State-resolved Studies Research Articles

Electron Beam Induced Damage of NaNO3 Single Crystals: An Energy, Temperature, and Quantum State Resolved Study

Electron beam induced damage of NaNO{sub 3} single crystals is examined using laser resonance enhanced multiphoton ionization detection of the neutral desorption products, post-irradiation temperature-programmed desorption (TPD), secondary electron emission microscopy (SEEM), and Auger electron spectroscopy (AES). The damage initially involves destruction of the nitrate group and production of O ({sup 3}P{sub J}) and NO ({sup 2}II) fragments with nonthermal energy distributions. Specifically, the O ({sup 3}P{sub J}) J state distribution measured at 100 eV incident electron energy is 5:1.5:0.25 for J = 2:1:0, the NO ({sup 2}II) vibrational state distribution is 1:0.56:0.35:0.40:0.23 for {nu} = 0:1:2:3:4, and the NO ({sup 2}II{sub 1/2,3/2}) rotational distribution has a high population of the upper ({sup 2}II{sub 3/2}) spin-orbit component. Production and desorption of these nonthermal fragments are dominated by the decay of {l{underscore}brace}NO{sub 3}{sup {minus}}{r{underscore}brace}*. At higher electron fluences, thermalized NO, O{sub 2}, and NO{sub 2} are also produced and released, through the NO{sub 2} is a minor product. The authors suggest that the formation and desorption of thermalized NO and O{sub 2} both involve NO{sub 2}{sup {minus}} defect states and unimolecular dissociation of NO{sub 3}{asterisk}. This is supported by the observation that the NO and O{sub 2} yields have the samemore » temperature dependence which is well described by the sum of two Maxwell-Boltzmann type equations with activation energies of 0.16 {+-} 0.03 and 0.010 {+-} 0.004 eV. O{sub 2} gas is also released in post-irradiation thermal cycling from 110 to 440 K with peaks at {approximately}260 and {approximately}340 K. The authors associate the post-irradiation TPD of O{sub 2} with reactions involving O atoms released during thermal decomposition of {l{underscore}brace}NO{sub 2}{sup {minus}}{hor{underscore}ellipsis}O{r{underscore}brace} and ONOO{sup {minus}}. The SEEM image shows damage features, and the AES spectra indicate that the irradiated region is depleted in both nitrogen and oxygen relative to Na. The elemental composition shows Na{sub 2}O as a final product of the NaNO{sub 3} radiation decomposition. The 100 eV electron beam damage cross section is at least {approximately}10{sup {minus}16} cm{sup 2}.« less

Vibrational state-resolved study of the O−+H2 reaction: Isotope effects on the product energy partitioning

The deuterium isotope effect on product energy partitioning in the O−+H2 particle transfer reaction is investigated in a crossed molecular beam experiment. Vibrational-state-resolved angular distributions are measured at six collision energies between 0.20 and 0.77 eV for the O−+H2 reaction and at seven collision energies between 0.22 and 1.20 eV for the O−+D2 reaction. The fraction of the total available energy deposited into product vibration is significantly larger in the deuterium system than in the hydrogen system. This effect is greatest at the lowest collision energies where OD− products are formed with more than twice as much vibrational energy as OH− products. The isotopic systems display similar trends in the product angular distributions, which extend over the full range of scattering angles at low energies and shift towards the forward direction as the collision energy is increased. These observations are discussed in terms of a competition between reaction mechanisms. An insertion-migration mechanism, yielding products with moderate vibrational excitation, is especially important at the lower energies. The insertion process leads to the isotope effect in the product energy partitioning which is explained in terms of Franck–Condon factors. As the energy increases, larger impact parameter collisions are able to proceed through a direct mechanism, yielding more tightly forward-scattered, vibrationally excited products. Since direct mechanisms show isotopically independent energy partitioning, the overall isotope effect diminishes with increasing energy as more collisions become purely direct. Bimodal rotational state distributions help strengthen the claim that two distinct reaction mechanisms produce the particle transfer product.

