Electron bombardment fragmentation of size selected molecular clusters

(CO₂) _n , (NO) _n and (NH₃) _n clusters are generated in a supersonic molecular beam and size selected by scattering from an He beam. By measurements of angular dependent mass spectra, TOF distributions and the angular dependence of the scattered signal quantitative information on the fragmentation probability by electron impact is derived. The van der Waals systems (CO₂) _n and (NO) _n appear only at masses which are simply multiples of the monomer mass. The preferred cluster ion is the monomer ion for all investigated cluster sizes withn=2 to 4. The fragment pattern for the quasi-hydrogen bonded (NH₃) _n -cluster shows, beside a large number of fragment masses, a preference for protonated ions. The results are explained in terms of simple models based on the structural change from the neutral to the ionized configuration and the fragmentation pattern of the monomer followed by ionmolecule reactions.     

Chemisorption and chemical reactions on size selected clusters

Low energy ion beam techniques have been used to perform a detailed study of the reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ with a range of simple molecules (D₂, CH₄, O₂, C₂H₄, CO and N₂). The reactions were studied over a center of mass collision energy range from 0.2eV up to 7eV. Activation barriers for chemisorption onto the clusters were deduced from the experimental results. The activation barriers for chemisorption on Al ₂₅ ⁺ and Si ₂₅ ⁺ are generally similar and show a qualitative correlation with the electronic properties of the reactant molecule. However, the products of the chemical reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ which result from cluster fragmentation are quite different. Si ₂₅ ⁺ shows a tendency to undergo fission as observed in a number of recent studies of the dissociation of the bare clusters.     

IR-photodissociation of size selected molecular clusters and their structures

Infrared photodissociation spectra of (CH₃NH₂) _n clusters were measured fromn=2 ton=6 near the monomer absorption of the C-N stretching mode at 1044 cm^?1 using a cw-CO₂ laser. The clusters were size-selected by scattering from a helium beam. The spectrum of cold dimers shows a red (1038 cm^?1) and a blue (1048 cm^?1) shifted peak which is attributed to the non-equivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 cm^?1 and 1046.0 cm^?1 caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Structure calculations confirm these results. Secondly, the mixed complexes C₂H₄-CH₃COCH₃ and C₂H₄-(CH₃COCH₃)₂ were investigated. The dimer spectrum, measured around the monomer frequency of the out-of-plane bending mode of C₂H₄ at 949 cm^?1, shows two peaks at 946.2 cm^?1 and 961.3 cm^?1. This splitting is attributed to two different isomers that are found in configuration calculations. A similar behaviour is found for the trimer.     

IR-UV double resonance spectroscopy of guanine-H2O clusters

We present the IR-UV double resonance spectrum of guanine monohydrate in the region 3100 cm(-1) to 3800 cm(-1) along with the energies and frequencies of these structures calculated at the non-empirical correlated ab initio RI-MP2/cc-pVDZ level. We assign the structures of guanine-water clusters by comparing the experimental spectra with the ab initio calculations and with the IR spectra of the bare guanine monomer. We find two clusters with guanine in the enol-amino tautomeric form and one structure with guanine in the keto-amino form.     

Collisional energy transfer and fragmentation of size selected CO2 clusters

Carbon dioxide clusters are generated in a supersonic molecular beam and size selected by scattering from a He beam. By analyzing the measured time-of-flight spectra as a function of the deflection angle, differential energy loss spectra for (CO₂)₂ — He are obtained which show a rotational rainbow structure with a maximal energy transfer of ΔE/E=0.4. This result is compatible with the slipped parallel structure of dimer but not with theT-shaped geometry. The scattering analysis is also used to derive information about the pressure dependence of cluster formation and the fragmentation by electron impact ionisation. The latter process leads preferably to the monomer product ion CO ₂ ⁺ with a small but finite probability for other ionic channels.     

Concerning resistance network analogues for relaxation in multi-level systems and for double resonance experiments

The value of resistance network analogues as an aid to understanding relaxation in multi-level systems is re-emphasised. It is pointed out that analogues of the cross-saturation parameters which arise in the theoretical analysis of ENDOR and ELDOR experiments can be obtained from resistance measurements on an analogue circuit.     

Transient nutation and molecular echoes in microwave double resonance experiments

Transient phenomena occurring in time resolved microwave-microwave double resonance experiments when the pump power is switched on and off have been investigated both theoretically and experimentally. Special attention has been paid to the case when both fields are saturating and exactly resonant. A calculation using the evolution operator formalism has been carried out to explain the experimental results. Transient nutations similar to those observed by the Stark modulation method have been observed and theoretically explained. A new phenomenon, the driven induction decay, has also been observed when the pump power is switched on. Moreover, double resonance photon echoes have also been exhibited. The application of these methods to relaxation studies is discussed.     

