Radiative lifetimes vibrational variation

Figure 6.10 Variation of the CO a3n radiative lifetime versus J and Cl. The CO a3n state borrows its oscillator strength from A1 II and, to a much lesser extent, from X1E+. The effective lifetime of an entire vibrational level will depend in a complex way on pressure, temperature and the mode of excitation. From data of Fairbairn (1970) corrected by Slanger and Black (1971). Jongma, et al., (1997) measure r = 3.8ms for v = 0, Cl = 1, J = 2e and 2/ levels.

The absorption coefficient of Bt2 in the visible region is intermediate between those of Oi and l2- Hence for Bt2, fluorescence studies are more difficult, and bromine has only recently attracted attention. As for I2, large variations of lifetime and cross-sections as a function of v have been reported, and the short lifetime of the excited state (less than anticipated from the absorption coefficient) indicates that for Bt2 the radiative lifetime contributes little to the measured lifetime, and that non-radiative processes are very important. Capelle, Sakurai, and Broida suggest that the predissociation rate is proportional to the inverse square of the molecular mass. However, the relatively large bandwidth used by these workers (0.1—0.8 nm) makes such a conclusion doubtful, and the variation of lifetime with vibrational-rotational state would seem to be more complex, as recently found by McAfee and Hozack. ... 

The observed hfetime usually significantly depends on the temperature (whereas the radiative lifetime only depends on the small variation of the retractive index as a function of temperature if the coordination is unaffected by the lower temperature). This dependence comes from the fact that some nonradiative deactivations of the lanthanide ion are vibrationally assisted (i.e., needs some heat to take place, e.g., back-transfer to the triplet state of the ligand). Nevertheless, the observed lifetime of the lanthanide does not depend on the excitation wavelength. A direct excitation through an f-f transition or through a sensitizer results in the same observed lifetime. In other words, this hfetime, and thus the deactivation of the lanthanide excited state only depends on the chemical environment and on the temperature, not on how this excitation was achieved. 

Kriegel et al 1987). Vibrational excitation of molecular ions is more efficient in collisions with buffer gas atoms or molecules heavier tlian helium (e.g. N2, Ar, etc.). This is demonstrated below. However, the occurrence of vibrational excitation is unfortvmate from the viewpoint of astrochemistry, since vibrational excitation of ions in drift tubes can obscure the influence of kinetic excitation on the rate coefficients of molecular ion-neutral reactions. Such information is needed in some astrophysical situations such as in interstellar MHD shocks. In these situations, the mean free times between collisions of the ions with the ambient gas are usually longer than the radiative lifetimes of the vibrational states of the ions and therefore the ions will generally be vibrationally relaxed. Thus data are required on the variation of the rate coefficients with Ej. for the reactions of ions in their ground vibrational state. So, drift tube data on molecular ion reactions must be applied with caution to astrophysical situations except that is for data obtained at low e/N where vibrational excitation of the molecular ions is minimal. 

Generally, an increase in temperature results in a decrease in the fluorescence quantum yield and the lifetime because the non-radiative processes related to thermal agitation (collisions with solvent molecules, intramolecular vibrations and rotations, etc.) are more efficient at higher temperatures. Experiments are often in good agreement with the empirical linear variation of In (1/Op — 1) versus 1/T. 

It is interesting to note that the emission spectra of the terbium chlorides solvated with H20 and D20 show no discernible differences. Since the rare-earth chlorides solvated with D20 are isostructural with the chlorides solvated with H20 and since the emission spectra are essentially identical, Freeman et al believe that the variations in lifetime are not brought about by changes in the radiative-transition probabilities, but are a consequence only of changes in radiationless quenching efficiencies. They speculate that the decreased efficiency upon substitution of D20 for H20 must be related to the large changes in vibrational frequencies associated with substitution of the H atoms by the D atoms. 

At high pressures gaseous system most closely resemble the situation in condensed media, and it is instructive to determine the radiative rate constant for this system, and Its variation with temperature. Assuming that internal conversion from Sj is a nonquenchable process which occurs prior to vibrational relaxation from a nonequilibrated state and using results of and 0.j. determined by Cundall and Dunnicliff (105) and values of Tp due to Lockwood (114), the calculated kp values are presented in Table 7 for both CgHg and C Dg. The similarity of rate constants for the protonated and deuterated molecules indicates that the large differences in yield and lifetime for the two isomers are the result of an Isotope effect on a nonradlative transition. 

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