Collisional time delay

A second scenario is provided by barrier-type resonances (sometimes referred to as quantum bottleneck states [QBS]), which do not rely on the internal excitation of the collision complex for their existence. In fact, barrier resonances are observed even when there is no well in Vad(s n). Collisional time delay occurs near the barrier maximum simply because the motion along the s-coordinate slows down passing over the barrier, as in the lower... 

Figure 1.5. Femtosecond spectroscopy of bimolecular collisions. The cartoon shown in (a illustrates how pump and probe pulses initiate and monitor the progress of H + COj->[HO. .. CO]->OH + CO collisions. The huild-up of OH product is recorded via the intensity of fluorescence excited hy the prohe laser as a function of pump-prohe time delay, as presented in (h). Potential energy curves governing the collision between excited Na atoms and Hj are given in (c) these show how the Na + H collision can proceed along two possible exit channels, leading either to formation of NaH + H or to Na + H by collisional energy exchange.

Figure 12. A series of time-resolved spectra of HC1 emission taken after initiation of a chain reaction by laser photolysis of Cl2 in the presence of C2H6. At the earliest time delay shown here HC1 is highly excited, and relaxes by collisional deexcitation at later times. Reproduced with permission from Ref. 86.

One example of this pump-and-probe technique is the investigation of collision-induced vibrational-rotational transitions in the different isotopes HDCO and D2CO of formaldehyde by an infrared-UV double resonance [1041]. A CO2 laser pumpes the V6 vibration of the molecule (Fig. 8.18). The collisional transfer into other vibrational modes is monitored by the fluorescence intensity induced by a tunable UV dye laser with variable time delay. 

Fig. 29 Lifetime measurements with the Zeiss Plate Vision of DBO (= 2,3-diazabicyclo [2.2.2]oct-2-ene) for three different oligopeptide protease substrate. For the uncleaved substrates the signal decays fast (r = 59 ns, 95 ns and 35 ns) due to the collisional, dynamic quenching of the DBO by tryptophan or trypsin. Upon cleavage the quencher and DBO are separated, which results in a lifetime increase (r = 234 ns, 287 ns and 315 ns). This lifetime change allows a time-gated FTRF detection (delay = 100 ns and gate = 700 ns) of the enzymatic reaction with a high signal-to-background ratio. Data by courtesy of T. Enderle, Hoffmann-La Roche, Pharmaceuticals Division, Assay Development and HTS, Basel/Switzerland [192]...

Consider first the force power spectrum. At modest bath densities this is to be derived for particle pairs initially at R moving inward. The strong repulsive potential for R < Rp leads to a power sjjectrum 8/iBMk7BBM(o>) of a form discussed in Section III C (with di = 0 because the trajectories begin at Rp, not Rj, and do not exjjerience the acceleration effect). The delay in time between initiation at Rp and recoil is so small that collisional effects are... 

According to the relevant power and momentum balance, Eqs. (38) and (39), the electron kinetics in steady-state plasmas is characterized by tbe conditions that at any instant the power and the momentum input from the electric field are dissipated by elastic and inelastic electron collisions into the translational and internal energy of the gas particles. This instantaneous complete compensation of the respective gain from the field and the loss in collisions usually does not occur in time-dependent plasmas, and often the collisional dissipation follows with a more or less large delay—for example, the temporally varying action of a time-dependent field. Thus, the temporal response of the electrons to certain disturbances in the initial value of their velocity distribution or to rapid changes of the electric field becomes more complicated, and the study of kinetic problems related to time-dependent plasmas naturally becomes more complex and sophisticated. Despite this extended interplay between the action of the binary electron collisions and the action of the electric field, the electron kinetics in time-... 

LIBS-LIF spectra are very different from usual LIBS spectra (Fig. 6.9). Estimated LIF decay time under VIS excitation is as rapid as the excitation OPO pulse width of 4 ns. The cause of such emission decay time shortening may be collisional quenching of the molecular excited states in LIP or thermal quenching of the excited states at high plasma temperature. Due to such short emission lifetime, MLIF measurements were done with a short gate width of W= 10 ns a delay of... 

In beam-foil experiments the velocities would be so great that no decay would be observed in any apparatus of convenient laboratory size. Similarly in the single-photon delayed-coincidence technique, the time required to obtain sufficient data would become quite prohibitive. The few reliable lifetime measurements that do exist have been made by the static afterglow technique. This was originally developed for experiments on the collisional destruction and diffusion of metastable atoms, which are discussed in detail in section 7.6. The difficulties encountered in the application of the afterglow and other methods to the experimental determination of the transition probabilities of forbidden lines have been reviewed by Corney (1973) and Corney and Williams (1972). 

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