State-resolved collisional quenching of highly vibrationally excited pyridine by water: The role of strong electrostatic attraction in V→RT energy transfer

We report experiments that investigate the influence of long-range attractive forces on collisional energy loss from highly vibrationally excited molecules. State-resolved studies of energy transfer from highly vibrationally excited pyridine (μ=2.2 D) to water (μ=1.8 D) in a low-pressure environment at 298 K have been performed using high-resolution transient absorption spectroscopy of water at λ≈2.7 μm. Pyridine in its ground electronic state with 37 900 cm−1 of vibrational energy was prepared by absorption of pulsed ultraviolet light (λ=266 nm) to the S1 state, followed by rapid internal conversion to the S0 state. Collisions between vibrationally excited pyridine and water that result in rotational and translational excitation of the ground vibrationless state of H2O (000) were investigated by monitoring the populations of individual rotational states of H2O (000) at short times following pyridine excitation. The infrared probe of water was the highly allowed asymmetric stretching (000→001) transition. The nascent distribution of rotationally excited H2O (000) states is well described by a thermal distribution with a rotational temperature of Trot=770±80 K. Doppler-broadened transient linewidth measurements yield the velocity distributions of the recoiling H2O (000) molecules that correspond to center-of-mass translational temperatures of Ttrans∼515 K for all water rotational states investigated. Additionally, rate constants for energy gain in individual water states were determined, yielding an integrated rate constant of k2int=1.1×10−11 cm3 mol−1 s−1 for the appearance of H2O (000) with Erot=1000–2000 cm−1. These results are compared with previous relaxation studies of excited pyrazine (μ=0 D) with water and of excited pyridine with CO2 (μ=0 D), and the influence of electrostatic attraction on the relaxation dynamics is discussed.

State-Resolved Studies of Collisional Quenching of Highly Vibrationally Excited Pyrazine by Water: The Case of the Missing V → RT Supercollision Channel

The quenching of highly vibrationally excited pyrazine through collisions with H2O at 300 K in a low-pressure environment was investigated using high-resolution transient absorption spectroscopy of water at λ ≈ 2.7 μm. Highly vibrationally excited pyrazine with Evib = 37 900 cm-1 was prepared by absorption of 266 nm light to the electronically excited S2 state, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in rotational and translational excitation of the vibrationless ground state of H2O (000) were investigated by measuring the state-resolved appearance of individual rotational states of H2O (000). Transient absorption measurements have been made on numerous rotational states to determine the nascent distribution of rotational energy gain in water. Doppler-broadened transient absorption line shapes were collected for a number of rotational levels in the (000) state in order to measure velocity distributions of the scattered water molecules. The nascent distribution of water rotational states with Erot > 1000 cm-1 is well described by Trot = 920 K, and the velocity distributions correspond to Ttrans ≈ 560 K, independent of the rotational state. Rate constants for energy gain into individual quantum states of H2O (000) from collisions with hot pyrazine provide a measure of the high-energy part of the energy-transfer probability distribution function. The quenching of hot pyrazine through collisions with water displays a significant reduction in the bath translational energy gain when compared to earlier studies on the quenching of hot pyrazine (Evib = 37 900 cm-1) by CO2 {Wall, M. C.; Mullin, A. S. J. Chem. Phys. 1998, 108, 9658}. A comparison of the two systems provides insights into the molecular properties that influence the relaxation of highly vibrationally excited molecules.

Independent Determination of Supercollision Energy Loss Magnitudes and Rates in Highly Vibrationally Excited Pyrazine with Evib = 36000−41000 cm-1

We report results of the first state-resolved study of large ΔE energy transfer resulting from collisions of highly excited pyrazine with initial vibrational energies of 36000−41000 cm-1. We have used transient infrared absorption spectroscopy to monitor the rotational and translational energy gain in a CO2 bath at short times following collisions with highly excited pyrazine. We have measured nascent distributions of high-energy rotational states, J = 56−84, for CO2 in its ground vibrationless (0000) state, the translational energy gain associated with these excited rotational states, and state-resolved rate constants for excitation of the high-energy bath states. Our results show that for the CO2 rotational states investigated, the nascent rotational distributions and translational energies change very little for a 13% change in pyrazine energy. In contrast, the rate constants for these events are strongly influenced by the donor internal energy content and increase by an order of magnitude for a 5000 c...

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.

EXPERIMENTAL STUDIES OF RESONANCES IN UNIMOLECULAR DECOMPOSITION

▪ Abstract In recent years we have witnessed tremendous progress in our understanding of unimolecular reactions on a fully state-resolved level. Here we describe recent state-resolved experimental studies of resonances in unimolecular reactions, focusing on the transition from isolated to overlapping resonances. Depending on the well depth and extent of intramolecular vibrational energy redistribution, the resonances can exhibit properties ranging from mode- and state-selective to statistical behavior. In the statistical limit the resonances are usually overlapped, and interference effects may become prominent. We use recent studies of HCO, HFCO, and CH3O to examine the transition from mode-selective to statistical behavior in the isolated regime. Experimental and theoretical studies of NO2, including photofragment yield spectra, fully resolved NO quantum state distributions, and decomposition rates are used to examine unimolecular decomposition in the regime of overlapping resonances.

