Double ionization and Coulomb explosion of the formic acid dimer by intense near-infrared femtosecond laser pulses

Ionization and fragmentation of formic acid dimers (HCOOH)(2) and (DCOOD)(2) by irradiation of femtosecond laser pulses (100 fs, 800 nm, ~1 × 10(14) W/cm(2)) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H(+), (HCOOH)HCOO(+), and H(3)O(+), which were produced via the dissociative ionization of (HCOOH)(2). In addition, we found that the TOF signals of COO(+), HCOO(+), and HCOOH(+) have small but clear side peaks, indicating fragmentation with large kinetic energy release caused by Coulomb explosion. On the basis of the momentum matching among pairs of the side peaks, a Coulomb explosion pathway of the dimer dication, (HCOOH)(2)(2+) → HCOOH(+) + HCOOH(+), was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH)(2)(2+) were also performed, and the kinetic energy release of the metastable dication was estimated to be 3.40 eV, showing good agreement with the observation. COO(+) and HCOO(+) signals with kinetic energies of 1.4 eV were tentatively assigned to be fragment ions through Coulomb explosion occurring after the elimination of a hydrogen atom or molecule from (HCOOH)(2)(2+). The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.     

Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21 fs laser pulses

Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH(3)OH(2+)-->CH(3) (+)+OH(+), exhibits a peak deflected from the laser polarization direction by 30 degrees -45 degrees , and the corresponding angular distribution for the migration pathway, CH(2)OH(2) (+)-->CH(2) (+)+H(2)O(+), in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.     

Visualizing hydrogen atoms migrating in acetylene dication by time-resolved three-body and four-body Coulomb explosion imaging

The visualization of ultrafast isomerization of deuterated acetylene dication (C(2)D(2)(2+)) is demonstrated by time-resolved Coulomb explosion imaging with sub-10 fs intense laser pulses (9 fs, 0.13 PW cm(-2), 800 nm). The Coulomb explosion imaging monitoring the three-body explosion process, C(2)D(2)(3+)→ D(+) + C(+) + CD(+), as a function of the delay between the pump and probe pulses revealed that the migration of a deuterium atom proceeds in a recurrent manner; One of the deuterium atoms first shifts from one carbon site to the other in a short timescale (～90 fs), and then migrates back to the original carbon site by 280 fs, in competition with the molecular dissociation. Correlated motion of the two deuterium atoms associated with the hydrogen migration and structural deformation to non-planar geometry are identified by the time-resolved four-body Coulomb explosion imaging, C(2)D(2)(4+)→ D(+) + C(+) + C(+) + D(+).     

Ultrafast hydrogen scrambling in methylacetylene and methyl-d3-acetylene ions induced by intense laser fields

Three-body Coulomb explosion processes of triply charged positive ions of methylacetylene (CH(3)-C≡C-H) and its isotopomer, methyl-d(3)-acetylene (CD(3)-C≡C-H), induced by an ultrashort intense laser field (790 nm, ～40 fs, 5.0 × 10(13) W cm(-2)) are investigated by the coincidence momentum imaging method. Two types of three-body decomposition processes accompanying the ejection of a proton are identified for methylacetylene, and six types of three-body decomposition processes accompanying the ejection of a proton or a deuteron are identified for methyl-d(3)-acetylene. From the observed momentum vectors of all the three fragment ions for each decomposition pathway, the proton and deuteron distributions are constructed in the coordinate space, and the hydrogen migration processes are investigated. It was shown that the hydrogen migration proceeds more efficiently from the methyl group than from the methine group. In addition to the decomposition pathways accompanying the migration of one H (or D) atom, the decomposition pathways accompanying the migration of two light atoms (H/D exchange and 2D migration) are identified. Furthermore, the decomposition pathways ascribable to the migration of three light atoms (H/D exchange followed by D migration) are identified, showing the high intramolecular mobilities of H and D atoms within methylacetylene and methyl-d(3)-acetylene in an intense laser field, resulting in the H/D scrambling.     

