Ultraviolet electron line excitation

The intensity ratios of recombination lines are almost independent of temperature. On the other hand, intensity ratios of optical and ultraviolet collisional lines are strongly dependent on electron temperature if the excitation levels differ. 

Such power levels can be quite adequate for Doppler-free saturation spectroscopy, as demonstrated in a series of recent studies of ultraviolet transitions of neutral helium in our laboratory at Stanford. (35-37) Fig. 5 shows as an example a spectrum of the 2 S - 5 P transition of He near 294.5 nm, recorded by intermodulated fluorescence spectroscopy. The ultraviolet radiation was generated by a yellow cw ring cavity dye laser with cavity-enhanced external ADA (ammonium dihydrogen arsenate) frequency doubler. The absorbing metastable He atoms were produced by electron impact excitation of He gas at about 0.04 torr. The spectrum shows a cluster of resolved line components which could be assigned after the fine and hyperfine Hamiltonian had been diagonalized in an uncoupled representation. We were surprised to learn that the hyperfine structure of the 5 P state of this simple 3-body system had been neither measured nor calculated before. 

The study of gases has recently been facilitated in our laboratory by the completion of a new ESCA instrument, see Fig,21. It shows a side view of the new instrument for free molecules and condensed matter. The instrument is UHV-compatible and includes four different excitation modes monochromatized AlKa, monochromatized and polarized ultraviolet, electron impact and monochromatized electron impact. Two recent studies will be reported here 1) The resolution of the vibrational fine structure in the Cls core line of methane nd 2) Post collisional interaction in electron excited LMM Auger electron emission from argon >. ... 

The question now is, what role do the K, L, M,. . . electrons play in generating the K, L, M,. . . series The answer is not obviously predictable from a knowledge of visible or ultraviolet spectra. Neither hydrogen nor helium has a K series, although each has K electrons. Why Because the K series is generated only when the K shell contains a hole that is filled by an electron that leaves one of the outer (L, M,. . . ) shells or the generation of the K series requires (1) the absence of a K electron, (2) the presence of an outer-shell electron whose transition to the K shell is permitted by the selection rules. This picture explains why—no matter what the method of excitation—all K lines have the same excitation threshold so that all K lines appear together if they appear at all. 

The discovery of two other series of emission lines of hydrogen came later. They are named for their discoverers the Lyman series in the ultraviolet range and Paschen series in the infrared region. Although formulas were devised to calculate the spectral lines, the physics behind the math was not understood until Niels Bohr proposed his quantized atom. Suddenly, the emission spectrum of hydrogen made sense. Each line represented the energy released when an excited electron went from a higher quantum state to a lower one. 

Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy (Figure 4). The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band. 

The different series are shown in the energy level diagram below, which is not drawn to scale. These three series are named after their discoverers. The lines due to excited electrons falling back to the ground state (n = 1) are seen in the ultraviolet region. This is because the energy involved corresponds to wavelengths that are shorter than 400 nm. 

Many molecules which have absorption bands in the wavelength region of existing laser lines can be excited by absorption of laser photons into single isolated rotational-vibrational levels of the electronic ground state 1W>-103) (jn the case of infrared laser lines) or of an excited electronic state (with visible or ultraviolet lines)... 

All the above-mentioned experiments dealt with vibrational excitation of molecules by infrared laser lines. Inelastic collision processes in excited electronic states of molecules can be investigated in a similar way by means of visible or ultraviolet laserlines. 

There are numerous needs for precise atomic data, particularly in the ultraviolet region, in heavy and highly ionized systems. These data include energy levels, wavelengths of electronic transitions, their oscillator strengths and transition probabilities, lifetimes of excited states, line shapes, etc. [278]. 

Lyman Series This series is formed when excited electrons in a hydrogen atoms fall from a higher energy level to first level. This series of lines is observed in ultraviolet region. 

Here v and //, the frequency and transition dipole moment, refer to a specific transition, but a given level can usually decay by spontaneous emission to a number of different levels. However if we take the frequency to be 10 GHz, and the dipole matrix element to be 3 x 10 V) Cm, we obtain a natural line width from (6.350) of 10-9 Hz. In the microwave region, therefore, this contribution is negligible, but in the near ultraviolet the natural line width of an excited electronic state is of the order 1 MHz, unless the state is metastable. 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

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