Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

The SH + CH(3) product channel for the photodissociation of CH(3)SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH(3) products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH(3) formed in the ground and excited vibrational states (v(2) = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH(3) channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH(3) (v(2) = 0, 1, and 2) products is indicative of a fast dissociation process for C-S bond cleavage. Two kinds of slower CH(3) products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH(3)S (X(2)E) radical via cleavage of the S-H bond in CH(3)SH, followed by probe laser photodissociation of nascent CH(3)S radicals yielding CH(3)(X(2)A(1), v(2) = 0-2) + S((3)P(j)/(1)D) products.     

Photodissociation of vibrationally excited SH and SD radicals at 288 and 291 nm: the S(1D2) channel

Ultraviolet photodissociation of SH (X 2Pi, upsilon"=2-7) and SD (X 2Pi, upsilon"=3-7) has been studied at 288 and 291 nm, using the velocity map imaging technique to probe the angular and speed distributions of the S(1D2) products. Photodissociation cross sections for the A 2Sigma+<--X 2Pi(upsilon") and 2Delta<--X 2Pi(upsilon") transitions have been obtained by ab initio calculations at the CASSCF-MRSDCI/aug-cc-pV5Z level of theory. Both the experimental and theoretical results show that SH/SD photodissociation from X 2Pi (upsilon"相似文献   

Heavy hydrides: H2Te ultraviolet photochemistry

The room-temperature ultraviolet absorption spectrum of H2Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high-n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH(2Pi1/2) selectively relative to the TeH(2Pi3/2) ground state. This is attributed to the role of the 3A' state, which has a shallow well at large R(H-TeH) and correlates to H+TeH(2Pi1/2). Note that the 2Pi1/2 state is analogous to the 2P1/2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3A' state is crossed at large R only by 2A", with which it does not interact. The character of 3A' at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, 2Pi1/2 has a higher degree of spherical symmetry than does 2Pi3/2 (recall that I(2P1/2) is spherically symmetric), and consequently 2Pi1/2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3A'<--X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3A'. The main absorption feature that is peaked at approximately 240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4A" surface. The 4A"<--X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0. For H2Te and TeH, these are 65.0+/-0.1 and 63.8+/-0.4 kcalmol, respectively, in good agreement with predictions that use high-level relativistic theory.     

Product pair correlation in CH3OH photodissociation at 157 nm: the OH + CH3 channel

The OH + CH(3) product channel for the photodissociation of CH(3)OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH(3) radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH(3) formed in the ground and excited states (v(2) = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH(3) channel is clearly inverted. Anisotropic distributions for the CH(3) (v(2) = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C-O bond cleavage. A slower CH(3) product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH(3)O(X (2)E) radical via the cleavage of the O-H bond in CH(3)OH, followed by probe laser photodissociation of the nascent CH(3)O radicals yielding CH(3)(X (2)A(1), v = 0) products.     

Electronic quenching of OH A 2Sigma+ radicals in single collision events with H2 and D2: a comprehensive quantum state distribution of the OH X 2Pi products

A pump-probe laser-induced fluorescence technique has been used to examine the nascent OH X (2)Pi product state distribution arising from non-reactive quenching of electronically excited OH A (2)Sigma(+) by molecular hydrogen and deuterium under single-collision conditions. The OH X (2)Pi products were detected in v'=0, 1 and 2; the distribution peaks in v'=0 and decreases monotonically with increasing vibrational excitation. In all vibrational levels probed, the OH X (2)Pi products are found to be highly rotationally excited, the distribution peaking at N'=15 when H(2) was used as the collision partner and N'=17 for D(2). A marked propensity for production of Pi(A') Lambda-doublet levels was observed, while both OH X (2)Pi spin-orbit manifolds were equally populated. These observations are interpreted as dynamical signatures of the nonadiabatic passage of the OH + H(2)/D(2) system through the seams of conical intersection that couple the excited state (2 (2)A') and ground state (1 (2)A') surfaces.     