Metastable decomposition and hydrogen migration of ethane dication produced in an intense femtosecond near-infrared laser field

We investigated a formation channel of triatomic molecular hydrogen ions from ethane dication induced by irradiation of intense laser fields (800 nm, 100 fs, ～1 × 10(14) W∕cm(2)) by using time of flight mass spectrometry. Hydrogen ion and molecular hydrogen ion (H,D)(n)(+) (n = 1-3) ejected from ethane dications, produced by double ionization of three types of samples, CH(3)CH(3), CD(3)CD(3), and CH(3)CD(3), were measured. All fragments were found to comprise components with a kinetic energy of ～3.5 eV originating from a two-body Coulomb explosion of ethane dications. Based on the signal intensities and the anisotropy of the ejection direction with respect to the laser polarization direction, the branching ratios, H(+):D(+) = 66:34, H(2)(+):HD(+):D(2)(+) = 63:6:31, and H(3)(+):H(2)D(+):HD(2)(+):D(3)(+) = 26:31:34:9 for the decomposition of C(2)H(3)D(3)(2+), were determined. The ratio of hydrogen molecules, H(2):HD:D(2) = 31:48:21, was also estimated from the signal intensities of the counter ion C(2)(H,D)(4)(2+). The similarity in the extent of H∕D mixture in (H,D)(3)(+) with that of (H,D)(2) suggests that these two dissociation channels have a common precursor with the C(2)H(4)(2+)...H(2) complex structure, as proposed theoretically in the case of H(3)(+) ejection from allene dication [A. M. Mebel and A. D. Bandrauk, J. Chem. Phys. 129, 224311 (2008)]. In contrast, the (H,D)(2)(+) ejection path with a lower extent of H∕D mixture and a large anisotropy is expected to proceed essentially via a different path with a much rapid decomposition rate. For the Coulomb explosion path of C-C bond breaking, the yield ratios of two channels, CH(3)CD(3)(2+)→ CH(3)(+) + CD(3)(+) and CH(2)D(+) + CHD(2)(+), were 81:19 and 92:8 for the perpendicular and parallel directions, respectively. This indicates that the process occurs at a rapid rate, which is comparable to hydrogen migration through the C-C bond, resulting in smaller anisotropy for the latter channel that needs H∕D exchange.     

Hydrogen scrambling in ethane induced by intense laser fields: statistical analysis of coincidence events

Two-body Coulomb explosion processes of ethane (CH(3)CH(3)) and its isotopomers (CD(3)CD(3) and CH(3)CD(3)) induced by an intense laser field (800 nm, 1.0 × 10(14) W/cm(2)) with three different pulse durations (40 fs, 80 fs, and 120 fs) are investigated by a coincidence momentum imaging method. On the basis of statistical treatment of the coincidence data, the contributions from false coincidence events are estimated and the relative yields of the decomposition pathways are determined with sufficiently small uncertainties. The branching ratios of the two body decomposition pathways of CH(3)CD(3) from which triatomic hydrogen molecular ions (H(3)(+), H(2)D(+), HD(2)(+), D(3)(+)) are ejected show that protons and deuterons within CH(3)CD(3) are scrambled almost statistically prior to the ejection of a triatomic hydrogen molecular ion. The branching ratios were estimated by statistical Rice-Ramsperger-Kassel-Marcus calculations by assuming a transition state with a hindered-rotation of a diatomic hydrogen moiety. The hydrogen scrambling dynamics followed by the two body decomposition processes are discussed also by using the anisotropies in the ejection directions of the fragment ions and the kinetic energy distribution of the two body decomposition pathways.     

