Spectral lines Stark broadening

Furthermore, isotopic structure and hyperfine structure, and also resonance broadening, resulting from the interaction between radiating and non-radiating atoms of the same species, and Stark broadening, resulting from interaction with electrical fields, contribute to the physical widths of the spectral lines. 

Stark broadening occurs if there are a considerable number of ions and electrons in the excitation source. Excited atoms therefore are subject to strong electrical fields from nearby ions and electrons and may collide with them. Spectral radiation emitted under these conditions may be sufficiently affected to cause the line to broaden. Since the local electrical fields are continually changing and are not of uniform intensity, a broad, diffuse line results. This effect was observed experimentally by early spectroscopists and led to their identification of certain spectral line series as being diffuse and fundamental. ... 

The main applications covered in this study are the accurate determination of rotation and rotation-vibration molecular energies the determination of the molecular geometry of simple molecules the evaluation of force field and of the vibration- and rotation-vibration interactions the measurement of pressure broadening and pressure shift of the spectral lines the determination of electric dipole moments via laser-Stark spectroscopy the studies of intramolecular dynamics the calculation of rate constants, equilibrium constants and other thermodynamic data the evaluation of relaxation times. 

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

In order to measure an absorption cross section, it is necessary to have a source of tunable radiation (or spectrometer) that has a spectral resolution narrower than the width of the molecular line (natural and/or Doppler) being measured. Inadequate spectral resolution will yield a too small (lower bound) value of ° i- An effective strategy is to pressure broaden the molecular line so that it becomes broader than the instrumental resolution (see Stark, et al., 1992 and Murray, et al., 1994). 

As a host crystal a p-terphenyl crystal, with a few micrometers in diameter, has been used, which has been doped with a low concentration (10 ) of ter-rylene molecules. The tip of the optical fiber has been cooled to 1.4 K in order to avoid broadening of the fluorescence line via collective phenomena such as interaction with the phonons of the host crystal. The observed linewidth in fact is the natural linewidth (a few tens of MHz Fig. 9.9). The spectral response of fhese individual molecules can now be used for a local analysis of the surface, albeif with relatively low resolution (180 nm) (Fig. 9.10). Further experimental tricks such as electrical field induced Stark shifts can be applied to determine at least the position of the investigated molecules to within a few 10 nm. 

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