Time constants photochemical

TRIR investigation of photochemical trans—f cis isomerization of [Re(f-styrylpyridine) (CO)3(bpy)]+ has revealed that the %7t state of planar /-styrylpyridine first undergoes a half-rotation (by -90°) around the -CH=CH- bond with a 12 ps time constant to form a perpendicular intermediate [31]. The reaction is then completed by further 90° rotation, which is much slower, 17-28 ns [ 119,127], For [Re(f-phenylazopyridine) (CO)3(bpy)]+, the first half-rotation around the -N=N- bond takes less than 40 ps, similarly to the styrylpyridine complex. The second step, which completes the isomerization, is much faster, 100-120 ps [32],... 

This tractability depends upon a rather unique and fortuitous combination of time constants which results in characteristic chemical response times being considerably shorter than dynamical time constants for many of the key mechanisms. Experimentally this means that pure chemical experiments can be executed in situ, to test photochemical hypotheses on the microscale within the atmosphere. On the other hand, dynamical experiments addressing the longer time scales can be conducted from Earth orbit via satellite to establish vertical and meridional transport maps. 

The photochemical charge separation in Rp. viridis was studied by Dressier, Umlauf, Schmidt, Hamm, Zinth, Buchanan and Michel " by directly exciting the primary donor with femtosecond pulses at 955 nm. In addition to the previously reported time constants of 3.5 and 200a subpicosecond time constant of 0.65 0.2 ps was also revealed in the spectral region belonging to the and Qy bands of the monomeric bacteriochlorophyll molecule. Except for the slightly smaller value of the decay time (0.65 ps vs. 0.9ps), the charge-separation and electron-flow sequences were interpreted the same way as for Rb. sphaeroides. 

Femtosecond kinetics of photochemical charge separation in photosynthetic bacteria at low temperatures (25 K) was studied by Lautwasser, Finkele, Scheer and Zinth using Rb. sphaeroides RCs depleted of quinones. Fig. 9 (C, a) shows the initial formation of P at 920 nm followed by a very rapid relaxation with a Ti of 1.4 0.3 ps. At 794 nm and 25 K, the absorption increases very rapidly to a maximum in about 0.1 and then decays to a minimum at tD 0.5 ps. This is followed by a slow rise and a plateau after 5 ps. The early rapid-decay component with a time constant of 0.3 0.15 ps appears to be the counterpart of the 0.9-ps component at room temperature. The data points in Fig. 9 (C, b) can be fitted by a model with three time constants, namely 0.3ps, l.Aps and 1 ns. 

In Chapter 2, the time constant appropriate to photochemical processes (rchem) was discussed, and it was shown that this time constant can be readily derived from knowledge of the rate of loss of chemical species. The time constants for dynamical effects on chemical species are somewhat more difficult to evaluate. 

Tchem r( yil. Under these circumstances, the effect of dynamics can be quite large, and the species distribution depends critically on both dynamics and chemistry. Chemical processes tend to introduce or enhance spatial gradients in the distribution of the tracers, while dynamics (e.g., mixing) tends primarily to reduce these gradients. The time constants for meridional and vertical transport, are, for example, comparable to the photochemical lifetime of N20 in the stratosphere, so that transport in the meridional plane is expected to be quite important in determining its density, in contrast to zonal transports, as discussed above. 

Plumb and Ko (1992) have shown that, for two chemical compounds whose local photochemical lifetimes are longer than the quasi-horizontal transport time constant, the slope of the correlation curve is provided by the ratio between the net fluxes of the two species through their respective mixing ratio isopleths (see also Murphy and Fahey, 1994). In addition, they calculated that for two long-lived source gases (e.g., N2O, CH4, etc.) that are emitted at the surface and are photochemically... 

The definition of the odd oxygen family clearly produces a substantial increment in the photochemical time constant of the equation to be considered, enabling us to solve it more readily. The very fast reactions, such as... 

This analysis allows us to obtain the concentrations of the odd oxygen family, but it must also be partitioned into its constituent parts. This can be done by examining the photochemical equilibrium expressions for certain members of the family. If the family is composed of N members, we must write photochemical equilibrium expressions for the shortest lived N — 1 members of the family and use these to establish ratios between family members. Below the mesopause, the photochemical time constant for atomic oxygen is relatively short and one can assume that it is essentially in photochemical equilibrium. Similar conditions apply to the excited 0(1D) atom. Neglecting minor terms, Eq. (5.26) becomes... 

The photochemical time constant r(O) for atomic oxygen is about 4 hours at 70 km, a day near 80 km and about a week at 100 km. Therefore, the distribution of atomic oxygen above the mesopause is dependent on dynamical conditions. It does not exhibit much diurnal variation above about 85 km, since its lifetime exceeds a day. At lower altitudes, on the other hand, atomic oxygen disappears rapidly after sunset, forming ozone through the recombination reaction (5.13). In... 