Near-UV photodissociation of oriented CH3I adsorbed on Cu(110)-I

Methyl iodide adsorbed on a Cu(110)-I surface has been found to be highly orientationally ordered. We have exploited this orientation to select different CH(3)I excited states for photodissociation by using polarized near-UV light at wavelengths of 308, 248, and 222 nm. Using p-polarized light at all three wavelengths, we find that dissociation proceeds largely via the (3)Q(0) state, consistent with the picture from gas-phase photolysis. In contrast, using s-polarized light we find contributions from the (3)Q(1) state at lambda=308 nm, the (1)Q(1) state at lambda=248 nm, and the (E,1) state at lambda=222 nm-the latter being a state that has not been implicated in gas-phase studies of CH(3)I A-band photolysis. We also note the contribution to surface photodissociation from low-energy photoelectrons causing dissociative electron attachment to adsorbed CH(3)I and have identified the promotion of direct photodissociation pathways during lambda=308 nm photolysis.     

Reaction dynamics of Y + O2--> YO(X,A',A)+ O(3P(J)) studied by the crossed-beam technique

The dynamics of the reaction, Y + O2--> YO + O was studied by using the crossed-beam technique at several collision energies from 10.3 to 52.0 kJ mol(-1). The Y atomic beam was generated by laser vaporization and crossed with the O2 beam at a right angle. Among the energetically accessible electronic states of YO, the formation of the A2Pi and A'2Delta states was observed by their chemiluminescence at all collision energies. By analyzing the chemiluminescence spectra of YO(A2Pi(1/2,3/2)-X2Sigma+), vibrational state distributions and relative populations of spin-orbit states were determined for YO(A2Pi(1/2,3/2)). At low collision energies, the vibrational distributions agree quite well with those expected from the statistical energy partitioning, while a little deviation from the statistical expectation was observed at the highest energy, 52.0 kJ mol(-1). The populations of two spin-orbit states are in good agreement with the statistical expectations at all collision energies. The vacuum ultraviolet laser-induced fluorescence (VUV-LIF) technique was employed to determine the distributions of spin-orbit states of the product O(3P(J)) at two collision energies, 20.7 and 52.0 kJ mol(-1). The line shapes of the O atom transitions were analyzed to determine relative branching ratio of the ground state to the excited states of YO, i.e. YO(X2Sigma+)+ O(3P(J))vs. YO(A2Pi and A'2Delta)+ O(3P(J)). The results showed that the electronically excited YO was formed with comparable amount with the ground state which is statistically more favorable, and suggested the occurrence of two mechanisms taking place in the title reaction.     

Quantum chemical and statistical rate investigation of the CF2(a3B1)+NO(X2Pi) reaction: a fast chemical quenching process

The reaction of CF2(a3B1) with NO(X2Pi) was theoretically investigated using the B3LYP, MP2, CCSD(T), G2M, CASSCF, and CASPT2 quantum chemical methods with various basis sets including 6-31G(d), 6-311G(d), 6-311+G(3df), cc-pVDZ, and cc-pVTZ. In agreement with the experimental kinetic data, the CF2(a3B1)+NO(X2Pi) reaction is found to proceed via a fast, barrier-free combination. This process, occurring on the doublet potential energy surface, leads to the electronically excited adduct F2C-NO(22A'), which readily undergoes a surface hopping to the 12A' electronic surface, with a Landau-Zener transition probability estimated to be close to 90% per C-N vibration. The metastable adduct F2C-NO(12A') can then either spontaneously decompose into CF2(X1A1)+NO(X2Pi) in a direct chemical quenching mechanism or relax to its ground-state equilibrium structure F2CNO(X2A'). The product distribution resulting from the latter, chemically activated intermediate was evaluated by solution of the master equation (ME), under different reaction conditions, using the exact stochastic simulation method; microcanonical rate constants were computed using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, based on the potential energy surfaces (PESs) constructed using both G2M and CASPT2 methods. The RRKM/ME analysis reveals that the hot F2CNO(X2A') rapidly fragments almost exclusively to the same products as above, CF2(X1A1)+NO(X2Pi), which amounts to an indirect chemical quenching mechanism. The reaction on the quartet PES is unlikely to be significant except at very high temperatures. The high crossing probability (up to 90%) between the two "avoided" doublet PESs points out the inherent difficulty in treating chemically activated reactions with fast-moving nuclei within the Born-Oppenheimer approximation.     

