Free lifetime

A more recent work by Paech ct al. (789) on the collision-free lifetimes of N02 excited by a tunable laser near 4880 and 5145 A states that although only a single level is excited, three different lifetimes of fluorescence, 3, 28, and 75 /rsec, have been observed. The results lead them to conclude that the initially formed 2B, state crosses over rapidly to another state, 2B2, with higher level density. The 2B2 state can have two different lifetimes (28 and 75 ftscc), depending on the extent of interaction with the ground state. The short life observed, 3 //sec, is determined primarily by the rate of internal conversion from the 2B, to 2B2 state. The results of some reported collision-free fluorescence lifetimes are given in Table VI-6 . 

Fluorescence from the A /l2 state has been observed by absorption of light wavelength 4550 A and above [McDonald and Brus (674)]. The collision-free lifetime of the lluorescenee is about 40 psec. The breaking-off of the emission... 

The collision free lifetime of the V =43 level of the B state is on the order of a few microseconds. The pressure in the laser cavity was estimated to be only lm torr. The He pressure was less than 5 torr for all runs. At these conditions neither self-quenching nor electrgnic quenching by He will significantly alter the lifetime.- A lifetime of a few microseconds is nearly ideal to use as a tracer in our supersonic flow fields in which velocities of 10s cm/sec are typical. This means that the fluorescing I2 would "light up" a region down-... 

The development of a theory of unimolecular reactions proceeded rapidly in the mid-1920s, initiated by Hinshelwood with an A whose collision-free lifetime for reaction was approximated by an energy-independent one. The analysis was much elaborated by Rice and Ramsperger [60] and Kassel [61], known later as the RRK theory, where now the lifetime was, as it is in modern times, energy-dependent [62]. These theoretical works of the 1920s stimulated many measurements of the unimolecular rates of dissociation of organic compounds as a function of the gas pressure. Within a few years, however, this entire field collapsed or, more precisely, evolved into a new field It was shown experimentally that the unimolecular reactions , assumed originally to consist of only one chemical step, in-... 

High-energy ( MeV) monoenergetic positron beams have been built and used in recent years. MV accelerators used in this work have been of Pelletron or Van de Graaff type. Source-free lifetime measurements in a... 

Mort and Lakatos (1970), Mort (1972), and Mort et al. (1972) reported hole mobilities in PVK of 10-7 cm2/Vs at 1.0 x 105 V/cm. The mobilities were field dependent and described by trap-free lifetimes of the order of 10-1 s The activation energy was 0.50 eV. It was reported that the width of the transport band was less than 0.10 eV. Pai (1970) described hole transport in PVK via a transit time with a logit)-1 pE1/2 field dependence. The temperature dependence was described by a field-independent activation energy of 0.36 eV. It was suggested that the field dependence of the transit time was due to the field dependence associated with the time carriers spend in localized states. Bulk trapping was reported for low fields. Mort and Lakatos (1970) and Pai (1970) are the first literature reference to dispersive transport in polymers. 

A method for measuring fluorescence quantum yields and cascade free lifetimes for open shell cations has been reported. The lifetimes are calculated from the coincidence between undispersed fluorescence photons and energy selected photo-electrons. A similar system has been used to evaluate the fluorescence lifetimes of and and fluorobenzene cations." ... 

In the collision-free conditions the fluorescence lifetime and yield are unchanged (initially reported data suggesting a shortening of the collision free lifetime are not confirmed in further works ° ). On the other hand, cross-sections for the collision-induced intersystem crossing are increased by about 40% for all collision partners,... 

Fig. 6.93 Cascade-free lifetime measurements despite the simultaneous excitation of many levels (a) preexcitation by collisions in a gas cell with subsequent laser excitation (b) decay of level k) without cascading curve a), its feeding by cascades curve b), and resulting population Nk t) with cascading and decaying curve c). (c) The fluorescence I x,X) is measured alternately with and without selective laser excitation...

Fig. 6.94 Experimental arrangement for cascade-free lifetime measurements in fast beams of ions or neutrals with fluorescence collection by conically shaped optical-fiber bundles...

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