Radiative rate constant, pressure

At high pressures gaseous system most closely resemble the situation in condensed media, and it is instructive to determine the radiative rate constant for this system, and Its variation with temperature. Assuming that internal conversion from Sj is a nonquenchable process which occurs prior to vibrational relaxation from a nonequilibrated state and using results of and 0.j. determined by Cundall and Dunnicliff (105) and values of Tp due to Lockwood (114), the calculated kp values are presented in Table 7 for both CgHg and C Dg. The similarity of rate constants for the protonated and deuterated molecules indicates that the large differences in yield and lifetime for the two isomers are the result of an Isotope effect on a nonradlative transition. 

A plot of the log of the LF(720nm)/CT(550nm) intensity ratio versus P proved linear (Fig. 15). An apparent volume difference of + 4.2cm3 mol-1, with the LF state being larger, was calculated according to Eqs. (14) and (15) from the slope and the assumption that the ratio of the radiative rate constants is pressure independent. 

Only the total lifetime r of a level can be measured, r is related to the rate of decrease of the number of molecules initially in a given level via both radiative and nonradiative routes. Let kr be the radiative rate constant (the probability per unit time that a molecule will leave the level as a result of emission of a quantum of light) and knr the predissociation rate (the dissociation probability per unit time). Recall that the pressure is assumed to be low enough that the rates are not affected by collisions. The number of molecules leaving the initial state during the time interval dt is given by... 

We now discuss the lifetime of an excited electronic state of a molecule. To simplify the discussion we will consider a molecule in a high-pressure gas or in solution where vibrational relaxation occurs rapidly, we will assume that the molecule is in the lowest vibrational level of the upper electronic state, level uO, and we will fiirther assume that we need only consider the zero-order tenn of equation (BE 1.7). A number of radiative transitions are possible, ending on the various vibrational levels a of the lower state, usually the ground state. The total rate constant for radiative decay, which we will call, is the sum of the rate constants,... 

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],... 

The emission spectrum consists of a series of weak bands starting at about 220 nm and then growing into a continuum from about 240 to 400 nm, with a maximum at approximately 270 nm as shown in Figure 5. Halstead and Thrush estimated that =65% of the emission occurs from the B2 state, =15% from the 3B3, and =20% from a combination of the A2 and Bi states [24, 28, 29] with a rate constant of 2 X 1CT31 cm6 molec 2 s 1 using argon as the bath gas at 300 K [53], As with the reaction of SO + 03 discussed above, collisional coupling results in a radiative lifetime that is pressure dependent. 

Standard hydrocarbon estimation. Measurement of the low-pressure radiative association rate constant is sufficient for assignment of if k and k are independently known, from the relation, ... 

Quenching half-pressure is equal lo (, t) 1 where kt is the rate constant for quenching reaction and r is the mean lifetime (radiative and predissociative) of excited NO. 

Excited-State Kinetics. A principal emphasis of this chapter is concerned with how the application of hydrostatic pressures influences rates of ES processes such as those illustrated in Figure 9. In this simple model, it is assumed that electronic excitation leads efficiently to the formation of a single, bound state, which can decay by unimolecular radiative decay (rate constant kr), nonradiative decay (fc ), or chemical reaction to give products (kp). Alternatively, there may be bimolecular quenching of the ES dependent on the nature and concentration of some quencher Q (fcq [Q]). Each of these processes may be pressure dependent. 

Rhodium(lll) complexes Collaborative studies between van Eldik, Ford and coworkers have led to thorough parameterization of pressure effects on photosolvolysis of the rhodium(III) halopentaammines Rh(NH3)sX + (Eq. 6.18) [39-45]. For these systems LF excitation is followed by rapid intersystem crossing (cDisc 1) to the lowest energy LF state E from which reactive (kp), radiative (k,) and non-radiative (k ) deactivation occur competitively (Fig. 6.10) [41, 46]. Rate constants for individual excited state processes were calculated from phosphorescence quantum yields fl>r, lifetimes r and quantum yields for halide ([Pg.198]

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