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The infrared photodissociation spectra of [(CO(2))(n)(H(2)O)(m)](-) (n=1-4, m=1, 2) are measured in the 3000-3800 cm(-1) range. The [(CO(2))(n)(H(2)O)(1)](-) spectra are characterized by a sharp band around 3570 cm(-1) except for n=1; [(CO(2))(1)(H(2)O)(1)](-) does not photodissociate in the spectral range studied. The [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2) species have similar spectral features with a broadband at approximately 3340 cm(-1). A drastic change in the spectral features is observed for [(CO(2))(3)(H(2)O)(2)](-), where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm(-1). Ab initio calculations are performed at the MP2/6-311++G(**) level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the [(CO(2))(n)(H(2)O)(m)](-) species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in [(CO(2))(n)(H(2)O)(m)](-): (1) the incorporated H(2)O is bonded to either CO(2) (-) or C(2)O(4) (-) through two equivalent OH...O hydrogen bonds to form a ring structure in [(CO(2))(n)(H(2)O)(1)](-); (2) two H(2)O molecules are independently bound to the O atoms of CO(2) (-) in [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2); (3) a cyclic structure composed of CO(2) (-) and two H(2)O molecules is formed in [(CO(2))(3)(H(2)O)(2)](-).

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.

Photodissociation spectra of mass-selected Mg(CH2=CHCN)n+ cluster ions were investigated in the wavelength region from 415 to 225 nm and 495 to 225 nm for n=1 and 2, respectively, by monitoring the total yield of fragment ions. The absorption bands exhibit large shifts from the 2P–2S resonance line of Mg+. In the spectrum of n=1, there are two bands at 26 400 and 40 800 cm−1. On the other hand, three absorption bands at 22 600, 28 800, and 37 500 cm−1 appear in the spectrum of n=2. The most stable structures in the ground state for n=1 and 2 were obtained by DFT(B3LYP/6-31+G*) calculations, and transition energies from these structures were obtained by using configuration interaction singles approach with the same basis set. The calculated excitation energies show good agreement with the experimental results. In addition, fragment ions of Mg(CH2=CHCN)m+ with m=4 and 5 are found to have high intensities from the parent ions of n=6–10 at a dissociation wavelength of 355 nm. From the result of theoretical ca...

We study vibrational predissociation of the HF dimer both by a full coupled channels treatment as well as in the Fermi golden rule approximation. Photodissociation cross sections, linewidths, and rotational state distributions are computed for excitations from the ground state with rotational quantum numbers J=1, K=0 to monomer stretch excited states with J=K=0, both for even and odd permutation symmetry. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the dimer stretch and dimer geared-bending modes. We find that dissociation is sufficiently slow for the Fermi golden rule approximation to be valid. The resonance positions and line strengths are compared with quasibound state calculations. The agreement with experimental data is fairly good for the photofragment angular distributions that were determined from the rotational state distributions, less good for some of the linewidths. Since we carefully checked that the results are converged with respect to the number of vibrational and rotational channels included, the remaining discrepancies are almost certainly due to small deficiencies in the SO-3 potential used in the calculations.

The state-resolved and isotope-specific detection of nascent ClO generated from the photodissociation of Cl2O parent molecules is performed by observing single-color (2+1) resonance enhanced multiphoton ionization (REMPI) spectra following excitation in the wavelength range from 336 to 344 nm; additionally state-resolved detection of nascent ClO is performed by observing single-color two photon laser-induced fluorescence. The REMPI spectrum is assigned to the ClO(C 2Σ−,v′=0←X 2ΠΩ,v=0) transition. The population of rotational states up to J=130 is evidence of large rotational excitation induced by a strong dependence of the excited potential energy surface (PES) on the Jacobi angle γ. Formation of the 2Π3/2 spin-orbit state is preferred: P(2Π1/2):P(2Π3/2)=0.30±0.05 suggesting significant radiationless transfer from the excited PES to closely lying neighboring states. The anisotropy parameter for the transition is determined to be β=0.35 independent of the ClO quantum state. The principal excited electronic...

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene 1CH2 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.

