Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.     

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C(120)O(2-)

C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.     

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.     

Channel switching effect in photodissociating N2O+ ion at 312.5 nm

A experimental observation is presented on the N2O+ photodissociation process, which exhibits a complete channel switching effect in a narrow energy range. The N2O+ ions, prepared at the X2Pi (000) state by (3+1) multiphoton ionization of neutral N2O molecules at 360.6 nm, were excited to different vibrational levels in the A2Sigma+ state in a wavelength range of 275-328 nm. Based on the estimates of total released kinetic energies from the time-of-flight mass spectrum, it was found that the dissociation pathway of N2O+ (A2Sigma+), NO+ (X1Sigma+) + N(4S) with lower dissociation limit, changes abruptly and completely to NO+ (X1Sigma+) + N(2D) with higher dissociation limit, in a excitation energy range of merely 250 cm(-1) at lambda approximately 312.5 nm. This phenomenon was explained by competition between the two dissociation pathways across the special excitation energy region.     

Photoelectron spectroscopy of higher bromine and iodine oxide anions: electron affinities and electronic structures of BrO(2,3) and IO(2-4) radicals

This report details a photoelectron spectroscopy (PES) and theoretical investigation of electron affinities (EAs) and electronic structures of several atmospherically relevant higher bromine and iodine oxide molecules in the gas phase. PES spectra of BrO(2)(-) and IO(2)(-) were recorded at 12 K and four photon energies--355 nm/3.496 eV, 266 nm/4.661 eV, 193 nm/6.424 eV, and 157 nm/7.867 eV--while BrO(3)(-), IO(3)(-), and IO(4)(-) were only studied at 193 and 157 nm due to their expected high electron binding energies. Spectral features corresponding to transitions from the anionic ground state to the ground and excited states of the neutral are unraveled and resolved for each species. The EAs of these bromine and iodine oxides are experimentally determined for the first time (except for IO(2)) to be 2.515 ± 0.010 (BrO(2)), 2.575 ± 0.010 (IO(2)), 4.60 ± 0.05 (BrO(3)), 4.70 ± 0.05 (IO(3)), and 6.05 ± 0.05 eV (IO(4)). Three low-lying excited states along with their respective excitation energies are obtained for BrO(2) [1.69 (A (2)B(2)), 1.79 (B (2)A(1)), 1.99 eV (C (2)A(2))], BrO(3) [0.7 (A (2)A(2)), 1.6 (B (2)E), 3.1 eV (C (2)E)], and IO(3) [0.60 (A (2)A(2)), 1.20 (B (2)E), ～3.0 eV (C (2)E)], whereas six excited states of IO(2) are determined along with their respective excitation energies of 1.63 (A (2)B(2)), 1.73 (B (2)A(1)), 1.83 (C (2)A(2)), 4.23 (D (2)A(1)), 4.63 (E (2)B(2)), and 5.23 eV (F (2)B(1)). Periodate (IO(4)(-)) possesses a very high electron binding energy. Only one excited state feature with 0.95 eV excitation energy is shown in the 157 nm spectrum. Accompanying theoretical calculations reveal structural changes from the anions to the neutrals, and the calculated EAs are in good agreement with experimentally determined values. Franck-Condon factors simulations nicely reproduce the observed vibrational progressions for BrO(2) and IO(2). The low-lying excited state information is compared with theoretical calculations and discussed with their atmospheric implications.     

Photodissociation dynamics of N2O at 130 nm: the N2(A 3Sigmau +,B 3Pig)+O(3PJ = 2,1,0) channels

Oxygen Rydberg time-of-flight spectroscopy was used to study the vacuum ultraviolet photodissociation dynamics of N(2)O near 130 nm. The O((3)P(J)) products were tagged by excitation to high-n Rydberg levels and subsequently field ionized at a detector. In agreement with previous work, we find that O((3)P(J)) formation following excitation to the repulsive N(2)O D((1)Sigma(+)) state produces the first two electronically excited states of the N(2) counterfragment, N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)). The O((3)P(J)) translational energy distribution reveals that the overall branching ratio between N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)) formation is approximately 1.0:1.0 for J = 1 and 2, with slightly less N(2)(B (3)Pi(g)) produced in coincidence with O((3)P(0)). The angular distributions were found to be independent of J and highly anisotropic, with beta = 1.5+/-0.2.     