Determination of the internal state distribution of the SD product from the S(1D)+D2 reaction

The S(1D)+D2-->SD+D reaction has been studied through a photolysis-probe experiment in a cell. S(1D) reagent was prepared by 193 nm photolysis of CS2, and the SD(X 2Pi) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Pi) product was determined. This reaction, previously studied theoretically and in a crossed molecular beam experiment, is known to proceed through formation and decay of a long-lived collision complex involving the deep well in the H2S ground electronic state. The determined SD rotational state distribution in the v=0 vibrational level was found to be approximately statistical, with a small preference for formation of the F1 (Omega=3/2) fine-structure manifold over F2 (Omega=1/2). The branching into the Lambda doublet levels was also investigated, and essentially equal populations of levels of A' and A" symmetry were found. The present results are compared with previous investigations of this reaction and the analogous O(1D)+D2 reaction.     

An ab initio study of the CH3I photodissociation. II. Transition moments and vibrational state control of the I* quantum yields

Multireference spin-orbit configuration interaction calculations of transition moments from the X A1 ground state to the 3Q0+, 3Q1, and 1Q excited states responsible for the A absorption band of CH3I are reported and employed for an analysis of the photofragmentation in this system. Contrary to what is usually assumed, the 3Q0+(A1), 3Q1(E), and 1Q(E)<--X A1 transition moments are found to be strongly dependent on the C-I fragmentation coordinate. The sign of this dependence is opposite for the parallel and perpendicular transitions, which opens an opportunity for vibrational state control of the photodissociation product yields. The computed absorption intensity distribution and the I* quantum yield as a function of excitation energy are analyzed in comparison with existing experimental data, and good agreement between theory and experiment is found. It is predicted that significantly higher I* quantum yield values (>0.9) may be achieved when vibrationally hot CH3I molecules are excited in the appropriate spectral range. It is shown that vibrational state control of the I*/I branching ratio in the alkyl (hydrogen) iodide photodissociation has an electronic rather than a dynamic nature: Due to a different electron density distribution at various molecular geometries, one achieves a more efficient excitation of a particular fragmentation channel rather than influences the dynamics of the decay process.     

Diminished cage effect in solid p-H2: Infrared absorption of CH3S observed from photolysis in situ of CH3SH, CH3SCH3, or CH3SSCH3 isolated in p-H2 matrices

We report the infrared absorption spectrum of the methylthio (or thiomethoxy) radical, CH(3)S (X (2)E(3/2)), produced via photodissociation in situ of three precursors CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3) isolated in solid p-H(2). The common absorption features observed with similar intensity ratios in each experiment are assigned to CH(3)S. The wavenumbers of these features agree satisfactorily with those predicted with a spin-vibronic Hamiltonian accounting for the anharmonic effects and the Jahn-Teller effects to the quartic term [A. V. Marenich and J. E. Boggs, J. Chem. Theory Comput. 1, 1162 (2005)]. In addition to an absorption line at 724.2?cm(-1), corresponding to a transition of 3(1) previously determined to be 727?cm(-1) from fluorescence spectra of gaseous CH(3)S, we identified fundamental transitions 6(1)(a(1)) at 771.1, 6(1)(e) at 1056.6, 5(1)(a(1)) at 1400.0, 4(1)(a(1)) at 2898.4?cm(-1), and several combination and overtone transitions. In contrast, photolysis of CH(3)SSCH(3) isolated in solid Ar produces mainly H(2)CS, CH(3)SH, and CS(2), but no CH(3)S. These results demonstrate the feasibility of using photolysis in situ of precursors isolated in solid p-H(2) to produce free radicals by taking advantage of the diminished cage effect of the matrix.     