The photoionizationdynamics of rotationally hot CO, photodissociated from OCS, have been studied using laser photoelectron spectroscopy via the intermediate B^1Σ^+Rydberg state leading to the X^2Σ^+ of the ion. The photodissociation of OCS near 230 nm produces rotationally hot, but vibrationally cold CO (X^1Σ^+,N″,v″=0,1) fragments along with S (^1D) atoms. These high rotational levels show photoelectron spectra with a very strong ΔN=0 transition and weaker ΔN=±1, ±2, and ±3 transitions. Agreement between measured and calculated spectra is good and suggests that there is significant angular momentum coupling in the photoelectron orbital. In the ionization step not only Δv=0, but also off-diagonal, non-Franck–Condon (Δv≠0) transitions are observed. The intensities of these transitions vary strongly within the region studied and can be explained by the excitation of superexcited Rydberg states with an A^2Π core.

The photoionizationdynamics of rotationally hot molecular nitrogen are studied employing resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy.Photodissociation of N_2O at ∼203 nm results in highly rotationally excited N2 fragments in X ^1∑^+_g (N″,v″=0,1) states and O atoms in the excited ^1D_2 state. Photoelectron detection of the rotationally hot N_2 states is performed by a two-photon excitation to the lowest a″ ^1∑^+_g Rydberg state followed by one-photon ionization. The large number of observed rotational levels, from N′=49 up to N′=94, results in improved rotational parameters for a″ ^1∑^+_g (v′=0). In addition, experimental and theoretical rotationally resolved photoelectron spectra of the a″ ^1∑^+_g (v′=0,1;N′) state are presented. In these spectra only ΔN = N^+ − N′ = even transitions are observed, with a dominant ΔN=0 peak and rather weak ΔN = ±2 peaks. The one-photon ionization is dominated by ejection of electrons in p and f partial waves. The agreement between experimental and calculated spectra is excellent.

The OH stretching vibration of jet-cooled phenol-N2 in the neural and cationic ground states was observed by using infrared–ultraviolet double resonance spectroscopy and infrared photodissociation spectroscopy, respectively. The OH vibration showed a small but significant low-frequency shift of 5 cm−1 upon the cluster formation in the neutral, while the shift drastically increased up to 159 cm−1 in the cation. These results represent the direct evidence of the in-plane cluster structure, in which phenolic OH is hydrogen bonded to N2, as was proposed in the zero kinetic energy photoelectron study [S. R. Haines et al., J. Chem. Phys. 109, 9244 (1998)].

The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nm using velocity map imaging to probe the angle–speed distributions of the D(2S) and O(3P2) products. Both experiment and complementary first principle calculations demonstrate that photodissociation occurs by promotion of OD from high vibrational levels of the ground X 2Π state to the repulsive 1 2Σ− state.

The speed and angular distribution of O atoms arising from the photofragmentation of C(5)H(8)-O(2), the isoprene-oxygen van der Waals complex, in the wavelength region of 213-277 nm has been studied with the use of a two-color dissociation-probe method and the velocity map imaging technique. Dramatic enhancement in the O atoms photo-generation cross section in comparison with the photodissociation of individual O(2) molecules has been observed. Velocity map images of these "enhanced" O atoms consisted of five channels, different in their kinetic energy, angular distribution, and wavelength dependence. Three channels are deduced to be due to the one-quantum excitation of the C(5)H(8)-O(2) complex into the perturbed Herzberg III state ((3)Δ(u)) of O(2). This excitation results in the prompt dissociation of the complex giving rise to products C(5)H(8)+O+O when the energy of exciting quantum is higher than the complex photodissociation threshold, which is found to be 41740 ± 200 cm(-1) (239.6±1.2 nm). This last threshold corresponds to the photodissociation giving rise to an unexcited isoprene molecule. The second channel, with threshold shifted to the blue by 1480 ± 280 cm(-1), corresponds to dissociation with formation of rovibrationally excited isoprene. A third channel was observed at wavelengths up to 243 nm with excitation below the upper photodissociation threshold. This channel is attributed to dissociation with the formation of a bound O atom C(5)H(8)-O(2) + hv → C(5)H(8)-O(2)((3)Δ(u)) → C(5)H(8)O + O and/or to dissociation of O(2) with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C(5)H(8)*-O(2) + hv → C(5)H(8)*-O(2)((3)Δ(u)) → C(5)H(8) + O + O. The kinetic energy of the O atoms arising in two other observed channels corresponds to O atoms produced by photodissociation of molecular oxygen in the excited a (1)Δ(g) and b (1)Σ(g)(+) singlet states as the precursors. This indicates the formation of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)) after excitation of the C(5)H(8)-O(2) complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners (1)X-(3)O(2) + hν → (3)X-(1)O(2) → (3)X + (1)O(2) is suggested as a source of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)). This cooperative excitation is in agreement with little or no vibrational excitation of O(2)(a (1)Δ(g)), produced from the C(5)H(8)-O(2) complex as studied in the current paper as well as from the C(3)H(6)-O(2) and CH(3)I-O(2) complexes reported in our previous paper [Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. The formation of O(2)(a (1)Δ(g)) from C(5)H(8)-O(2) was observed at λ(pump) = 213-277 nm with the yield going down towards the long wavelength edge of this interval. This spectral profile is interpreted as the red-side wing of the band of a cooperative transition (1)X-(3)O(2) + hν → (3)X(T(2))-(1)O(2)(a (1)Δ(g)) in the C(5)H(8)-O(2) complex.

