H. Helm

Mass spectrometric determination of partial electron impact ionization cross sections of He, Ne, Ar and Kr from threshold up to 180 eV

A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section function thus obtained was compared with ionization tube measurements of the total ionization cross section curve. Good agreement with the data of Rapp and Englander‐Golden was found. This comparison allows the normalization of the measured relative partial cross section functions, yielding absolute partial cross section values for all of the processes mentioned above. The experimental results presented are compared with previous determinations where available.A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section...

Chemical recognition in terahertz time-domain spectroscopy and imaging

In this paper, we present an overview of chemical recognition with ultrashort THz pulses. We describe the experimental technique and demonstrate how signals for chemical recognition of substances in sealed containers can be obtained, based on the broadband absorption spectra of the substances. We then discuss chemical recognition in combination with THz imaging and show that certain groups of biological substances may give rise to characteristic recognition signals. Finally, we explore the power of numerical prediction of absorption spectra of molecular crystals and illuminate some of the challenges facing state-of-the-art computational chemistry software.

Far infrared properties of electro-optic crystals measured by THz time-domain spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO3, LiTaO3, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm−1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.

Electro-optic detection of THz radiation in LiTaO3, LiNbO3 and ZnTe

Freely propagating THz pulses are detected in electro-optic (eo) crystals by monitoring the phase retardation (PR) of an infrared femtosecond probe pulse. This technique permits the determination of the temporal shape of the THz pulse in the subpicosecond time domain. We present measurements in LiTaO3, LiNbO3, and ZnTe and compare their signal performance as eo crystals with theoretical calculations for the PR signal. ZnTe shows the best performance for eo detection.

Double ionization of van der Waals dimers: NeXe and ArXe

Simple consideration of atomic first and second ionization potentials show that for certain atomic combinations stable diatomic ions carrying two positive charges should exist below the lowest dissociation limit. One such ion NeXe++ has been observed experimentally by double ionization of the van der Waals dimer NeXe. Similar experiments also resulted in production of a long‐lived ArXe++ ion. The stability of these doubly charged molecular ions is rationalized using semiquantitative potential curves derived from the isoelectronic neutral molecules XeO and ICl. The appearance potentials found experimentally for both doubly charged molecular ions are consistent with the predictions from the potential energy curves. Some observations of other investigators on the kinetics of the low lying states of Xe++ in Ar and Ne are reinterpreted.

N(4S0), N(2D0), and N(2P0) yields in predissociation of excited singlet states of N2

Predissociation in several singlet valence and Rydberg states of molecular nitrogen has been investigated using photofragment spectroscopy. We report here measurements of the yields of the atomic fragment products N(4S0), N(2D0), and N(2P0) from predissociation of specific rotational levels in the b’ 1Σu+ (v=9–13), c’ 1Σu+ (v=3,4), c 1Πu (v=3,4), and o 1Πu (v=3) states of N2. These states are prepared by laser excitation from the metastable a‘ 1Σg+ (v=0) level in a fast (3 keV) molecular beam. Correlated atomic fragments from single molecular dissociation events are monitored using a position‐ and time‐sensitive detector to obtain a complete and sensitive scheme for all possible N2 dissociation products. Dissociation of the N2 states is found to occur to N(2D0)+N(4S0), and N(2P0)+N(4S0) products; production of N(4S0)+N(4S0) is found not to occur from any of the states investigated here. Branching of the dissociation products between the two active limits is found to be strongly correlated with the energy ...

Product branching in predissociation of the e 1Πu, e’ 1Σ+u, and b’ 1Σ+u states of N2

A measurement of the N(2D0) and N(2P0) yields in predissociation of the e 1Πu, e’ 1Σ+u, and b’ 1Σ+u states of molecular nitrogen is reported using photofragment spectroscopy. These states are prepared by laser excitation from the long‐lived a‘ 1Σ+g (v=0) level in a fast molecular beam. A position‐ and time‐sensitive detector is used to monitor the two correlated fragments produced by single molecular dissociation events. The translational energy imparted in the fragments establishes the electronic energy of the resulting nitrogen atoms and hence the branching among the available dissociation limits of nitrogen. Extremely pronounced rotational dependence in the branching yields is observed for the e and e’ members. The dependence mimics, in part, known molecular perturbation of the Rydberg states, and it reveals a complex competition among continuum states for the population in the Rydbergs.

Photodissociation measurement of rovibrational energies and populations of molecules in fast beams

Using as examples H2 and H+2, we show that position‐ and time‐sensitive detection of the two correlated photofragments can be used to determine the rovibrational energies and populations of molecules in a fast beam by observing the product distribution from bound–free photodissociation.

Photoexcitation and predissociation intensities of the c′ 1∑u+(v=3 and 4), c 1∏u(v=3 and 4), and b′ 1∑u+(v=10, 12, 13, and 15) states of N2

Photofragment spectroscopy is applied to investigate perturbations in the photoexcitation and predissociation of excited singlet states of N2. Discrete rovibrational levels in the Rydberg states c′ 1∑u+ (v=3 and 4) and c 1∏u (v=3 and 4) and in the valence state b′ 1∑u+ (v=10, 12, 13, and 15) are prepared by laser excitation from the metastable a″ 1∑g+ (v=0) state in a fast (3 keV) molecular beam. Fragment atoms produced by predissociation are monitored as a function of exciting laser frequency using a position- and time-sensitive detector to measure the photofragment intensity distribution in the rovibrational bands. The photofragment spectra show extreme departures from normal rovibronic intensity distributions due to strong perturbations in the two highly mixed complexes: c′(3)/c(3)/b′(10) and c′(4)/c(4)/b′(13). The measured photofragment intensities are compared to photoexcitation line strengths calculated using a comprehensive model of the Rydberg-valence state mixing [Stahel, Leoni, and Dressler, J. ...

Electron impact ionization of (N2)2: Appearance energies of N+3 and N+4

Using a molecular beam ionization mass spectrometer apparatus the appearance energies for the processes (N2)2+e→N+4+2e and (N2)2+e→N+3+N+2e were measured. From these measurements, the exothermicity of the ion clustering reaction N+2+N2 to form N+4 was deduced to be (0.9±0.2) eV and the lower limit for the dissociation energy of N+3 into N2(X 1∑+g)+N+(3P) was established to be (3.4±0.2) eV.