Laser spectroscopy transition probabilities

Although Dispersed Fluorescence (DF) spectroscopy is probably better classified as a form of double resonance spectroscopy, DF is discussed here because it is a form of emission spectroscopy where all of the emission originates from a single, laser-populated, upper electronic-vibrational-rotational level, (e, v, J ). A DF spectrum typically contains two [R J" = J — 1), P(J" = J + 1)] or three [i ( J — 1), Q(J ), P J +1)] rotational transitions per electronic-vibrational e",v" level. Often there is a progression of vibrational bands, [ v, v" = n), (v, v" = n + 1),. .. (v, v" = n + to)] where v" = n is the lowest vibrational level (band farthest to the blue) and v" = n + m is the highest vibrational level observable (limited either by the detector response or Franck-Condon factors) in the DF spectrum (see Fig. 1.8 and Fig. 1.15). 

Absorption.—The studies reported here are conventional one-photon absorption studies on ground-state molecules and atoms. The use of a rapidly tuneable CW dye laser for direct absorption spectroscopy has been described.98 Transition probabilities in the spectra of Ne (I),97 and the classification of the 650.4 nm line of Xe97 have been discussed. Pressure-broadening coefficients for the atomic iodine 2P4-2Pj transition for C02, N2, He, Ne, Ar, Kr, and Xe have been measured as 7.4 0.7, 6.2 0.8, 3.6 0.3, 4.3 0.4, 5.1 0.5, 4.4 0.4, and 3.0 0.3, respectively.98 Measurements of the polarizabilities of alkali-metal atoms have been described,99 and hyperfine interactions in the excited states of sodium discussed.100 Excitation parameters in the vacuum-u.v. region... 

The primary object of Raman spectroscopy is the determination of molecular energy levels and transition probabilities connected with molecular transitions that are not accessible to infrared spectroscopy. Linear laser Raman spectroscopy, CARS, and hyper-Raman scattering have very successfully collected many spectroscopic data that could not have been obtained with other techniques. Besides these basic applications to molecular spectroscopy there are, however, a number of scientific and technical applications of Raman spectroscopy to other fields, which have become feasible with the new methods discussed in the previous sections. We can give only a few examples. 

The book begins with a discussion of the fundamental definitions and concepts of classical spectroscopy, such as thermal radiation, induced and spontaneous emission, radiation power and intensity, transition probabilities, and the interaction of weak and strong electromagnetic (EM) fields with atoms. Since the coherence properties of lasers are important for several spectroscopic techniques, the basic definitions of coherent radiation fields are outlined and the description of coherently excited atomic levels is briefly discussed. 

The first experiments on Doppler-free two-photon spectroscopy were performed on the alkali atoms [7.43-7.47] because their two-photon transitions can be induced by cw dye lasers or diode lasers in convenient spectral ranges. Furthermore, the first excited P state is not too far away from the virtual level in Fig. 7.26b. This enlarges the two-photon transition probabilities for such near-resonant transitions. Meanwhile, there are numerous further applications of this sub-Doppler technique in atomic and molecular physics. We shall illustrate them by a few examples only. 

In the proposed book there is an emphasis cm luminescence lifetime, which is a measure of the transition probability and non-radiative relaxation from the emitting level. Luminescence in minerals is observed over a time interval of nanoseconds to milliseconds. It is therefore a characteristic and a unique property and no two luminescence emissions will have exactly the same decay time. The best way for a combination of the spectral and temporal nature of the emission can be determined by time-resolved spectra. Such techniques can often separate overlapping features, which have different origins and therefore different luminescence lifetimes. The method involves recording the intensity in a specific time window at a given delay after the excitation pulse where both delay and gate width have to be carefully chosen. The added value of the method is the energetic selectivity of a laser beam, which enables to combine time-resolved spectroscopy with powerful individual excitation. 

Fundamental quantities, such as wavelengths and transition probabilities, determined using spectroscopy, for atoms and molecules are of direct importance in several disciplines such as astro-physics, plasma and laser physics. Here, as in many fields of applied spectroscopy, the spectroscopic information can be used in various kinds of analysis. For instance, optical atomic absorption or emission spectroscopy is used for both qualitative and quantitative chemical analysis. Other types of spectroscopy, e.g. electron spectroscopy methods or nuclear magnetic resonance, also provide information on the chemical environment in which a studied atom is situated. Tunable lasers have had a major impact on both fundamental and applied spectroscopy. New fields of applied laser spectroscopy include remote sensing of the environment, medical applications, combustion diagnostics, laser-induced chemistry and isotope separation. 

P. Hartmetz, H. Schmoranzer Lifetime and absolute transition probabilities of the 2PiQ ( Sj) level of Nel by beam-gas-dye laser spectroscopy. Z. Physik A 317, 1 (1984)... 

There are, however, several advantages to using TDLs for measurement of gas-phase flame species. These include high resolution (typically better than lxl0 cm" ), good spatial resolution (200 to 1 mm), reasonable output power ( 1 mW), and the ability to scan over their spectral range on a millisecond or better timescale. Probably the most widely studied molecular flame species by tunable diode laser spectroscopy is CO. In addition to the reasons for study outlined above in the discussion of broadband source methods, CO possesses several fundamental (v = 0-l) and hot-band transitions (v = 1-2, V = 2-3) which occur within several line widths (approximately 0.05 cm" ) of each other. At room temperature, populations of states from which hot-band transitions occur are very low. However, at flame temperatures, populations of vibrational states other than the v = 0 state may become appreciable. When temperatures (and also species concentrations) are calculated from simultaneous measurement of a fundamental and a hot-band transition, the technique is referred to as two-line thermometry. 

Optogalvanic spectroscopy is an excellent and simple technique to perform laser spectroscopy in gas discharges. Assume that the laser beam passes through part of the discharge volume. When the laser frequency is tuned to a transition E. - E between two levels of atoms or ions in the discharge, the population densities (E. ) and n (E ) are changed by optical pumping. Because of the different ionization probabilities from the two levels, this population... 

The application range of laser-induced fluorescence (LIF) extends from the assignment of molecular spectra and the measurements of molecular constants, transition probabilities, and Franck-Condon factors to the study of collision processes or the determination of internal state populations in the reaction products of chemical reactions. Another aspect of LIF concerning sensitive detection of small concentrations of absorbing molecular components was discussed in Sects.8.2-5. Let us first briefly consider the relevance of LIF to molecular spectroscopy. 

In the higher atmosphere the aerosol density decreases rapidly with altitude and other detection schemes may become more advantageous. Raman spectroscopy or detection of laser-induced fluorescence excited by frequency-doubled pulsed lasers has been utilized [14.22]. Both Raman and fluorescence intensities excited by the laser at a location x are proportional to the density n. (x) of scattering particles. However, because of the high pressure (p latm) the fluorescence is quenched if the collisional deactivation na v becomes faster than the spontaneous decay A. = 1/t. (see Sect. 12.2). Transition probabilities and quenching cross sections must therefore be known if quantitative results are to be obtained from measurements of the fluorescence intensity. 

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