Laser-stimulated desorption from surfaces

Recent experimental and theoretical results focus on nonthermal laser-induced desorption after UV-laser excitation with the aim of obtaining insights into microscopic processes, including the relevant potential energy surfaces and lifetimes of the excited states that are involved. Results range from measurements and interpretation of quantum state resolved studies to femtochemistry.

Infrared laser transient absorption spectroscopy of the ethyl radical

The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 μm is reported. The band origin is found to be at 528.1 cm−1. The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G12 symmetry, qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm−1 in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1°. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical.

State-Resolved Studies of Reactions in the Gas Phase

During the first years of The Journal of Physical Chemistry, chemists were just beginning to understand chemical reactions in gases as sequences of elementary reaction steps. Basic models of reaction dynamics were developed in succeeding decades which provided powerful qualitative insight into reaction mechanisms and rate constants. In recent decades, the development of molecular beam and laser technologies has allowed us to look into the transition state region at the dynamics of chemical bond formation and breaking. State-selective preparation of reactants and state-resolved detection of products with velocity and angle resolution permit exacting quantitative test of dynamical theories. Thanks to the ability to calculate accurate ab initio potential energy surfaces and solve Schroedinger's equation for reactive scattering dynamics, the simplest reactions are now understood quantitatively at the most fundamental level possible. The concepts and models thus developed, along with ever more powerful experim...

State resolved studies of photochemical dynamics at surfaces

Photochemical surface processes are initiated by the absorption of photons by an adsorbate or the substrate, and followed by various energy transfer and relaxation processes that lead to nuclear motion along the reaction coordinates. A powerful way to gain insight into the molecular dynamics of such processes involves the determination of final state distributions in the translational and internal degrees of freedom of desorbed product molecules. These distributions include the angular distribution, the translational, vibrational, and rotational energy distributions, the spatial alignment of angular momentum vectors, and populations of spin-orbit and lambda doublet states. Additional information is contained in correlations between two or more degrees of freedom, e.g., the velocity distribution as a function of desorption angle or internal state, or correlations between spin-orbit and rotational state populations. In this Report we review state resolved studies of surface photochemistry ( hν ⩽ 6.4 eV) for which the final state distributions have contributed to our understanding of the microscopic dynamics, focusing on recent progress in our ability to explain the observed distributions with simple dynamical models. After briefly reviewing basic photochemical excitation and reaction mechanisms at surfaces, we discuss experimental techniques, including time-of-flight measurement by mass spectrometer and laser spectroscopic techniques. We then review vibrational distributions of photochemically desorbed molecules, interpreting them in terms of a simple quantum dynamics model. A section on translational distributions discusses dynamical models explaining observed velocity distributions of photochemically desorbed species for both weakly and strongly quenched systems. Next we review rotational distributions of photochemically desorbed molecules, and present a dynamical model that can explain many of the observed distributions, as well as the ubiquitous positive correlations between the rotational and translational degrees of freedom. After discussing recent measurements of the spatial alignment of angular momentum vectors of photodesorbed molecules, we review models explaining spin-orbit state propensities and spin-orbit-rotational correlations for photodesorption and surface photodissociation systems.

Ion Reaction Dynamics

This article reviews recent progress in the study of ion reaction dynamics in which product energy disposal studies provide detailed information about the potential energy surfaces mediating chemical rea ction. We emphasize product state-resolved studies of the proton tran sfer and hydro­ gen atom tran sfer reactions of 0-. We also discuss several critical experi­ ments that test the validity of the double minimum potential energy surf ace model in describing the bimolecular nucleophilic substitution (SN2) reac­ tion. Photo ionization methods for producing vibrational and vibrational­ rotational state selecti ons of ions are also discussed, and recent examples with Hi and Nt are presented. The role of these state-selection methods in elucidating mode-s elective chemistry in the reactions of NHt and C2Ht are also discussed.

Quantum state-resolved study of NH 3 photodesorbed from GaAs(1 0 0)

The laser-induced desorption of ammonia from GaAs(1 0 0)-(4 × 6) has been studied using (2 + 1)-photon REMPI as a state-specific probe. Ammonia and its deuterated sibling exhibit surprisingly large isotope effects in their relative cross sections for photodesorption from GaAs. This finding has led to the proposition of a vibrationally mediated UV photodesorption model. We find that NH 3 desorbs with a mean translational energy, 〈 E trans〉, of 300 K, a rotational temperature of about 520 K, and vibrational excitation in the ν 2 mode corresponding to about 930 K.

