Albert Danon

Dissociation and ionization in hyperthermal 1-iodopropane-diamond scattering

Abstract The dissociation of 1-iodopropane due to scattering from single crystal diamond (111) was studied at hyperthermal kinetic energies (1–10 eV). 1-Iodopropane was accelerated in a hydrogen seeded supersonic molecular beam. Dissociation probabilities up to 4% at a kinetic energy of 8 eV were measured using a quadrupole mass spectrometer (QMS). The dissociation was mostly into iodine atoms and not through the lower energy channel, namely the elimination reaction into propylene and hydrogen iodide. The angular distribution of the scattered iodine atoms was shifted from the specular angle into the grazing direction (supraspecular scattering). This shift and angular difference from the scattered molecular angular distribution, as well as the fragment identity, preclude statistical dissociation after vibrational excitation and energy redistribution. We have also found dissociative ionization resulting in the generation of positive propyl and negative iodine ions. The kinetic energy dependence of both the natural iodine atoms and positive propyl ions could be fitted to the same type of empirical formula resulting in an identical threshold of 3.7 eV. This effective threshold which is much higher than that of the elimination (0.84 eV), or the direct dissociation (2.3 eV), suggests that the dissociation occurs after collision induced electronic excitation and predissociation, or ionization and subsequent neutralization. The kinetic energy of the scattered undissociated molecules, as well as that of the generated atomic iodine, was measured using a time-of-flight technique. This information, combined with internal vibrational temperature measurement of the scattered molecules, resulted in the determination of relative energy transferred to the surface and to the vibrational-rotational degrees of freedom.

Hyperthermal surface ionization: a novel ion source with analytical applications

Abstract We have found that a wide range of organic molecules with hyperthermal kinetic energy (1–20 eV) can undergo an efficient molecular ionization or dissociative ionization upon scattering from a surface. The molecular kinetic energy is obtained in a simple supersonic expansion of the organic heavy molecule seeded in hydrogen (or helium) carrier gas through a pinhole nozzle. Hyperthermal surface ionization (HSI) is a sensitive, selective and informative ionization method. It is characterized by several properties that make it attractive for analytical applications. The HSI mass spectra (negative and positive ions) exhibit unique fragmentation patterns. These HSI mass spectra are different from electron impact (EI) ionization mass spectra, and reveal structural and isomeric information. The ionization yield is determined by the molecular mass, electron affinity or ionization potential and thus extreme selectivity against common light gases is observed. The absence of the vacuum residual gases in the mass spectra combined with the high molecular ionization efficiency contributes to an improved signal-to-noise ratio and detection sensitivity. The supersonic jet chamber serves as an efficient high-load jet separator in the tail-free coupling of a gas chromatograph (GC) to a mass spectrometer (MS) and allows the use of beam modulation and lock-in amplification. The HSI ion source itself can serve as a selective and sensitive GC detector for many functional groups of molecules such as I, Br, Cl, F, PO2, NO2, CN, NH2 polycyclic aromatic hydrocarbons and organometallics. Most notable among these are the organofluorine molecules whose mass spectra are demonstrated and discussed. A simple GC detector based on HSI is described. Intra-nozzle chemical reactions can be used for the selective and sensitive determination of olefins among paraffins.

Ceramic nozzle for molecular acceleration and its temperature measurement

An all ceramic nozzle is described, based on a watch jewel and having a very small heated volume (V<10−3 cc). Using this nozzle, molecular catalytic and thermal decomposition is minimized; thus, high molecular kinetic energies can be obtained for ‘‘unstable’’ organic molecules. The real nozzle temperature is measured by the variation of its backing pressure with its temperature, keeping constant gas flux conditions. The nozzle is capable of being heated to over 1500 °K with less than 20 W and is thermally isolated from the organic molecule oven that is differentially heated and separately temperature controlled.

Isotope, molecular and surface effects on hyperthermal surface induced dissociated ionization

Abstract Organic molecules acquired with hyperthermal (1–20 eV) kinetic energy undergo efficient surface ionization. This hyperthermal surface ionization (HSI) may produce both positive and negative ions. The dissociative ionization of alkyl halides results in the production of negative ions of the functional group having high electron affinity, and positive ions of the alkyl radical, which can further dissociate into smaller fragments. The effect of the alkyl chain lenght and the bromine isotope effect on the obtained HSI mass spectra were studied in several alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positive ion formation yield strongly increases with the size of the C n H 2n + i radical for n = 1–4 and then a quasi saturation is observed. The observed radical fragmentation increases with the incident molecular kinetic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy brominem isotope increased ionization is observed at intermediate molecular kinetic energies. Piperidine HSI on a rhenium filament exhibits a single (M - 1) ion while its HSI from an oxidized rhenium filament having a much higher work function is characterized by a much richer fragmentation pattern. The dissociative ionization mechanism is rationalized in terms of a surface—molecule electron transfer followed by an immediate dissociation into a negative halogen ion and an alkyl radical. This radical, which usually has a low ionization potential, can transfer an electron to the surface and scatter as a positive ion. At high kinetic energies, the radicals or positive ions can further dissociate near the surface, and then scatter away as lower mass ions with ion yield which depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemicl and electron transfer processe and not by gas phase unimolecular ion dissociation, as found with large polyatomic molecules.