Fragmentation channels of K-shell excited rare-gas clusters studied by multiple-ion coincidence momentum imaging

Multiple-ion coincidence momentum imaging experiments were carried out for K-shell (1s) excited Ar clusters containing about 130 atoms and Kr clusters containing about 30, 90, and 160 atoms. The time-of-flight spectra reveal that the major products of the Coulomb explosion are singly charged ions. With increasing the number of charges generated in clusters, the momentum of monomer ions such as Ar(+) and Kr(+) increases, while that of cluster ions such as Ar(3) (+), Kr(2) (+), and Kr(3) (+) decreases. This observation indicates the site-specific decay process that the heavier ions appear in the central part of clusters. We have also investigated the momentum distribution in various fragmentation channels and the branching ratio of each channel at the Coulomb explosion. When the number N(coin) of coincidently detected ions is four, for example, the most frequent channel from Kr clusters containing 30 atoms is to emit simply four Kr(+) ions, but Kr(2) (+) ions participate in the fragmentation from the larger Kr clusters. The fragmentation channel in which two Ar(2) (+) ions are emitted becomes dominant with increasing N(coin), and the average momentum of Ar(2) (+) ion in this channel is larger than that in the channels where only single Ar(2) (+) is emitted.     

Ionization and dissociation of CH3I in intense laser field

The ionization-dissociation of methyl iodide in intense laser field has been studied using a reflection time-of-flight mass spectrometry (RTOF-MS), at a laser intensity of < or =6.6x10(14) W/cm(2), lambda=798 nm, and a pulse width of 180 fs. With the high resolution of RTOF-MS, the fragment ions with the same M/z but from different dissociation channels are resolved in the mass spectra, and the kinetic energy releases (KERs) of the fragment ions such as I(q+) (q=1-6), CH(m) (+) (m=0-3), C(2+), and C(3+) are measured. It is found that the KERs of the fragment ions are independent of the laser intensity. The fragments CH(3) (+) and I(+) with very low KERs (<1 eV for CH(3) (+) and <0.07 eV for I(+)) are assigned to be produced by the multiphoton dissociation of CH(3)I(+). For the fragments CH(3) (+) and I(+) from CH(3)I(2+), they are produced by the Coulomb explosion of CH(3)I(2+) with the interaction from the covalent force of the remaining valence electrons. The split of the KER of the fragments produced from CH(3)I(2+) dissociation is observed experimentally and explained with the energy split of I(+)((3)P(2)) and I(+)((3)P(0,1)). The dissociation CH(3)I(3+)-->CH(3) (+)+I(2+) is caused by Coulomb explosion. The valid charge distance R(c) between I(2+) and CH(3) (+), at which enhanced ionization of methyl iodide occurs, is obtained to be 3.7 A by the measurements of the KERs of the fragments CH(3) (+) and I(2+). For the CH(3)I(n+) (n> or =3), the KERs of the fragment ions CH(3) (p+) and I(q+) are attributed to the Coulomb repulsion between CH(3) (p+) and I(q+) from R(c) approximately 3.7 A. The dissociation of the fragment CH(3) (+) is also discussed. By the enhanced ionization mechanism and using the measured KER of I(q+), all the possible Coulomb explosion channels are identified. By comparing the abundance of fragment ions in mass spectrum, it is found that the asymmetric dissociation channels with more charges on iodine, q>p, are the dominant channels.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. IV. Coulomb explosion of molecular heteroclusters