Low-energy electron stimulated desorption of neutrals from multilayers of SiCl4 on Si(111)

The interaction of low-energy electrons with multilayers of SiCl(4) adsorbed on Si(111) leads to production and desorption of Cl((2)P(32)), Cl((2)P(12)), Si, and SiCl. Resonant structure in the yield versus incident electron energy (E(i)) between 6 and 12 eV was seen in all neutral channels and assigned to dissociative electron attachment (DEA), unimolecular decay of excited products produced via autodetachment and direct dissociation. These processes yield Cl((2)P(32)) and Cl((2)P(12)) with nonthermal kinetic energies of 425 and 608 meV, respectively. The Cl((2)P(12)) is produced solely at the vacuum surface interface, whereas the formation of Cl((2)P(32)) likely involves subsurface dissociation, off-normal trajectories, and collisions with neighbors. Structure in the Cl((2)P(32)) yield near 14 and 25 eV can originate from excitation of electrons in the 2e, 7t(2) and 6t(2), 6a(1) levels, respectively. Although the 14 eV feature was not present in the Cl((2)P(12)) yield, the broad 25 eV feature, which involves complex Auger filling of holes in the 6t(2) and 6a(1) levels of SiCl(4), is observed. Direct ionization, exciton decay, and DEA from secondary electron scattering all occur at E(i)>14 eV. Si and SiCl were detected via nonresonant ionization of SiCl(x) precursors that are produced via the same states and mechanisms that yield Cl. The Si retains the kinetic energy profile of the desorbed precursors.     

Quantum chemical modeling of photoadsorption properties of the nitrogen-vacancy point defect in diamond

Quantum chemical calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with the first absorption band calculated at approximately 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0), we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (approximately 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (approximately 3.2 eV and higher).     

Two photon dissociation dynamics of NO2 and NO2 + H2O

To explore the dynamics of OH formation from two photon absorbed NO(2) with H(2)O, a high-level multiconfigurational perturbation theory was used to map the potential energy profiles of NO(2) dissociation to O ((1)D) + NO (X(2)Π), and subsequent hydrogen abstraction producing 2OH (X(2)Π) + NO (X(2)Π) in the highly excited S(PP) (?(2)A', (2)ππ*) state. The ground state NO(2) is promoted to populate in the S(NP1) (?(2)A", (2)nπ*) intermediate state by one photon absorption at ~440 nm, one thousandth of which is further excited to S(PP) (?(2)A', (2)ππ*) state and undergoes a medium-sized barrier (~11.0 kcal/mol) to give rise to OH radicals. In comparison with the hydrogen abstraction reaction in highly vibrationally excited NO(2) ground state, two photon absorption facilitates NO(2) dissociation to O ((1)D) and O ((1)D) + H(2)O → 2OH (X(2)Π) but results in low quantum yield of NO(2)** since there is a weak absorption upon the second beam light at ~440 nm. It can be concluded that the reaction of two photon absorbed NO(2) with H(2)O makes negligible contributions to the formation of OH radicals. In contrast, single photon absorption at <554 nm is a possible process on the basis of the present and previous computations.     

Photodissociation channels for N2O near 130 nm studied by product imaging

The photodissociation of N(2)O at wavelengths near 130 nm has been investigated by velocity-mapped product imaging. In all, five dissociation channels have been detected, leading to the following products: O((1)S)+N(2)(X (1)Sigma), N((2)D)+NO(X (2)Pi), N((2)P)+NO(X (2)Pi), O((3)P) + N(2)(A (3)Sigma(+) (u)), and O((3)P) + N(2)(B (3)Pi(g)). The most significant channel is to the products O((1)S) + N(2)(X(1)Sigma), with strong vibrational excitation in the N(2). The O((3)P) + N(2)(A,B):N((2)D,(2)P) + NO branching ratio is measured to be 1.4 +/- 0.5, while the N(2)(A) + O((3)P(J)):N(2)(B) + O((3)P(J)) branching ratio is determined to be 0.84+/-0.09. The spin-orbit distributions for the O((3)P(J)), N((2)P(J)), and N((2)D(J)) products were also determined. The angular distributions of the products are in qualitative agreement with excitation to the N(2)O(D (1)Sigma(+)) state, with participation as well by the (3)Pi(v) state.     