Until now, photodissociation studies on free complex protonated peptides were limited to the UV wavelength range accessible by intense lasers. We have studied photodissociation of gas-phase protonated leucine-enkephalin cations for vacuum ultraviolet (VUV) photons energies ranging from 8 to 40 eV. We report time-of-flight mass spectra of the photofragments and various photofragment-yields as a function of photon energy. For sub-ionization energies our results are in line with existing studies on UV photodissociation of leucine-enkephalin. For photon energies exceeding 10 eV we could identify a new dissociation scheme in which photoabsorption leads to a fast loss of the tyrosine side chain. This loss process leads to the formation of a residual peptide that is remarkably cold internally. (C) 2011 American Institute of Physics. [doi:10.1063/1.3515301]

Photodesorption of O(2)(X (3)Σ(g) (-)) and O(2)(a (1)Δ(g)) from amorphous solid water at 90 K has been studied following photoexcitation within the first absorption band at 157 nm. Time-of-flight and rotational spectra of O(2) reveal the translational and internal energy distributions, from which production mechanisms are deduced. Exothermic and endothermic reactions of OH+O((3)P) are proposed as plausible formation mechanisms for O(2)(X (3)Σ(g) (-) and a (1)Δ(g)). To examine the contribution of the O((3)P)+O((3)P) recombination reaction to the O(2) formation following 157 nm photolysis of amorphous solid water, O(2) products following 193 nm photodissociation of SO(2) adsorbed on amorphous solid water were also investigated.

Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules H2O and D2O, in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen deuterium atom formed by photodissociation of a neighboring water molecule. Desorption of D2O following 193 nm photoirradiation of a D2O /H2S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism. © 2010 American Institute of Physics. doi:10.1063/1.3386577

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH3(v = 0) and OH(v = 0, 1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH3 and OH radicals from solid methanol are discussed. The formation of O(1D and 3P) atoms and H2O was investigated, but the yield of these species was found to be negligible. CH3 products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice.

Translationally and internally hot H(2) molecules are produced from the 157 nm photodissociation of amorphous solid methanol at 90 K by two distinct mechanisms: exothermic recombination of two H-atom photoproducts bound to the surface and unimolecular molecular elimination of H(2) from the photoexcited methanol. The vibrationally hot H(2)(v=2-5) products are characterized by high translational and rotational temperatures. A third mechanism, the almost thermoneutral abstraction of a hydrogen atom from methanol parent molecule by the photolytically produced hydrogen atom, yields translationally and rotationally cold H(2)(v=0 and 1) products. Comparison with the results of the vacuum ultraviolet photolysis of water ice is discussed. Production of translationally hot and cold hydrogen atoms is also confirmed.

Electronic structures of trans- and cis-dichloroethylenes and tetrachloroethylene were studied using symmetry-adapted cluster configuration interaction theory. Basis sets up to the aug-cc-pVTZ of Dunning, Jr., augmented with appropriate Rydberg functions were used for the calculations. The results derived in the present study show good agreement with the available experimental values. In all cases, the main bright excitation was the * transition. The other vertical excitations, *, n*, and n*, which have not been studied before, were also investigated. First Rydberg series involving transitions from the orbitals to one 3s, three 3p, and five 3d orbitals were identified clearly. Several new assignments and reassignments of features in the experimental spectra were suggested. Contrary to earlier prediction, two n-* states, along with a -* state in the dichloroethylenes, were calculated to be located above the main -* state. Accordingly, crossing between both the n-* states with the bright -* state is highly likely, unlike conclusions made in the earlier studies. This indicates that the photodissociation mechanism proposed by the earlier calculations warrants revision. Several low-lying triplet excited states were also studied. Electronic spectra of trans-1-chloro-2-fluoroethylene and cis-1-chloro-2-fluoroethylene were also calculated. The * transitions of these haloethylenes are compared and interpreted in terms of the inductive and resonance effects.