In this paper we present a theoretical and computational study of the temporal dynamics and energetics of Coulomb explosion of (CD4)(n) and (CH4)(n) (n=55-4213) molecular heteroclusters in ultraintense (I=10(16)-10(19) W cm(-2)) laser fields, addressing the manifestation of electron dynamics, together with nuclear energetic and kinematic effects on the heterocluster Coulomb instability. The manifestations of the coupling between electron and nuclear dynamics were explored by molecular dynamics simulations for these heteroclusters coupled to Gaussian laser fields (pulse width tau=25 fs), elucidating outer ionization dynamics, nanoplasma screening effects (being significant for I< or =10(17) W cm(-2)), and the attainment of cluster vertical ionization (CVI) (at I=10(17) W cm(-2) for cluster radius R(0)< or =31 A). Nuclear kinematic effects on heterocluster Coulomb explosion are governed by the kinematic parameter eta=q(C)m(A)/q(A)m(C) for (CA(4))(n) clusters (A=H,D), where q(j) and m(j) (j=A,C) are the ionic charges and masses. Nonuniform heterocluster Coulomb explosion (eta >1) manifests an overrun effect of the light ions relative to the heavy ions, exhibiting the expansion of two spatially separated subclusters, with the light ions forming the outer subcluster at the outer edge of the spatial distribution. Important features of the energetics of heterocluster Coulomb explosion originate from energetic triggering effects of the driving of the light ions by the heavy ions (C(4+) for I=10(17)-10(18) W cm(-2) and C(6+) for I=10(19) W cm(-2)), as well as for kinematic effects. Based on the CVI assumption, scaling laws for the cluster size (radius R(0)) dependence of the energetics of uniform Coulomb explosion of heteroclusters (eta=1) were derived, with the size dependence of the average (E(j,av)) and maximal (E(j,M)) ion energies being E(j,av)=aR(0) (2) and E(j,M)=(5a/3)R(0) (2), as well as for the ion energy distributions P(E(j)) proportional to E(j) (1/2); E(j)< or =E(j,M). These results for uniform Coulomb explosion serve as benchmark reference data for the assessment of the effects of nonuniform explosion, where the CVI scaling law for the energetics still holds, with deviations of the a coefficient, which increase with increasing eta. Kinematic effects (for eta>1) result in an isotope effect, predicting the enhancement (by 9%-11%) of E(H,av) for Coulomb explosion of (C(4+)H(4) (+))(eta) (eta=3) relative to E(D,av) for Coulomb explosion of (C(4+)D(4) (+))(eta) (eta=1.5), with the isotope effect being determined by the ratio of the kinematic parameters for the pair of Coulomb exploding clusters. Kinematic effects for nonuniform explosion also result in a narrow isotope dependent energy distribution (of width DeltaE) of the light ions (with DeltaE/E(H,av) approximately 0.3 and DeltaE/E(D,av) approximately 0.4), with the distribution peaking at the high energy edge, in marked contrast with the uniform explosion case. Features of laser-heterocluster interactions were inferred from the analyses of the intensity dependent boundary radii (R(0))(I) and the corresponding average D+ ion energies (E(D,av))(I), which provide a measure for optimization of the cluster size at intensity I for the neutron yield from dd nuclear fusion driven by Coulomb explosion (NFDCE) of these heteroclusters. We infer on the advantage of deuterium containing heteronuclear clusters, e.g., (CD4)(n) in comparison to homonuclear clusters, e.g., (D2)(n/2), for dd NFDCE, where the highly charged heavy ions (e.g., C4+ or C6+) serve as energetic and kinematic triggers driving the D+ ions to a high (10-200 keV) energy domain.     

A new series of luminescent phosphine stabilised platinum ethynyl complexes

A series of cis-platinum ethynyl complexes with the general formula cis-[Pt(dppe)(C[triple bond]CR)2](dppe = 1,2-bis(diphenylphosphino)ethane; R = C6H4-p-NO2 1, C6H4-p-CH3 2, C6H4-p-C[triple bond]CH 3 and C6H4-p-C6H4-p-C[triple bond]CH 4) have been prepared by the coupling reaction of cis-[Pt(dppe)Cl2] with two equivalents of the appropriate alkyne. The new complexes have been fully characterized by spectroscopic techniques, and the cis square planar arrangement at the platinum centre has been confirmed by single-crystal X-ray diffraction studies of complexes 1, 2 and 4. The absorption spectra of the complexes 1-4 are dominated by a pi-->pi* band that contains some platinum (n + 1) p orbital character. The position of the band is dependent on the electron donating or withdrawing properties of the ethynyl substituents, R. Complex 1 displays a triplet emission in the green, at room temperature, while complexes 2-4, display singlet emissions in the blue. Again, the difference can be attributed to the nature of the R substituents.     

