Inelastic collisional transitions

Fig. 8.6 Schematic term diagram for illustration of different possible inelastic collisional transitions of an optically pumped level k) = J ) of...

Inelastic scattering produces a pennanent change in the internal energy and angrilar momentum state of one or both structured collision partners A and B, which retain their original identity after tire collision. For inelastic = (a, P) — /= (a, P ) collisional transitions, tlie energy = 1 War 17 of relative motion, before ( ) and after... 

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 8.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

The depopulation cross sections of the Rb nd states of 25 < n < 40 are 1000 A2, which is the same as the cross section of the Rb ns state if the ns —> (n - 3)1,1 > 3 contribution is subtracted. For the Rb nd states the calculated contribution of the scattering of the nd state to nl S 3 and (n—1)1 s 3 states with no change in the rotational state of the CO is <100 A2, so 90% of the cross section is due to the inelastic transitions leading to rotational excitation. Presumably it is because the resonant transfer accounts for 90% of the observed cross section that the structure in the cross section is more visible in the nd cross sections than in the ns cross sections. For both the ns and nd states minimal collisional ionization is observed and calculated in this n range, principally because there are too few CO molecules with energetic enough A/ = -1 rotational transitions. For example, only CO 7 > 18 states can ionize an n = 42 Rydberg state by a A7 = -1 transition, and only 3% of the rotational population distribution is composed of 7 > 18 states. 

In the classical model the inefficiency of purely disorienting elastic collisions can easily be understood if one takes into account the fact that such disorientation is connected with a turn of the angular momentum J by a tangible angle (collisional randomization of 3(0,rotational level, i.e. the collision becomes an inelastic one. 

If the excess energy is electronic, there is a chance that it may be emitted within a time of 10 to 10 sec as radiation if the transition is possible. Otherwise such electronic energy must be degraded by inelastic collisions to other forms of energy, usually vibrational. Generally it is found that the electronic states produced in chemical reactions are metastable, so that we may expect electronically excited states to follow the path of collisional dcexcitation. The rare cases of allowed transitions result in what are referred to as chemiluminescent reactions. ... 

In addition to LIF resonant two-photon ionization (Sect. 1.4) can also be used for the sensitive detection of collision-induced rotational transitions. This method represents an efficient alternative to LIF for those electronic states that do not emit detectable fluorescence because there are no allowed optical transitions into lower states. An illustrative example is the detailed investigation of inelastic collisions between excited N2 molecules and different collision partners [995]. A vibration-rotation level (v, J ) in the a Jig state of N2 is selectively populated by two-photon absorption (Fig. 8.10). The collision-induced transitions to other levels v + An, / + AJ) are monitored by resonant two-photon ionization (REMPI, Sect. 1.2) with a pulsed dye laser. The achievable good signal-to-noise ratio is demonstrated by the collisional satellite spectrum in Fig. 8.10b, where the optically pumped level was v = 2, J = 7). This level is ionized by the P(l) parent line in the spectrum, which has the signal height 7.25 on the scale of Fig. 8.10b. 

In summary, Miller s 1970 papers [1, 2] on classical 5-matrix theory had a profound influence on the theory of molecular collisions and related topics such as photodissociation. Following earlier work on elastic scattering, they demonstrated how the results of classical mechanics can be built into a quantum mechanical framework of inelastic collisions. In my view the greatest asset of the classical 5-matrix theory is its interpretative power. The general shape of transition probabilities or collisional cross sections can be easily understood in terms of classical trajectories and their quantum mechanical interference. Exact quantum mechanical programs are like black boxes and the results are often difficult to understand without the help of classical mechanics or semiclassical analyses. The new developments such as the IVR are likely to become major tools for systems consisting of many atoms. 

Inelastic collisions of A with molecules B of the liquid host may cause radiationless transitions from the level Ei populated by optical pumping to lower levels En. These radiationless transitions shorten the lifetime of Ei and cause collisional line broadening. In liquids the mean time between successive inelastic collisions is of the order of 10 to 10 s. Therefore the spectral line Ei Ek is greatly broadened with a homogeneously broadened profile. When the line broadening becomes larger than the separation of the different spectral lines, a broad... 

Contents J.M.Bowman Introduction. - D.Secrest Inelastic Vibrational and Rotational Quantum Collisions. -G. C.Schatz Quasiclassical Trajectory Studies of State to State Collisional Energy Transfer in Polyatomic Molecules. - R. Schinke, J. M. Bowman Rotational Rainbows in Atom-Diatom Scattering. - M.Baer Quantum Mechanical Treatment of Electronic Transitions in Atom-Molecule Collisions. - Subject Index. 

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