Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. II. Electron dynamics and outer ionization of the nanoplasma

We explore electron dynamics in molecular (CD4)(1061) clusters and elemental Xen (n=249-2171) clusters, responding to ultraintense (intensity I=10(16)-10(19) W cm(-2)) laser fields. Molecular dynamics simulations (including magnetic field and relativistic effects) and analyses of high-energy electron dynamics and nuclear ion dynamics in a cluster interacting with a Gaussian shaped laser field (frequency 0.35 fs(-1), photon energy 1.44 eV, phase 0, temporal width 25 fs) elucidated the time dependence of inner ionization, the formation of a nanoplasma of unbound electrons within the cluster or its vicinity, and of outer ionization. We determined the cluster size and the laser intensity dependence of these three sequential-parallel electronic processes. The characteristic times for cluster inner ionization (tau(ii)) and for outer ionization (tau(oi)) fall in the femtosecond time domain, i.e., tau(ii)=2-9 fs and tau(oi)=4-15 fs for (CD4)(1061), tau(ii)=7-30 fs and tau(oi)=5-13 fs for Xe(n) (n=479,1061), with both tau(ii) and tau(oi) decreasing with increasing I, in accord with the barrier suppression ionization mechanism for inner ionization of the constituents and the cluster barrier suppression ionization mechanism for outer ionization. The positive delay times Deltatau(OI) between outer and inner ionization (e.g., Deltatau(OI)=6.5 fs for Xen at I=10(16) W cm(-2) and Deltatau(OI)=0.2 fs for (CD4)(1061) at I=10(19) W cm(-2)) demonstrate that the outer/inner ionization processes are sequential. For (CD4)(1061), tau(ii)tau(oi), reflecting on the energetic hierarchy in the ionization of the Xe atoms. Quasiresonance contributions to the outer ionization of the nanoplasma were established, as manifested in the temporal oscillations in the inner/outer ionization levels, and in the center of mass of the nanoplasma electrons. The formation characteristics, dynamics, and response of the nanoplasma in molecular or elemental clusters were addressed. The nanoplasma is positively charged, with a high-average electron density [rho(P)=(2-3)10(22) cm(-3)], being characterized by high-average electron energies epsilon(av) (e.g., in Xe(1061) clusters epsilon(av)=54 eV at I=10(16) W cm(-2) and epsilon(av)=0.56-0.37 keV at I=10(18) W cm(-2), with epsilon(av) proportional, variant I(1/2)). Beyond the cluster boundary the average electron energy markedly increases, reaching electron energies in the range of 1.2-40 keV for outer ionization of Xe(n) (n=249-2171) clusters. The nanoplasma exhibits spatial inhomogeneity and angular anisotropy induced by the laser field. Femtosecond time scales are predicted for the nanoplasma production (rise times 7-3 fs), for the decay (decay times approximately 5 fs), and for the persistence time (30-10 fs) of a transient nanoplasma at I=10(17)-10(18) W cm(-2). At lower intensities of I=10(16) W cm(-2) a persistent nanoplasma with a "long" lifetime of > 50 fs will prevail.     

Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=0.35 fs(-1)). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of [Xe(q+)](n) with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) W cm(-2)), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the [Xe(q+)](n) ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe(2171) at tau=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.     

532 nm纳秒激光电离产生Xez＋(z ≤ 11)高价离子

利用25 ns脉冲Nd-YAG 532 nm激光,在1011 W•cm－2的光场强度下,研究了Xe原子团簇的激光电离过程,观察到较强的高价离子信号,其中最高价态达＋11.不同脉冲束位置和束源压力的实验表明,仅当激光作用于脉冲束中段时才能观察到高价离子,且高价离子信号强度随束源压力的增加而迅速增强,说明束中大尺寸团簇的存在与高价离子的形成密切相关.通过实验,认为高价离子可能来源于电离原子团簇而形成的纳米尺度等离子体小球对激光光场的共振吸收.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. I. Extreme multielectron ionization

