Measuring absolute dissociative electron attachment cross sections . 101

Crossed-beams experiments, such as Wippi, allow for the determination of absolute disso-ciative electron attachment cross-sections, σ_DEA. The measured ion yield, i_ion, is related toσ_DEA, by the following expression [42]:

i_ion =σ_DEA·i_e·N_t·l (6.9)

where ie is the incident electron current, Nt is the density of neutrals in the interaction region, and l is the interaction length between electrons and neutrals within, in this case, the HEM. In crossed-beam experiments, the σDEA may be determined by comparing the

6.1. Wippi

measured ion yield for a fragment anion formed from a sample with well-known cross sections occurring at∼0.8 eV for Cl^– from CCl₄ (σ_DEA,Cl⁻_/CCl4 = 5.0×10⁻²⁰m²) [162] or at 5.2 eV for F^– from SF6 (σ_DEA,F⁻_/SF6 = 5.0×10⁻²² m²) [163].

The different experimental conditions were considered by normalizing the anion signal intensity with respect to the partial pressures, as well as to the intensity of the incident electron current by using the following relation:

i_ion

i_Cl⁻_/CCl4 = σ_DEA,ion

σ_DEA,Cl⁻_/CCl4 × i_e,ion

i_e,Cl⁻_/CCl4 × N_t,ion N_t,Cl⁻_/CCl4 ×

l_ion

l_Cl⁻_/CCl4 (6.10) The interaction length between the electrons and the neutrals either from the sam-ple, l_ion, or calibrant,l_Cl⁻_/CCl4 is assumed to be the same. Furthermore, the target density N_t is directly proportional to the partial pressure value, P. The partial pressure of both sample, P_ion, and calibrant, P_CCl4, is defined as the difference between the working pres-sure and the base prespres-sure meapres-sured by a ionization gauge connected to the chamber. The target density of sample delivered by a capillary directly into the interaction region of the HEM is, however substantially higher than the target density of calibrant introduced in the chamber via the stagnant gas inlet. At the same incident electron current, and partial pressure of calibrant compound, through a comparison between the intensity of the ion yield obtained when the compound was introduced (i) via the stagnant gas inlet, or (ii) via the capillary; it was determined that the target density of neutrals may be increased by 25 times when the sample is introduced as in the latter case. Thus, this correction factor must be taken into account when calculating the cross section.

Absolute dissociative electron attachment cross sections, σ_DEA,ion, can be thus de-termined through the following relation:

σ_DEA,ion=σ_DEA,Cl⁻_/CCl4 × i_ion

i_Cl⁻_/CCl4 × i_e,Cl⁻_/CCl4 i_e,ion × 1

25 P_CCl4

P_ion (6.11) The experimental uncertainty of the cross section values of about one order of magnitude results from the several sources of experimental errors in the experiment. Namely, the estimation of the partial pressure ratio, the efficiency of ion extraction towards the QMS,

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6.1. Wippi

which in turn exhibits a non-constant transmission for different masses, and at last the channeltron detection efficiency.

Partial pressure ratio

The partial pressure ratio between the calibrant and the sample, P_CCl4/P_ion, was deter-mined from the pressure values measured with the ionization gauge connected to the cham-ber. In ionization gauges, the measured pressure value depends on the ion current, which in turn depends on the electron current emitted by the filament, as well as the ionization cross section of the gas, at electron energies of 70 eV, in the gauge [164]. Therefore, the error associated with the partial pressure ratio results from the difference between the ionization cross sections at 70 eV of either the calibrant compound, and of the sample of interest.[43]

Ion extraction field

In Wippi, a weak electric field extracts the ions from the HEM, in order to avoid extraction of ∼0 eV electrons. The electric field extracts, however less efficiently fragment anions which are formed with high kinetic energy release (KER) in respect to Cl^–/CCl₄ or F^–/SF₆ anions used for calibration. By simulating the extraction field and the geometry of the interaction region of a trochoidal electron monochromator (TEM), Engmann et al. [165]

have found that the extraction efficiency for fragment anions formed with KER values between 50 and 100 meV is reduced from about 30 up to 60% when compared to thermal ions, i.e. formed at rest (KER=0 eV). In Wippi, this effect, and how it contributes to the accuracy of the determined cross sections is yet to be investigated.