Two-photon dissociation of the NO dimer in the region 7.1-8.2 eV: excited states and photodissociation pathways

A study of excited states of the NO dimer is carried out at 7.1-8.2 eV excitation energies. Photoexcitation is achieved by two-photon absorption at 300-345 nm followed by (NO)(2) dissociation and detection of electronically excited products, mostly in n=3 Rydberg states of NO. Photoelectron imaging is used as a tool to identify product electronic states by using non-state-selective ionization. Photofragment ion imaging is used to characterize product translational energy and angular distributions. Evidence for production of NO(A (2)Sigma(+)), NO(C (2)Pi), and NO(D (2)Sigma(+)) Rydberg states of NO, as well as the valence NO(B (2)Pi) state, is obtained. On the basis of product translational energy and angular distributions, it is possible to characterize the excited state(s) accessed in this region, which must possess a significant Rydberg character.     

B2(BO)2(2-)-diboronyl diborene: a linear molecule with a triple boron-boron bond

We have produced and investigated an unique boron oxide cluster, B4O2(-), using photoelectron spectroscopy and ab initio calculations. Relatively simple and highly vibrationally resolved PES spectra were obtained at two photon energies (355 and 193 nm). The electron affinity of neutral B4O2 was measured to be 3.160 +/- 0.015 eV. Two excited states were observed for B4O2 at excitation energies of 0.48 and 0.83 eV above the ground state. Three vibrational modes were resolved in the 355 nm spectrum for the ground state of B4O2 with frequencies of 350 +/- 40, 1530 +/- 30, and 2040 +/- 30 cm(-1). Ab initio calculations showed that neutral B4O2 (D(infinity h), 3sigma(g)-) and anionic B4O2(-) (D(infinity h), 2pi(u)) both possess highly stable linear structures (O[triple bond]B-B=B-B[triple bond]O), which can be viewed as a B2 dimer bonded to two terminal boronyl groups. The lowest nonlinear structures are at least 1.5 eV higher in energy. The calculated electron detachment energies from the linear B4O2- and the vibrational frequencies agree well with the experimental results. The three observed vibrational modes are due to the B-B, B=B, and B[triple bond]O symmetric stretching vibrations, respectively, in the linear B2(BO)2. Chemical bonding analyses revealed that the HOMO of B2(BO)2, which is half-filled, is a bonding pi orbital in the central B2 unit. Thus, adding two electrons to B2(BO)2 leads to a B[triple bond]B triple bond in [O[triple bond]B-B[triple bond]B-B[triple bond]O]2-. Possibilities for stabilizing B2(BO)2(2-) in the form of B2(BO)2Li2 are considered computationally and compared with other valent isoelectronic, triple bonded species, B2H2Li2, B2H2(2-), and C2H2. The high stability of B2(BO)2(2-) suggests that it may exist as a viable building block in the condensed phase.     

Cluster-enhanced X-O2 photochemistry (X=CH3I, C3H6, C6H12, and Xe)

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X-O(2) (X=CH(3)I, C(3)H(6), C(6)H(12), and Xe). A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O((3)P(J),J=2,1,0) atom photoproduct via (2+1) resonance enhanced multiphoton ionization. The kinetic energy distribution (KED) and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full ("crush") or partial ("slice") detection of the three-dimensional O((3)P(J)) atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X-O(2) complex compared to a free O(2) molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X-O(2)(A'(3)Delta(u)) with excitation localized on the O(2) subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X(+)-O(2) (-) charge transfer (CT) state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X-O(2) complexes with X=CH(3)I and C(3)H(6), involves direct excitation into the (3)(X(+)-O(2) (-)) CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O(2) (-), which then photodissociates, providing fast (0.69 eV) O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O(2)(b (1)Sigma(g) (+)) are also observed when the CH(3)I-O(2) complex was irradiated. Potential energy surfaces (PES) for the ground and relevant excited states of the X-O(2) complex have been constructed for CH(3)I-O(2) using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O((3)P(J)) atom production channels.     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

