Atomic spectra line spectrum

An atomic spectrum line 5536 ACyellow green) always appears clearly, but is not intensive enough to disturb the flame colour. A molecular spectrum band BaO appears in the flame which contains little or no chlorine or hydrogen chloride gas. On the contrary another molecular spectrum band BaCl appears with chlorine or hydrogen chloride gas. 

The discussion of emission spectroscopy will be concluded by a description of a rather unusual application. Bay and Steiner have measured atomic hydrogen concentrations in the presence of molecular hydrogen by microwave excitation of the atomic hydrogen line spectrum. With low power fed into the gas (ca. 5 watts), there is not enough energy available for the dissociation of molecular hydrogen and subsequent excitation. Thus the measured intensities of the atomic hydrogen lines correspond to the concentrations of atoms already present in the reaction mixture. The method is curiously similar to that adopted to detect atoms and free radicals by mass spectrometry (see Section 3). 

The Balmer series for hydrogen atoms—a line spectrum... 

The Brackett series lines in the atomic spectrum of hydrogen result from transitions from n > 4 to n = 4. 

To explain why hydrogen atoms emit the line spectrum, we must seek a model with the... 

Historically, the visible emission lines shown in Figure 15-3 were the first atomic hydrogen lines discovered. They were found in the spectrum of the sun by W. H. Wollaston in 1802. In 1862, A. J. Angstrom announced that there must be hydrogen in the solar atmosphere. These lines were detected first because of the lesser experimental difficulties in the visible spectral region. They are called the "Balmer series because J. J. Balmer was able to formulate a simple mathematical relation among the frequencies (in It S). The ultraviolet series shown in Figure 15-3 was... 

All atoms display line spectra. In general these spectra are much more complicated than the atomic hydrogen spectrum shown in Figure 15-3. Nevertheless, these spectra can be interpreted in terms of the concepts we have developed for the hydrogen atom. 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

This splitting of the energy levels by the magnetic field leads to the splitting of the lines in the atomic spectrum. The wave number v of the spectral line corresponding to a transition between the state /i mi) and the state nihm2) is... 

The atomic spectrum of singly ionized helium He+ with i = 4, 2 = 5, 6,. .. is known as the Pickering series. Calculate the energy differences, wave numbers, and wavelengths for the first three lines in this spectrum and for the series limit. 

It is important to note that we have tried to avoid carbon-rich stars, because they have a rich molecular line spectrum, mostly CN, CH and C2, obliterating many interesting atomic lines of rare elements. This is why we had in our sample a star, CS 31082-001, in which we were able to measure the 385.97 nm line of U II, whereas in the similar r-process element enriched star CS 22892-052, but carbon rich, a CN line obliterates the U II line. 

Eventually, other series of lines were found in other regions of the electromagnetic spectrum. The Lyman series was observed in the ultraviolet region, whereas the Paschen, Brackett, and Pfund series were observed in the infrared region of the spectrum. All of these lines were observed as they were emitted from excited atoms, so together they constitute the emission spectrum or line spectrum of hydrogen atoms. 

The idea of a transition between two energy levels suggests that the transition will occur at only one precise frequency as a sharp spike in the absorption or emission spectrum. This is not the case and, in fact, the transitions have an intrinsic width and shape containing information about the local environment of the atoms. The line profile of an atomic transition has contributions from three effects ... 

The spectrum of the Sun contains the absorption lines associated with the atomic spectrum of heavier elements such as Fe (Figure 4.2), which indicates that the Sun is a second-generation star formed from a stellar nebula containing many heavy nuclei. The atomic spectra of heavier atoms are more complex. The simple expression for the H atom spectrum needs to be modified to include a quantum defect but this is beyond the scope of this book. Atomic spectra are visible for all other elements in the same way as for H, including transitions in ionised species such as Ca2+ and Fe2+ (Figure 4.2). 

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