Atomic systems fluorescence intensity

To show this, we consider the fluorescence intensity detected at a point R in the far-field zone of the radiation emitted by the atomic system. The intensity is proportional to the first-order correlation functions of the atomic dipole operators as [7,8]... 

An alternative approach is to produce known concentrations of O, Br, or I atoms, using rapid reactions of established stoicheimnetry. This ai Hoadi is useful, for instance in calibrating Br-atom resonance fluorescence intensity, where the concentration of Br atoms in the system may be varied simply by altering the flow rate of Bt2 added to an excess concentration ( 10 cm ) of O i atoms. The reactions occurring are both fast and give the overall stoicheiometry ... 

Heavy atom enhancement of intersystem crossing has been used to determine the mechanism of acridine photoreduction in ethanol.115 It was found that addition of sodium iodide decreased the fluorescence intensity and the rate of disappearance of acridine to the same extent, confirming that the singlet state is responsible for photoreduction. From the increase in triplet state absorption upon addition of iodide it was found that Of for acridine was 0.76. Thus the short singlet lifetime (0.8 nsec) of acridine is due to rapid inter-system crossing to unreactive triplet states. 

This represents a formidable practical problem, as one is very unlikely to find isolated atoms with two nonorthogonal dipole moments and quantum states close in energy. Consider, for example, a V-type atom with the upper states 11), 3) and the ground state 2). The evaluation of the dipole matrix elements produces the following selection rules in terms of the angular momentum quantum numbers J — J2 = 1,0, J3 — J2 = 1,0, and Mi — M2 = M3 — M2 = 1,0. Since Mi / M3, in many atomic systems, p12 is perpendicular to p32 and the atomic transitions are independent. Xia et al. [62] have found transitions with parallel and antiparallel dipole moments in sodium molecules (dimers) and have demonstrated experimentally the effect of quantum interference on the fluorescence intensity. We discuss the experiment in more details in the next section. Here, we point out that the transitions with parallel and antiparallel dipole moments in the sodium dimers result from a mixing of the molecular states due to the spin-orbit coupling. 

Thus, at low intensities of the squeezed vacuum field (N atomic system. For large intensities (N > 1), the thermal fluctuations of the atomic dipoles dominate over the squeezed fluctuations, resulting in a reduction of squeezing in the fluorescence field. 

Potassium iodide (KI) is commonly used as fluorescent quenching agent because of the heavy atom effect. Therefore, it has wide applications in the study of the interaction mechanism between fluorescence probes and biomacromolecules. But in the study on the interaction mechanism of morin-fish serum nucleic acid system,10 it was found that KI can enhance the fluorescence intensity and lifetime of a system. 

The first reason for the enhancement effect was the formation of the more favorable structure for luminescence. The studies on the effect of different halogen anion compounds (KC1, KBr and KI) on the morin-hsDNA systems were conducted. The results showed that at the same concentrations and conditions (<1.0x10 3mol/L), KCI weakened the fluorescent intensity of the system, while both KBr and KI enhanced it, and the enhancement order was KI > KBr. It is proposed that the lone electron pair of electronegative I (or Br ), bonds to the conjugated system of morin, and that the combined system then interacted with fsDNA. With the increase of atomic radius from Br to I, the attraction between nucleus and outer electrons of... 

The fluidity in the neighborhood of probe molecules can be tested by use of probes capable of intramolecular excimer formation. The probe molecules contain the two excimer-forming moieties linked by an alkyl chain. The extent of excimer formation depends on the viscosity of the environment and can be monitored by measuring the excimer/monomer fluorescence intensity ratio. The dependence of this ratio on reciprocal viscosity for the probe molecule dipyrenylpropane is shown in Fig. 18, in which the obtained microfluidities for surfactant systems are indicated. The fluidities decrease in the order SHS microemulsion, SDS, CTAC, Triton X-100 cf. Ref. 167 (for abbreviations see Tables 6 and 7). The same sequence order was found by Kano et al. (68). In systems containing heavy counterions the method leads to data that must be evaluated carefully, since heavy atom interactions may be different with excited monomers and excimers. The intramolecular excimer technique is also useful in biological studies. For instance, Almeida et al. investigated the sarcoplasmic reticulum membrane in which the activity of the Ca -pumping enzyme is modulated by the membrane fluidity (197). 

Since the dissociation time is very short compared to the lifetime of the excited sodium atom Na (3p), the dissociating (Nal) emits nearly exclusively at the atomic resonance fluorescence. The atomic fluorescence intensity /piCNa, At), monitored in dependence on the delay time At between the pump-and-probe pulse, gives the probability for finding the excited system [Nal] at a certain internuclear separation R, where Vi(/ ) — V2(R) = ho)2 (Fig. 6.99b). 

Atomic and molecular species identification via motional resonance-based detection is also possible in more complex systems, namely in large multispecies ion crystals of various size, shape, and symmetry [47,52,70] (Figure 18.19). The basic principle of the method is as follows. The radial motion of the ions in the trap is excited using an oscillating electric field of variable frequency applied either to an external plate electrode or to the central trap electrodes. When the excitation field is resonant with the oscillation mode of one species in the crystal, energy is pumped into the motion of that species. Some of this energy is distributed through the crystal, via the Coulomb interaction. This, in turn, leads to an increased temperature of the atomic coolants and modifies their fluorescence intensity, which can be detected. 

When atoms, ions, or molecules in a fast beam are excited and the fluorescence intensity is monitored as a function of the distance z downstream of the excitation point, the time resolution At = Az/v is determined by the particle velocity v and the resolvable spatial interval Az from which the fluorescence is collected [12.37]. In this case, detection systems can be used that integrate over the intensity and measure the quantity... 

In competition with radiationless deactivation, energy can also be lost in the form of radiation (F, P) [3], [28], Fluorescence occurs with molecules that either (1) have extended n-systems (such as polycondensed aromatics) (2) do not permit deactivation by torsional or rotational motion of parts of the molecule or (3) have no heavy atoms as substituents [32], 33]. In addition to these molecular properties, the environment also plays a part. Thus, the fluorescence intensity increases at low temperature and in solid matrices. This is even more important in phosphorescence, where the T] -> So transition is in fact spin-forbidden. If a higher vibrational level (v >0) is occupied in Ti, in accordance with the Boltzmann equation (Eq. 5),. so-called delayed fluorescence [28] can occur by backward intersystem crossing [T, S,(b = 0) S ]. 

The principles of the laser-excited atomic fluorescence (LEAF) technique are very simple. A liquid or solid sample is atomized in an appropriate device. The atomic vapor is illuminated by laser radiation tuned to a strong resonance transition of an analyte atom. The excited analyte atoms spontaneously radiate fluorescence photons and a recording. system registers the intensity of fluorescence (or total number of fluorescent photons). The extremely high spectral brightness of lasers makes it possible to saturate a resonance transition of an analyte atom. Therefore, the maximum fluorescence intensity of the free analyte atoms can be achieved while the effect of intensity fluctuations of the excitation source are minimized. Both factors provide the main advantage of LEAF— extremely high sensitivity. The best absolute detection limits achieved in direct analysis by LEAF... 

The atom is in an excited state after an absorption event, with one of the core electron levels left empty (a so-called core hole), and a photoelectron is emitted. The excited state will eventually decay typically within a few femtoseconds after the absorption event. Two main processes occur for the decay of the excited atomic state following an X-ray absorption event (Figure 6.2). The first is X-ray fluorescence, in which a higher energy core-level electron fills the deeper core hole. The fluorescence energies emitted in this way are characteristic of the atom, and can be used to identify the atoms in a system and quantify their concentrations by the fluorescence intensity. For example, an L shell electron dropping into the K level gives the Ka fluorescence line. 

There are many situations in which the X-ray absorption spectrum is most easily measured (indirectly) by monitoring the fluorescence produced following absorption of X-rays. One measures variations in the fluorescence intensity of a particular atomic species as the energy of incident photons is varied over an absorption edge of a selected element. This fluorescence excitation spectrum , distinct from the fluorescence spectrum, provides an indirect measurement of the X-ray absorption coefficient, albeit one that is subject to several well-known instrumental effects. If care is taken in sample preparation, systematic errors can be minimized. Fluorescence detection is the method of choice for dilute systems, because it provides an improved signal-to-noise ratio. 

The general expression for the intensity I of fluorescence with polarization vector I2 following excitation by absorption of light with frequency w and polarization vector is given by the Breit formula [2.21]. Let a = (a", J", M") be the initial state of the atomic system, b = (a , J, M ) the intermediate state and c = (a, J, M) the final state of the absorption-emission sequence a-> b-> c. We then obtain... 

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