the PES utilizing a distancelike function, and it reproduces well the ab initio energies up to 20 kcal/mol above the dissociation threshold with the root-mean-square (rms) deviation equal to 1.5 kcal/mol. Two types of classical trajectory studies, i.e., power spectra calculations and product-state distribution analysis, were performed to assess the reliability of the present potential function. The results were found to be in good agreement with the available experimental ones.

Quantum state-resolved dynamics study for the reaction O(1D)+H2→OH+H(J=0)

This paper presents an exact quantum state-to-state dynamics calculation for the prototype insertion reaction O(1D)+H2→OH+H with the total angular momentum J=0. By extending the Peng et al. previous study of total reaction probabilities [Chem. Phys. Lett. 248, 37 (1996)], the state-to-state reaction probabilities from the initial ground rovibrational state (v=j=0) have been obtained for 500 evenly spaced energy points covering the total energy from 0.3 to 0.8 eV. An individual state-to-state reaction probability is heavily oscillatory as a function of the collision energy, while the total reaction probability remains flat in the high-energy region, which implies that the reaction is predominantly direct. In the low-energy region, several possible resonance energies have been identified and compared to the reduced-dimension counterpart. The product-state distribution clearly demonstrates an inverted rotational distribution in which highly excited rotational states are excessively populated. The vibrational distribution is also in good agreement with the three-dimensional trajectory calculations as well as the reduced-dimension calculation.

Classical trajectory study of product state vector correlations: A model for the photodissociation of CF3NO

The photodissociation dynamics of CF3NO are modeled by three-dimensional classical trajectory calculations on the S0 and T1 electronic potential surfaces that include all vibrational and rotational degrees of freedom. The intramolecular potential energy surface for CF3NO was obtained by optimizing parameters in an analytical potential function to best fit spectroscopic data and the results of UMP2/6-31+G* level ab initio calculations. Initial parent molecule energy and angular momentum distributions are computed using a new method that simulates the conditions of molecular beam experiments. The calculations are directed at interpreting experimental measurements of correlated NO product state distributions in the photodissociation of the nitrosoalkanes. The results show that the v–j correlation of NO photofragment is primarily determined by the C–N–O bending force constant and exit channel barrier on the T1 surface.

State selected unimolecular dissociation of HOCl

The unimolecular decomposition of HOCl is investigated by exciting the molecule to the region of the sixth overtone of its OH stretching vibration (7νOH) using overtone–overtone double resonance. The excitation scheme is sufficiently selective to allow preparation of a single angular momentum quantum state within the 7νOH vibrational manifold lying ∼2500 cm−1 above the dissociation limit. From the measured linewidths associated with the rotational features appearing in the action spectrum, we obtain an upper limit estimate for the dissociation rate of kuni&amp;lt;9.4×109 s−1 which is substantially slower than that expected on the basis of RRKM theory. The nascent OH fragment product state distribution exhibit strong oscillations which depend on the rotational quantum numbers of the parent HOCl molecule. From the measured energy release associated with the OH fragment, the heat of formation of HOCl is estimated to be ΔH00(0 K)=−16.7±0.6 kcal/mol.

A global A-state potential surface for H2O: Influence of excited states on the O(1D)+H2 reaction

In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angular distributions with 1A′′ included, especially at translational energies of 4 kcal/mol and above. A rough estimate of the influence of the 2A′ surface indicates that this state also makes an important contribution to vibrational and angular distributions at high energies. The 1A′′ and 2A′ surfaces also measurably affect the energy dependence of the integral cross sections at energies above 2 kcal/mol, and the temperature dependence of thermal rate coefficients for O(1D)+H2 at temperatures of 1000 K and above.

Perturbative approach to potential surface inversion for bound and half-scattering problems

A recently proposed inverse perturbation with singular value decomposition (IPSVD) method to correct potential energy surfaces (PES) of bound molecular systems is generalized in two significant ways. Firstly, in the current generalization, any given primitive PES of a bound system can in principle serve as a zeroth order (starting) PES which can be corrected to approach the desired PES by successive use of the perturbative inversion method. Secondly, the perturbative approach is further generalized and the mathematical equations are derived for correcting dissociative PES of half-scattering problems by using experimentally measured product state distribution of the fragments or spectrum as input variables. The specific mathematical treatments for the above two generalizations are explicitly given and discussed in the paper. An example is shown by application to HCN bound state potential energy surface.

Vibrational predissociation of the ND3-Ar Van der Waals complex: Comparison with NH3-Ar

We study the vibrational predissociation of ND3-Ar through excitation of the ν2 “umbrella” mode of ND3, with the same (scaled or unscaled) ab initio potential and the same quantum mechanical method as used earlier for NH3-Ar [J. Millan et al., J. Chem. Phys. 103, 4138 (1995)]. Just as in that case, we find that a simultaneous excitation of one of the Van der Waals modes (stretch, Σ or Π bend, of symmetry A1, A2, or E) has a strong effect on the lifetime of the complex and on the product state distribution. In particular, the excitation of the Van der Waals stretch mode leads to shorter lifetimes and colder rotational state distributions of the emerging ND3 fragment. With the original (unscaled) ab initio potential, which produces the most realistic lifetimes (line widths in the infrared spectrum) for NH3-Ar, the spread of the lifetimes for different initial quasi-bound states of ND3-Ar is much larger than for NH3-Ar. This may be related to the more localized nature of the (quasi-)bound states of ND3-Ar and the shorter average distance 〈R〉. The rotation-inversion (jk±) state distributions of the emerging ND3 fragment are highly non-statistical, as in ND3-Ar, but this fragment contains (absolutely and relatively) less rotational energy than the NH3 fragment in the photo-dissociation of NH3-Ar. Most of the (quasi-)bound levels of the ND3-Ar complex show a clear preference for an ND3 inversion-tunneling state of definite + or − parity, but the distribution of the emerging ND3 fragment over its +/− states does not reflect this preference.

