Significant Vibrational Excitation Research Articles

Chemiluminescent reactions of manganese with fluorine: Influence of dynamics on product energy partitioning in vibration and rotation of MnF*(b,c)

Chemiluminescent exit channels of Mn+F2-->MnF*+F were investigated using the molecular beam technique in a beam-gas configuration with an array detector. Two uncongested regions, corresponding to MnF transitions c5Sigma+(b)-a5Sigma+(b) and b5Pi(i)(int)-a5Sigma+(b) were fit for vibrational and rotational populations, which were used to develop a microscopic reaction mechanism for these chemiluminescent exit channels. In both MnF* electronic states, significant vibrational excitation but little rotational excitation was found. Significant vibrational excitation has been attributed to early energy release as Mn loses an electron at long range to the lowest unoccupied molecular orbital on F2. The incipient bond is strengthened as backbonding from the F2- to Mn+ increases the covalent character of the intermediate. Finally, no strict geometric constraints are placed on the exit channel and hence there is no significant repulsive energy release into product rotation. Our proposed mechanism exhibits dynamic control in that the course of the reaction is determined by both geometric factors and dynamic factors.

Unimolecular Dissociation of Formyl Halides HXCO → CO + HX (X= F, Cl): An Ab Initio Direct Classical Trajectory Study

The dynamics of the unimolecular dissociation of formyl halides, HXCO (X = F, Cl), have been studied by classical trajectory calculations at the MP2/6-31G(d,p) level of theory and have been compared to similar calculations for H2CO. The calculated transition states are planar with elongated C−X and H−X bonds. Trajectories were started from the transition state with 12 kcal/mol of energy above the potential barrier, corresponding to the zero-point energy plus 2.25 and 2.87 kcal/mol of excess energy in the transition vector of formyl fluoride and formyl chloride, respectively. The CO fragments are produced rotationally hot with 〈J〉 = 40−50 but vibrationally cold with only 10−20% in v = 1. The HX fragment shows significant vibrational excitation with 〈v〉 = 1.0 for HF and 〈v〉 = 2.3 for HCl, compared to 〈v〉 = 1.16 for H2. The average rotational quantum number for HCl, 〈J〉 = 12.8, is considerably higher than for HF, 〈J〉 = 9.5, or H2, 〈J〉 = 3.3. Product translation receives about 42% of the available energy in formyl fluoride and about 31% in formyl chloride compared to 70% for formaldehyde. The results show good qualitative agreement with the available experimental energy partitioning for HFCO; HF is observed to be vibrationally hot, but the distribution has not been measured. No experiments are available for HClCO dissociation.

Atom-radical reaction dynamics of O(3P)+C3H5→C3H4+OH: Nascent rovibrational state distributions of product OH

The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.

Full-dimensional quantal initial state-selected reaction probabilities ( J=0) for the reaction OH( v=0, j=0)+CO( v=0, j=0)→CO 2+H

We present six-dimensional (6D) quantum dynamics calculations of initial state-selected reaction probabilities for the reaction of OH( v=0, j=0) with CO( v=0, j=0), for total angular momentum J=0. The results are compared to five-dimensional (5D) calculations in which the potential was averaged over the vibrational ground state wavefunction of CO. The 6D reaction probabilities are in good overall agreement with the 5D results up to a collision energy E i of 0.47 eV, but they exceed the 5D values at larger energies, by up to 40% at E i=0.8 eV. Significant vibrational excitation of the CO fragment occurs for E i⩾0.4 eV.

Glyoxal photodissociation. II. An ab initio direct classical trajectory study of C2H2O2→CO+H2CO

Photodissociation of glyoxal via the H2CO+CO channel has been investigated by ab initio classical trajectory calculations using Becke’s three-parameter hybrid functional method with split valence and polarized basis set [B3LYP/6-311G(d,p)]. To model the experimental conditions, trajectories were started from a microcanonical ensemble at the transition state with 8.5 kcal/mol excess energy distributed among the vibrational modes and the transition vector. The CO product was produced with a broad rotational distribution but with almost no vibration excitation. When combined with the results from the H2+2CO channel, the calculated vibrational and rotational distributions of CO are in excellent agreement with the experimental observations. The rotational distribution of H2CO was very broad ranging up to J=85. The H2CO product has significant vibrational excitation in the out-of-plane bending, CH2 rocking, CH2 scissoring, and CO stretching modes. For both the H2+2CO and the CO+H2CO channels, the majority of available energy was partitioned into translations.

