Collision time scale

P, Multiphoton ionization rate, 1/cm -ps Collision time scale, s... 

Depending on the time and length scales, different transport laws can be used. When the objects have comparable size to the wavelength of energy carrier, wave phenomena, that is, reflection, refraction, diffraction, etc., dominate the energy transport mechanism. When the time scale of interest (t) is of the order of collision time scale (t ), time-dependent wave mechanics must be used. Schrodinger s equation must be used for electrons and phonons. Maxwell s equation must be used for photons ... 

Spectroscopic detemiination of the HE rotational distribution is another story. In both the chemical laser and infrared chemiluminescence experiments, rotational relaxation due to collisions is faster or at least comparable to the time scale of the measurements, so that accurate detemiination of the nascent rotational distribution was not feasible. However, Nesbitt [40, 41] has recently carried out direct infrared absorption experiments on the HE product under single-collision conditions, thereby obtaining a fiill vibration-rotation distribution for the nascent products. 

CFIDF end group, no selective reaction would occur on time scales above 10 s. Figure B2.5.18. In contrast to IVR processes, which can be very fast, the miennolecular energy transfer processes, which may reduce intennolecular selectivity, are generally much slower, since they proceed via bimolecular energy exchange, which is limited by the collision frequency (see chapter A3.13). 

The scope of this section restricts the discussion. One omitted topic is the collision and interaction of molecules with surfaces (see [20, 21] and section A3.9). This topic coimects quantum molecular dynamics in gas and condensed phases. Depending on the time scales of the interaction of a molecule witli a surface, the... 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

One attraction of MD simulation is the possibility of computer animation. The mobility of ions inside a charged cylindrical pore can be visualized. Some movie clips of EMD and NEMD are downloadable at http //chem.hku.hk/ kyc/movies/. mpg. Some features that escape statistical averages can be learned in watching the animation. While the coions are present mainly in the center of the pore, occasional collisions with the wall do occur, as observed in the movie. The time scale of a coion staying near the wall is of the order of 1 ps, compared to 10 ps for the counterion. While the averaged equilibrium distributions indicate an infinitesimal concentration of coion at the wall, reaction of coion with the wall can occur within a time scale of 1 ps. From the video, it can also be observed that the radial mobility of the counterion is more significant compared to the coion s and compared to the axial mobility. It is consistent with the statistical results. 

After completion of this diffusional process within a few /us, in other words after the consumption of S04 and formation of Py + or G+, interestingly, a larger amount of Py+ was formed for PyODNn with a shorter distance separation between the G-region and Py moiety in the time scale of 100 /is (Fig. 3a). This secondary formation of Py + was attributed to the hole transfer from G + to Py in DNA (Scheme 3, path b). Since the hole transfer from the G-region to the Py moiety is considered to be much slower for Py-ODN5 compared to the PyODNn (n=l 4), in the case of PyODN5 Py + was formed only from the initial collision process the time profile of Py+ at 470 nm for PyODN5 was subtracted from those for PyODNn (n=l 4). This... 

As discussed above, the solution environment provides for a set of time scales different from the gas phase environment. In solution, there are typically 1013 collisions second"1 of a solute molecule with solvent molecules. Thus, if a photolytically generated species is expected to have a large cross section for reaction with solvent and it is desired to monitor that reaction, both generation and monitoring must be done on a picosecond (psecond) or even sub-psecond timescale. That monitoring this rapid is necessary has been confirmed in an experiment on Cr(CO)6 in cyclohexane solution where psecond photolysis and monitoring was not rapid enough to detect the naked Cr(CO)5 that existed before coordination with cyclohexane (55). 

---