Electronics with molecules

The interaction of electrons with molecules gives molecular ions, some of which can break down to give smaller fragment ions. The collection of molecular and fragment ions is separated by a mass analyzer to give a chart relating ion mass and abundance (a mass spectrum). 

All Cl plasmas contain electrons with low energies, issued either directly from the filament but deactivated through collisions, or mostly from primary ionization reactions, which produce two low-energy electrons through the ionization reaction. The interaction of electrons with molecules leads to negative ion production by three different mechanisms [5] ... 

Similar problems are encountered in a description of elastic or rotationally inelastic collisions of the electrons with molecules that have permanent dipole moment. However in this case K is never zero because k0 and ki have different norms due to an energy transfer to the vibrational excitation. 

In the first half of the twentieth century, positive-ion molecule reactions and the interaction of hyperthermal electrons with molecules were emphasized. Some thermal electron molecule reactions in flames and electron swarms were investigated [3]. Prior to 1950 only the electron affinities of hydrogen and the halogen atoms had been measured. A 1953 review on electron affinities noted... 

This book is based on the study of reactions of thermal electrons with molecules using the ECD, negative ion chemical ionization (NICI) mass spectrometry in the gas phase and polarographic reduction in aprotic solvents [18]. Only the complementary studies related to our research are considered here. 

Lovelock often simply tried to measure the response of every volatile compound in the laboratory to characterize a detector. This method of inquiry was a Lovelock trademark later adopted by the Wentworth group. In the first half of the twentieth century few reactions of thermal electrons with molecules were studied. Some studies of electrons and ions in flames and electron swarms were conducted during this time. H. S. W. Massey described the work on negative ions in a series of monographs [3]. 

Steelhammer and Wiley began their research as undergraduate students. Steel-hammer, Han, Ristau, and Wiley eventually obtained their Ph.D. s at other universities. These students established the ECD model for obtaining fundamental information on the reactions of thermal electrons with molecules. 

In this chapter the experimental ECD and NIMS procedures for studying the reactions of thermal electrons with molecules and negative ions are described. Gas phase electron affinities and rate constants for thermal electron attachment, electron detachment, anion dissociation, and bond dissociation energies are obtained from ECD and NIMS data. Techniques to test the validity of specific equipment and to identify problems are included. Examples of the data reduction procedure and a method to include other estimates of quantities and their uncertainties in a nonlinear least-squares analysis will be given. The nonlinear least-squares procedure for a simple two-parameter two-variable case is presented in the appendix. 

The objective of the ECD and NIMS experiments is to measure the molar response of different compounds as a function of temperature. From these data the fundamental kinetic and thermodynamic properties of the reaction of thermal electrons with molecules and negative ions can be determined. The measurement is carried out in the same manner as the calibration of any detector. Known amounts of a compound are injected into the chromatograph and purified on a column, they then enter the detector. The response of the detector is normalized to the number of moles injected. When obtaining physical parameters, the detector temperature is changed and the procedure repeated. Since the molar response can vary by three to four orders of magnitude, the concentrations of the test molecule and the conditions in the detector at different temperatures must be taken into account. 

More than a century ago Thompson determined the mass-to-charge ratio of the electron and established its fundamental nature. It remains the only one of the subatomic particles that has not been subdivided. Simultaneously, Tswett initiated the study of modern chromatography. Fifty years later Lovelock observed that the reaction of molecules with thermal electrons greatly perturbed ionization currents generated by radioactivity in air. This led to the electron capture detector (ECD) and inextricably bound chromatography and the reactions of thermal electrons with molecules. 

In 1972 Wentworth, Chen, and Steelhammer set out to write a monograph entitled Negative Ions Reaction and Formation in the Gas Phase, so scientists could plan future research using the ECD. At the time few fundamental properties of thermal electron reactions had been measured. Now many molecular electron affinities and rate constants for thermal electron attachment have been measured. Currently electron affinities and bond dissociation energies can be verified using theoretical SCF calculations on desktop computers. It is especially timely to review the techniques for studying reactions of thermal electrons with molecules and to evaluate the results. 

This book is based on the reactions of thermal electrons with molecules. The ECD, negative-ion chemical ionization (NICI) mass spectrometry, and polaro-graphic reduction in aprotic solvents methods are used to determine the kinetic and thermodynamic parameters of these reactions. The chromatograph gives a small pure sample of the molecule. The temperature dependence of the response of the ECD and NIMS is measured to determine fundamental properties. The ECD measurements are verified and extended by correlations with half-wave reduction potentials in aprotic solvents, absorption spectra of aromatic hydrocarbons and donor acceptor complexes, electronegativities, and simple molecular orbital theory. 

The present book was partially inspired by a conversation the authors had with Dr. Alan Marchand. When reminded that 1997 was the 100th anniversary of the discovery of the electron, he laughed out loud. He noted that it was a bit presumptuous of modern scientists to state that the electron had been discovered. According to Marchand it is and always has been a fundamental particle, but indeed it remains the only member of the triad—the electron, proton, and neutron—that has not been further divided. Once we established that this groundwork had already been established, an interesting discussion of the ECD and its use in determining the fundamental properties of the reaction of thermal electrons with molecules evolved. Professor Marchand was unaware of the efforts in this field and believed that the only method of measuring electron affinities was powerful and expensive mass spectrometers. From this conversation came the impetus for our book. 

Can the present-day electronics based on sihcon and related inorganic semiconductor materials, in particular micro- to nanoelectronics, be extended in the not-too-distant future to include electronics with molecules which makes use of molecular functional units and applies their specific molecular properties. Can molecular devices be integrated into circuits based on silicon. The fascinating aspect of such ideas is a conceivable further miniaturisation beyond that possible with inorganic semiconductors, combined with the enormous variability which is offered by organic chemistry, and perhaps also the greater ease of development and fabrication of devices based on such materials. 

Ingolfsson, O., F. Weik and E. Illenberger. 1996. The reactivity of slow electrons with molecules at different degrees of aggregation Gas phase, clusters and condensed phase. Int JMass Spectrum Ion Process 155 ... 

The extensive experimental information available on mass spectrometry is very important to radiation chemistry since a mass spectrum reflects the probabilities for the formation of various fragment ions in collisions of electrons with molecules at moderate temperatures. 

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