Photodissociation spectroscopy of Mg+–acetaldehyde

We have investigated the photodissociation spectroscopy of the Mg+–acetaldehyde bimolecular complex over the spectral range 220–400 nm. We find evidence for four distinct absorption bands in the near ultraviolet that correlate with Mg+-based and acetaldehyde-based transitions. Our results suggest that the Mg+-centered 3pπ(A″)←3sσ(A′) and acetaldehyde-centered π*(A″)←n(A′) transitions are mixed, and result in significant vibrational excitation in the complex leading to broad and unresolved absorption bands. In contrast, the predominantly Mg+-based excitation bands, assigned as 3pπ(A′)←3sσ(A′) and 3pσ(A′)←3sσ(A′), each show prominent vibrational progressions identified with intermolecular wagging modes of the complex. These intermolecular wag progressions are short and anharmonic indicating low barriers to isomerization. In the 3pσ(A′)←3sσ(A′) band several higher frequency vibrational modes are also active and have been assigned to the Mg–OCHCH3 intermolecular stretch, a CCO deformation mode, and the C–O stretch.

Impulsive IR-multiphoton dissociation of acrolein: observation of non-statistical product vibrational excitation in CO ( v=1–12) by time resolved IR fluorescence spectroscopy

On IR-multiphoton excitation, vibrationally highly excited acrolein molecules undergo concerted dissociation generating CO and ethylene. The vibrationally excited products, CO and ethylene, are detected immediately following the CO 2 laser pulse by observing IR fluorescence at 4.7 and 3.2 μm, respectively. The nascent CO is formed with significant vibrational excitation, with a Boltzmann population distribution for v=1–12 levels corresponding to T v=12 950±50 K. The average vibrational energy in the product CO is found to be 26 kcal mol −1, in contrast to its statistical share of 5 kcal mol −1, available from the product energy distribution. The nascent vibrationally excited ethylene either dissociates by absorbing further infrared laser photons from the tail of the CO 2 laser pulse or relaxes by collisional deactivation. Ethylene IR-fluorescence excitation spectrum showed a structure in the quasi-continuum, with a facile resonance at 10.53 μm corresponding to the 10P(14) CO 2 laser line, which explains the higher acetylene yield observed at a higher pressure. A hydrogen atom transfer mechanism followed by C–C impulsive break in the acrolein transition state may be responsible for such non-statistical product energy distribution.

Molecular beam study of the chemiluminescent reaction of manganese and ozone

The electronically chemiluminescent reaction Mn+O3→MnO*+O2 was investigated using a beam-gas configuration. Light from the MnO A 6∑+–X 6∑+ transition was collected by a charge coupled device (CCD) array detector with resolutions of 0.5 and 0.1 nm. The spectrum at lower resolution (500–655 nm) encompassed the Δv=−3 to +2 sequences, while that at higher resolution (555.5–583.5 nm) encompassed only the Δv=0 sequence. These two spectra were separately fitted with a nonlinear least-squares program to obtain vibrational and rotational distributions of the nascent MnO*. The limited vibrational-state coverage of the higher-resolution spectrum made it unrealiable for determining the vibrational state distribution, and it was useful only for characterizing the rotational distribution when v′=0. The best-fit vibrational excitation is somewhat less than for the Prior model, but the rotational excitation is considerably greater. A consideration of the electronic structure of reactants and products indicates that principal changes occurring in the chemiluminescent reaction are σ–electron donation from the sdz2 hybridized Mn orbital to the O3 lowest unoccupied molecular orbital (LUMO) (2b1) and π–electron backdonation from the O–O 4b2 orbital to the Mn 3dπ orbital. Correlation of the orbitals involved indicates that direct access is allowed to the MnO A 6∑+(10σ*18σ1) state. This mechanism favors Mn approach perpendicular to the O3 plane and suggests that the product’s rotational excitation may originate in O2–OMn repulsion arising from removal of electron density from the slightly bonding 4b2 orbital of O3. However, some rotational excitation could also be attributed to conservation of angular momentum arising from a sizable reactive impact parameter. The lack of significant vibrational excitation is a consequence of the short-range nature of the partial charge transfer in this reaction channel.

