Laser excited atomic state

In view of the fundamental nature of the system more extensive and more precise data would be highly desirable as a test for bound state QED calculations. With the exception of the 1S-2S energy difference measurement, which is limited in precision by the laser wavelength measurement, all present experiments suffer from low statistics due to the small number of Ps atoms. It seems therefore worthwile discussing recent advances of increased Ps production and in parti-cular methods to populate the metastable 2 Sj state, which could serve as a basis for excited state spectroscopy. While laser excitation of states above n=2 from the ground state requires wavelengths between 205 nm (Lg) and 182 nm (ionization limit), which are very difficult to obtain with sufficient power, the corresponding wave-... 

Even this double-resonance spectroscopy has already been applied to the study of atomic transitions before lasers were available. In these pre-laser experiments incoherent atomic resonance lamps served as pump sources and a radio frequency field provided probe transitions between Zeeman levels of optically excited atomic states [509]. However, with tunable lasers as pump sources, these techniques are no longer restricted to some special favorable cases, and the achievable signal-to-noise ratio of the double-resonance signals may be increased by several orders of magnitude [510]. 

Similar measurements of d-f and d-g intervals in lithium Rydberg states by Cooke et yielded fine structure intervals in accordance with the hydrogenic theory. Ruff and co-workers measured n = 15-17 f-g intervals in Cs and determined values for the effective Cs dipole and quadrupole polarizabilities. To reach the Rydberg levels the first laser excited atoms from the 6si/2 ground state to the lp-i/2 state, from which they cascaded to the 5ds/2 level. The second laser then induced the 5ds/2-16/7/2 transition. This excitation technique was first used by Lundberg and Svanberg for lifetime measurements. ... 

Section 4 contains an analysis of the proper description of the state of the system which is initially prepared in a collision experiment involving a laser-excited atom. Here an adiabatic analysis will be used to point out several inadequacies in simple semiclassical treatment of these spin-changing transitions. Finally, section 5 is devoted to a presentation of the orbital-locking models of Hertel and co-woikers [10-12]. The insights gained in our more exact quantum treatment will be used to examine critically the validity of these models. A brief conclusion follows. 

The primary difficulty arises from the fact that although it is most convenient to carry out the scattering calculation in a coupled basis [section 2] in which the total angular momentum is a good quantum number, the wavefunction of two atoms at infinite separation can best be expressed in an uncoupled basis. To illustrate this point, consider the initial state of the diatomic (atom+laser-excited atom) prior to collision. Prior to the collision the relative oibital angular momentum d is always oriented peipendicular to the collision plane, in other words d is always perpendicular to the collision-frame z-axis, which, as discussed in section 3, is coincident with Vj i, the initial relative velocity vector. If the electric field vector of the pump laser, which defines the laboratoiy-fixed Z axis, is chosen to lie parallel to Vi i(Fig. 3), and if we consider a P<- S excitation process, then, as discussed in section 3, only the P =o(ij=l ttij=0>) atomic state is prepared [13-15,31]. Since Z and Vj i are coincident pnor to the collision, the collision-frame and laboratory-frame z-axes are identical This we shall refer to as parallel... 

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

Fig. 6.12 Level scheme with two stable atomic states pi) and q2) and a decaying excited atomic state e). By lasers the states pi) and q2) are coupled to e). The excited state amplitudes arising from the two lasers can interfere desrtuctively under certain circumstances. In this case no excitation and thus no spontaneous emission of photons takes place and we have created a dark state .

Pulsed lasers (Chapter 9) may be used both for photolysis and as a source. Since the pulses can be extremely short, of the order of a few picoseconds or less, species with comparably short lifetimes, such as an atom or molecule in a short-lived excited electronic state, may be investigated. 

An Xc2 excimer laser has been made to operate in this way, but of much greater importance are the noble gas halide lasers. These halides also have repulsive ground states and bound excited states they are examples of exciplexes. An exciplex is a complex consisting, in a diatomic molecule, of two different atoms, which is stable in an excited electronic state but dissociates readily in the ground state. In spite of this clear distinction between an excimer and an exciplex it is now common for all such lasers to be called excimer lasers. 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

A second simplihcation results from introducing the Born-Oppenheimer separation of electronic and nuclear motions for convenience, the latter is most often considered to be classical. Each excited electronic state of the molecule can then be considered as a distinct molecular species, and the laser-excited system can be viewed as a mixture of them. The local structure of such a system is generally described in terms of atom-atom distribution functions t) [22, 24, 25]. These functions are proportional to the probability of Ending the nuclei p and v at the distance r at time t. Building this information into Eq. (4) and considering the isotropy of a liquid system simplifies the theory considerably. 

A dilute I2/CCI4 solution was pumped by a 520 nm visible laser pulse, promoting the iodine molecule from its ground electronic state X to the excited states A,A, B, and ti (Fig. 4). The laser-excited I2 dissociates rapidly into an unstable intermediate (I2). The latter decomposes, and the two iodine atoms recombine either geminately (a) or nongeminately (b) ... 

Uranium enrichment using LIS has been exhaustively studied and the conceptual outlines of two different methods can be found in the open literature. These methods are multi-photon dissociation of UF6 (SILEX, or Separation of Isotopes by Laser Excitation) and laser excitation of monatomic uranium vapor (Atomic Vapor Laser Isotope Separation, or AVLIS). Following an enormous investment, AVLIS was used by the United States DOE in the 1980s and early 1990s, but due to the present oversupply of separated uranium, the plant has been shut down. 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

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