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.     

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.     

A study of the radical-radical reaction dynamics of O(3P)+t-C4H9 --> OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.     

State-resolved dynamics of the CN(B2Sigma+) and CH(A2Delta) excited products resulting from the VUV photodissociation of CH3CN

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2Sigma+-X2Sigma+) and CH(A2Delta-X2Pi) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2Sigma+) and CH(A2Delta) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2Sigma+) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2Sigma+)) ranges from 375 to 600 K at lower K' and from 1840 to 7700 K at higher K' depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2Sigma+) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2Delta) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2Delta))=4895+/-140 K in v'=0 and 2590+/-110 K in v'=1. From thermodynamic calculations, it is evident that CH(A2Delta) is produced along with CN(X2Sigma+)+H2. These products can be formed by a two step mechanism (via excited CH3* and ground state CN(X2Sigma+)) or a process similar to the "roaming" atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2Delta) and CN(B2Sigma+) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2Delta):CN(B2Sigma+) ratio of (1.2+/-0.1):1 at 16 eV, thereby indicating that production of CH(A2Delta) is significant at 16 eV.     

Generation and detection of alkyl peroxy radicals in a supersonic jet expansion

Alkyl peroxy radicals are synthesized in a supersonic jet expansion by the initial production of alkyl radicals and subsequent reaction with molecular oxygen. Parent ions CH3OO+/CD3OO+ are observed employing vacuum ultraviolet (VUV) single photon ionizationtime-of-flight mass spectroscopy (TOFMS). Employing infrared (IR) + VUV photofragmentation detected spectroscopy, rotationally resolved infrared spectra of jet-cooled CH3OO and CD3OO radicals are recorded for the A 2A' <-- X 2A" transition by scanning the IR laser frequency while monitoring the CH3 + and CD3 + ion signals generated by the VUV laser. The band origins of the A 2A'<--X 2A" transition for CH3OO and CD3OO are identified at 7381 and 7371 cm(-1), respectively. Rotational simulation for the CH3OO and CD3OO 0(0) 0 transitions of A<--X yields a rotational temperature for these radicals of approximately 30 K. With the aid of ab initio calculations, two and five vibrational modes for the A 2A' excited electronic state are assigned for CH3OO and CD3OO radicals, respectively. Both experimental and theoretical results suggest that the ground electronic state of the ions of ethyl and propyl peroxy radicals are not stable although their ionization energies (IE) are less than 10.5 eV. The C2H5OO+/C3H7OO+ cations can readily decompose to C2H5 +/C3H7 + and O2. This is partially responsible for the inability of IR+VUV photofragmentation spectroscopy to detect the near IR A<--X electronic transition for these radicals.     

The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH)

Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1Sigma+ and 1Pi stationary points, and for the bent ground-state X 1A', and excited-states 2 1A' and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas-Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol(-1) for the 2 1A' state and 52.6 kcal mol(-1) for the 1 1A" state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1 1A" state (53.1 kcal mol(-1)) than those for the corresponding 2 1A' state (57.6 kcal mol(-1)), suggesting that the 1 1A" state might be the first singlet excited state while the 2 1A' state might be the second singlet excited state.     

UV photodissociation of the van der Waals dimer (CH3I)2 revisited: pathways giving rise to ionic features

The CH(3)I A-state-assisted photofragmentation of the (CH(3)I)(2) van der Waals dimer at 248 nm and nearby wavelengths has been revisited experimentally using the time-of-flight mass spectrometry with supersonic and effusive molecular beams and the "velocity map imaging" technique. The processes underlying the appearance of two main (CH(3)I)(2) cluster-specific features in the mass spectra, namely, I(2)(+) and translationally "hot" I(+) ions, have been studied. Translationally hot I(+) ions with an average kinetic energy of 0.94+/-0.02 eV appear in the one-quantum photodissociation of vibrationally excited I(2)(+)((2)Pi(32,g)) ions (E(vib)=0.45+/-0.11 eV) via a "parallel" photodissociation process with an anisotropy parameter beta=1.55+/-0.03. Comparison of the images of I(+) arising from the photoexcitation of CH(3)I clusters versus those from neutral I(2) shows that "concerted" photodissociation of the ionized (CH(3)I)(2)(+) dimer appears to be the most likely mechanism for the formation of molecular iodine ion I(2)(+), instead of photoionization of neutral molecular iodine.     

