Laser-fired

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

As we pointed out above, an attractive feature of the velocity selected K collisions is that we can examine the region o) 2n < 1/r. We consider first the case in which there is no control over the phase of the field at which the collisions occur. This case is exemplified by Figs. 15.11(b)-(d), which show the effect of adding progressively larger amplitude 1 MHz rf fields of uncontrolled phase. Since the laser fires at an uncontrolled phase of the rf field, the observed cross section can be calculated using Eq. (15.34). As shown by Fig. 15.11 the cross section is broadened in approximately the manner shown in Fig. 15.7. As shown by a... 

Figure 11.4. Comparison of data acquired for same sample at different laser firing frequencies (10 and 1400 Hz). The data acquisition time is much more rapid and measurable sensitivity is improved.

A second, older type of X-ray laser uses a powerful Nd-YAG pumping laser (e.g., Nova) to excite plasmas (e.g., Ne-like Ti+12 ions), which in turn can emit soft X rays by now even tabletop soft X-ray lasers exist (using the chirped-amplification of a Nd YAG laser fired once every 3 to 4 minutes, and Ni-like Pd1 4 ions). 

In a typical experimental measurement a Nd YAG laser fires a 5 ns pulse on to the target electrode. It is convenient to use a polymeric material for the electrode, made conductive by the addition of conductive carbon black, to achieve intimate contact with the specimen. Adding a thin layer of a volatile liquid to the electrode surface just prior to firing the laser greatly enhances the magnitude of the signal, by helping to increase the size of the pressure pulse... 

Each time the photolysis laser fires a complete profile of the OH decay is obtained and recorded. A number of such decays are stored and averaged to reduce the signal to noise and bimolecular rate coefficients are obtained at a number of different temperatures by the procedures outlined above. 

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

In an extremely inventive approach, PDT with liposomes has been used to achieve localized delivery of highly toxic, short-lived, free radicals to neovascular tissue. A dye, photoporphorin, is encapsulated in liposomes which are injected intravenously. A laser fired into the eye simultaneously destabilizes the dye molecule causing it to form a free radical and destabilizes the liposome causing it to release the free radical. The free radical interacts with the first tissue it encounters, in this case the neovascular tissue. PDT is one of the only treatments currently available for age-related macular degeneration (see Chapter 6). 

The nitrogen pump lasers are triggered by a common master control unit with delay lines arranged so that each laser fires at predetermined and well-controlled times with respect to the others. The dye laser pulses were monitored by a fast vacuum photodiode and oscilloscope. The dye lasers provided 5-10 ns pulses having 0.5-2.0 cm spectral linewidths with less than 5 ns jitter. A boxcar integrated the signal received from the particle multiplier. 

The firing of the dissociation excimer laser is delayed by 570-640 /is with respect to the triggering pulse for opening the pulsed valve. We find that the intensity remains nearly constant when the delay between the photodissociation and ionization laser firings is varied in the range of 30-100 ns. 

The time behavior of the C2(a n ) population under typical conditions is illustrated in Figure 2. The very fast rise occurs when the excimer laser fires and the C2 is produced. There is some... 

What " she asked. By then they had reached the conference office anteroom. The partizan bodyguards were all. sprawled on the floor with limbs twitching erratically. Six Edenists stood over them, their TIP pistols pointing down. Three scorch lines slashed the walls where laser fire had. burned the composite. A pair of spent nerve short-out grenades rolled around on the carpet. 

For more concentrated samples, the variation of the remission function before and after the laser firing is given by... 

Figure 4 - Insert shows the effect of blocking the confining laser beams 15 ms after the YAG laser fires. The horizontal trace is the scattered 1ight level. The fraction of atoms remaining is plotted a function of the time-off period.

To determine the incident beam velocity distribution, a time-of-f light spectrum shown in figure 3 was taken with the ultraviolet probe laser positioned in the incident beam. Since the infrared laser fires for a few nanoseconds during the 600 msec duration of the molecular beam pulse, a short pulse of NO (v=l,J=3/2,0=1/2) is produced on top of a 600 microsecond background pulse of NO v=0 distributed over the lowest few rotational states. The molecular beam flux-velocity distribution, I(v)dv = number of particles/area/sec with velocities in the range v - v+dv, can be characterized by ... 

In laser SNMS, there are two operating modes [292] on the one hand, with a short ion pulse the laser volume can just be filled when the laser fires. This ensures minimal sample consumption but results in fractionation effects because of the different velocities of different masses. For high accuracy, a standard sample that is sufficiently similar in regard to the velocity distributions is required [282]. The other mode uses a longer ion pulse (>1 ps) to achieve a steady-state composition... 

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