Short delay time

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

However, at very short delay time, some of the hot toluene which has not dissociated into fragments could absorb the VUV photon and result in dissociative ionization. 

The current gas supply consists of two capillaries of different diameters that satisfy the previously mentioned pressure values (30 and 180 pm). Thus, a rapid pressure drop across a short length is accomplished with a short delay time, which is about 350 ms in this arrangement. The pressure drop of the sample gas for the plasma chamber in operation mode is shown in Fig. 18. 

Eoor fitting to the biexponential decay function in the short delay times, the estimated uorescence decay times obtained from the measurements at the magic angle were about 10 and 220 ps. From the fitting of the polarized fluorescence in the long delay times, the rotational relaxation time was obtained to be 124 ps. Reliable value for r(0) was not obtained for this case. 

In fact this reaction has been suggested by Benson and Buss as a replacement for reaction (31). If reaction (48) is in equilibrium and reaction (49) replaces reaction (31), then the rate coefficient attributed to reaction (31) is really 49 48,-48- Nicholas and Norrish have re-examined the flash photolysis of CI2-O2 mixtures at room temperature and monitored the products at short delay times before [CIO] became large. Their results support reaction (49) and they report it-48/ 49 15 or 7.7, depending on the extinction coefficient of CIO. They also found = (6.2+1.l)x 10 l. mole . sec with N2 as a chaperone. 

Methyl radicals were indeed detected at short delay times in flash photolysis , and ESR studies have demonstrated the presence of CHj and NO2 radicals . In addition, large yields of CH are formed in the photolysis, presumably by abstraction reactions of CHj . Recently, experiments by kinetic flash photolysis have shown that the primary step leads to C-N cleavage and that the formation of methyl nitrite is explained by step (b). In contrast to previous results the... 

In the flash photolysis of methyl nitrite, NO was observed at short delay times ... 

All vibrational levels of the ground electronic state are populated up to at least u" = 6 (3). The relative population of the first five levels has been measured as a function of time and of pressure in the range C2N2 = 0.14 to 56 mm., CNBr = 3.6 X 10 to 36 mm., and CNI = 2.5 X 10 2 to 0.27 mm. At short delay times and moderately low pressures, these populations are approximately equal— i.e., there is an infinite vibrational temperature. In about 100 fUsec, all excited levels decay, but the zeroth level can be followed for 600 jusec. At very low pressures, the population of the fomih excited level can exceed that of the zeroth level by a factor of 4 and the decay is much slower. 

Fig. 10.5 (a) A schematic drawing of CO bound atop on the step and terrace sites of the Pt(533) surface, (b) Sum-frequency spectra for CO bound atop on the step and terrace sites exhibiting resonances at 2,080 and 2,100 cmrespectively, (c) Experimental (grey) and calculated (black, normalized to experimental curves) pump-probe spectra at short delay times for a surface partially covered with CO (75% of steps occupied and 20% of terrace sites) for excitation with a laser pulse with a fluence of 60 Jm . Reprinted with permission from [1]. Copyright 2005 AAAS... 

Ne recently studied the formation of the solvated electron in pure ethane-1,2-diol by photo-ionisation of the solvent [18,32]. The results showed that the excess electron presents a wide absorption band in the visible and near-IR domains at short delay times after the pump pulse, and that the red part ofthe absorption band drops rapidly in the first few picoseconds while the blue part increases slightly (Fig. 9). The time resolved spectra were fitted correctly by either one of two solvation models a stepwise mechanism involving several distinct species and a continuous relaxation model. In Figure 10 are reported, as an example, the kinetics and spectra of the three successive species (the weakly bound the strongly bound e and the solvated electron e/) involved in the electron solvation dynamics according to the stepwise model. 

A second important observation is that the temporal shape of the emission rise follo vs a step function. This is demonstrated in Fig. 11.8. It sho vs the results of fitting an exponential increase convoluted with the crosscorrelation (a), a delayed step like rise convoluted with the crosscorrelation (b) and the complete model function (c) to an experimental trace of HBT at a probe wavelength of 564 nm where the oscillatory contributions are quite weak. The exponential increase and the delayed step function give almost the same ESIPT time [33]. However, the exponential increase deviates significantly from the data at short delay times whereas the step function matches quite accurately the essential shape of the trace. 

Fluorescence applied to oil identification has been an active field, with 17 papers presented on the subject at the last three Pittsburgh Conferences. A number of interesting developments for fluorescence and low-temperature luminescence (LTL) are described by Eastwood et al. (58). These include synchronous scanning, difference spectrofluorometry, synchronous difference spectroscopy, derivative spectroscopy, and total luminescence (or contour) spectroscopy and combinations of these techniques. In a recent presentation, Eastwood and Hendrick (59) reported an extension of their low-temperature luminescence studies to include polarized excitation and emission spectroscopy, and time-resolved phosphorescence. Preliminary studies of polarization effects indicate that differences exist in low-temperature polarized luminescence spectra of oils, which may aid in oil identification. In the time-resolved phosphorescence spectra of oils, the most significant difference observed was enhancement of the vanadyl porphyrin signal at approximately 700 nm for short delay times (20 fxsec). 

To combine the advantages of both types, Nellcor designed a sensor see also Figure 23-20) which uses only 1 m of sample line and an extremely small sample cell and offers the advantage of short delay times of about 1 s, low flows of SO mL/min and short rise times of about S5 ms [23]. These features also offer acceptable monitoring of neonates. In practical use. the features arc slightly influenced by the normal problems in sidestream sensors with condensed water also which arc suposed to be overcome by (he proximal-diverting monitor. 

From the simulation data, the correlation time is found by integrating the time correlation function as shown in Eq. (6). While the correlation functions may be easily computed from the trajectories, statistical errors due to finite trajectory length limit the useful data to short delay times r [16,17]. The variance of a Gaus-... 

In the pulsed mass-spectrometer ion-source experiments, the distribution of internal states of the ions is not defined. Reliable results may only be obtained for short delay times at low pressure, thereby restricting the application to fast reactions. Difficulty may also be experienced in identifying reaction pathways. 

FIG U RE 15.27 A diagram of High-Q Pulsed CID. Precursor ions are excited at a higher q -value, ca 0.8, for a short period, then after a short delay time, for example 1 ms, the trapping RF level is reduced to a lower value of q, for example, 0.1. 

In Fig. 7.38 the auto-correlation function G(t) of dye molecules in a liquid sample excited by polarized light is shown [934]. The step increase for short delay times r is due to the anti-bunching effect . When a molecule is excited at time t it can be only again excited after a is has returned into the initial ground state by spontaneous... 

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