Collisional decay

The fluorescence signal can be used in a number of ways. Most simply it provides a measure of the population of the excited state or states through Equation 1. In addition, if a relationship can be found between the number density of all the quantum states under excitation conditions, then the total number density of the species can be deduced. Unfortunately, collisional decay process can cause redistribution of population from the excited level, complicating interpretation. 

Our previously elaborated approach, [Kofman 2000 Kofman 2001 (a)], to dynamical control of states coupled to an arbitrary zero-temperature "bath" or continuum has reaffirmed the intuitive anticipation that, in order to suppress their decay, we must modulate the system-bath coupling at a rate exceeding the spectral interval over which the coupling is significant. The spectra of baths (continua) corresponding to vibrational or collisional decay or decoherence typically allow dynamical suppression, using realistic rates of modulation, [Kofman 2000 Kofman 2001 (a)]. 

In an optical lattice the state of the system is expanded using the single particle Bloch states. In this picture the collisional decay of strongly driven condensates is then calculated, leading to a remarkable splitting / shift of the collisional products, away from the usual spherical s-wave. 

In order to treat this system theoretically, the Bloch states of the strong optical lattice must be considered. In this picture we develop a model to explain the collisional decay of the oscillations as a two-particle collision of Bloch-states (and no longer free atoms). There are various quantum paths for this collision (since every lattice momentum has several relevant branches), leading to a destructive interference of the central s-wave collisional sphere, and a splitting in the resulting collisional shell, related to the observed spectral splitting. The matrix elements for this process lead to a suppression of the outward driven shell and enhance the centrally driven collisional shell, which is no longer... 

B12 and 21 are the transition probabilities for absorption and stimulated emission, respectively, and 1 n2, n, and wT the analyte atom densities for the lowest state, the excited state, the ionized state and the total number densities, gi and g2 are the statistical weights, A2i is the transition probability for spontaneous emission and 21 the coefficient for collisional decay. Accordingly,... 

The second stage of the Bloch-Bradbury mechanism includes collision with the third-body particle M (density no), leading to relaxation and stabilization of O2 or collisional decay of the unstable ion ... 

Another interesting question is whether more complex ultracold molecules can be created than simple dimers. A first step into this direction is the observation of scattering resonances between ultracold dimers [101] which have been found in the collisional decay of an optically trapped sample of CS2 molecules and interpreted as a result of a resonant coupling to tetramer states. 

Significant spin-orbit effects have also been observed in a number of the excited inert gas nonreactive collisional decay channels. In excitation transfer collisions between metastable argon and hydrogen atoms. 

To detect tlie initial apparent non-RRKM decay, one has to monitor the reaction at short times. This can be perfomied by studying the unimolecular decomposition at high pressures, where collisional stabilization competes with the rate of IVR. The first successful detection of apparent non-RRKM behaviour was accomplished by Rabinovitch and co-workers [115], who used chemical activation to prepare vibrationally excited hexafluorobicyclopropyl-d2 ... 

The high yields—about 50%—which were observed in all cases, indicate the strong involvement of secondary fission products (i.e., those produced by /S-decay of precursors). A consideration of mechanisms of formation of the organometallic products led to the conclusion (13) that the j8-decay itself must be the cause of the molecule formation. Neither purely mechanical collisional substitution, nor thermal chemical reactions, nor radical reactions, nor radiation-induced reactions seemed to be responsible for the synthesis reactions. 

Both collisional activation (in ion traps) and post source decay (in curved field reflectron TOF analyzers) have been used successfully to obtain sequence ions from peptides prepared in situ on the sample holder. Single Dalton mass windows are advantageous for precursor selection, as are realized in ion-trap and trap-TOF configurations. Publicly available search algorithms can be used... 

Fragmentation Region metastable decay collisional induced dissociation photodissociation... 

It is important to notice that a change in lifetime is not a necessary result of a change in fluorescence intensity. For instance, the Ca2+ probe Fluo-3 displays a large increase in intensity on binding Ca2+, but there is no change in lifetime. This is because the Ca-free form of the probe is effectively nonfluorescent, and its emission does not contribute to the lifetime measurement. In order to obtain a change in lifetime, the probe must display detectable emission from both the free and cation-bound forms. Then the lifetime reflects the fraction of the probe complexed with cations. Of course, this consideration does not apply to collisional quenching, when the intensity decay of the entire ensemble of fluorophores is decreased by diffusive encounters with the quencher. 

An important class of luminescence sensors are those based on the decrease of luminescence intensity and lifetime of the probes as function of analyte concentration. Assume that the probe intensity decays by a single exponential with an unquenched lifetime tq. If quenching occurs only by a dynamic (collisional) mechanism, then the ratio to/t is equal to Fq/F and is described by the classic Stern-Volmer equation... 

The Dp and Dq are the diffusion coefficients of probe and quencher, respectively, N is the number molecules per millimole, andp is a factor that is related to the probability of each collision causing quenching and to the radius of interaction of probe and quencher. A more detailed treatment of fluorescence quenching including multiexponential intensity decays and static quenching is given elsewhere/635 There are many known collisional quenchers (analytes) which alter the fluorescence intensity and decay time. These include O2/19 2( 29 64 66) halides,(67 69) chlorinated hydrocarbons/705 iodide/715 bromate/725 xenon/735 acrylamide/745 succinimide/755 sulfur dioxide/765 and halothane/775 to name a few. 

If a collisional quencher of the fluorophore is also incorporated into the membrane, the lifetime will be shortened. The time resolution of the fluorescence anisotropy decay is then increased,(63) providing the collisional quenching itself does not alter the anisotropy decay. If the latter condition does not hold, this will be indicated by an inability to simultaneously fit the data measured at several different quencher concentrations to a single anisotropy decay process. This method has so far been applied to the case of tryptophans in proteins(63) but could potentially be extended to lipid-bound fluorophores in membranes. If the quencher distribution in the membrane differed from that of the fluorophore, it would also be possible to extract information on selected populations of fluorophores possibly locating in different membrane environments. 

As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

---