Abstract In this chapter, we consider the factors that lead to complexity in atomic lines. We will learn that all spectral lines have a fundamental (natural) width and exist as a spread of frequencies rather than a single frequency. The width of atomic lines can also be affected by external factors such as pressure and laser power. This will be related to what we observe when probing an atom with a laser. In addition, we will learn about how this fundamental width is related to the lifetime of a state, how many photons per second an atom can absorb from a laser, and the two major types of experimental plots known as absorption plots and transmission plots.
Abstract In this chapter, we consider the factors that lead to complexity in atomic lines. We will learn that all spectral lines have a fundamental (natural) width and exist as a spread of frequencies rather than a single frequency. The width of atomic lines can also be affected by external factors such as pressure and laser power. This will be related to what we observe when probing an atom with a laser. In addition, we will learn about how this fundamental width is related to the lifetime of a state, how many photons per second an atom can absorb from a laser, and the two major types of experimental plots known as absorption plots and transmission plots.
The advent of lasers has revolutionized the methods and possibilities of spectroscopy. Their monochromaticity permits the study of extremely narrow spectral features which would not have been resolved using classical techniques. Moreover, their extreme spectral power density, in giving rise to the field of nonlinear optics, which will be studied in Chapter 7, has resulted in the development of new and powerful spectroscopic techniques. These techniques, of which we describe briefly some examples in this complement, have as their basis the fact that an atom does not respond linearly when subjected to a very intense light field. First of all we shall describe the broadening mechanisms that explain the fact that the experimentally measured widths of atomic or molecular resonances are larger than those that would be observed if the atoms were isolated and at rest. We shall introduce in Section 3G.1 the concept of homogeneous width, which can be considered as the intrinsic width of a transition, and inhomogeneous width which concerns the variation of the frequency of a given transition that occurs because of the state of motion of the atom, or because of factors in its environment. We shall explain why it is possible, using nonlinear optical techniques to eliminate the effects of inhomogeneous broadening on recorded spectra, resulting in an improvement over the resolution of conventional techniques. We shall then concentrate more specifically on Doppler broadening , which is the most commonly encountered source of inhomogeneous broadening, describing two nonlinear optical techniques, saturated absorption spectroscopy (in Section 3G.2) and two-photon spectroscopy (in Section 3G.3), which enable it to be overcome.
Backward, turn backward, O time in your flight. Make me a child again, just for tonight. –Elizabeth Akers Allen Since the time of Kirchhoff it has been known that, when light is passed through an atomic gas, those wavelengths are observed that would be emitted if the gas were incandescent. If the gas is sufficiently cold, then the wavelengths observed are limited to ground-state transitions. As the temperature of the sample is elevated, wavelengths corresponding to transitions between excited states become absorbing, and balances between emission and absorption occur. The study of the central wavelengths of emission lines or absorption notches is known as first-order spectroscopy, and it provides information on the energy-level structure of the atom. The study of the shape of these lines in frequency space is known as second-order spectroscopy, and provides information on the lifetime of the level and the collision rates and temperature of the gas. Thus, whereas first-order spectroscopy shows that emission and absorption measurements yield the same central wavelengths, second-order spectroscopy shows that the natural linewidth for emission and absorption are both specified by the level lifetime, and that the intensity of emission and absorption features both involve the linestrength factor (through the emission transition probability rate and the absorption oscillator strength). The connection between the lifetime and the linewidth can be made plausible by a simple semiclassical model. When an electron is excited to a specific orbit in an atom, its binding energy is established through the exchange of virtual photons with the effective central core. If the perturbations that eventually cause the electron to make a transition from that orbit are weak, the meanlife will be long.
This chapter shows that several non-linear features can be demonstrated and the line shape of sub-natural line width can be obtained by manipulation of the powers of the two interacting laser radiations. The interaction of the laser radiation can produce a coherent superposition of the atomic states such that atoms are trapped in one state so that the atomic system may be transparent even in the presence of radiation frequency that is on resonance. Saturation spectroscopy is more conveniently performed in three level atomic systems interacting with two laser radiations. One of the radiations, called pump beam, has higher intensity and frequency on resonance or close to resonance with the transition frequency across two levels of the atom. The second laser radiation, called probe beam, is weaker and interacts with two other energy levels, one of which is common to the pump transition. A few hyperfine transitions are within the envelope of one Doppler profile.