Photoelectron spectroscopy of anions at 118.2 nm: observation of high electron binding energies in superhalogens MCl4- (M=Sc, Y, La)

High energy photon is needed for photoelectron spectroscopy (PES) of anions with high electron binding energies, such as superhalogens and O-rich metal oxide clusters. The highest energy photon used for anion PES in the laboratory has been 157 nm (7.866 eV) from F2 eximer lasers. Here, we report an anion PES experiment using coherent vacuum ultraviolet radiation at 118.2 nm (10.488 eV) by tripling the third harmonic output (355 nm) of a Nd:YAG laser in a XeAr cell. Our study focuses on a set of superhalogen species, MCl(4) (-) (M=Sc, Y, La), which were expected to possess very high electron binding energies. While the 157 nm photon can only access the ground state detachment features for these species, more transitions to the excited states at binding energies higher than 8 eV are observed at 118.2 nm. The adiabatic detachment energies are shown to be, 6.84, 7.02, and 7.03 eV for ScCl(4) (-), YCl(4) (-), and LaCl(4) (-) eV, respectively, whereas their corresponding vertical detachment energies are measured to be 7.14, 7.31, and 7.38 eV.     

Evidence of CH2O (a3A2) and C2H4 (a3B1u) produced from photodissociation of 1,3-trimethylene oxide at 193 nm

We investigated the dissociative ionization of formaldehyde (CH(2)O) and ethene (C(2)H(4)) produced from photolysis of 1,3-trimethylene oxide at 193 nm using a molecular-beam apparatus and vacuum-ultraviolet radiation from an undulator for direct ionization. The CH(2)O (C(2)H(4)) product suffers from severe dissociative ionization to HCO(+) (C(2)H(3) (+) and C(2)H(2) (+)) even though photoionization energy is as small as 9.8 eV. Branching ratios of fragmentation of CH(2)O and C(2)H(4) following ionization are revealed as a function of kinetic energy of products using ionizing photons from 9.8 to 14.8 eV. Except several exceptions, branching ratios of daughter ions increase with increasing photon energy but decrease with increasing kinetic energy. The title reaction produces CH(2)O and C(2)H(4) mostly on electronic ground states but a few likely on triplet states; C(2)H(4) (a(3)B(1u)) seems to have a yield greater than CH(2)O (a(3)A(2)). The distinct features observed at small kinetic energies of daughter ions are attributed to dissociative ionization of photoproducts CH(2)O (a(3)A(2)) and C(2)H(4) (a(3)B(1u)). The observation of triplet products indicates that intersystem crossing occurs prior to fragmentation of 1,3-trimethylene oxide.     

Neutral photodissociation of superexcited states of molecular iodine

The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I(2) formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I*(2)[R(B (2)Sigma(g) (+))] converging on the I(2) (+)(B (2)Sigma(g) (+)) state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*[R((3)P(J))]+I((2)P(32)), where the I*[R((3)P(J))] are Rydberg atoms converging on the I(+)((3)P(J)) states of the ion with J=2, 1, and 0. In the case of the I*[R((3)P(2))] fragments, the average Rydberg lifetime is observed to be 325+/-25 micros. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25-50 cm(-1) below the ionization limit.     

H-bonded network rearrangements in the S0, S1 and D0 states of neutral and cationic p-cresol(H2O)(NH3) complexes

The H-bonded network rearrangements in the S(0), S(1) and D(0) states of the neutral and cationic p-CreOH(H(2)O)(NH(3)) complexes were studied experimentally by means of (1 + 1)/(1 + 1') REMPI (Resonantly Enhanced MultiPhoton Ionization) and time resolved LIF (Laser Induced Fluorescence) spectroscopies combined with DFT (Density Functional Theory) calculations at the B3LYP/6-311G++(d,p) level. A comparison of the rearrangement process of the H-bonded network in the three states is given. Two cyclic H-bonded isomers were found on the S(0) potential energy surface and the results indicate that the rearrangement in this state is unlikely at the temperature of the supersonic expansion due to the presence of a high-energy barrier (7503 cm(-1)). On the other hand, the re-determination of the S(1) excited state lifetimes confirms that neither the H-bonded rearrangement nor the excited state hydrogen transfer (ESHT) reaction takes place in the S(1) state at the excitation energies of this work. Thus, it is concluded that the absorption of the second photon to reach the D(0) state takes place from the S(1) state of the cyclic-(OH-OH(2)-NH(3)) isomer. A preferential evaporation of H(2)O upon vertical ionization of the cyclic-(OH-OH(2)-NH(3)) isomer is observed which is consistent with a statistical redistribution of the internal energy. Nevertheless, our theoretical calculations suggest that initial excitation of the H-bonded network rearrangement modes may also play a role to leave the H(2)O molecule as a terminal moiety in a chain-(OH-NH(3)-OH(2))(+) isomer. The reaction pathway for the solvent rearrangement involves a double proton transfer process with a very low energy barrier (575 cm(-1)) that is overcome at the vertical ionization energy of the complex.     

Spectroscopic properties of novel aromatic metal clusters: NaM4 (M=Al,Ga,In) and their cations and anions

The ground- and several excited states of metal aromatic clusters, namely NaM(4) and NaM(4) (+/-) (M=Al,Ga,In) clusters have been investigated by employing complete active-space self-consistent-field followed by multireference singles and doubles configuration interaction computations that included up to 10 million configurations and other methods. The ground states NaM(4) (-) of aromatic anions are found to be symmetric C(4nu) ((1)A(1)) electronic states with ideal square pyramid geometries. While the ground state of NaIn(4) is also predicted to be a symmetric C(4nu) ((2)A(1)) square pyramid, the ground state of the NaAl(4) cluster is found to have a C(2nu) ((2)A(1)) pyramid with a rhombus base, and the ground state of NaGa(4) possesses a C(2nu) ((2)A(1)) pyramid with a rectangle base. In general, these structures exhibit two competing geometries, viz., an ideal C(4nu) structure and a distorted rhomboidal or rectangular pyramid structure (C(2nu)). All of the ground states of the NaM(4) (+) (M=Al,Ga,In) cations are computed to be C(2nu) ((3)A(2)) pyramids with rhombus bases. The equilibrium geometries, vibrational frequencies, dissociation energies, adiabatic ionization potentials, adiabatic electron affinities for the electronic states of NaM(4) (M=Al,Ga,In), and their ions are computed and compared with experimental results and other theoretical calculations. On the basis of our computed excited states energy separations, we have tentatively suggested assignments to the observed X and A states in the anion photoelectron spectra of Al(4)Na(-) reported by Li et al. [X. Li, A. E. Kuznetov, H. F. Zheng, A. I. Boldyrev, and L. S. Wang, Science 291, 859 (2001)]. The X state can be assigned to a C(2nu) ((2)A(1)) rhomboidal pyramid. The A state observed in the anion spectrum is assigned to the first excited state ((2)B(1)) of the neutral NaAl(4) with the C(4nu) symmetry. The assignments of the excited states are consistent with the experimental excitation energies and the previous Green's function-based methods for the vertical transition energy separations between the X and A bands.     

Unimolecular neutral and ion kinetics by variable-time neutralization—reionization mass spectrometry

A new method is described for the determination of rate parameters of unimolecular dissociations of neutral intermediates and ions produced by collisional neutralization and reionization at kiloelectronvolt kinetic energies. The method utilizes variable time-scales for neutral and ion dissociations to obtain time-dependent survivor and product ion yields. Kinetic analysis then provides phenomenological rate constants for both neutral and ion dissociations. Neutralization with CH₃I, NO, (C₂H₅)₂O, C₆H₆ and Xe of methyl iodide cation radicals is shown to produce the intermediate neutral molecules with different internal energies, resulting in different rates of neutral and ion dissociations. Vibrational excitation in neutralized CH₃I results in neutral dissociations with rate parameters in the (1–3) × 10⁵ s^–1 range. The origin of neutral excitation by fast collision is explained by the intermediate formation of highly excited Rydberg states that decay by photon emission to the vibrationally excited ground state of the molecule.