We present a combined computational and threshold photoelectron photoion coincidence study of two isotopologues of dimethyl ether, (DME - h6)n and (DME - d6)n n = 1 and 2, in the 9-14 eV photon energy range. Multiple isomers of neutral dimethyl ether dimer were considered, all of which may be present, and exhibited varying C-H⋯O interactions. Results from electronic structure calculations predict that all of them undergo barrierless proton transfer upon photoionization to the ground electronic state of the cation. In fact, all neutral isomers were found to relax to the same radical cation structure. The lowest energy dissociative photoionization channel of the dimer leads to CH3OHCH3 (+) by the loss of CH2OCH3 with a 0 K appearance energy of 9.71 ± 0.03 eV and 9.73 ± 0.03 eV for (DME - h6)2 and deuterated (DME - d6)2, respectively. The ground state threshold photoelectron spectrum band of the dimethyl ether dimer is broad and exhibits no vibrational structure. Dimerization results in a 350 meV decrease of the valence band appearance energy, a 140 meV decrease of the band maximum, thus an almost twofold increase in the ground state band width, compared with DME - d6 monomer.

A new approach to detecting the spatial and energetic distribution of photodissociation products is demonstrated, in which an inhomogeneous electric field is used for spatially selective field ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a supersonic beam are excited to high-n Rydberg states and allowed to expand through the inhomogeneous field provided by an octopole. The field in the octopole results in the field ionization of those fragments that have moved to large radial distances (∼5 mm) from the beam axis, and therefore this device transmits only those Rydberg molecules with a low velocity component in the direction perpendicular to the beam. A detailed characterization of the properties of this “steradiancy analyzer” is carried out and factors limiting the energy resolution (currently approximately 100 cm−1 at an excess energy of 250 cm−1), along with possible improvements, are discussed in detail.

A novel experiment for near-threshold photodissociation studies is presented. State-selective excitation of the molecular photofragments to high-n Rydberg states is used in a variation of the ion imaging technique, allowing for undistorted detection of slow fragments produced close to the channel dissociation threshold. As a first demonstration of this method, the angular anisotropy parameter β for production of NO (J = 17/2) and O3P2 in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for β as a function of excess energy is presented, accounting for the decrease of anisotropy in the angular photofragment distribution upon approaching the channel threshold. The experimental values of β fluctuate substantially around the values predicted by the model, indicating strong underlying fluctuations in the state-to-state rate constant. This experiment offers in principle a unique route to measuring state-to-state reaction rate constants in situations where existing time- or frequency-resolved methods are inappropriate. © 2000 American Institute of Physics.

The first comprehensive high-resolution photoabsorption spectrum of (14)N(15)N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet and span 100 000-109 000 cm(-1) (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N(2) that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schrödinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

Velocity-aligned, superthermal O(1D) atoms generated via the photodissociation of N2O have been employed to investigate the stereodynamics of the title reaction. The power of this experimental technique, when coupled with Doppler-resolved, polarized laser-induced fluorescence probing of the reaction products, is demonstrated by reference to the specific reaction channel leading to NO(v′=0)+NO(v′=16,17), which is shown to proceed via direct stripping dynamics. Furthermore, the observed product-state selective linear and angular momenta disposals imply that the reaction is stereodynamically constrained to occur via collinear collision geometries. © 1992 American Institute of Physics.

Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O(1D) and OH(ν = 0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O(1D) atoms from the ice surface. One is unimolecular dissociation of H2O to H2+O(1D) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H2O2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice. © 2009 American Institute of Physics.

Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), via the (3)Sigma(u) (-) ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O(+) and O(-) ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda approximately 205 nm) was found to cause three-photon pumping of O(2) to a level at 18.1 eV. The parallel character of the observed O(+) and O(-) images allowed us to conclude that dissociation takes place on the (3)Sigma(u) (-) ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) products. This was revealed by the observed depletion of the yield of the O(-) anion and the appearance of a new O(+) cation signal with a kinetic energy E(transl)(O(+)) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O(+),O(-)) interatomic distance from 12 to about 900 A. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments.