Multiphoton ionization studies of xenon clusters

Non-resonant multiphoton ionization of xenon clusters has revealed the same magic numbers as found in the case of electron impact ionization. Large dissociation rates are found for the trimer through pentamer ion, measured on a time scale of ≈ 10⁻⁷ s after ionization.     

Multiphoton ionization mass spectroscopic detection of ammonia clusters

Ammonia cluster ions (NH₃)_n?2NH⁺₄ (n = 2?8), havevbeen detected by multiphoton ionization (MPI) mass spectroscopy, incorporating both a supersonic nozzle source and an ArB excimer laser. The MPI method reveals that the intensity ratio of NH⁺₄/NH⁺₃ is more than an order of magnitude greater than that in the electron impact mass spectrum.     

Multiphoton ionization detection of photodissociation fragments: no from NO2

The multiphoton ionization (MPI) of NO is used to probe the dynamics of the photodissodation of NO₂ in a one-color experiment. The resulting MPI spectra clearly indicate the high internal rotational energy in the NO fragment. MPI shows promise as a highly sensitive, information-rich diagnostic for internal state determinations of nitric oxide.     

The photodissociation of NO(2) by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.     

Multiphoton dissociation and ionization of nickelocene

In this paper we report the results of an experimental study of collision-free molecular multiphoton dissociation (MPD) and molecular multiphoton ionization (MPI) of nickelocene (NiC₁₀H₁₀), induced by the light of a tunable dye laser in the wavelength region 3750–5200 A. The spectral dependence of the ion signal reveals a multitude of narrow (fwhm from <0.5 cm^?1 to 1.5 cm^?1) intense peaks superimposed on a very weak background (relative amplitude ratio for peaks/background ≈ 10³). The sharp resonances in the ion signal are attributed, on the basis of spectroscopic analysis, to two-photon resonant three-photon ionization of Ni(I) and to one-photon resonant three-photon ionization of Ni(I), the Ni(I) being produced by MPD of nickelocene. The ion signal in the spectral range 3750–3950 A reveals enhanced continuous background due to MPI of nickelocene. This ion signal spectra, together with studies of the intensity dependence of the overall (nickelocene MPD) - (Ni(I) MPI) processes, as well as the (nickelocene molecular MPI) reaction, reveal four reactive processes. (a) Two-photon molecular MPI for hω ? 3.10 eV photons. (b) Three-photon molecular MPI for hω = 3.10-2.10 eV. (c) Two-photon MPD at hω ? 2.86 eV. (d) Three-photon MPD for hω = 2.8-1.9 eV. The overall dissociation energy of nickelocene (Nicp₂) to give Ni + 2cp was determined to be 5.76 ± 0.60 eV and the two-photon ionization potential of this molecule is 6.29 ± 0.015 eV. Our results provide dynamic evidence concerning a simultaneous “explosive” photodissociation mechanism of Nicp₂ by process (c) and for the dominating role of the dissociative channel, characterized by a branching ratio of ?50 in favor of predissociation over autoionization, for process (c) at 6.3–6.6 eV. The MPD processes (c) and (d) are expected to exhibit intramolecular erosion of phase coherence effects. Processes (c) and (d) are of high efficiency ≈0.01 ions/molecule at saturation exhibited at laser power of ≈ 10⁸ W cm^?2.     

Multiphoton ionization of VOCl3

VOCl₃ is studied by resonance-enhanced multiphoton ionization in the region 430 to 440 nm. Mass spectrometric detection reveals the production of both V⁺ and VO⁺ with the molecular ion having the larger intensity. The V⁺ ion current spectrum shows sharp atomic resonance peaks while the VO⁺ spectrum is structureless.     

Multiphoton ionization of many-electron atoms

We review the main experimental results obtained in the multielectron multiphoton ionization of rare gases. Multiple ionization is interpretated as a sequential process. We study the influence of many-electron effects in multiphoton ionization within the framework of the random phase approximation. This is applied to a calculation of the two-photon ionization cross section of xenon. Finally, the role of screening effects in the multiphoton stripping of heavy atoms (Xe) is estimated.     