In this paper we present a theoretical and computational study of extreme multielectron ionization (involving the stripping of all the electrons from light, first-row atoms, and the production of heavily charged ions, e.g., Xe(+q) (q< or =36) from heavy atoms) in elemental and molecular clusters of Xe(n),(D(2))(n), and (CD(4))(n) (n=55-1061) in ultraintense (intensity I=10(15)-10(19) W cm(-2)) laser fields. Single atom or molecule multielectron ionization can be adequately described by the semiclassical barrier suppression ionization (BSI) mechanism. Extreme cluster multielectron ionization is distinct from that of a single atomic or molecular species in terms of the mechanisms, the ionization level and the time scales for electron dynamics and for nuclear motion. The novel compound mechanism of cluster multielectron ionization, which applies when the cluster size (radius R(0)) considerably exceeds the barrier distance for the BSI of a single constituent, involves a sequential-parallel, inner-outer ionization. The cluster inner ionization driven by the BSI for the constituents is induced by a composite field consisting of the laser field and inner fields. The energetics and dynamics of the system consisting of high energy (< or =3 keV) electrons and of less, similar 100 keV ions in the laser field was treated by molecular dynamics simulations, which incorporate electron-electron, electron-ion, ion-ion, and charge-laser interactions. High-energy electron dynamics also incorporates relativistic effects and includes magnetic field effects. We treat inner ionization considering inner field ignition, screening and fluctuation contributions as well as small [(< or =13%)] impact ionization contributions. Subsequent to inner ionization a charged nanoplasma is contained within the cluster, whose response to the composite (laser+inner) field results in outer ionization, which can be approximately described by an entire cluster barrier suppression ionization mechanism.     

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.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

The dynamics of endohedral complex formation in surface pick-up scattering as probed by kinetic energy distributions: experiment and model calculation for Cs@C60+

Endohedral Cs@C60 molecules were formed by implanting low energy (E0 = 30-220 eV) Cs+ ions into C60 molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C(60) layer provide clear evidence for a submonolayer coverage. The Cs+ penetration and Cs@C60 ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C60 left on the surface following the Cs+ impact. The Cs@C60 formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C60+ were measured for two different Cs+ impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the Cs+ impact energy. Both KEDs peak around 1.2 eV with similar line shapes. A simple model for the formation/ejection/fragmentation dynamics of the endohedral complex is proposed. The model leads to a strong correlation between the vibrational and kinetic energy of the outgoing Cs@C60. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C60 emitted from the surface, fragmentation of the Cs@C60+ ion, and radiative cooling. It is concluded that the measured KEDs are heavily biased by the experimental breakdown function. Good agreement between experimental and calculated KEDs is obtained.     

Multiply charged cluster ions of Ar,Kr, Xe,N2, O2, CO2, SO2 and NH3: Production mechanism,appearance size and appearance energy

Clusters of Ar, Kr, Xe, N₂, O₂, CO₂, SO₂ and NH₃ formed by supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry (up to 15000 amu). Besides mass spectra of singly charged ions showing the characteristic anomalous distributions, we have in particular investigated the properties of multiply charged cluster ions. Critical appearance sizes of doubly and triply charged cluster ions, n₂ and n₃ respectively, found in the present study confirm recent theoretical predictions about n₃/n₂ and their dependence on the properties of the cluster constituents. The appearance energies of multiply charged cluster ions determined are shifted way below the appearance energies of the respective monomer ions. These huge red shifts together with the observed linear threshold laws and large maximum ionization efficiencies indicate that multiply charged cluster ions are produced by sequential single ionization events of one incoming electron at different cluster sites. Furthermore, we have also obtained for the first time clear evidence that (for electron energies above the appearance energy of doubly charged ions) an appreciable amount of singly charged (also fragment) ions is produced via Coulomb explosion of unstable doubly charged ions in the ion source.     

RgBF2(+) complexes (Rg = Ar, Kr, and Xe): the cations with large stabilities

Rare gas containing cations with general formula [Rg, B, 2F](+) have been investigated theoretically by second-order Mo?ller-Plesset perturbation, coupled cluster, and complete active space self-consistent field levels of theory with correlation-consistent basis sets. Totally two types of minima, i.e., boron centered C(2) (v) symmetried RgBF(2) (+) (Rg = Ar, Kr, and Xe) which can be viewed as loss of F(-) from FRgBF(2) and linear FRgBF(+) (Rg = Kr and Xe) are obtained at the CCSD(T)∕aug-cc-pVTZ∕SDD and CASSCF(10,8)∕aug-cc-pVTZ∕SDD levels, respectively. It is shown that the RgBF(2) (+) are global minima followed by FRgBF(+) at 170.9 and 142.2 kcal∕mol on the singlet potential-energy surfaces of [Rg, B, 2F](+) (Rg = Kr and Xe) at the CASPT2(10,8) ∕aug-cc-pVTZ∕SDD∕∕CASSCF(10,8)∕aug-cc-pVTZ∕SDD, respectively. The interconversion barrier heights between RgBF(2) (+) and FRgBF(+) (Rg = Kr and Xe) are at least 39 kcal∕mol. In addition, no dissociation transition state associated with RgBF(2) (+) and FRgBF(+) can be found. This suggests that RgBF(2) (+) (Rg = Ar, Kr, and Xe) can exist as both thermodynamically and kinetically stable species, while linear FRgBF(+) (Rg = Kr and Xe) can exist as metastable species compared with the lowest dissociation limit energies just like isoelectronic linear FRgBO and FRgBN(-). From natural bond orbital and atoms-in-molecules calculations, it is found that the positive charge is mainly located on Rg and boron atoms for both types of minima, the Rg-B bonds of ArBF(2) (+), KrBF(2) (+), and XeBF(2) (+) are mostly electrostatic, thus can be viewed as ion-induced dipole interaction; while that of linear FKrBF(+) and FXeBF(+) are covalent in nature. The previous experimental observation of ArBF(2) (+) by Pepi et al. [J. Phys. Chem. B. 110, 4492 (2006)] should correspond to C(2) (v) minimum. The presently predicted spectroscopies of KrBF(2) (+), XeBF(2) (+), FKrBF(+), and FXeBF(+) should be helpful for their experimental identification in the future.     