Asymmetric(e,2e)measurement of vibrational intensities in the 100-eV electron-impact ionization ofN2to theN2+X2Σg+andA2Πustates

Asymmetric $(e,2e)$ experiments have been carried out on ${\mathrm{N}}_{2}$ for incident energy 100 eV, incident electron scattering angle 4\ifmmode^\circ\else\textdegree\fi{}, secondary-electron energy 5 eV, and secondary-electron ejection angles from -32\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{}. The experiments have sufficient energy resolution (\ensuremath{\sim}0.13 eV) to resolve the vibrational structure in the coincidence energy-loss spectrum. Ionization transitions to the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ and $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ states were observed. When the final state of the product ion was $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+},$ no vibrational excitation was observed at secondary electron ejection angles of 100\ifmmode^\circ\else\textdegree\fi{} (recoil) or -80\ifmmode^\circ\else\textdegree\fi{} (binary). However, at -32\ifmmode^\circ\else\textdegree\fi{}, near the peak of the binary lobe of the cross section, a strong increase in the production of the ${v}^{\ensuremath{'}}=1$ state was observed. In contrast, production of the $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ state was characterized by the expected Frank-Condon vibrational intensities at 100\ifmmode^\circ\else\textdegree\fi{} and 80\ifmmode^\circ\else\textdegree\fi{} on the recoil side but a decrease in the relative intensities of the ${v}^{\ensuremath{'}}=1,$ 2, and 3 vibrational states at -32\ifmmode^\circ\else\textdegree\fi{}. The A state deviations occur where the cross section is small, so we conclude that 100-eV electron impact does not produce significant vibrational excitation in the $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ state. On the other hand, the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ state is produced with significant vibrational excitation in the binary lobe where the cross section is large. This excitation is important for both a theoretical understanding of the ionization process as well as plasma and atmospheric phenomena.

Total electron scattering cross section for Cl2

Absolute measurements of the total electron scattering cross section, σsc,t(ε), for chlorine, Cl2, are reported for electron energies, ε, ranging from 0.3 to 23 eV. The present data are in reasonable agreement with previous measurements of the cross sections for total electron scattering and total rotational excitation, and indicate significant indirect vibrational excitation due to negative ion resonances.

Detection of benzaldehyde and formaldehyde in the UV photolysis of gas-phase methyl benzoate

The excitation of gas-phase methyl benzoate at 240 nm leads to the observation of phosphorescence. The dispersed phosphorescence spectrum has an assigned origin of 25 270 cm −1 and a prominent CO progression of 1720 cm −1, consistent with literature reports of gas-phase benzaldehyde spectroscopy. Weaker bands, which correspond to formaldehyde ν 17 and ν 25, are also evident. Time-resolved IR diode laser absorption spectroscopy has been used to probe formaldehyde. Excitation of methyl benzoate at 222 nm clearly indicates the generation of formaldehyde as a photoproduct. The temporal profile of the formaldehyde signal is consistent with significant nascent vibrational excitation in this product. The ratio of formaldehyde initially in the ground vibrational state to that in the excited vibrational states is estimated to be 0.6 ± 0.1. The proposed elimination mechanisms are analogous to those postulated for the formation of CO 2 and acetaldehyde from pyruvic acid.

Imaging of the HOMO electron density in Cr(CO) 6, Mo(CO) 6 and W(CO) 6 by electron momentum spectroscopy: a comparison with Hartree-Fock and DFT calculations

Studies of the electron density distributions in the t 2g HOMO orbitals (essentially metal valence d character) of the transition metal carbonyls Cr(CO) 6, Mo(CO) 6 and W(CO) 6 have been made using electron momentum spectroscopy (EMS). The EMS measurements provide a test of basis set effects and the quality of theoretical methods in the description of these large systems. The experimental momentum profiles are compared with theoretical spherically averaged momentum profiles calculated using the plane wave impulse approximation (PWIA) at the level of the target Hartree-Fock approximation (THFA) with a range of basis sets. Electron density maps for the HOMO orbitals in both momentum and position space have also been calculated for the oriented molecules at the Hartree-Fock level. Further comparisons of the measured momentum profiles to PWIA calculations are made in the target Kohn-Sham approximation (TKSA) using density functional theory (DFT) with the local density approximation and also with gradient corrected exchange correlation potentials. In addition to momentum profiles, other electronic properties have also been calculated by the various theoretical methods and compared to experimental values. The experimental momentum profiles for the HOMO orbitals of all three hexacarbonyls show significant cross-section at momenta below ∼0.5 au including a large contribution near p = 0 which is not expected from symmetry considerations or predicted by either the Target Hartree-Fock or the DFT target Kohn-Sham PWIA calculations. The discrepancies may be due to the significant vibrational excitation of the initial state metal carbonyl molecules expected at room temperature. However, theoretical studies of the Cr 3d and Mo 4d atomic systems show that significant low momentum components are introduced into the EMS cross-section when the PWIA treatment is replaced with the distorted wave impulse approximation (DWIA). While DWIA calculations are not possible for molecules the results for atomic Cr 3d and Mo 4d strongly suggest that the unusual effects observed experimentally in the M(CO) 6 HOMO (essentially Cr 3d, Mo 4d and W 5d in character) cross-sections are due to distortion of the electron waves in the collision process.