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.     

Velocity map imaging of dissociative ionization and coulomb explosion of CH3I induced by a femtosecond laser

The dissociative ionization and the Coulomb explosion of CH3I irradiated by a 35 fs 800 nm laser with a laser intensity of 4 x 10(13) to 6 x 10(14) W/cm2 was studied. In a relatively weak laser field (about 10(13) W/cm2), the dissociative ionization of CH3I took place. The speed distributions of the CH3+ and I+ fragments were measured and fitted using multiple Gaussian functions. Different product channels were found for CH3+ and I+, respectively. In a strong laser field (about 10(14) W/cm2), the multiply ionized fragment ions of Iq+ (q 相似文献   

An experimental and theoretical investigation of gas-phase reactions of Ca2+ with glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. III. Coulomb explosion of deuterium clusters

In this paper we present a theoretical and computational study of the energetics and temporal dynamics of Coulomb explosion of molecular clusters of deuterium (D2)n/2 (n = 480 - 7.6 x 10(4), cluster radius R0 = 13.1 - 70 A) in ultraintense laser fields (laser peak intensity I = 10(15) - 10(20)W cm(-2)). The energetics of Coulomb explosion was inferred from the dependence of the maximal energy EM and the average energy Eav of the product D+ ions on the laser intensity, the laser pulse shape, the cluster radius, and the laser frequency. Electron dynamics of outer cluster ionization and nuclear dynamics of Coulomb explosion were investigated by molecular dynamics simulations. Several distinct laser pulse shape envelopes, involving a rectangular field, a Gaussian field, and a truncated Gaussian field, were employed to determine the validity range of the cluster vertical ionization (CVI) approximation. The CVI predicts that Eav, EM proportional to R0(2) and that the energy distribution is P(E) proportional to E1/2. For a rectangular laser pulse the CVI conditions are satisfied when complete outer ionization is obtained, with the outer ionization time toi being shorter than both the pulse width and the cluster radius doubling time tau2. By increasing toi, due to the increase of R0 or the decrease of I, we have shown that the deviation of Eav from the corresponding CVI value (Eav(CVI)) is (Eav(CVI) - Eav)/Eav(CVI) approximately (toi/2.91tau2)2. The Gaussian pulses trigger outer ionization induced by adiabatic following of the laser field and of the cluster size, providing a pseudo-CVI behavior at sufficiently large laser fields. The energetics manifest the existence of a finite range of CVI size dependence, with the validity range for the applicability of the CVI being R0 < or = (R0)I, with (R0)I representing an intensity dependent boundary radius. Relating electron dynamics of outer ionization to nuclear dynamics for Coulomb explosion induced by a Gaussian pulse, the boundary radius (R0)I and the corresponding ion average energy (Eav)I were inferred from simulations and described in terms of an electrostatic model. Two independent estimates of (R0)I, which involve the cluster size where the CVI relation breaks down and the cluster size for the attainment of complete outer ionization, are in good agreement with each other, as well as with the electrostatic model for cluster barrier suppression. The relation (Eav)I proportional to (R0)I(2) provides the validity range of the pseudo-CVI domain for the cluster sizes and laser intensities, where the energetics of D+ ions produced by Coulomb explosion of (D)n clusters is optimized. The currently available experimental data [Madison et al., Phys. Plasmas 11, 1 (2004)] for the energetics of Coulomb explosion of (D)n clusters (Eav = 5 - 7 keV at I = 2 x 10(18) W cm(-2)), together with our simulation data, lead to the estimates of R0 = 51 - 60 A, which exceed the experimental estimate of R0 = 45 A. The predicted anisotropy of the D+ ion energies in the Coulomb explosion at I = 10(18) W cm(-2) is in accord with experiment. We also explored the laser frequency dependence of the energetics of Coulomb explosion in the range nu = 0.1 - 2.1 fs(-1) (lambda = 3000 - 140 nm), which can be rationalized in terms of the electrostatic model.     