As light passes an atom, it exerts forces on the charges, electrons, and nuclei that alter the atomic structure. These may be slight distortions (perturbations) of the electron cloud or they may be more severe, as described in subsequent sections of this monograph. We wish to determine those changes, given the radiation field, or to devise a radiation field that will produce specified changes. The changes to the atomic structure also affect the radiation that subsequently passes the atom. To describe those effects we must consider wave equations for radiation in the presence of altered atomic structure. More generally we must find self-consistent equations for the atoms and the field together, as discussed in Chaps. 21 and 22. Here we consider mathematical descriptions of the influence of coherent radiation on individual atoms, molecules, or other single quantum systems. Individual atoms Traditional sources of emission and absorption spectra, though revealing energy states of the constituent atoms and molecules, are macroscopic samples. One observes averaged characteristics of many individual particles, see Chap. 16. Quantum theory offers the basic formalism for dealing with individual atoms exposed to controlled radiation fields. Several experimental techniques provide acceptable approximations to this ideal. The following paragraphs note some of these examples. Vapors Neutral atoms or molecules in a vapor move freely along straight-line paths, interrupted by brief collisions that redirect the two collision partners. When the kinetic energies of the two partners are small, there can be no transfer of kinetic energy into internal energy of either particle – the collision is elastic .
Summary form only given, as follows. We investigated the hidden alignment of Ne plasma in a magnetic field observing the optical characteristics of the medium-absorption and refraction. A tunable single frequency laser beam was used as a probe beam. Depending on the direction of the applied magnetic field and the geometry of observation, the dependence of the plasma birefringence or dichroism on the magnetic field was registered. The dye laser was tuned to /spl lambda/=607.4 nm (2p/sub 3/-1s/sub 4/ Ne transition). The dependence on the external magnetic field of the laser beam intensity passed through the Ne discharge cell was registered, for various laser frequencies in the limits of the absorption line. At the edge of the absorption line, the signal of the dichroism consists of two Lorentzian components while at the center of the absorption line the wide component is absent. The wide component width depends on neon pressure as: H(P)=(1.64/spl plusmn/0.03) gauss + P(torr) (0.33/spl plusmn/0.08) gauss/torr. This value is in a good agreement with other results concerning the lifetime of the 1s/sub 4/ state. The mechanism of alignment formation in this state is not clear. This state could not be aligned by light absorption following a quantitative calculation. The irradiation of atoms is isotropic within the limits of all the absorption line because of the high magnitude of absorption for the 1s/sub 4/-1s/sub 0/ transition. We consider the optical pumping as the most probable mechanism for alignment of this state. The magnitude of absorption on the s-p neon transitions are not high. The intensity of the spontaneous emission beam along the tube is greater than in the perpendicular direction. If the quantization axis coincides with the tube axis, this beam empties the magnetic sublevels with M=/spl plusmn/1, thus creating longitudinal alignment. The absence of a signal at the center of the absorption line can be explained by the high selfabsorption on the s-p transitions. The width of the narrow component of the signal is approximately 0.7 gauss. Its pressure dependence is small. We have established experimentally the absence of signals of such width in the spontaneous emission from 2p levels. Hence, all optical mechanisms of alignment transmission from higher or from lower states to the 1s/sub 4/ state are excluded. Apparently this signal arises from alignment of metastable state 1s/sub 5/. In our opinion the collisions are the most probable mechanism of alignment transmission.
In Chapter 3 the interaction Hamiltonian which determines the way light interacts with matter is derived. Simple perturbative analysis is applied to see if basic dynamics of atoms can be explained. The partial successes of perturbation theory are compared with predictions of an “exact” method of calculating the occupation probabilities of various atomic states. Even the “exact” method is shown to fail, establishing the need for improved approaches that yield correct results in later chapters. Some elementary results for the transition rates, including Fermi's golden rule are covered. Then the density matrix is introduced as a tool for describing not only the populations of atomic energy levels but coherence that can be created and lost during the dynamic evolution of atoms in time. A vector model based on the Bloch vector is presented as a useful way of picturing coherent atom–field interactions in an optical “spin” space, particularly multiple‐pulse interactions. Mechanisms are described that cause line‐broadening in optical spectroscopy, such as the Doppler effect. The chapter closes by showing that it is usually possible to reduce the complexity of real atoms on an experimental basis to facilitate direct comparisons with theoretical calculations based on simple models with only two or three energy levels.