钪-硫团簇的形成和光解

报道了用激光直接溅射法产生钪硫团簇, 并用串级飞行时间质谱仪研究了所产生的团簇离子的分布及紫外激光光解规律。钪硫二元团簇正负离子都是由周边硫原子包围团簇骨架而构成的, 骨架是由包含着不同数目的Sc2S3这样的组份单元组成, 它们结合紧密, 构成了稳定的钒硫团簇的核心。稳定的团簇正离子为ScS(Sc2S3)n^+和Sc2S2(Sc2S3)n^+。稳定的团簇负离子为ScS2(Sc2S3)n^-,S3(Sc2S3)n^-, (Sc2S3)n^-。周边硫原子数目随样品中硫的摩尔含量的增加而增多, 它们结合较弱, 易于剥离。在紫外光解时往往以失去S2, S4, S6的方式解离。通过分析认为具有组份单元的Sc对于S团簇的结构可能是一种笼状结构。     

Multiphoton ionization and photodissociation at the second-order space focus in a time-of-flight mass spectrometer: The amino acid tryptophan

Neutral IR-desorbed molecules of the amino acid tryptophan entrained in a supersonic beam were post-ionized by multiphoton ionization (MUPI) in the gas phase. Variations in the intensity of the ionization laser produced different degrees of fragmentation observed in the time-of-flight mass spectra of tryptophan according to the ladder-switching model for MUPI fragmentation. From the number of absorbed photons the upper values for the appearance energies of specific fragment ions were obtained. A complete fragmentation pathway of tryptophan was established by using photodissociation of molecular ions and fragment ions formed by MUPI. Photodissociation was successfully performed by secondary excitation with a UV laser at the second-order space focus of a reflection time-of-flight instrument. A fragmentation tree for a laser-desorbed molecule obtained in a laser tandem time-of-flight instrument is shown for the first time.     

Multiphoton ionization translational spectroscopy (MITS) of I2

A new technique is introduced based on the measurement of kinetic energies and angular distributions of fragments in multiphoton ionization of molecule Both positive and negative ions are measured as well as neutrals. Information is obtained about highly excited repulsive states and about the multiphot ionization process itself. Examples are reported for MPI of I₂. In a one-laser experiment dissociative autoionization is observed. In a two-laser experiment processes are distinguised, that have different intermediate states. Lifetimes of those states are measured. Information is also obtained ab an ion pair formation process, leading to a reinterpretation of potential curves.     

Multiphoton ionization spectra of two caged amines

The multiphoton ionization spectra of quinuclidine (ABCO) and triethylenediamine (DABCO) have been measured. All of the observed resonances are two-proton transitions to low-lying localized Rydberg states. For ABCO the lowest energy transitions are assigned as ¹A₁ (3s) ← ¹A₁ (n) 4.84 eV, and ¹E (3p_xy) ← ¹A₁ (n), 5.42 eV. In the case of DABCO, orbitals localized on the nitrogens interact and are split into two new orbitals. In terms of the split orbitals the low energy resonances in DABCO are assigned as ¹E′[3p_xy(?)] ← ¹A₁ (n+_, 4.44 eV, and ¹E″ [3p_xy(+)] ← ¹A₁ (n+), 4.94 eV.     

Intra-cluster ion-molecule reactions and photodissociation of molecular clusters

Zeitschrift für Physik D Atoms,Molecules and Clusters - Intra-cluster ion-molecule reactions in benzene clusters and benzene-water binary clusters were observed in Resonance-Enhanced...     

Chemical ionization (NO) spectra of n-alkenoic acids and their esters

It is shown that the position of one double bond in long-chain unsaturated fatty acids and their esters can be determined by chemical ionization (NO) mass spectra. Homoconjugated dienoic acids (as linoleic acid) do not give a characteristic framentation pattern, in contrast to those which contain three or more homoconjugated double bonds.     

Multiphoton ionization detected by fourier-transform mass spectrometry

Ions produced by multiphoton ionization (MPI) of naphthalene, fluoranthene and triphenylene have been detected by Fourier-transform mass spectrometry (FT MS). Paret ions are produced very efficiently at 250 and 222 nm with pulse energies as low as 1 mJ. With FT MS a complete, high-resolution mass spectrum is obtained for each laser pulse.     

