Product dissociation rates monitoring

Scanning the frequency of the dissociation laser and collecting the total OH fluorescence, while the state-selection and probe frequencies are kept fixed on specific transitions, produces a PHOFEX spectrum an example is displayed in the right-hand panel of Fig. 10. The lines correspond to specific resonance states with rotational quantum number J and projection quantum number if = 2 in vibrational state (6,0,0). If the individual lines are broader than the resolution of the laser system, one can determine the width from fitting the spectrum and thus determine the state-specific dissociation rate. If the true linewidth caused by dissociation is smaller than the resolution of the laser system, the rates can be extracted from time-resolved measurements. All three laser frequencies are fixed, and the OH probe laser used to detect a particular state of OH is delayed with respect to the dissociation laser. In this way one can monitor the appearance of the OH products as function of the delay time, in the same way as described above for N02- In contrast to NO2, however, the rate is a state-specific rate rather than an average rate, because of the high selectivity of the overtone... 

The NO2 dissociation rate was measured by a two-color picosecond pump-probe method in which the product NO was monitored by LIF. Of particular significance in this study is that the NO2 density of states at the dissociation limit of 25,130.6 cm is relatively well established from an extrapolation of experimentally determined densities at an energy of 18,500 cm . This density (for cold samples where the rotations do not contribute significant densities) is 0.3 states per cm , (Miyawaki et al., 1993) which leads to a minimum rate constant l/h p( ) = 1 x 10 sec . The experimentally measured rate increases from 0 to 1.6 x 10 sec at the dissociation limit. It is interesting that the subpicosecond laser pulses with their transform limited resolution of about 20 cm do not excite individual NO2 resonance states (see section 8.3, p. 284) but, instead, prepare a superposition of those states that are optically accessible within the laser bandwidth. It is thought that all resonance states in this bandwidth are... 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

In this work, we have demonstrated that the CH radical can be generated with sufficiently high concentrations by means of the multiphoton dissociation of CHBr at 193 nm for kinetic measurements. The formation and decay of the CH radical was monitored by the laser-induced fluorescence technique using the (A2 b — X2ir) transition at 430 nm. Several rate constants for the reactions relevant to high temperature hydrocarbon combustion have been measured at room temperature. One of the key reactions, CH + N2, has been shown to be pressure-dependent, presumably due to the production of the CHN2 radical at room temperature. 

If the a-chymotrypsin-catalysed hydrolysis of 4-nitrophenyl acetate [10] is monitored at 400 nm (to detect 4-nitrophenolate ion product) using relatively high concentrations of enzyme, the absorbance time trace is characterised by an initial burst (Fig. 5a). Obviously the initial burst cannot be instantaneous and if one uses a rapid-mixing stopped-flow spectrophotometer to study this reaction, the absorbance time trace appears as in Fig. 5b. Such observations have been reported for a number of enzymes (e.g. a-chymotrypsin [11], elastase [12], carboxypeptidase Y [13]) and interpreted in terms of an acyl-enzyme mechanism (Eqn. 7) in which the physical Michaelis complex, ES, reacts to give a covalent complex, ES (the acyl-enzyme) and one of the products (monitored here at 400 nm). This acyl-enzyme then breaks down to regenerate free enzyme and produce the other products. The dissociation constant of ES is k2 is the rate coefficient of acylation of the enzyme and A 3 is the deacylation rate coefficient. Detailed kinetic analysis of this system [11] has shown... 

Figure 7.16 Energy level diagram for the NCNO NC + NO dissociation including the scheme for monitoring the NCNO decay rate and the product NO appearance rate (which were shown to be equal). Taken with permission from Khundkar et al. (1987).

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