A coherent transition model for photodesorption from metals: NH3/Cu

A new time-dependent quantum mechanical scheme is proposed to study the uv photodesorption dynamics of ammonia from Cu(111). The desorption is considered here as the result of substrate-mediated electronic excitation and subsequent de-excitation. The transition from a short-lived excited electronic state to the ground state is described in a coherent manner. The dynamics of the system is represented by nuclear wave packets on two quasicoupled potential energy surfaces with two degrees of freedom (the desorption and N–H3 inversion modes). The desorbed molecules are found to have significant vibrational excitation and their translational energy distributions are highly structured due to the dominance of a predesorption mechanism. The desorption yield and the isotope effect are found to depend sensitively on the excited state lifetime. The results are compared with previous wave packet models and with experiments.

Quantum mechanical studies of photodesorption of ammonia from a metal surface: Isotope effects, final state distributions, and desorption mechanisms

The photodesorption dynamics of ammonia from a copper surface is studied quantum mechanically using empirical potential energy surfaces. The desorption is facilitated by substrate-mediated electronic excitation and subsequent de-excitation of the adsorbate, which are simulated in our model as Franck–Condon pump and dump between two electronic states. The delayed de-excitation populates metastable predesorption states which lay above the desorption limit. The slow decay of these resonances via energy transfer from an internal mode to the desorption mode results in incomplete and rather slow desorption. The desorbed molecules have significant vibrational excitation and their translational energy distributions are highly structured, due to the dominance of the predesorption mechanism. The desorption yield depends sensitively on the time delay between the excitation and de-excitation. Strong isotope effects are observed, consistent with experimental findings. The anomalously large NH3 yield relative to ND3 is attributed to its faster motion along the inversion coordinate on the excited state.

Reaction of Cl with vibrationally excited CH4 and CHD3: State-to-state differential cross sections and steric effects for the HCl product

The mechanism for the reaction of atomic chlorine with vibrationally excited methane is investigated by measurement of correlated state and scattering distributions using the method of core extraction (see preceding paper). Laser photolysis of molecular chlorine creates monoenergetic chlorine atoms (≳98% Cl 2P3/2) that react with vibrationally excited methane molecules prepared by linearly polarized infrared laser excitation. The resulting HCl product population distributions are determined by (2+1) resonance-enhanced multiphoton ionization (REMPI), and the differential cross section for each product rovibrational state is measured by core extraction. Approximately 30% of the product is formed in HCl(υ=1,J) with a cold rotational distribution; the remaining population is formed in HCl(υ=0,J) and is more rotationally excited. We observe a rich variation of the scattered flux that is dependent on the internal-energy state of the product. The HCl(υ=1) product is sharply forward scattered for low J and becomes nearly equally forward–backward scattered for high J; the HCl(υ=0,J) product is back and side scattered. The reactions of Cl with C–H stretch-excited methane (CH4) and C–H stretch-excited CHD3 are found to have similar angular and internal-state distributions. Observation of the spatial anisotropy of the HCl(υ=0, J=3) product shows that significant vibrational excitation of the methyl fragment does not occur. The measured spatial anisotropy is most consistent with a model in which backscattered HCl(υ=0, J=3) is formed in coincidence with slight methyl vibrational excitation and the forward-scattered HCl(υ=0, J=3) is formed in coincidence with no methyl excitation. The approach of the attacking chlorine atom with respect to the C–H stretch direction can be varied by rotating the plane of polarization of the infrared excitation. A marked steric effect is observed in which Cl atoms approaching perpendicular to the C–H stretch preferentially yield forward-scattered HCl(υ=1) product. On the other hand, the reaction is weakly dependent on the rotational quantum state of CH4(υ3=1,J), and on the rotational polarization. The data are consistent with a model that has a widely open ‘‘cone of acceptance’’ in which the impact parameter controls the internal-state and scattering distributions of the HCl product.

Transport of diatomic ions in atomic gases

A classical kinetic theory is presented for the drift and diffusion of diatomic ions in atomic gases under the influence of an electrostatic field. The theory is restricted to non-vibrating ions (rigid rotors) and to electric fields of small and intermediate strengths, where the average energy gained by the ions from the field is insufficient to cause significant vibrational excitation.