Photostimulated desorption of CO from geologic calcite following 193-nm irradiation

We report the results of a photostimulated desorption study of neutral CO product from geologic calcite using very low fluence 193-nm excimer laser radiation. Detailed product state distributions are obtained using (2+1) resonance enhanced multiphoton ionization. The CO translational energy can be characterized by a temperature (T=110{plus_minus}15K) significantly lower than that of the substrate (T=295K). The CO rotational state distribution is non-Boltzmann and there is step structure every four or five rotational states. The evolution of CO emission as a function of laser exposure shows no incubation period or enhanced CO photodesorption over time. The CO yield is linear with laser power and displays no detectable threshold. The low kinetic energy of the desorbed CO and the nonthermal CO distributions implicate the decay of a surface species or state. {copyright} {ital 1997} {ital The American Physical Society}

Photofragment imaging of HNCO decomposition: Angular anisotropy and correlated distributions

Photodissociation of jet-cooled isocyanic acid has been examined by photofragment ion imaging of H(D) from H(D)NCO and CO from HNCO, and by laser induced fluorescence (LIF) of NH(a 1Δ) from HNCO. Only modest recoil anisotropy is observed in the H+NCO channel at 243.1 nm (β=−0.13±0.05), while the D+NCO channel at approximately the same wavelength reveals no anisotropy (β=0.00±0.05), confirming that the dissociation of H(D)NCO from the opening of the H(D) channel proceeds via vibrational predissociation on the S0(1A) surface. In contrast, substantial anisotropy (β=−0.66±0.08) is observed in the NH(a 1Δ)+CO channel at 230.1 nm, but this value can correspond to dissociation on either S0 or S1. The photolysis region between 243 and 230 nm thus appears important in providing clues to the dissociation mechanism and the competition between different potential energy surfaces. At 217.6 nm, product state distributions exhibit clear dynamical biases. CO is produced in both ν=0 and ν=1, while NH(a 1Δ) distributions correlated with different rovibrational levels of CO, although different in shape, are always cold, consistent with the global NH distribution measured by LIF. The NH distributions indicate dissociation on S1(1A′′), and can be described by Franck–Condon mapping of transition state wave functions in the HNC bending coordinate without additional torque, implying little anisotropy in the potential along that coordinate. On the other hand, a larger torque is manifest in the CO rotational distribution. Although at 217.6 nm the dissociation is likely to be dominated by decomposition on S1, competition with radiationless decay is still manifest. From analysis of the CO photofragment velocity distribution at 230.1 nm, the NH(a 1Δ)+CO dissociation threshold is determined at 42 765±25 cm−1.

A quasiclassical trajectory study of product state distributions from the CN+H2→HCN+H reaction

An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a “spectator bond” in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles.

Consequences of Conical Intersections in the H + O2 → OH + O Reaction?

Doppler-resolved laser-induced fluorescence spectroscopy of the OH reaction products of the photoinitiated reaction H + O{sub 2} {yields} OH + O has been applied at collision energies near 1.8 eV, where an anomalous peak in the total reaction cross section has been reported by Kessler and Kleinermanns. Although the product state distributions show no remarkable variation as the collision energy varies through this region, analysis of the Doppler line shapes suggests a sideways scattering component to the differential cross sections near 1.8 eV that is not present at higher collision energies. We propose that the qualitative change in the differential cross sections as well as the local peak in the integral cross section can be a consequence of the dynamics in the vicinity of the C{sub 2v} conical intersection. 36 refs., 4 figs.

Fourier transform infrared emission study of the mechanism and dynamics of HOI formed in the reaction of alkyl iodides with O(3P)

Vibrationally excited hypoiodous acid (HOI) is observed as a product in the reaction of alkyl iodides with O(3P). Fourier transform infrared emission techniques are used to detect the excited ν1, OH, stretch of the HOI product, to determine the mechanism of HOI production, and to measure the vibrational product state distributions. The HOI product is formed by O atom reaction with two-carbon and larger straight or branched chain alkyl iodides and cyclic alkyl iodides, e.g., C2H5I, n-C3H7I, i-C3H7I, (CH3)3CI, n-C6H13I, and c-C6H11I, but not with CH3I. Experiments with selectively deuterated ethyl iodides provide direct evidence that HOI is formed in a beta-elimination mechanism involving a five-membered ring transition state. The O atom attacks the iodine and then abstracts a hydrogen from the beta carbon during the lifetime of the complex. Time-resolved experiments allow the extraction of nascent vibrational state distributions for the ν1 stretch of HOI (v=1:v=2:v=3) using different alkyl iodides and assuming the radiative rates are given by Aν→ν−1=vA1→0: C2H5I, 0.53(4):0.39(3):0.08(3); n-C3H7I, 0.61(6):0.34(5):0.05(2); and i-C3H7I, 0.54(6):0.38(4):0.08(3). These distributions are nonstatistical with the v=2 states having only slightly less population than those with v=1. For product molecules with up to three quanta of ν1 excitation, more than 50% of the reaction exothermicity is deposited into the OH stretch.