Beam based lifetime measurements

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

Bimodal or not bimodal—critical comments Is the bimodal distribution as observed by beam based positron lifetime analysis (BPALS) real or a systematic effect of the data analysis To date the answer cannot be given with certainty. Arguments could be made why such a distribution is not observed by SAXS and is observed by BPALS in data shown here [62], and in work by Gidley et al.[46] Positrons are implanted at specific depths and only after measuring at different mean depth can one... 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

Based on lifetime measurements in various A-state, v = 0 and 1, rotational levels (NH3 excitation by electron beam) [1], Franck-Condon (FC) factors r-centroids r, oscillator... 

Figure 16-4. Scheme proposed, based on CASPT2 calculations, for the main decay pathways of adenine (A), measured in molecular beams with intrinsic lifetimes t, < 100 fs and t2 1 ps. Energies in kcal mol-1 referred to the lowest conical intersection. A similar scheme is proposed for guanine (G). (Reproduced from Ref. [50] with permission from Wiley-VCH Verlag)... 

This chapter will begin by looking at some of the hardware requirements for positron-based experiments and then move on to their application in the measurement of angular correlation, positron lifetimes and Doppler broadening parameters. We shall then look at the generation and application of beams of mono-energetic positrons. 

Information of a different sort is obtained in a molecular beam experiment, although the means for producing the species undergoing unimolecular decomposition is also chemical activation. Whereas the conventional kinetic studies yield reaction rates for direct comparison with RRKM lifetimes, the beam technique yields product recoil energy distribution which, in principle, contain information regarding exit channel dynamics specifically ignored in RRKM. Comparison of experimental results with RRKM theory is indirect, requiring additional assumptions whose validity must be determined. Fortunately, however, statistical theories of a different sort exist which base their predictions on asymptotic (and therefore measureable) properties of the... 

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