IR dissociation of ammonia clusters

Dissociation spectra of NH₃ clusters have been recorded using a cw CO₂ laser. For the dimer two absorption bands have been found at 979 and 1004 cm⁻¹, which originate from the excitation of two non-equivalent NH₃ molecules. A tunneling motion is held responsible for the observed structure on one of these bands. The symmetry group of the NH₃ dimer is presented considering the tunneling motion solely. Heavier NH₃ clusters dissociate at frequencies between 1020 and 1100 cm⁻¹. The dissociation spectrum of the SiH₄-NH₃ complex shows one peak centered at 972.3 cm⁻¹.     

Optical spectroscopy on size selected gold clusters deposited in rare gas matrices

We report on the successful “soft-landing” of size selected Au trimers in solid Krypton matrices. The Au cations are produced by sputtering, mass-selected in a quadrupole mass spectrometer, co-deposited with Krypton on a cooled CaF₂ substrate, and neutralized by low energy electrons. The deposition of low kinetic energy cations (10 eV) gives rise to strong excitation bands, detected by the emitted fluorescence light, which are unambiguously attributed to Au trimers. The deposition and fragmentation process is qualitatively discussed.     

Infrared-infrared double resonance spectroscopy of the isomers of acetylene-HCN and cyanoacetylene-HCN in helium nanodroplets

Infrared-infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene-HCN and cyanoacetylene-HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene-HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene-HCN isomer. However, population transfer is not observed for the cyanoacetylene-HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement.     

The isomers of small carbon clusters

Absolute cross-sections for photodetachment of negative carbon clusters are reported for C _n ^? (n=3,...,8). These measurements are made using different types of ion sources, which create different isomers. These new results indicate that various negative and neutral isomers exist, some with electron affinities as low as ～1 eV.     

IR laser-induced decomposition of volatile uranyl complexes II. New double resonance results

New IRIR double resonance measurements are presented for the 10.6 μm CO₂-laser-induced dissociation of UO₂[(CF₃CO)₂CH]₂·(CH₃O)₃PO (UO₂L₂TMP). The results confirm that loss of base, TMP, is preceded by isomerisation of the parent molecule to a species with a red-shifted UO₂ absorption profile; but the red shift is much less than previously deduced. A red shift is also reported for product UO₂L₂, and is attributed, like that of the isomer, to partial detachment of one of the bidentate ligands. Isomerisation and dissociation yields are lower than deduced from the previous double resonance measurements, but agree satisfactorily with results from a refined analysis of molecular beam experiments (paper I), demonstrating that the uranyl absorption profile must have a significant inhomogeneous component. The present isomerisation yields also disagree profoundly with laser-induced fluorescence depletion measurements, and it is argued that the latter measure a photophysical, rather than a photochemical, process.     

Mass spectrometric identification of double bond positional isomers of hexadecenyl acetate without chemical derivatization

A method for the identification of the double bond positional isomers of hexadecenyl acetate has been established by analysing similarity of the mass spectra patterns on a fuzzy classification, in which the intensity ratios of six diagnostic pairs of the predominant ions were selected as standard parameters for the characterization of the double bond position. The procedure was tested with △2 to △15-isomers of chemically unmodified hexadecenyl acetate, and the original double bond position in the acetates was located unambiguously.     

Polarization dependence of transition intensities in double resonance experiments: unresolved spin doublets

The polarization dependence of transition intensities in multiple resonance spectroscopic experiments can provide information useful for making rotational assignments. A formalism to describe the polarization dependence of transition intensities in multiple resonance experiments, particularly for cases when two rotational/fine structure quantum numbers are needed to specify the state of the system, is presented. The formalism is presented in a form usable both when the transitions between the underlying fine structure components are experimentally resolved, as well as when they are unresolved, to form composite lines. This sort of treatment is necessary for cases when the two quantum numbers that specify the fine structure differ significantly, such as is the case at low N, when the difference between J and N becomes comparable to the value of J. Ratios of transition intensities in different experimentally convenient polarization arrangements are evaluated for the case of composite N transitions formed by combining the spin components of a doublet system. The formalism is expressed in a form easily extendable to accommodate experimental cases of more than two excitation steps, or a combination of excitation steps and an external static electric field. This polarization diagnostic has been experimentally applied to assign spectral features in double resonance Rydberg spectra of CaF.     