Collisionally-activated dissociation in hyperthermal surface ionization of cholesterol

Abstract Cholesterol in a hydrogen-seeded supersonic molecular beam was scattered from a continuously oxidized rhenium foil. The hyperthermal surface scattering exhibited efficient molecular ionization with a controlled amount of molecular ion dissociation. At 5.3 eV incident molecular kinetic energy the hyperthermal surface ionization mass spectrum was dominated by the parent molecular ion. Upon the increase of the molecular kinetic energy, a gradual increase in the degree of ion dissociation was observed. At 22eV incident kinetic energy the parent ion was completely dissociated and the mass spectrum was dominated by an extensive consecutive fragmentation. An efficient kinetic-vibrational energy transfer was observed, and it is extimated to be over 18% of the available incident kinetic energy. The implication for surface collisionally-activated dissociation of polyatomic ions is discussed. Rhenium oxide is suggested as an optimal surface for this purpose, as well as for the hyperthermal surface ionization of neutral species.

Temperature programmed desorption-mass spectrometer with supersonic molecular beam inlet system

A novel atmospheric temperature programmed desorption-mass spectrometer (TPD-MS) device based on supersonic molecular beams (SMB) is demonstrated. The coupling between SMB and TPD-MS enables one to obtain highly sensitive TPD measurements with very fast time response (less than 1 s), that makes it possible to distinguish between desorptions from energetically neighboring sites. Quantitative adsorption measurements of adsorbing gas and vapors could be easily done. High detection sensitivity of 4 ng/s for H2O16 and 100 pg/s for H2O18 is achieved. The high sensitivity TPD measurements are demonstrated by the ability to monitor simultaneous physical adsorption of nitrogen, oxygen, and water on various sites on carbon fibers at room temperature.

Surface-molecule proton transfer in the scattering of hyperthermal DABCO from H/Pt(111)

Abstract Hyperthermal DABCO is scattered from H-covered Pt(111). The ionization by electron and proton transfer is measured as a function of the incident molecular kinetic energy and the angle of incidence. The positive ions produced are energy analyzed showing hyperthermal distributions which are dependent on the incident energy. This means that no equilibration occurs at the surface and therefore protonation at hyperthermal energy is an Eley-Rideal reaction. The protonation yield depends linearly on the incident DABCO flux and the H-coverage on the Pt(111) surface. The ionization yield for both electron and proton transfer depends on the incident energy as k ( E i − E tr ) n with a similar threshold energy of 1.5 eV. From this we learn that direct protonation occurs by electron transfer and a subsequent binding of an H-atom. Within the error bars we found no difference between H-atom and D-atom transfer. Protonation of other molecules with a high proton affinity such as dimethylaniline at H-covered Pt(111) was measured as well. Analytical applications for both surface analysis and molecular detection are discussed.

Water coadsorption effect on the physical adsorption of N2 and O2 at room temperature on carbon molecular sieve fibers

The effect of water vapours on the physical adsorption of nitrogen and oxygen from air, on the surface of an activated carbon molecular sieve fibre, was studied by temperature-programmed desorption mass spectrometry with a supersonic molecular beam inlet system. During exposure of the carbon surface at room temperature to moist air, the adsorption kinetics for oxygen and nitrogen are fast, with oxygen being slightly faster, however, the water adsorption kinetics are much slower. When water starts to fill the pore volume, the adsorbed nitrogen and oxygen are removed, with oxygen being more easily removed. At high relative humidity, hydrogen bonding between neighbouring water molecules leads to the formation of clusters of adsorbed water, which trap the nitrogen molecules and form new sites for nitrogen adsorption. The effect of surface modification of carbon molecular sieves, by evacuation at high temperatures, on the adsorption sites of nitrogen and oxygen was also explored.

Chemically induced hyperthermal surface ionization

Hyperthermal beams of cyclohexane and carbon tetrachloride were scatte from a 2% W−Th filament. Chlorine negative ions generated by chemically induced hyperthermal surface ionization were monitored by quadrupole mass spectrometer. The surface temperature of the filament was 2400K. (AIP)

Interaction of O2, N2 and He at room temperature with carbon molecular sieves sensed by adsorption measurements

Abstract Heats of adsorption of N 2 and O 2 on carbon molecular sieve fibers (CMSFs) have been determined by flow microcalorimetry (FMC), by static microcalorimetry (SMC), and by temperature programmed desorption mass spectrometry (TPD-MS). The FMC determined the heats of displacement of He, used as a carrier gas, by the more strongly adsorbed O 2 and N 2 . The heats and the amounts of adsorption were determined concurrently and the molar heats of adsorption calculated. Generally, these determinations revealed that the displacement of He by O 2 and by N 2 is protracted and occurs in two stages. Re-arrangement of the adsorbates occurs in the second stage with little or no adsorption. The heats of adsorption were found to be surprisingly high and He difficult to displace. The determinations of the heats of adsorption on the CMSF were accomplished by two independent methods: microcalorimetry and TPD. Both of these methods produced closely similar heats of adsorption. He adsorption near constrictions increases the adsorption selectivity between O 2 and N 2 and increases the heat of adsorption of N 2 due to a better fit of the N 2 molecules to the constriction sites. The heat of adsorption of O 2 is decreased by He adsorption because He occupies some of the low energy sites of O 2 but not that of N 2 .