Metastable atom excitation spectroscop

Electron spectroscopic techniques are based on determination of the energy distribution of electrons released in the ionization process. Two of these techniques became very popular among chemists and molecular physicists, namely photoelectron spectroscopy (PES) and X-ray electron spectroscopy also termed electron spectroscopy for chemical analysis (ESCA) . Penning ionization electron spectroscopy (PIES) is related to PES, but the target molecule is ionized by electronically excited (metastable) atoms of a noble gas, mostly He, Ne and Ar instead of the photons used in PES. PIES is not such a widely used technique as PES and ESCA, but probably the most attractive one for vdW molecules. 

The development of techniques to prepare well-characterized targets of species in excited states (long-lived metastable atoms and molecules, electronically excited atoms and molecules, and vibrationally excited, hot molecules) for electron and ion collisions, including attachment and dissociative attachment studies and spectroscopic measurements. 

In testing the capability of the mass spectrometer to detect metastable components, we excited helium in a discharge and looked for metastable He(2 S) atoms. The ionization curve in Figure 13 shows the presence of metastable He atoms. A rough value of the ionization potential obtained from these data was 5 e.v., which correlates with the spectroscopically calculated ionization potential of He(23S) atoms of 4.77 e.v. In order to observe these atoms it is necessary to maintain the discharge close to the sampling orifice, indicating very rapid destruction of the metastables by wall collision. 

The recombination process is governed by the same selection rules as spectroscopic transitions. Let us consider the recombination of a carbon ion in its metastable state, Is 2s 2p This means that we have to study the spectroscopic transition from the highly excited atomic state Is 2s 2p ( P)nx, where n is a large number and x is s, p, d, f,..., to one of the low states of the carbon atom. The ground state is Is 2s 2p P and the other low states are Is 2s 2p and and Is 2s 2p As the highly excited atom is in a triplet or quintet state, only transitions to P or are allowed. Therefore the recombination of the metastable carbon ion must leave the carbon atom in the P or state. The selection rules have been discussed in more detail earlier. ... 

In our laboratory a series of laser-microwave studies has been performed on metastable and short-lived excited states of heliumlike Li. Singly ionized lithium, like all members of the two-electron He isoelectronic sequence, belongs to the fundamental systems in atomic physics. Many of its spectroscopic and quantum-mechanical characteristics have been calcu-... 

Detection of atoms and molecules which have transitions in the vacuum UV region is a serious challenge for any spectroscopic technique. One way to solve the problem is excitation of a species, not from the ground state, but from a higher metastable state which can be efficiently populated by electron impact in a gas discharge. The population of a metastable level by electron impact followed by LEAF detection was successfully applied for the detection of Ne and N2 traces in He, and for detection of NO and NO2 traces in He, Ar, N2. and air. LODs of ca. 10" - 10 vol % were obtained. These LODs are 100- 1000 times better as compared with any traditional analytical technique. 

The observation of optical emission from electronically excited reaction products (chemiluminescence, CL) and the formation of ions in encounters between neutrals (chemiionization, Q) provide many beautiful examples of branching among the multitude of accessible states. Beam-gas or cross beam arrangements have been used, the reactants usually being atoms or atomic ions (in their ground or metastable states) plus diatomic target molecules. CL is highly product specific thanks to the spectroscopic detection, in Cl mass analysis is sometimes employed. 

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