Electron bombardment induced fragmentation of size selected neutral (D2O) n clusters

The fragmentation behaviour of size selected neutral (D₂O) _n clusters withn4 after ionization with 70 eV electrons is subject of this work. Size selection by scattering the cluster beam from a He target beam in combination with a quadrupole mass filter and time resolved measurements at specific laboratory angles enables us to determine the neutral precursor masses of the detected ions. The measured fragment pattern is dominated by deuterated ions of the form (D₂O) _n–x D⁺ withx1. The dimer fragmentation which leads with a probability of 62.5% to the D₃O⁺ ion and with 37.5% to D₂O⁺ can be explained by fast intracluster ion-molecule reactions of charged monomer fragments reacting with the partner molecule. For larger clusters the fragmentation process can be rationalised by the creation of an initially highly excited D₃O⁺ (D₂O) _x complex which is stabilized by evaporating additional monomer units with the main fragment channel (D₂O)D⁺ forn=3 and (D₂O)₂D⁺ forn=4. With increasing cluster size an increasing tendency of evaporation of more than one water monomer unit has been observed.     

Electron impact ionization of size selected hydrogen clusters (H2)N: ion fragment and neutral size distributions

Clusters consisting of normal H2 molecules, produced in a free jet expansion, are size selected by diffraction from a transmission nanograting prior to electron impact ionization. For each neutral cluster (H2)(N) (N=2-40), the relative intensities of the ion fragments Hn+ are measured with a mass spectrometer. H3+ is found to be the most abundant fragment up to N=17. With a further increase in N, the abundances of H3+, H5+, H7+, and H9+ first increase and, after passing through a maximum, approach each other. At N=40, they are about the same and more than a factor of 2 and 3 larger than for H11+ and H13+, respectively. For a given neutral cluster size, the intensities of the ion fragments follow a Poisson distribution. The fragmentation probabilities are used to determine the neutral cluster size distribution produced in the expansion at a source temperature of 30.1 K and a source pressure of 1.50 bar. The distribution shows no clear evidence of a magic number N=13 as predicted by theory and found in experiments with pure para-H2 clusters. The ion fragment distributions are also used to extract information on the internal energy distribution of the H3+ ions produced in the reaction H2+ + H2-->H3+ +H, which is initiated upon ionization of the cluster. The internal energy is assumed to be rapidly equilibrated and to determine the number of molecules subsequently evaporated. The internal energy distribution found in this way is in good agreement with data obtained in an earlier independent merged beam scattering experiment.     

IR spectroscopy of hydrogen-bonded 2-fluoropyridine-methanol clusters

The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.     

Improvement of double pulsed field gradient spin‐echo ROESY experiments for identification of bound waters

A novel pulse sequence incorporating the double pulsed field gradient spin‐echo (DPFGSE) and the gradient‐tailored excitation WATERGATE techniques is presented that has particular use for identifying bound waters in ¹⁵N‐labeled macromolecules. This sequence, DPFGSE–ROESY–HSQC, affords greater spectral sensitivity than the DPFGSE–ROESY–HMQC experiment which was previously presented and is consequently useful for rapidly obtaining reliable information for characterizing macromolecular bound water molecules. A significant enhancement in the sensitivity is achieved by using the gradient‐tailored excitation WATERGATE sequence in the reverse INEPT step as it allows the use of much higher receiver gains. Since coherence selection is not used, the sequence has improved sensitivity together with less spectral artifacts. The advantage of this pulse sequence is illustrated using ¹⁵N‐labeled ribonuclease T₁. Copyright © 2003 John Wiley & Sons, Ltd.     

Dramatic size effects and evidence of structural isomers in the reactions of rhodium clusters, Rhn+/-, with nitrous oxide

The reactions of gas phase rhodium clusters, Rhn+/- (n<30), with nitrous oxide, N2O, have been investigated under single collision conditions by Fourier transform ion cyclotron resonance mass spectrometry. The only significant reaction observed is the sequential generation of oxides. Absolute rate constants for the reactions of all clusters have been determined and, in the case of the cationic clusters especially, they exhibit large fluctuations as a function of cluster size with local minima observed for n=5, 19, 28. Striking similarities are observed with the variation in rate constants for these clusters in reactions with small hydrocarbons (C. Adlhart and E. Uggerud, J. Chem. Phys., 2005, 123, 214709). Corresponding size effects are also observed but are less marked in the reactions of the anionic clusters. The reactions of several clusters exhibit marked deviations from simple pseudo-first-order kinetics suggesting the presence of multiple isomeric forms: Rh11+, Rh12+ and Rh8- exhibit characteristic biexponential decays which are interpreted in terms of the existence of different structural forms of the cluster which have markedly different reactivity. By contrast, Rh6+, Rh7+ and Rh8+ show rates which apparently increase with time, probably due to collisional activation. Thermalisation of the clusters prior to reaction by exposure to pulses of argon results in changes to the kinetics of these anomalous systems which can be explained in terms of collision induced isomerisation.