Collisional depopulation

Fig. 11.20 The n dependence of the rate constants for collisional depopulation of K nd atoms by SF6 ( ) (ref. 93) and for formation of free SF6 ions in collisions of SF6 with K nd (A) (ref. 93), K nd (A) (ref. 91), Na np( ) (ref. 92), Xe nf ( ) (ref. 85). Also shown are rate constants calculated with (—) and without (-------) taking into account postattach-...

The simplest method consists of investigating the collisional depopulation of a laser excited rovibronic level, i.e. of measuring the rates and cross-sections of the collisional relaxation of its population bPo- The relaxation rate Tk of polarization moments bPQ of various rank K may be represented, in the case of isotropic collisions, as follows ... 

There are two features of these observations that are striking. First, collisional depopulation persists even when the relative kinetic energy of the collision partners is sensibly zero, as shown by the existence of... 

Fig. 6.86 Collisional depopulation of the excited level 7 of a molecule and example of a Stern-Vollmer plot for the NaK level v = 1, 7 = 13) depopulated by collisions with...

Nonradiative processes (knr) can occur with a wide range of rate constants. Molecules with high knr values display low quantum yields due to rapid depopulation of the excited state by this route. The measured lifetime in the absence of collisional or energy transfer quenching is usually referred to as To, and is given by to = (kr + knr). ... 

In Fig. 11.10 the energy separation used for the d state, the d-s interval, suggests that the d state is depopulated by collisional transitions to primarily the s... 

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. 

Fig. 11.15 Principal quantum number dependence of the experimental rate constants for the total depopulation of the Xe nf states by NH3 ( ) and for collisional ionization (O). Also shown is the calculated ionization rate constants of Rundel (R) (ref. 69), Latimer (L) (ref. 10), and Matsuzawa (M) (ref. 70) (from ref. 64).

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. 

We have examined the nature of LIFS in some detail. The response of an atomic or molecular system is described in terms of appropriate rate (or balance) equations whose individual terms represent the rate at which individual quantum states are populated and depopulated by radiative and collisional processes. Given the response of a system to laser excitation, one may use the rate equations to recover information about total number density, temperature and collision parameters. 

Dynamic quenching occurs within the fluorescence lifetime of the fluorophore, i.e., during the excited-state lifetime. This process is time-dependent. We have defined fluorescence lifetime as the time spent by the fluorophore in the excited state. Collisional quenching is a process that will depopulate the excited state in parallel to the other processes already described in the Jablonski diagram. Therefore, the excited-state fluorescence lifetime is lower in the presence of a collisional quencher than in its absence. 

Nonradiative Processes. The nonradiative pathways which depopulate are still a matter of some discussion. It has been pointed out that the short lifetime of precludes collisional quenching of this electronic state, and the failure to observe sensitized triplet emission from biacetyl (92) indicates that the... 

Perturbations affect the rate of absorption and emission of radiation in a fully understood and exactly calculable manner. They also affect the rates of chemical and collisional population/depopulation processes, but in a less easily estimated way. Perturbation effects on steady-state populations can be very large and level-specific. Although collision-induced transitions and chemical reactions are not governed by rigorous selection rules as are electric dipole transitions and perturbation interactions, some useful propensity rules have been suggested theoretically and confirmed experimentally. Gelbart and Freed (1973) suggested that the cross sections for collision-induced transitions between two different electronic states, E and E, are... 

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