QMS transmission

The mass dependent transmission of the QMS can lead to a less efficient detection of ions with heavier masses. For positive ions, Engmann et al. [165] have reported a lin-ear dependency between transmission through the QMS and mass. This was achieved by normalizing the ion counts obtained for the formation of N₂⁺/N₂, Ar⁺/Ar, Kr⁺/Kr and Xe⁺/Xe against the well-known ionization cross sections for which case. To the best of my

6.2. Experimental Setup - Notre Dame University

knowledge, the mass dependency of QMS transmission for negative ions was not investi-gated so far. Therefore, in this context, it was considered a constant transmission for the different fragment anions. Consequently, the exact contribution of the QMS transmission to the inaccuracy of the determined cross section values is yet to be estimated.

Channeltron detection efficiency

For a given incident particle, the channeltron detection efficiency is defined as the proba-bility of this particle or photon producing an output pulse. In the case of the channeltrons manufactured by Dr. Sjuts Optotechnik GmbH, detection efficiency for electrons, positive ions and UV light have been investigated.[130] However, to the best of my knowledge, the mass dependency for both positive and negative ions on the detection efficiency was not investigated so far. In this thesis, the channeltron detection efficiency for different anions was considered to be constant.

6.2 Experimental Setup - Notre Dame University

At the Radiation Laboratory at the University of Notre Dame, the experiments were carried out in a crossed electron-molecule beam apparatus coupled with a QMS (HAL/3F PIC Hiden Analytical, Inc.) operated in RGA (residual gas analyzer) mode, as shown in figure 6.10.[166] An electron impact source is installed at the front of the QMS, as represent in figure 6.11. A description of the mode of operation of QMS was already provided in the section 6.1.4.

In the electron impact ion source, the electron emission occurs from one of two filaments made of oxide coated iridium. The incident electron current, i.e., the current of electrons which pass the grid, is set by the software and it may range from 0.2 µA up to a few µA. For the studies carried out with benzaldehyde (see section 7.2), a constant incident current of 2 µA was used. The positive bias voltage,V_cage, is applied to the cage and to the filament power supply (Vf ilament) to simultaneously attract the electrons to the cage, as well as to repel them from the grounded side walls. This cage can transmit the electrons, because it is made of a fine metal mesh. The ions are thereby formed within the

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6.2. Experimental Setup - Notre Dame University

cage due to interactions with the electrons emitted by a heated filament. The electrons are accelerated to a given kinetic energy by a positive voltage drop,V_electron, between the cage and the filament. The electron energy scale was calibrated by measuring the well-known resonances of SF₆^– and F^– from SF₆, see section 6.3. The electron energy resolution was estimated to be approximately 500 meV (FWHM) for an incident electron current of 2µA.

There is also a focus lens right before the QMS entrance.

Figure 6.10. Crossed beam setup at the Notre Dame University. Please note that the electron gun was not used in the present studies. Adapted from [167].

The installed QMS enables mass-analysis of ions with unit mass resolution up to m/z 300. A channel electron multiplier was used for ion detection (see section 3.3 for details on the working principle).

The base pressure of 10⁻⁹ mbar was achieved by a turbomolecular pump baked by a oil-free scroll pump. An ionization gauge was used to monitor to the pressure. Furthermore, the chamber was heated by a set of halogen bulbs, in order to reduce the adsorption of material on the colds surfaces of setup.

During a study, the vapor of a liquid sample or gas was introduced in the chamber by an external gas line coupled with a needle precision valve and a bellow-sealed valve.

On the vacuum-side, the sample’s vapor enters in the ion source of the QMS through a 1 mm-diameter stainless-steel capillary. Before performing the studies, the liquid samples