Emission resulting from excited states

In the past two years, three new fluorescent protein variants have been generated that allow the selective activation or color conversion of fluorescent signal after specific illumination. The first variant, PA-GFP (photoactivatable GFP) [20] is based on wild-type Aequorea GFP, which has a bimodal absorption or excitation spectrum with two peak maxima, at 395 and 475 nm, corresponding to the protonated and the deprotonated states of the chromophore, respectively. When excited at 475 nm, wild-type Aequorea GFP emits maximal fluorescence at 503 nm, while excitation at 395 nm yields a maximum at 508 nm [2]. The latter large Stokes shift results from excited state deprotonation of the chromophore, as phenols become greatly more acidic in their excited states. Thus, excitation of the protonated chromophores gives emission at greater than 500 nm, similar to the direct excitation of the deprotonated chromophore. 

The basic research in our fields is now done largely in universities. It can have incredibly important practical results, but those results cannot normally be predicted in advance. Who would have thought that the basic study of induced energy emission from excited states of atoms and molecules that led to the laser would wind up giving us a better way to record music, or read supermarket prices Would a music company have funded that research Who would have thought that our increased understanding of the chemistry of life would have led to the creation of biotechnology as an entirely new industry The industry that benefited from the basic research could not have funded it, since it did not yet exist. 

The brilliant emissions resulting from the oxidation of certain oxalic acid derivatives, especially in the presence of a variety of fluorophores, are the bases of the most active area of current interest in CL. This group of chemiluminescent reactions has been classified as peroxyoxalate chemistry because it derives from the excited states formed by the decomposition of cyclic peroxides of oxalic acid derivatives called dioxetanes, dioxetanones, and dioxetanediones. 

Chemiluminescence is believed to arise from the 2Bj and the 2B2 electronic states, as discussed above for the reaction of NO with ozone [17]. The primary emission is in a continuum in the range =400-1400 nm, with a maximum at =615 nm at 1 torr. This emission is significantly blue-shifted with respect to chemiluminescence in the NO + 03 reaction (Xmax = 1200 nm), as shown in Figure 2, owing to the greater exothermicity available to excite the N02 product [52], At pressures above approximately 1 torr of 02, the chemiluminescence reaction becomes independent of pressure with a second-order rate coefficient of 6.4 X 10 17 cm3 molec-1 s-1. At lower pressures, however, this rate constant decreases and then levels off at a minimum of 4.2 X 1(T18 cm3 molec-1 s-1 near 1 mtorr, and the emission maximum blue shifts to =560 nm [52], These results are consistent with the above mechanism in which the fractional contribution of (N02 ) to the emission spectrum increases as the pressure is decreased, therefore decreasing the rate at which (N02 ) is deactivated to form N02. Additionally, the radiative lifetime and emission spectrum of excited-state N02 vary with pressure, as discussed above for the NO + 03 reaction [19-22],... 

Eximer Fluorescence. Since Forster and Kasper discovered concentration-dependent long-wavelength emission resulting from association of an electronically excited pyrene molecule with another ground state pyrene molecule,39 the phenomenon of excimer fluorescence has been studied extensively.40 The mechanism for excimer formation and emission can be represented by... 

Sulfur Dioxide. Both flame photometric and pulsed fluorescence methods have been applied to the continuous measurement of S02 from aircraft. In the flame photometric detector (FPD), sulfur compounds are reduced in a hydrogen-rich flame to the S2 dimer. The emission resulting from the transition of the thermally excited dimer to its ground state at 394 nm is measured by using a narrow band-pass filter and a photomultiplier tube. 

Electronically excited NO has been observed as a product of the reaction of ground state oxygen atoms with nitric oxide. In the first [447], the emission results from the two-body radiative recombination reaction... 

Figure 15 The kinetic scheme illustrating the interplay between exciton (S) and charge carrier (q) trapping by crystal defects (S0t)-The PL spectrum of the crystal contains the excitonic emission (kr, hvm) and the trap center emission (kj., hi ). the latter being controlled by the number of the defect sites available for excitation. The exciton capture process (yst) competes directly with charge carrier trapping (yqt). The defects filled with charge reduce the emission resulting from radiative relaxation of the excited states produced at defect sites. For further explanations, see text.

The photoluminescence of dipyridophenazine complexes of ruthenium ) in the presence and absence of DNA has been well-characterized (38-40, 46-52). Excitation of the dppz complexes with visible light (440 nm) leads to localized charge transfer from the metal center (39, 40). In aqueous solution, the emission resulting from the metal-to-ligand charge-transfer excited state is deactivated via nonradiative energy transfer... 

Figure 2 Experimental arrangement for measurements of the Fe nuclear resonance at the Advanced Photon Source (APS). In the standard fill pattern, electron bunches with a duration of 100 ps are separated by 153 ns. X-ray pulses are generated when alternating magnetic fields in the undulator accelerate these electron bunches. The spectral bandwidth of the X-rays is reduced to 1 eV by the heat-load monochromator and to 1 meV by the high-resolution monochromator. At the sample, the flux of the beam is about 10 photons/s. APD indicates the avalanche photodiode used to detect emitted X-rays. The lower right inset illustrates that counting is enabled only for times weU-separated from the X-ray pulse, so that only delayed photon emission resulting from decay of the nuclear excited state contributes to the experimental signal...

Another major chemical phenomenon related to ultrasonic cavitation is sonolumi-nescence, by which a tiny light is formed in a cool liquid. This form of light emission results from the high-temperature formation of reactive chemical species in excited electronic states. Emitted light from such states provides a spectroscopic probe for the cavitation effect. Some electrical and thermal theories on this phenomenon have been reported [25]. 

The figure below represents part of the emission spectrum for a one-electron ion in the gas phase. All the lines result from electronic transitions from excited states to the n — 3 state. 

---