Multiphoton ionization and Coulomb explosion of C2H5Br clusters: a mass spectrometric and charge density study

Using time‐of‐flight mass spectrometry (TOFMS), laser‐induced photochemistry of ethyl bromide clusters has been investigated at three different wavelengths (viz. 266, 355 and 532 nm) utilizing nanosecond laser pulses of ~5 × 10⁹ W/cm². An interesting finding of the present work is the observation of multiply charged atomic ions of carbon and bromine at 355 and 532 nm, arising from the Coulomb explosion of (C₂H₅Br)_n clusters. At 266 nm, however, the (C₂H₅Br)_n clusters were found to exhibit the usual multiphoton dissociation/ionization behaviour. The TOFMS studies are complemented by measuring the total charge density of the ionized volume at 266, 355 and 532 nm, using the parallel plate method, and the charge densities were found to be ~2 × 10⁹, 6 × 10⁹ and 2 × 10¹¹ charges/cm³, respectively. The significantly higher charge density and the presence of energetic, multiply charged atomic ions at 532 nm are explained by the higher ponderomotive energy of the 532 nm photon, coupled with the Coulomb stability of the residual multiply charged ethyl bromide clusters generated upon laser irradiation, due to their larger effective cluster size at 532 nm than at 355 and 266 nm. Copyright © 2011 John Wiley & Sons, Ltd.     

Multiphoton ionization and two-photon excitation spectroscopy of nitric oxide

The multiphoton ionization and two-photon excited fluorescence action spectra of nitric oxide are compared. At low fluence the two spectra differ when more than three photons are required for ionization. This may be due to the presence of several energy acquisition routes, providing new spectroscopic information on intermediate slates.     

Infrared photodissociation spectroscopy of protonated acetylene and its clusters

The protonated acetylene cation, C2H3+, (also known as the vinyl cation) and the proton-bound acetylene dimer cation (C4H5+) are produced by a pulsed supersonic nozzle/pulsed electrical discharge cluster source. The parent ions are also generated with weakly attached argon "tag" atoms, e.g., C2H3+Ar and C4H5+Ar. These ions are mass selected in a specially designed reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy in the 800-3600 cm-1 region. Vibrational resonances are detected for both ions in the C-H stretching region. C2H3+ has a strong vibrational resonance near 2200 cm-1 assigned to the bridged proton stretch of the nonclassical ion, while C4H5+ has no such free-proton vibration. Instead, C4H5+ has resonances near 1300 cm-1, consistent with a symmetrically shared proton in a di-bridged structure. Although the shared proton structure is not the lowest energy isomer of C4H5+, this species is apparently stabilized under the supersonic beam conditions. Larger clusters containing additional acetylene units are also investigated via the elimination of acetylene. These species have new IR bands indicating that rearrangement reactions have taken place to produce core C4H5+ ions with the methyl cyclopropane cation structure and/or the protonated cyclobutadiene isomer. Ab initio (MP2) calculations provide structures and predicted spectra consistent with all of these experiments.     

Infrared photodissociation spectroscopy of Na(NH3)n clusters: probing the solvent coordination

The first mass-selective vibrational spectra have been recorded for Na(NH3)n clusters. Infrared spectra have been obtained for n = 3-8 in the N-H stretching region. The spectroscopic work has been supported by ab initio calculations carried out at both the DFT(B3LYP) and MP2 levels, using a 6-311++G(d,p) basis set. The calculations reveal that the lowest energy isomer for n or= 7 is indicative of molecules entering a second solvation shell, i.e., the inner solvation shell around the sodium atom can accommodate a maximum of six NH3 molecules.     

Multiphoton ionization photoelectron spectroscopy: xenon and nitric oxide

Photoelectron spectra (≈50 mcV, full width at half maximum) following resonantly enhanced multiphoton ionization of xenon and nitric oxide have been recorded and compared to the previous low-resolution (≈120 mcV) results. Several new features have been resolved and additional experiments, which include iso topic substitution and two-color multiphoton ionization. have been performed.