Time-resolved gated detection

Fig. 1.13. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A Ti02 (0.3M) modified with ascorbic acid (0.08 M). The lower trace was obtained with a 550 nm laser (laser intensity 10 mJ per pulse, 10 ns pulse duration, 20 scans), 1 (is after the laser pulse. Both spectra were recorded at 8 K. Insert schematic presentation of events in time-resolved direct detection. The spectrum is taken at the time x after the laser pulse for each magnetic field using gate integrators. Magnetic field (H) is not modulated.

Unusual behavior of the luminescence line at 417 nm has been detected by time-resolved spectroscopy. It is usually ascribed to Tb ", but sometimes in spectra with a narrow gate this line remains strong, while other lines of Tb disappear (Fig. 4.8b,c). The supposition that those lines are connected with Nd " " was confirmed by our study of CaW04 Nd, where, besides the known IR, the group of UV and violet lines with short decay times are detected, while in CaW04 Tb such lines are absent (Fig. 5.7). 

Figures 4.31a,b represent narrow luminescence hnes detected in barite by time-resolved spectroscopy. Much weaker lines at 446 and 672 nm accompany the strongest one at 588 nm. They have a relatively short decay time of 5 ps and emphasized in the spectrum with short gate. Such a combination of spectral and kinetic properties is not suitable for any trivalent REE besides P j2 f9/2...

The main feature is the green emission corresponding to transitions from state S j2 to the ground state At the same time, an intense luminescence may be detected at 1.5 pm, which is caused by resonance transitions 13/215/2- The presence of green luminescence indicates that the de-activation of the high level accompanied by IR emission is not complete, but it results in a relatively short decay time of Er " green emission. Thus the luminescence of Er is easier to detect in time-resolved spectra with a narrow gate. In order for correct identification of Er " " lines in minerals several of them were synthesized and artificially activated by Er (Fig. 5.19). Besides that, comparison has been made with CL spectra of synthetic minerals artificially activated by Er (Blank et al. 2000). 

Thulium displays in minerals an intense UV and blue visible luminescence with a line spectrum near 360 and 450 nm, correspondingly. They are connected with electron transitions from different excited levels D2 and at 360-365 and 450-455 nm. The liuninescence of Tm " is more easily detected in time-resolved spectra with a narrow gate, because it usually has a relatively short decay time. The UV Hne usually has a much shorter decay time compared with the blue line. Different decay times from these levels are evidently connected with nonradiative relaxation due to the presence of high frequency vibrations in the lattice. The best excitation is at 355 nm, which is connected with transition... 

Another evidently radiation-induced band occurs in the orange part of spectrum. Under long waved UV and visible excitations the band peaking at 600 nm is detected with half-width of 95 nm (Fig. 5.66a). Excitation spectrum of this emission contains for maxima peaking at 345,360 and 410 nm (Fig. 5.66b). The band is evidently not symmetrical with shoulder at 625 nm, but such form remains in all time-resolved spectra with different delays and gates and does not resolved to several emission bands. This band can be detected with extremely narrow gate width, which is a strong evidence that its decay time is very short, approximately 10-12 ns, which is on the border of our experimental system alrility. At 40 K the band becomes extremely intensive, while its spectrum and decay time remain practically the same. 

The luminescence spectrum of the Canada apatite contains the yellow band, which is similar to Mn + emission in the Ca(II) site (Fig. 5.71). Nevertheless, this band has short decay time, which is not suitable for strictly forbidden d-d transitions in Mn +. It dominates in the time-resolved spectrum with a delay of 10 ps and gate of 100 ps when the shorter-lived centers are quenched, while the longer-Hved ones are not detected. A change in the lifetime may be indicative of the energy transfer from Mn + by a radiationless mechanism. A condition necessary for this mechanism is coincidence or a close distance between energy level pairs of the ion sensitizer and the ion activator. Here, the process of luminescence is of an additive nature and a longer duration and greater quantum yield of the activator luminescence accompany a reduced... 

The Experimental Technique chapter describes our experimental setup with the following main parts laser source (Ar, excimer, Nd-YAG, nitrogen, dye, OPO), imaging monochromator, gated detector (Intensified Charge Coupled Device) and computer with corresponding software. The main features of the experimental devices are described, which enable us to accomphsh time-resolved detection. 

The first measurements of Na nd fine structure intervals using quantum beats were the measurements of Haroche et al41 in which they detected the polarized time resolved nd-3p fluorescence subsequent to polarized laser excitation for n=9 and 10. Specifically, they excited Na atoms in a glass cell with two counterpropa-gating dye laser beams tuned to the 3s1/2—> 3p3/2 and 3p3/2— ndj transitions. The two laser beams had orthogonal linear polarization vectors et and e2 as shown in Fig. 16.9. 

Figure 28. The 300 K time-resolved luminescence spectra of Mn2+ ZnS nanocrystals in polyvinyl butyral (PVB) films, collected at the various delay times indicated following a 248-nm excimer laser pulse (pulse width 40 ns, 2 Hz, excitation density = 5.6 mJ/cm2, detection gate width = 2 ps). [Adapted from (133).]...

Some degree of temporal resolution of emission may be obtained by incorporating a phosphoroscope attachment in the simple apparatus described above. A mechanical or electronic device is used to allow periodic and out-of-phase excitation and detection of luminescence. In the simplest case a mechanical shutter interrupts the excitation beam periodically and the detection system is gated so that emission is observed only after a fixed interval of time has elapsed after excitation. Under these conditions short-lived processes such as prompt fluorescence will have decayed to zero intensity and only longer-lived emission will be recorded. For mechanical devices the limit of measurable lifetime is of the order of 1 ms, thus allowing time resolved studies to be made of certain phosphorescence and delayed emission procesres (see ... 

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