Collisional energy transfer quenching

Collisional energy transfer quenching. In this case, simultaneous exchange of two electrons occurs, requiring overlap of two pairs of orbitals, and orbital overlap criteria are more demanding than for electron transfer quenching [63] (Fig. 1.25) ... 

Nonradiative processes (knr) can occur with a wide range of rate constants. Molecules with high knr values display low quantum yields due to rapid depopulation of the excited state by this route. The measured lifetime in the absence of collisional or energy transfer quenching is usually referred to as To, and is given by to = (kr + knr). ... 

COLLISIONAL ENERGY-TRANSFER SPECTROSCOPY WITH LASER-EXCITED ATOMS IN CROSSED ATOM BEAMS A NEW METHOD FOR INVESTIGATING THE QUENCHING OF ELECTRONICALLY EXCITED ATOMS BY MOLECULES... 

The results obtained so far with collisional energy-transfer spectroscopy are restricted to excited sodium atoms A = Na(32/,3/2) and quenching by a variety of simple polar and nonpolar molecules. The technique is applicable to any vaporizable molecule and will be available for a number of other atoms as well in due course with the progress of laser technology. The E-V-R transfer processes from and to sodium atoms have a number... 

Collisional Energy-transfer Spectroscopy with Laser-excited Atoms in Crossed Atom Beams A New Method for Investigating the Quenching of Electronically Excited Atoms by Molecules... 

Free radicals readily quench phosphorescence. This phenomenon may be due to a variety of factors. Explain in terms of short-range collisional energy transfer. 

The Ar(3P(, Pt) levels are 11.623 and 11.827 eV, respectively, above the ground (1S) level. The lifetimes are 8.4 and 2.0 nsec (33), respectively. The Ar(3P,1 Pj) states are formed by absorption of the Ar resonance lines at 1067 and 1048 A. In the 1 to 100 mtorr concentration range the lifetime of Ar(3P, P() atoms is of the order of 10 /tsec [Hurst et al. (494)], which is 1000 times as long as that of isolated atoms because of imprisonment of resonance radiation. If the ionization potential ofa molecule is below 11.6 eV, it is possible to increase the photoionization yield (sensitize) by adding Ar to the sample. The increase of the ionization yield is caused by collisional energy transfer between Ar(3P, Pi) atoms and the molecule before the excited atoms return to the ground state by resonance emission. Yoshida and Tanaka (1065) have found such an increase in the Ar propane, and Ar-ammonia mixtures when they are excited by an Ar resonance lamp. Boxall et al. (123) have measured quenching rate constants for Ar(3P,) atoms by N2) 02, NO, CO, and H2. They are on the order of the gas kinetic collision rate. 

Although flames are convenient sources of MOH molecules, they suffer from serious drawbacks for spectroscopic and dynamical studies. The high temperature ( 2000 K) of flames causes numerous vibrational and rotational levels to be populated resulting in very dense spectra. The high pressure (1 atm) broadens the rotational lines (>0.1 cm ) and increases the overlap of the lines. In addition, resonant laser-induced fluorescence is difficult to detect because of quenching and the overwhelming presence of nonresonant fluorescence caused by rapid collisional energy transfer. The luminescence of the flame itself also interferes with measurements. 

In addition to unimolecular decay, photoexcited molecules may also exhibit bimolecular decay resulting from interactions with other (ground state) molecules. The interaction may take the form of a collisional energy transfer or ser itization process (equation 12.16) or as a quenching interaction, in which neither product is in the excited state (equation 12.17). 

Figure 7.3 Radiative (absorption, stimulated emission, fluorescence) and non-radiative (quenching, collisional energy transfer, elastic scattering) processes in a molecular system with electronic, vibrational and rotational energy levels...

In addition to the non-radiative quenching mentioned further above, an addition collisional energy transfer process can be observed, namely the transfer from the laser-excited level to neighbouring quanmm levels within the excited-state manifold. Hence, under the right conditions, one observes lines from levels that were not directly populated by the laser excitation. 

Figure IX-D-2. Semilog plot of the Stern-Vohner constants (k /ki) versus 1/X for quenching of excited acetone by collisional energy transfer with air or nitrogen molecules T 298 K). The trend line is the least-squares fit to the data excluding the most divergent points at l/X = 0.00403 and 0.00303 nm logio(kq/ d) = 10.27- 3761 x (1/k).

Michaels C A, Lin Z, Mullin A S, Tapalian H C and Flynn G W 1997 Translational and rotational excitation of the C02(00°0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene evidence for impulsive collisions accompanied by large energy transfers J. Chem. Phys. 106 7055-71... 

Quenching is the reduction in fluorescence intensity and can be caused by various processes. It occurs either during the lifetime of the excited state or in the ground state. Quenching processes that happen in the excited state are collisional quenching, charge transfer reactions, or energy transfer. The latter is the basis for FRET probes but the other events happen as well under certain conditions and it is important to consider them. 

Energy transfer, as described by Forster [78], requires a long range dipole-dipole interaction between the donor and the acceptor fluorophore. This energy transfer is possible at distances between 2 and 10 nm. Contrary to what happens in collisional quenching, there is no need for physical contact between the two molecules. 

Silver ions cause strong quenching of protein fluorescence by at least two distinct mechanisms collisional quenching and energy transfer to Ag+-mercaptide absorption bands.415 The effect was studied in detail for both sulfhydryl and non-sulfhydryl proteins and had a number of practical applications including the determination of SH groups and as a probe of binding sites. 

The fluorescence is quenched by collisional quenching and fluorescence resonance energy-transfer (FRET). When the probe sequence anneals to the target sequence fluorescence... 

Excited molecules can release some of their energy to molecules located nearby. This energy transfer occurs with a rate constant kq (collisional quenching), or with a rate constant kt (energy transfer at distance). 

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