溴甲烷在强激光场中的电离和解离

利用自制的反射式飞行时间质谱仪(RTOF-MS)研究了多原子分子CH₃Br在强激光场中的电离解离. 得到了溴甲烷在强激光场中电离解离的飞行时间质谱, 基于RTOF-MS的高分辨率(M/ΔM>2000), 测量了分子库仑爆炸产生的系列碎片离子的动能释放(KER), 用多光子解离和库仑爆炸解释了实验结果. 与碘甲烷在强场中的实验结果对比发现: (1) 在相同的激光场强下, 碘甲烷电离解离的最高价碎片离子为I⁶⁺而溴甲烷为Br³⁺; (2) 溴甲烷质谱中存在母体离子的脱氢产物CH_mBr⁺ 和CH_mBr²⁺, 而对于碘甲烷, 没有检测到这些通道, C-I键首先断开; (3) 质谱中存在H⁷⁹Br⁺和H⁸¹Br⁺, 而碘甲烷的电离解离中不存在HI产物; (4) 溴甲烷库仑两体爆炸的有效电荷间距随着两碎片电荷乘积的增大而增大, 而对于碘甲烷此间距几乎不随电荷乘积变化; (5) CH_m⁺(m=0, 1, 2)的主要生成通道可能与碘甲烷不同, 不是来自CH₃⁺的顺序脱氢, 而是来自脱氢母体离子的直接解离.     

An enormous vibrational motion: the gas-phase structure of dimethyl-bis(methoxyethynyl) germanium

The structure of dimethyl-bis(methoxyethynyl) germanium has been determined in the gas phase by electron diffraction utilising flexible restraints from quantum chemical calculations. Theoretical methods (B3LYP/6-311+G* and MP2/6-311+G*) predict a low barrier to rotation of the methoxy groups in the molecule in addition to low-frequency vibrations of the long ethynyl chains. In the equilibrium structure the Ge-C[triple bond]C angles of the two methoxyethynyl fragments in the molecule are computed to deviate by up to 4 degrees from the linear arrangement. As a consequence of low-frequency large-amplitude vibrational motion the experimental structure of these fragments without applying vibrational corrections deviates considerably from linearity, while the structure corrected for vibrational effects using the harmonic approximation and taking into account a non-linear transformation between internal and Cartesian coordinates (r(h1)) shows closer agreement with theory. The main experimental structural parameters of dimethyl-bis(methoxyethynyl) germanium (r(h1)) are: r(Ge-C)(mean), 192.5(1) pm; DeltaGeC =r(Ge-C(methyl))-r(Ge-C(ethynyl)), 4.5(5) pm, r(C[triple bond]C)(mean), 122.8(2) pm; r(C-O)(mean), 138.9(3) pm; DeltaCO =r(C(methyl)-O)-r(C(ethynyl)-O), 14.5(2) pm, r(C-H)(mean), 109.1(4) pm; [angle](X-C-H)(mean)(X = Ge,O), 109(1) degree; [angle]C(ethynyl)-Ge-C(ethynyl), 108.1(4) degree; [angle]C(methyl)-Ge-C(methyl), 113.4(5) degree; [angle]Ge-C[triple bond]C, 163(1) degree; [angle]C[triple bond]C-O, 176(2) degree; [angle]C-O-C, 115.2(6) degree; methoxy group torsion, tau, 36(9) degree from the position in which the C-O bond eclipses the further Ge-C(ethynyl) bond.     