Figure 5.14 shows the photochemical lifetime for atmospheric methane as a function of altitude along with the time constants associated with transport by the winds and vertical mixing. Since the stratospheric lifetime of this compound against photochemical destruction is the same order of magnitude as the transport time, it provides an excellent tracer to study transport processes. 

Figure 5.14- Photochemical lifetime of CH4, and the time constants associated with transport processes.

Figure 5.18. Photochemical lifetime of carbon monoxide, and the time constants for transport processes.

Figure 5.23 shows the photochemical lifetime of water vapor, and the time constants characterizing atmospheric transport. The production of water vapor by methane oxidation is essentially complete by about 50 km. Because the photochemical and vertical transport lifetimes for this gas are comparable above about 50 km, and because there is no known chemical source of water vapor in this region, it provides an excellent tracer for mesospheric transport processes (Allen et al., 1981 Bevilacqua et al., 1983 Le Texier et al, 1988). In addition to... 

Figure 5.23. Photochemical lifetimes of water vapor and molecular hydrogen, and the time constants for atmospheric transport processes in the middle atmosphere.

Figure 5.27 presents calculated altitude profiles of the photochemical lifetimes of the HOx family and its constituent members, as well as H202, and the time constants for dynamical processes. As in the case of Ox, the lifetime of the family is several orders of magnitude greater than those of... 

Figure 5.27. Photochemical lifetime of odd hydrogen radicals, as well as the time constants for transport by the zonal and meridional winds, and a onedimensional diffusive time constant.

Figure 5.32 shows the photochemical time constant of N2O and those appropriate to transport processes. Like CH4, N2O is an excellent tracer for transport in the middle stratosphere, where its lifetime is comparable to those for advection by the mean meridional circulation. At higher altitudes in the upper stratosphere and lower mesosphere, the N2O lifetime remains close to the mean meridional transport lifetimes, making it a more sensitive tracer than CH4 in this region. 

Figure 5.37 displays the altitude profiles of the photochemical lifetimes of the NOy family and its constituent members (gas phase chemistry only), as well as the time constants for transport at middle latitudes. Note that the photochemical lifetimes for NOx and HN03 are both comparable to the transport lifetimes in the lower stratosphere, so that the distribution of these species will be quite dependent on atmospheric transport. It is important to treat them separately in numerical model... 

Figure 5.46 displays early observations of NO in the thermosphere and mesosphere. The observed variability indicates the strong sensitivity of NO to dynamic processes in the atmosphere, as one might expect based on the comparison between its photochemical lifetime and the time constant for dynamics in this region (see Figure 5.37). The observed minimum near the mesopause is due to the photodissociation of NO and its subsequent recombination with N(4S). The depth of the minimum depends on the competition between downward transport and this photochemical loss. Its variability as depicted in the figure is probably related to both seasonal and short term temporal variations in mesospheric transport parameters.

Figure 5.55. Photochemical lifetimes of Clx, Cl, CIO, HOC1, CIONO2, HC1, CF2CI2, and CFCI3, as well as the time constants appropriate to atmospheric transport.

Since the time constant for negative ion formation by attachment (ae) is at most about an hour at the top of the middle atmosphere (and is much less at lower altitudes), photochemical steady state can be assumed ... 

According to Eq. (3-8), the photochemical steady state is approached with a time constant... 

In Section 1.4.2, in analogy to the thermal rate constants, photochemical quantum yields have been defined. It was mentioned that the amount of light absorbed varies according to Section 1.4.3 during the photoreaction. Accordingly the definitions given in Section 1.4.2 exhibit a dependence on time. 

The dynamics of the photochemical process involving the ZnTPPS-ZnTMPyP ion pair has been recently investigated by pump-probe transient absorption. The transient absorption responses shown in Figure 11.10 were obtained by pumping the porphyrins at 560 nm and probing at 510 nm. It should be indicated that the depopulation of the Si state for ZnTPPS and ZnTMPyP in their monomeric form exhibits time constants of 1.6 and 0.6 ns, respectively. As mentioned before, this relaxation mainly contains contribution from radiative... 

The photosynthetic apparatus of the purple bacterium Rhodobacter sphae-roides is composed of two light harvesting complexes, LHl and LH2, which surround and interconnect photochemical reaction centres. Picosecond absorption spectroscopy with weak picosecond laser pulses is a powerful technique to probe the excited state dynamics in antenna systems. For Rb. sphaeroides at 77K the measured picosecond absorption kinetics were interpreted to give the following sequence of energy transfer events and time constants (1-3). 

Laser techniques have developed rapidly. Picosecond pulses became possible at the beginning of the 1990s. The time resolution of very fast photochemical reactions was improved considerably. Here, we are talking about time constants of the same order of magnitude as vibration cycles. Fast biochemical reactions, for example, the... 

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