Collisional transfer

A may also return to the ground state via a radiationless transition, most commonly by collisional transfer of energy to a solvent molecule. 

Corsico, B., Cistola, D.P., Frieden, C. and Storch, J. (1998) The helical domain of intestinal fatty acid binding protein is critical for collisional transfer of fatty acids to phospholipid membranes. Proceedings of the National Academy of Sciences USA 95,12174-12178. 

Herr, F.H., Matarese, V., Bernlohr, D.A. and Storch, J. (1995) Surface lysine residues modulate the collisional transfer of fatty acid from adipocyte fatty acid binding protein to membranes. Biochemistry 34, 11840-11845. 

Forster (1959) classifies the qualitative features based on which one can distinguish the various modes of energy transfer. Mainly, only collisional transfer depends on solvent viscosity (vide infra), whereas complexing between the donor and acceptor changes the absorption spectrum. On the other hand, the sensitizer lifetime decreases for the long-range resonant transfer process, whereas it should be unchanged for the trivial process. 

Collisional transfer, in which donor and acceptor approach each other sufficiently closely to allow spatial overlap of the orbitals of the donor and acceptor. 

In our discussion above, it was pointed out that a molecule in the excited state can return to lower energy levels by collisional transfer or by light emission. Since these two processes are competitive, the fluorescence intensity of a fluorescing system depends on the relative importance of each process. The fluorescence intensity is often defined in terms of quantum yield, represented by (X This describes the efficiency or probability of the fluorescence process. By definition, XL is the ratio of the number of photons emitted to the number of photons absorbed (Equation 5.6). 

Eq. (11.37) is written assuming that after atoms are removed from the state excited by the laser, state A, they never return. In contrast, if the atoms initially excited to state A are collisionally transferred to a longer lived state R and do return to A, the fluorescence from state A exhibits a two exponential decay which is of the form25... 

One of the more well studied collision processes involving Rydberg atoms is collisional angular momentum mixing, or i mixing, the collisional transfer of population among the nearly degenerate states of the same n.28 The process has... 

The increased cross sections for these three states are attributed to resonant electronic to vibrational energy transfer. Table 11.1 identifies the three atomic transitions and the resonant molecular transitions in CH4 and CD4. For example the rapid depopulation of the Na 7s state by CD4 is attributed to the Na 7s — 5d transition. To verify this assignment the cross section for the 7s — 5d transfer was measured for both CH4and CD4 by observing the 5d-3p fluorescence as well as the 7s-3p fluorescence. The 7s — 5d cross sections are 215 A2 for CD4 and 15 A2 for CH4. As shown by Fig. 11.16, the 7s CD4 cross sections is —240 A2 above the smooth dotted curve in good agreement with the 7s — 5d cross section. Similar confirmations were carried out for the other two resonant collisional transfers. 

The +) and —) states of Beigman and Syrkin are Stark states, and their description of slow collisions is related to one given originally by Smith et al.3 They proposed that the collisional transfers occur if the field of the passing ion brings the initial state to an avoided crossing with a Stark state of the adjacent Stark manifold. This requirement is easily stated in a quantitative form,... 

Fig. 14.2 Energy level diagram for the Na 16p, 17s, and 17p states in a static electric field. The vertical lines are drawn at the four fields where the s state is midway between the two p states and the resonant collisional transfer occurs (from ref. 12).

The results show that collisional transfer is very efficient from a molecule in the highly excited and closely spaced quantum states, which behave as a classical system with little restriction due to quantisation. The monatomic and diatomic gases appear to be rather less effective than the polyatomic molecules, which may emphasise the importance of V-V transfer in this type of process. However, it is desirable to establish experimentally how the energy is distributed in both molecules following collision. 

If the OH undergoes rotational energy transfer, it will undergo vibrational energy transfer as well (22). Figure 5 shows the emission from the v7=l level following excitation of the N7 =4 rotational level in v/=0. This results from molecules which have been collisionally transferred upwards some 3000 cm"l. Also shown in Fig. 5, on the same intensity scale, is a small portion of the emission from v7=0. From a ratio of these intensities, we find that the v/=l population is about 3.5% of that in v7 0, N. ... 

Figure 5. Fluorescence scans of emission following excitation of N = 4, J — 9/2 of the V = 0 level in OH in a CHk-air flame. Top (1,0) band fluorescence, emitted by molecules collisionally transferred upwards to v = 1 bottom two rotational lines in the (0,0) band, emitted by molecules in the N = = 12 level of V — 0. Both scans are on the same intensity scale.

In actuality, the molecular populations are collisionally transferred among these internal levels at rates of the same general magnitude as the quenching, so that an appreciable fraction of the original population of the two levels connected by the laser can reside in the remainder of the levels. The fraction in the upper laser-pumped level, and thus S, is in general dependent on the detailed state-dependent rates of transfer within and between the two manifolds of vibrational and rotational levels. 

Figure 1. Four-level molecular model. QiS is the collisional-transfer rate constant from level i to level j, TV is the sum of the electronic quenching and spontaneous emission rate constants, W,t is the absorption rate constant, and Wlt is the stimulated emission rate constant. WIt and WtI are proportional to the laser power PL. The dashed vertical line separates levels le and 2e, which are treated as an isolated system, from those levels not affected directly by the laser radiation.

At laser intensities sufficient to saturate the le-2e transition, the stimulated emission and absorption processes which couple the levels are fast relative to collisional transfer processes, and a quasi-equilibrium balance (e)/Np(e)] is quickly established. If the total population of levels le and 2e is approximately constant during the laser pulse, the upper level population 112(e) can be reliably related to Np(e)0 using an analysis similar to a two level atomic model (1, 2, 3). ... 

A[N (e) + (e)], the net population transfer into or out of levels le and 2e during the laser pulse, will be nearly zero if the laser pulse length tl is less than or comparable to the characteristic collisional transfer time tc - (Q23+T24)- Ql4 > simply because few collisions will occur during the laser pulse. 

Manabe, T., Yabuzaki, T. and Ogawa, T. (1981). Observation of collisional transfer from alignment to orientation of atoms excited by a single-mode laser, Phys. Rev. Lett., 46, 637-640. 

As before, due to conservation of energy in the elastic Coulomb collisions, the total collisional transfer of energy between electrons and ions QT must fulfill ... 

ArF laser photolyis of H2S seeded in free-jet expansion of Ar used to form Sj. Analysis of LIF spectrum Lifetimes for two vib-rotational levels of Te2(A0 ). Cross-sections for deactivation by Tej Laser emissions from different excited Tcj isotopes Quantum yields for CI2 luminescence at wavelengths 308 nm following VUV excitation in the presence of Ar, Kr, and Xe f Three bound-free transitions observed in LIF from Brj following excitation at 158 nm. Collisional transfer by SF, N2, and He t Near-dissociation behaviour of l2(B n ) tLIF-Fourier transform study of the "near-dissociation behaviour of IjfBO I). Precise value for the well depth of the X state determined... 

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