Coulomb explosion of nitrogen and oxygen molecules through non-Coulombic states

We have systematically studied Coulomb explosion of nitrogen and oxygen molecules in intense 8 and 24 fs laser pulses. In the experiment, we explicitly separated all explosion pathways through coincident measurements. The high resolution kinetic energy releases (KERs) and the exotic angular distributions of atomic ions provide direct evidence that Coulomb explosion occurs through non-Coulombic states. In the theory, we calculated dissociation potential energy curves (PECs) of nitrogen and oxygen molecules and their multicharged molecular ions using multiconfiguration second-order perturbation theory. The results indicate that Coulomb potentials are close to the accurate PECs of multicharged molecular ions only when the internuclear distance is larger than 3 ?. In comparison with the experimental observations and the theoretical calculations, we determined the internuclear distance when Coulombic explosion occurs. It is near the equilibrium distance of the neutral molecules in the case of 8 fs laser pulses and expands gradually with the increase of the charge state of the molecular ions in the case of 24 fs laser pulses.     

Direct detection of polyynes formation from the reaction of ethynyl radical (C2H) with propyne (CH3-C[triple bond]CH) and allene (CH2=C=CH2)

The reactions of the ethynyl radical (C(2)H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C(2)H radical is prepared by 193-nm photolysis of CF(3)CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron-photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C(5)H(4) produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50-70% diacetylene (H-C[triple bond]C-C[triple bond]C-H) and 50-30% C(5)H(4), with a C(5)H(4)-isomer distribution of 15-20% ethynylallene (CH(2)=C=CH-C[triple bond]CH) and 85-80% methyldiacetylene (CH(3)-C[triple bond]C-C[triple bond]CH). The reaction of allene with ethynyl gives 35-45% ethynylallene, 20-25% methyldiacetylene and 45-30% 1,4-pentadiyne (HC[triple bond]C-CH(2)-C[triple bond]CH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan's atmospheric chemistry are discussed.     

Fragmentation of 3,7-dialkyl-1,5-diphenyl-3,7-diazabicyclo[3.3. 1]nonan-9-ones under electron ionization

A series of 3,7-dialkyl-1,5-diphenyl-3,7-diazabicyclo[3.3. 1]nonan-9-ones was prepared, and the details of their fragmentation under electron ionization (EI) were elucidated. The molecular ions of each compound under consideration were quite abundant in their EI spectra. Full-scan spectra exhibited a number of fragment ions which were clearly assigned using MS/MS and accurate mass measurements. The basic fragmentation of 3,7-dialkyl-1,5-diphenylbispidinones was due to the cleavage of C(1)-C(2) bond followed by a hydrogen migration similar to an odd-electron McLafferty rearrangement. Alternatively, the C(1)-C(2) bond cleavage was followed by the elimination of an imine molecule, Alk-N=CH(2). Further fragmentation resulted in ions at m/z 234 and 103, present in the spectra of all the compounds under study. The fragmentation pathways proposed in this paper are based on the substituent shifts, accurate mass measurements and collision-induced dissociation spectra of selected ions. The results of the present work can be useful in selecting the fragment ions suitable for identification and quantitation of bispidinones in biological matrices.     

Wavelength-dependent Coulomb explosion in carbon disulphide (CS2) clusters: generation of energetic multiply charged carbon and sulphur ions

A gigawatt laser-induced Coulomb explosion has been observed in carbon disulphide (CS(2)) clusters generating energetic, multiply charged [C](m+) (m = 1-4) and [S](n+) (n = 1-6) atomic ions of carbon and sulphur. The Coulomb explosion shows wavelength dependence. Comparison of these results with our earlier work shows that the polarizability and dipole moment might help in energy absorption from the laser field but they are not mandatory conditions for this low-intensity Coulomb explosion. The results show that in a field of 10(9) W/cm(2), absorption of 266 and 355 nm laser radiation by CS(2) clusters leads to multiphoton dissociation/ionization whereas at 532 nm the whole cluster explodes generating multiply charged atomic ions.