Approximations to complete basis set-extrapolated, highly correlated non-covalent interaction energies

The first-principles calculation of non-covalent (particularly dispersion) interactions between molecules is a considerable challenge. In this work we studied the binding energies for ten small non-covalently bonded dimers with several combinations of correlation methods (MP2, coupled-cluster single double, coupled-cluster single double (triple) (CCSD(T))), correlation-consistent basis sets (aug-cc-pVXZ, X = D, T, Q), two-point complete basis set energy extrapolations, and counterpoise corrections. For this work, complete basis set results were estimated from averaged counterpoise and non-counterpoise-corrected CCSD(T) binding energies obtained from extrapolations with aug-cc-pVQZ and aug-cc-pVTZ basis sets. It is demonstrated that, in almost all cases, binding energies converge more rapidly to the basis set limit by averaging the counterpoise and non-counterpoise corrected values than by using either counterpoise or non-counterpoise methods alone. Examination of the effect of basis set size and electron correlation shows that the triples contribution to the CCSD(T) binding energies is fairly constant with the basis set size, with a slight underestimation with CCSD(T)∕aug-cc-pVDZ compared to the value at the (estimated) complete basis set limit, and that contributions to the binding energies obtained by MP2 generally overestimate the analogous CCSD(T) contributions. Taking these factors together, we conclude that the binding energies for non-covalently bonded systems can be accurately determined using a composite method that combines CCSD(T)∕aug-cc-pVDZ with energy corrections obtained using basis set extrapolated MP2 (utilizing aug-cc-pVQZ and aug-cc-pVTZ basis sets), if all of the components are obtained by averaging the counterpoise and non-counterpoise energies. With such an approach, binding energies for the set of ten dimers are predicted with a mean absolute deviation of 0.02 kcal/mol, a maximum absolute deviation of 0.05 kcal/mol, and a mean percent absolute deviation of only 1.7%, relative to the (estimated) complete basis set CCSD(T) results. Use of this composite approach to an additional set of eight dimers gave binding energies to within 1% of previously published high-level data. It is also shown that binding within parallel and parallel-crossed conformations of naphthalene dimer is predicted by the composite approach to be 9% greater than that previously reported in the literature. The ability of some recently developed dispersion-corrected density-functional theory methods to predict the binding energies of the set of ten small dimers was also examined.     

Comparative study of multielectron ionization of alkyl halides induced by picosecond laser irradiation

The interaction of C2H5X, 1-C3H7X, 1-C4H9X, where X = I, Br, Cl, with strong (1 x 10(13)-1.2 x 10(14) W/cm2) 35 ps laser pulses at 1064 nm is studied by means of time-of-flight mass spectrometry. The multielectron ionization following the C-X bond elongation has been verified for the studied molecules. By combination of the intensity dependence of the ion yields, the estimated kinetic energies of the released fragment ions, and their angular distributions, we have identified the different dissociation channels of the transient multiply charged parent ions. From the dependence on the laser intensity of the ratio of the doubly charged halogen ions to the singly charged ones, it is concluded that the molecular coupling with the laser field increases with the molecular size.     

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.     

The size of neutral free clusters as manifested in the relative bulk-to-surface intensity in core level photoelectron spectroscopy

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or "bulk" cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.     