CH2BrCl在265 nm光解产生的氯甲基自由基的振动内能

利用离子速度影像技术研究了CH2BrCl在265nm附近的激光光解.利用2+1共振增强多光子电离分别获得光解产物Br(2P1/2)和Br(2P3/2)的离子速度图像,从而得出Br(2P1/2)和Br(2P3/2)的速度分布,以及光解碎片的总平动能分布.据此,运用角动量守恒碰撞模型获得了解离氯甲基自由基(·CH2Cl)的振动内能分布.研究结果表明:CH2BrCl+hv→Br(2P1/2)+CH2Cl通道产生的氯甲基自由基中被激发的振动模主要是v4、v3+v4、v2+v4和v2+v6;CH2BrCl+hv→Br(2P3/2)+CH2Cl通道产生的氯甲基自由基中被激发的振动模主要是v2+v6、v1+v3、v2+v5、v2+v3+v5和v1+v5;母体分子CH2BrCl在吸收光解光子后除有v5(CBrstretch)振动模被激发外,还有v7(CH2a-stretch)等其它振动模也被激发.     

Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

Methyl, methyl-d(3), and ethyl hydroperoxide anions (CH(3)OO(-), CD(3)OO(-), and CH(3)CH(2)OO(-)) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH(3)OO, X(2)A' '] = 1.161 +/- 0.005 eV, EA[CD(3)OO, X(2)A' '] = 1.154 +/- 0.004 eV, and EA[CH(3)CH(2)OO, X(2)A' '] = 1.186 +/- 0.004 eV. The photoelectron spectra yield values for the term energies: Delta E(X(2)A' '-A (2)A')[CH(3)OO] = 0.914 +/- 0.005 eV, Delta E(X(2)A' '-A (2)A')[CD(3)OO] = 0.913 +/- 0.004 eV, and Delta E(X(2)A' '-A (2)A')[CH(3)CH(2)OO] = 0.938 +/- 0.004 eV. A localized RO-O stretching mode was observed near 1100 cm(-1) for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Delta(acid)G(298)(CH(3)OOH) = 367.6 +/- 0.7 kcal mol(-1), Delta(acid)G(298)(CD(3)OOH) = 367.9 +/- 0.9 kcal mol(-1), and Delta(acid)G(298)(CH(3)CH(2)OOH) = 363.9 +/- 2.0 kcal mol(-1). From these acidities we have derived the enthalpies of deprotonation: Delta(acid)H(298)(CH(3)OOH) = 374.6 +/- 1.0 kcal mol(-1), Delta(acid)H(298)(CD(3)OOH) = 374.9 +/- 1.1 kcal mol(-1), and Delta(acid)H(298)(CH(3)CH(2)OOH) = 371.0 +/- 2.2 kcal mol(-1). Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH(298)(CH(3)OO-H) = 87.8 +/- 1.0 kcal mol(-1), DH(298)(CD(3)OO-H) = 87.9 +/- 1.1 kcal mol(-1), and DH(298)(CH(3)CH(2)OO-H) = 84.8 +/- 2.2 kcal mol(-1). We review the thermochemistry of the peroxyl radicals, CH(3)OO and CH(3)CH(2)OO. Using experimental bond enthalpies, DH(298)(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Delta(f)H(298)[CH(3)OO] = 4.8 +/- 1.2 kcal mol(-1) and Delta(f)H(298)[CH(3)CH(2)OO] = -6.8 +/- 2.3 kcal mol(-1).