Spiers Memorial Lecture. State-resolved studies of unimolecular reactions

During the last decade, the development of molecular beam and laser technologies has permitted a look into the transition-state region at the dynamics of chemical bond formation and breaking. State-selective preparation of reactants and state-resolved detection of products permits an exacting quantitative test of dynamical theories that are based on accurate ab initio potentials. The control of reaction rate and energy release to products by vibrations at the transition state is illustrated for ketene fragmentation over a simple barrier to yield triplet methylene and carbon monoxide. The barrier to exchange of carbon atoms in ketene has a shallow well at its top; this causes quantum dynamical resonances for motion along the reaction coordinate which are seen as peaks in the isomerization rate constant as a function of energy. For dissociation to singlet methylene and carbon monoxide, there is no barrier to the reverse reaction. The reaction rate is controlled by a variational transition state which tightens as energy increases. The transition state is defined on a separate vibrationally adiabatic potential-energy surface for each product vibrational state. The dynamics of energy flow between transition state and separated fragments are in the strongly coupled limit near threshold (0–200 cm–1). For higher energies the methylene rotations are significantly colder than predicted statistically and thus the dynamics must be partially adiabatic.

State-resolved studies of the laser-induced desorption of NO from Si(111) 7×7: Low coverage results

The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.

Quantum state-resolved study of the rovibrational excitation of OCS by hot hydrogen atoms

The vibrationally inelastic scattering of OCS by hot hydrogen atoms was studied using an excimer laser photolysis/diode laser probe technique. Nascent rotational distributions for the 1000 (2062 cm−1) and 0001 (859 cm−1) stretching vibrations are well fit to Boltzmann temperatures of 840±50 and 545±60 K, respectively. The total vibrational excitation probability of the 0001 state was about 2.3±0.3 times that of the 1000 state. Measurement of OCS linewidths immediately after hot atom collisions indicate translational recoil temperatures that vary from 360 to 730 K, with a positive dependence on rotational state. The results are interpreted in terms of repulsive, hard shell collisions with the different stretching vibrations arising from trajectories approaching different ends of the OCS molecule.

Quantum state-resolved study of pure rotational excitation of CO2 by hot atoms

Rotationally inelastic scattering of carbon dioxide by translationally hot H, D, and Cl atoms was studied by time-resolved diode laser absorption. The high J rotational distribution falls off quite rapidly between J=60 and J=80. D atom collisions have roughly twice the excitation cross section versus H atom collisions, with the H*/D* ratio decreasing with increasing J. These results are consistent with a constraint on the total reagent orbital angular momentum available for rotational excitation. Transient Doppler profiles measured immediately after hot atom/CO2 collisions indicate that CO2 molecules excited to high J levels have a larger recoil velocity than molecules excited to lower J levels. This result is consistent with predictions based on a simple model which treats the CO2 potential as a hard shell ellipsoid.

Controlling the pathways in molecular decomposition: the vibrationally mediated photodissociation of water

Vibrationally mediated photodissociation, in which vibrational overtone excitation prepares a single rovibrational state of water that an ultraviolet photon subsequently dissociates, permits a fully state resolved study of the photodissociation of water. Dissociating water from six different initial rotational states and detecting the OH products by laser-induced fluorescence shows that the distribution of the products among their rotational states depends strongly on the state initially selected in the vibrational overtone excitation step. These measurements also demonstrate the control of the dissociation pathway by selection of different intermediate states. Dissociating water molecules from one vibrational state ({vert bar}04>4{sup {minus}}) produces almost no vibrationally excited OH products, but dissociating another state ({vert bar}13>{sup {minus}}) that corresponds to a different nuclear motion produces roughly comparable amounts of OH(v = 1) and OH(v = O).

Classical theory of collisional depolarization and rotational relaxation in open-shell diatomic molecules

The semiclassical S-matrix theory of Miller and Marcus is utilized to derive an exact classical mechanical expression for the effective cross sections governing collisional depolarization and rotational relaxation in open-shell molecules with Σ electronic symmetry. These cross sections are expressed as a sum of products of classical dynamical terms (which may be evaluated by conventional Monte Carlo techniques) and geometrical terms that describe the quantum mechanical coupling of the angular momentum vectors in the open-shell molecule. All effects on rotational transitions due to the nonzero electronic spin angular momentum in the open-shell molecule are accounted for through the geometrical terms. Recently, sophisticated molecular beam and laser double resonance techniques have allowed the state-resolved study of rotational energy transfer in collisions involving open-shell molecules in their ground electronic state. The present extension of classical scattering theory provides the theoretical framework for an exact classical mechanical calculation of the collision dynamics in such experiments.