Measurement of initial translational energies of peptide ions in laser desorption/ionization mass spectrometry

The initial kinetic energy distribution of [Arg]-vasopressin molecular ions generated by matrix-assisted UV laser desorption/ionization was measured using a delayed ion extraction, linear time-of-flight mass spectrometer. Energy distributions of the nicotinic acid matrix ions, with or without the presence of peptide, were also measured. These were compared with the kinetic energy distribution of gramicidin-S ions using IR laser desorption. The measured molecular ion kinetic energy distribution from vasopressin is much broader than that from gramicidin-S, and is characterized by a high-energy tail that most likely results from entrainment of anlayte ions in the higher velocity matrix ions and fragments as they leave the surface.     

Photodissociation of pyrrole-ammonia clusters by velocity map imaging: mechanism for the H-atom transfer reaction

The photodissociation dynamics of pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (～0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.     

Ultrafast photodissociation dynamics of acetone at 195 nm: II. Unraveling complex three-body dissociation dynamics by femtosecond time-resolved photofragment translational spectroscopy

As a continuation of the preceding paper in this issue (J. Phys. Chem. A 2005, 109, 6805), we studied photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm using femtosecond time-resolved photofragment translational spectroscopy. The technique, which is implemented by the combination of fs pump-probe ionization spectroscopy and kinetic energy resolved time-of-flight mass spectrometry (KETOF), measured temporal evolutions of the product kinetic energy distributions (KEDs) with a time resolution limited only by the laser pulse widths. Two methyl product KED components were resolved and assigned to the primary and secondary methyl products on the basis of their temporal behaviors. The results support the mechanism in which the primary dissociation occurs on the acetone S1 surface and provide complementary dynamical information to that discussed in the preceding paper.     

Degradation of protein in nanoplasma generated around gold nanoparticles in solution by laser irradiation

We developed a method of protein degradation in an aqueous solution containing gold nanoparticles by irradiation of a pulse laser. In the present study, lysozyme was used as an example. Lysozyme degradation proceeded most efficiently when a pH of the solution was adjusted so that it was at the isoelectric point. The scheme of the lysozyme degradation is as follows: (1) Lysozyme molecules in the solution are neutralized and adsorbed on the gold nanoparticles with its pH value adjusted at the isoelectric point, (2) nanoplasma is generated in the close vicinity of a gold nanoparticle which is excited by an intense 532-nm laser, (3) lysozyme molecules in the nanoplasma are degraded into small fragments. Lysozyme degradation does not proceed efficiently at a pH value deviated from the isoelectric point because the lysozyme molecules are dissolved uniformly so that only a small portion of the lysozyme molecules are located in the vicinity of gold nanoparticles which create the nanoplasma.     

纳秒激光电离产生Kr17+的研究

Up to Kr17 + multicharged krypton ions have been observed in time-of-flight mass spectrum by a 25 ns Nd-YAG 1. 064 μm laser at laser intensity about 1012 W/ cm2 . Experimental results indicate that the multicharged ions appear only when the laser interacts with the middle part of the pulsed beam,and the intensities of the multicharged ions increase dramatically by increasing the backing pressure of Kr gas,which indicates that the clusters in the beam is essential to the production of multicharged ions. From the experimental results,it is concluded that the cluster is ionized via multiphoton ionization and forms a nanoplasma ball,which can absorb the laser resonantly to further ionize the single charge ion to the high charge state.     

A model for energy transfer in inelastic molecular collisions applicable at steady state or non-steady state and for an arbitrary distribution of collision energies

A new model for energy exchange between translational and internal degrees of freedom in atom-molecule collisions has been developed. It is suitable for both steady state conditions (e.g., a large number of collisions with thermal kinetic energies) and non-steady state conditions with an arbitrary distribution of collision energies (e.g., single high-energy collisions). In particular, it does not require that the collision energies be characterized by a quasi-thermal distribution, but nevertheless it is capable of producing a Boltzmann distribution of internal energies with the correct internal temperature under quasi-thermal conditions. The energy exchange is described by a transfer probability density that depends on the initial relative kinetic energy, the internal energy of the molecule, and the amount of energy transferred. The probability density for collisions that lead to excitation is assumed to decrease exponentially with the amount of transferred energy. The probability density for de-excitation is obtained from microscopic reversibility. The model has been implemented in the ion trap simulation program ITSIM and coupled with an Rice-Rampsberger-Kassel-Marcus (RRKM) algorithm to describe the unimolecular dissociation of populations of ions. Monte Carlo simulations of collisional energy transfer are presented. The model is validated for non-steady state conditions and for steady state conditions, and the effect of the kinetic energy dependence of the collision cross-section on internal temperature is discussed. Applications of the model to the problem of chemical mass shifts in RF ion trap mass spectrometry are shown.     

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (n> or =10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n approximately 10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations.