Excitation Efficiency

Collisions between the Na atoms and molecules will redistribute the atomic velocities and fill the velocity hole created by radiation pressure. These collisions, however, are relatively infrequent. Therefore, ample time exists for a long laser pulse to push the atoms out of resonance before collisions can rethermalize the vapor. This loss in excitation efficiency can be overcome with... 

Figure 3 demonstrates the simplifications in the spectrum of an optimized laser pulse that can be achieved through the application of the sifting technique [see Fq. (7)]. The excitation efficiency of the pulse is only minimally reduced due to the additional restrictions imposed in the sifting procedure. The example used in this case is for a vibrational-rotational excitation process, H2(v = 0,7 = 0) H2(v =1,/ = 2). 

The fluorescent components are denoted by I (intensity) followed by a capitalized subscript (D, A or s, for respectively Donors, Acceptors, or Donor/ Acceptor FRET pairs) to indicate the particular population of molecules responsible for emission of/and a lower-case superscript (d or, s) that indicates the detection channel (or filter cube). For example, / denotes the intensity of the donors as detected in the donor channel and reads as Intensity of donors in the donor channel, etc. Similarly, properties of molecules (number of molecules, N quantum yield, Q) are specified with capitalized subscript and properties of channels (laser intensity, gain, g) are specified with lowercase superscript. Factors that depend on both molecular species and on detection channel (excitation efficiency, s fraction of the emission spectrum detected in a channel, F) are indexed with both. Note that for all factorized symbols it is assumed that we work in the linear (excitation-fluorescence) regime with negligible donor or acceptor saturation or triplet states. In case such conditions are not met, the FRET estimation will not be correct. See Chap. 12 (FRET calculator) for more details. 

S is the output gray value after the s.e. channel detector scaling (gs) of the sum of the fractions of donor fluorescence in the s.e. channel (FJ,) and of acceptor fluorescence in the s.e. channel (F) ). The donor fluorescence depends on Qd, the excitation efficiency at lfx (that is, Fe ), the number of donors (ND), and the population of donors that lose their energy by FRET (ENs-). The fluorescence of acceptors depends on QA, the amount of acceptor molecules (NA) excited with 2dx ( szsA) and on the amount of acceptor molecules excited by FRET (ENs, which is linear to Fefj). 

Relating back to Eq. (7.8) from the main text, the sensitized emission gray scale image / is composed of the emission from ENs, which depends on the acceptor quantum yield QA, scaled by factors for sensitized emission channel gain (gs), fraction of acceptor fluorescence in the sensitized emission channel F, and donor excitation efficiency tdxD ... 

A microwave-assisted, high-temperature, and high-pressure UV digestion reactor has been developed by Florian and Knapp [44] for analytical purposes. The apparatus consists of the immersed electrodeless discharge lamp operating as a result of the MW field in the oven cavity (Fig. 14.8). An antenna fixed to the top of EDL enhanced the EDL excitation efficiency. Another interesting MW-UV reactor has... 

The low qantum efficiency (about 10-8) therefore contradicts Kellogg s suggestions, due to the low excitation efficiency, which may be explained in one of two ways. 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

Riehl et al. also characterized the CL system lucigenin-hydrogen peroxide-A-methylacridone in the presence of different cationic surfactants such as HTAC, S-ClV-dodecyl-A lV-dimethylammonio) propane-1-sulfonate, and DODAB [41], Enhancement factors (ratio between CL intensity in the presence of organized medium and CL intensity in the absence of organized medium) of CL intensity were found of 3.4, 2.5, and 1.6, respectively. The alterations in CL intensity are explained in terms of the effect of the different surfactants on the rate of the reaction and on excitation efficiency. 

The same authors studied the CL of 4,4,-[oxalylbis(trifluoromethylsulfo-nyl)imino]to[4-methylmorphilinium trifluoromethane sulfonate] (METQ) with hydrogen peroxide and a fluorophor in the presence of a, p, y, and heptakis 2,6-di-O-methyl P-cyclodextrin [66], The fluorophors studied were rhodamine B (RH B), 8-aniline-l-naphthalene sulfonic acid (ANS), potassium 2-p-toluidinylnaph-thalene-6-sulfonate (TNS), and fluorescein. It was found that TNS, ANS, and fluorescein show CL intensity enhancement in all cyclodextrins, while the CL of rhodamine B is enhanced in a- and y-cyclodextrin and reduced in P-cyclodextrin medium. The enhancement factors were found in the range of 1.4 for rhodamine B in a-cyclodextrin and 300 for TNS in heptakis 2,6-di-O-methyl P-cyclodextrin. The authors conclude that this enhancement could be attributed to increases in reaction rate, excitation efficiency, and fluorescence efficiency of the emitting species. Inclusion of a reaction intermediate and fluorophore in the cyclodextrin cavity is proposed as one possible mechanism for the observed enhancement. 

Short-wave UV radiation is strongly absorbed by the Eu + ion raising this ion into its c.t. state. The ion then relaxes to the Do level from which luminescence occurs. An example of the second case is YTa04-Tbs+ which can be excited efficiently by UV radiation into the Af Sd state (38). 

Although the activation parameters obtained from the thermal decomposition of a great number of diverse dioxetane derivatives have been interpreted on the basis of the biradical mechanism, no general interpretation of the excitation efficiencies has been given on fhe basis of fhis mechanism Furfhermore, most theoretical... 

The reason two-photon excitation efficiency depends on the spectral phase of the laser pulse is beyond the scope of the peak intensity argument given above. It requires a more detailed consideration of how the pulse contributes to the nonlinear optical transitions. Let us consider, e.g., the case of two-photon excitation at wavelengths corresponding to = 400 nm. In addition to a pair of 800-nm photons, there are multiple combinations of spectral components within the bandwidth of an ultrashort pulse that can combine to cause excitation equivalent to 400 nm. Their wavelengths satisfy the relation particular, one can have photons at... 

As a hrst experiment, we wanted to return to the fundamental premise of TPM, which states that the efficiency of the two-photon excitation should be inversely proportional to the pulse duration. For TL pulses it is equivalent to having two-photon excitation efficiency proportional to the spectral bandwidth of the laser pulses. 

FIGURE 10.9 (a) Spatial conhnement of the excitation efficiency of higher-order nonlinear effects. The hrst-order fnnction represents the distrihntion of the tip-enhanced held intensity as shown in Figures 10.2 and 10.3. (h) Energy diagram of the CARS process. 

In the photolysis of ozone, only emission from 02(1A9) can be detected,70 and the absence of 02(1S,+) is now understood in terms of the considerable reactivity of 02(123+) with ozone61 (see Sect. V-B-l). has been detected in the vacuum UV photolysis products of 0277 although absolute measurements of the excitation efficiency have not, so far, proved practicable. The limits of sensitivity of the photomultiplier detector to 1.27 [x emission suggest that reaction (19) could proceed perhaps 20 times more rapidly than reaction (20) work is at present in progress in an attempt to measure k12 directly. 

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