Deexcitation vibrational

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

When an excited state is converted by ejection of an atomic electron, a high positive charge can be produced through subsequent Auger electron emission. Within the period of molecular vibration this charge is spread throughout the molecule to all atoms, and a Coulomb explosion results. This primary phenomenon occurs, of course, not only as a result of [ decay, but must be taken into account in all cases of nuclear reaction when deexcitation by inner electron conversion occurs... 

Figure 15. Time-dependent behavior of OH(t> = 1-5) observed following production of the radical by the 0( D) + H2S reaction. The data, taken at 50- s intervals, clearly show the effects of vibrational cascade by collisional deexcitation of the initially produced inverted distribution. Reproduced with permission from Ref. 45.

Complex reaction kinetics often incorporate processes of the preceding type and the inverse. Modeling the earth s atmosphere necessitates a detailed knowledge of its photochemistry, including the vibrational excitation and deexcitation of N2, 02, OH, and so on in E-V-R transitions with atoms and molecules. This has been reviewed by a number of authors,6 9 and an informative survey is given in Chapter 6, of the first volume of this book.10... 

Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium conditions in the solid where the electronic system reaches several thousand degrees Kelvin on a sub-picosecond timescale, while the lattice (phonon) bath, stays fairly cold. As illustrated in Figure 3.22, photoexcited hot electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic structure may induce vibrational motions of the adsorbate-substrate bond. For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species. After 1-2 ps, the hot electron... 

Fig. 1. Energy levels of the antiproton in pHe+. The p is captured by replacing one of the Is electrons, which corresponds for the p to a state with principal quantum number no JW /m, where M is the reduced mass of the atomcule, and m the electron mass. About 3% of antiprotons are captured in metastable states (black lines) at high angular momenta L n — 1, for which deexcitation by Auger transitions is much slower than radiative transitions. The lifetimes of these states is in the order of /is. The antiprotons follow predominantly cascades with constant vibration quantum number v = n — L — 1 (black arrows) until they reach an auger-dominated short-lived state. The atomcule then ionizes within < 10 ns and the pHe++ is immediately destroyed in the surrounding helium medium. The overall average lifetime of atomcules is about 3 — 4 ps...

The second method has been applied to cw CO lasers it relies on the fact that CO(t> = 0) and a number of other vibrationally cold gases preferentially deexcite lower vibrational levels of CO, where vibration-vibration energy exchange is closest to resonance [256, 257]. Consequently, controlled addition of these gases can make the populations in neighboring vibrational levels more suitable for laser action and can enhance the power output [258, 259],... 

Vibrational and Rotational Excitation in Gaseous Collisions probability of excitation or deexcitation is determined from... 

Figure 3.10 Vibrational deexcitation of a classical Morse oscillator as a function of the orientation angle fl0 (see text), according to Kelley [98], for the case mA + mB mc = 2 + 1 - 1. Rotational energy is acquired via intramolecular V—R transfer. AirT0T is the net internal energy lost by the molecule BC.

The case 4+1 — 80, representing, for example, He + HBr, was shown by Kelley to illustrate the difficulty with which vibrational energy is transferred when the Br end is struck (m = 0.0008) as opposed to the values obtained when the H atom is struck (m = 3.8). It was also demonstrated that, considering all values of 0O for this case, the predominant deexcitation mechanism is intramolecular V-R transfer. Since this V-R process depends upon particle masses and collision energy in a manner different from the V-T process, a colinear collision model will not always lead to a proper description of vibrational deexcitation. This conclusion probably applies as well to more complicated systems, such as noble gas collisions with CH4. 

Vibrational Excitation and Deexcitation of the Ground Electronic State... 

Radiationless transitions such as those in Equations 4.6 and 4.7 involve deexcitations in which the excess energy is often first passed on to other parts of the same molecule. This causes the excitation of certain vibrational modes for other pairs of atoms within the molecule—we will discuss such vibrational modes in conjunction with the Franck-Condon principle (see Fig. 4-12). This energy, which has become distributed over the molecule, is subsequently dissipated by collisions with other molecules in the randomizing interchanges that are the basis of temperature. 

Figure 5-7. Energy level diagram including vibrational sublevels, indicating the principal electronic states and some of the transitions for carotenoids. The three straight vertical lines represent the three absorption bands observed in absorption spectra, the wavy lines indicate possible radiationless transitions, and the broad arrows indicate deexcitation processes (see Fig. 4-9 for an analogous diagram for chlorophyll).

Figure 3 NRVS data recorded on a single crystal of Fe(TPP)(l-Melm)(NO), oriented with the X-ray beam 13.8° from the planes of all porphyrin molecules, at two different temperatures, 119 K (blue) and 287 K (red). The two curves in the main panel are normalized according to Lipkin s first moment sum rule (equation 2) and scaled up by 200 times. It is apparent that increasing temperature leads to effective line broadening and to signals at negative energy resulting from vibrational deexcitations. The inset shows an expanded view of the recoiUess line, with shoulders due to low-frequency lattice vibrations...

Because of their different physical origin, T, and T2 require quite different theoretical approaches, which we briefly summarize. For energy relaxation, T, is summed over the different possible channels for vibrational level i to deexcite toward other vibrational levels j. The golden rule gives directly... 

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