Atomic systems selective excitation

Efficient and selective excitation of electronic target states in atoms and molecules lies at the heart of photochemical applications (see corresponding references in Section 6.1) as well as quantum information processing [102, 103]. Here we demonstrate the potential of SPODS, introduced in the previous sections, for ultrafast electronic switching in a multistate model system. In the previous... 

Soep and co-workers then investigated this system using Ca-HCl complexes [47]. Here, p-orbital orientation is controlled by state-selective excitation of complexes in the vicinity of the Ca Pi - So atomic transition. 

In using atomic spectroscopy analysis the sample introduction is an extension to sample preparation. To understand the limitations of practical sample introduction systems it is necessary to reverse the train of thought, which tends to flow in the direction of sample solution > nebulisation > spray chamber > excitation > atomisation. An introduction procedure must be selected that will result in a rapid breakdown of species in the atomiser to give reproducible results irrespective of the sample matrix. In designing an FI A system to carry out atomic emission and to generate efficient free atom production for excitation the following criteria must be adhered to as closely as possible ... 

Consider the Menon-Agarwal approach to the Autler-Townes spectrum of a V-type three-level atom. The atom is composed of two excited states, 1) and 3), and the ground state 2) coupled by transition dipole moments with matrix elements p12 and p32, but with no dipole coupling between the excited states. The excited states are separated in frequency by A. The spontaneous emission rates from 1) and 3) to the ground state 2) are Tj and T2, respectively. The atom is driven by a strong laser field of the Rabi frequency il, coupled solely to the 1) —> 2) transition. This is a crucial assumption, which would be difficult to realize in practice since quantum interference requires almost parallel dipole moments. However, the difficulty can be overcome in atomic systems with specific selection rules for the transition dipole moments, or by applying fields with specific polarization properties [26]. 

It has been shown [31] that a system of two identical two-level atoms may be prepared in the symmetric state s) by a short laser pulse. The conditions for a selective excitation of the collective atomic states can be analyzed from the interaction Hamiltonian of the laser field with the two-atom system. We make the unitary transformation... 

Chemical lasers are pumped by reactive processes, whereas in photodissociation lasers the selective excitation of certain states and the population inversion are directly related to the decomposition of an electronically excited molecule. Photolysis has been the only source of energy input employed in dissociation lasers, although it appears quite feasible to use other energy sources, e.g. electrons, to generate excited states. Table 4 lists the chemical systems where photolysis produces laser action. It is appropriate to begin the discussion of Table 4 with the alkali-metal lasers since Schawlow and Townes in 1958 35> chose the 5 f> 3 d transitions of potassium for a first numerical illustration of the feasibility of optical amplification. These historical predictions were confirmed in 1971 by the experimental demonstration of laser action in atomic potassium, rubidium and cesium (Fig. 14). 

If in atoms a transition /) ) can be selected, which represents a true two level system (i.e., the fluorescence from A ) terminates only in /)), the atom may be excited many times while it flies through the laser beam. At a spontaneous lifetime r and a travel time T through the laser beam, a maximum of n = Tl 2x) excitation-fluorescence cycles can be achieved (photon burst). With T — 10 s and r = 10 s... 

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

The factors governing the stereochemistry of the photoaddition of aP-unsaturated ketones to olefins have been fully discussed by Wiesner, with examples from his own work. The stereochemistry of the product may be predicted by assuming that the p-carbon atom of the excited system is pyramidal, and can select a most stable conformation as judged by the principles of conformational analysis. The product is formed by reaction of the most stable conformer with the olefin. For example, the most stable conformation of the excited state of the yP-unsaturated ketone (125) may be considered to be (126), rather than (127), and it is the most stable form (126) which gives rise to the product (128), by 2 + 2 addition to allene. 

Compared with the early models of Black and Dalgarno (1977), there have been a few developments in the molecular data relevant to the excitation calculations. Although the collisional excitation rates for H + H2 and H2 + H2 are still uncertain, more accurate values for the latter system have been provided by Schaefer (1985). The rotational excitation rates of CO by H2 have recently been recomputed independently by Schinke et af. (1985) and Flower and Launay (1985), and show good agreement. Monteiro and Flower (1987) have reminded us of a neglected selection rule for atomic line structure excitation, which suggests that the J=0— 1 excitation of C and O by H2 is in first order forbidden. Finally, Chambaud et af. (1988) have computed rotational excitation cross sections for C2 by H2. 

The simplest systems in which the effect of a selective vibrational excitation can be studied are those of reactions of free atoms with vibrationally excited diatomic molecules. The various channels for removal of the vibrationally excited molecules BC (v) may be written as... 

FIGURE 12. Schematic representation of the initial relative velocity vector Vj j, the initial relative orbital angular momentum d, and the initially excited p orbital in the case of perpendicular excitation. Since the prepared p orbital has mL= l, it lies perpendicular to but is not necessarily perpendiciiar to d. Yet, although Z and are coincident prior to the collision, the azimuthal orientation of d is not restricted - a unique collision plane is not selected. Also, the initially prepared p orbital can be either perpendicular to or in the plane of collision. Thus the initially excited electronic wavefimction can be either symmetric or antisymmetric with respect to reflection in the initial plane of rotation of the two atom system - both e and f A-doublet states are excited. 

For selected atomic transitions, the upper level may decay only into the initial lower level (true two-level system). In this case the same atom can be excited many times during its flight time T through the laser beam. If its upper-state lifetime is r, the number of fluorescence photons emitted per second may be as high as T/t (photon burst), if the laser is intense enough to saturate the transition. 

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