Diode laser probing of CO2 and CO vibrational excitation produced by collisions with high energy electrons from 193 nm excimer laser photolysis of iodine

Time domain diode laser absorption spectroscopy has been used to measure vibrational, rotational, and translational excitation of CO2 and CO following excimer laser photolysis of iodine in a low pressure mixture of CO2 and I2 or CO and I2. Nascent rotational population distributions have been measured in a number of low-lying CO2 vibrational levels, including 0001, 1000, 0200, 0220, and 0002 as well as the v=1 level of CO. In addition, measurements of CO2 translational excitation have been obtained for the majority of the rovibrational states probed. Significant vibrational excitation of CO2 has been observed with almost no increase in rotational and translational energy of the molecule. These results are consistent with the production of vibrationally excited CO2 via collisions with hot electrons which arise from multiphoton ionization of I2. Direct detection of the electrons has been accomplished using a time-resolved magnetic induction technique.

Molecule excitation in sputtering, scattering and electron or photon induced desorption

Simple theoretical considerations are shown to predict significant rotational and vibrational excitation of sputtered, scattered, and energetic desorbed molecules, alignment of the rotation axis, and orientation effects in dissociative and non-dissociative charge exchange cross-sections. Where available, experimental data verifying these predictions is presented. A theory of molecule-surface electronic interactions is outlined, and used to derive, from experimental data, the dependence on ionisation energy of the probability of occupation of a molecular state ( H 2 b 3 Σ u +) following scattering from a metal surface (Ni(111)). It is shown that vibrational excitation may occur as a consequence of non-dissociative charge capture. The possible role of substrate excitation (in the form of electron-hole pair creation) in vibrational quenching of ejected and scattered molecules is suggested.

Kinetic Investigation of (a) the Collisional Quenching of Mg(33PJ) by CO2 over the Temperature Range 600–1100 K by Time‐Resolved Atomic Emission (Mg(33P1) → Mg(31S0) + hv) and (b), E—(E, V) Transfer from Mg(33PJ) to MgO by Time‐Resolved Molecular Emission (MgO, B1Σ+ ‐ A1II and B1Σ+ ‐ X1Σ+) Following Pulsed Dye‐Laser Excitation at λ = 457.1 nm (Mg(33P1) ← Mg(31S0))

AbstractWe present a kinetic study of the collisional behaviour of Mg(33PJ) with CO2 and electronic energy transfer between Mg(33PJ) and MgO over the temperature range 600–1100 K. Mg(33P1) was generated by the pulsed dye‐laser excitation of magnesium vapour in a slow‐flow system at λ = 457.1 nm (Mg(33P1) → Mg(31S0)) and monitored by time‐resolved atomic emission at the resonance wavelength using pre‐trigger photomultiplier gating with Boxcar integration and computer interfacing. Absolute second‐order rate constants were measured for the removal of Mg(33PJ) by CO2, yielding the Arrhenius form:k1 = (2.4 ±0.50.7)·10−10 exp (−(47.6±5.8 kJ mol−1)/RT) cm3 molecule−1s−1.Chemiluminescence was monitored in the time‐domain for MgO, B1Σ+ ‐ A1∏ and B1Σ+ ‐ X1Σ+) for a number of vibrational levels. The molecular emission profiles were shown to be quantitatively consistent with electronic energy transfer from Mg(33PJ) to MgO, (X1Σ+ ‐ A1∏ yielding B1Σ+) yielding MgO(B1Σ+) by correlation of the atomic and molecular emission profiles. These measurements thus demonstrate the use of the B—A and B—X band systems of MgO in the time‐domain as spectroscopic markers for the kinetic behaviour of Mg(33P1), an aspect of the observations considered in detail. Vibrational temperatures were also measured for MgO(B1Σ+) using emission from vibrational levels ν′ = 0 to 4, demonstrating significant vibrational excitation on electronic energy exchange between Mg(33P1) and MgO(X1Σ+).

Chemistry of vibrationally excited ozone in the upper atmosphere

Recent atmospheric and laboratory experiments have shown that significant nonthermal vibrational excitation of O3 can occur near the mesopause. These results can be interpreted in terms of a steady state model in which O3(υ) is formed by radiative and chemical excitation and is destroyed by radiative and collisional deactivation. The only atmospheric chemical source of O3(υ) which can be clearly identified is the three‐body recombination of O with O2. Little is known about the detailed, level‐dependent kinetics of O3(υ), and several key assumptions are required for kinetic interpretation of the data. Predicted vibrational distributions, infrared band radiances, and diurnal effects are compared to in situ data in various assumed limits, and critical processes are discussed. Possible auroral sources of O3(υ) are also examined.