Stark line broadening

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). 

Figure 10.20. Field strength distribution in the cathode plasma sheath above a bare and a diamond-coated Si substrates for Kh = —200 V. The field strength was evaluated from the Stark splitting (broadening) of the Hoi and Hp lines [234].

Figure 6. The first single-molecule optical spectra, showing use of the FM/Stark technique for pentacene in />-terphenyl. (a) Simulation of absorption line with (power-broadened) linewidth of 65 MHz. (b) Simulation of FM spectrum for (a), com = 75 MHz. (c) Simulation of FM/Stark line-shape, (d) single-molecule spectra at 592.423 nm, 512 averages, 8 traces overlaid, bar shows value of 2o)m = 150 MHz. (e) Average of traces in (d) with fit to the in-focus molecule (smooth curve), (f) Signal far off line at 597.514 nm. (g) Traces of SFSatthe O2 line center, 592.186 nm. After Ref. 1.

If collisions with electrons and ions occur, as in discharge lamps. Stark broadening, due to the strong electrical fields experienced by the atoms during collisions, will also contribute to the total line broadening. 

One of the simplest examples of line interference is impact broadening of H atom La Stark structure, observed in plasmas [176] (Fig. 4.1.(a)). For a degenerate ground state the impact operator is linear in the S-matrix ... 

In solid state lasers the fluorescence lines are broadened 26) by statistical Stark fields of the thermal vibrating crystal lattice and furthermore by optical inhomogenities of the crystal. The corresponding laser lines are accordinglyjlarge at multimode operation 22)... 

The autput of a mode-locked ruby laser 729) producing a train of pulses of 5 psec duration with a maximum peak power of 5 GW was focused into a cell pressurized with the sample gas. Pulse-energy conversion efficiencies into the Raman lines of up to 70 % have been obtained. The induced rotational lines are broadened this could be due to a strong optical Stark effect 730)... 

Stark broadening occurs in the presence of an electric field, whereby the emission line is split into several less intense lines. At electron densities above 1013 the field is relatively inhomogeneous, the splitting is different for different atoms and the result is a single broadened line. 

Fig. 6.2 Stark structure and field ionization properties of the m = 1 states of the H atom. The zero field manifolds are characterized by the principal quantum number n. Quasidiscrete states with lifetime r > 10-6 s (solid line), field broadened states 5 x 10 10 s < x < 5 x 10-6 s (bold line), and field ionized states r < 5 x 10 10 s (broken line). Field broadened Stark states appear approximately only for W > ITC. The saddle point limit Wc = -2 /E is shown by a heavy curve (from ref. 3).

To observe a 7s — 9 transition requires that there be a 9p admixture in the 9 state. For odd this admixture is provided by the diamagnetic interaction alone, which couples states of and 2, as described in Chapter 9. For even states the diamagnetic coupling spreads the 9p state to all the odd 9( states and the motional Stark effect mixes states of even and odd (. Due to the random velocities of the He atoms, the motional Stark effect and the Doppler effect also broaden the transitions. Together these two effects produce asymmetric lines for the transitions to the odd 9t states, and double peaked lines for the transitions to even 9( states. The difference between the lineshapes of transitions to the even and odd 9i states comes from the fact that the motional Stark shift enters the transitions to the odd 9( states once, in the frequency shift. However, it enters the transitions to the even 9( states twice, once in the frequency shift and once in the transition matrix element. Although peculiar, the line shapes of the observed transitions can be analyzed well enough to determine the energies of the 9( states of >2 quite accurately.25... 

For an accurate data analysis, a detailed understanding of systematic effects is necessary. Although they are significantly reduced with the improved spectroscopy techniques described above, they still broaden the absorption line profile and shift the center frequency. In particular, the second order Doppler shift and the ac-Stark shift introduce a displacement of the line center. To correct for the second order Doppler shift, a theoretical line shape model has been developed which takes into account the geometry of the apparatus as well as parameters concerning the hydrogen atom flow. The model is described in more detail in Ref. [13]. 

In 1996, Nemet and Kozma showed the emission spectrometry of gold laser-produced plasma to be of interest for analytical purposes a delay time of 800-1000 ns was found to ensure nni/-thermal equilibrium and thorough atomization in the plasma. The line profiles obtained under such conditions (both resonant and Stark-broadened) were fitted to a symmetric Lorentzian curve [170]. Recently, LIBS was used in combination with effective chemometric tools to develop a determination method for gold in homogeneous samples that allows the characterization of jewellery products. The results confirmed the LIBS technique as an effective alternative to the hallmark official methods [143,144,171]. 

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