Seeded beams

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

If the gas is a mixture of several pure gases of different molecular weights, with one of the gases representing only a minor constituent of the mixture, then the minor constituent takes on the velocity of the major constituent. This is the seeded beam technique as first verified experimentally by Becker et al. (26, 27). Since the macroions constitute only about 10"8 mole % of the final gaseous mixture, intermediate energy beams of macroions can be produced with the macroions having narrow velocity distributions (2). 

The seeded-beam technique has a further advantage in that the final beam is enriched in the heavier constituent (25, 26). This is the so-called Mach focussing factor. ... 

Determination of the total excitation cross section is again done by observing the resonance-line emission. Both the initial relative kinetic energy and the N2 temperature may be varied. First experiments of this type have studied the excitation of Na(3p) by N2, H2, and D2, vibrationally excited at 2000 to 3000°K for initial relative velocities from 1000 to 4000 m/sec l25-126 as well as both potassium and sodium.127 Recent studies with seeded beams 128131 on K + N2 and K + CO involved lower initial vibrational states v. This type of experiment is very difficult to analyze and evaluation invokes a number of assumptions for fitting the experimental data. A special threshold law for the energy dependence of the partial cross sections is adopted ... 

Figure 17 S0 vs. 0 for ethylene interacting with Ag(41 0) (panel A) and Ag(2 1 0) (panel B). Filled symbols pure beams open symbols seeded beams. The lack of angular dependence is due to strong steering effects.

FIG. 11.4 Induced molecular orientation in the material after the all optical poling process. The difference in the amount of molecules ori ted upward or downward is the largest in the direction parallel to the polarization direction of the seeding beams. In the direction perpendicular to the polarization of the seeding beams the number of molecutes oriented In positive or negative sense remains the same, i.e.,it is centrosymmetric. ... 

If A denotes the phase difference between writing fields at co and 2co frequencies, for linear and parallel seeding beams polarizations, Equation 11.3 can be written as... 

The results of experiments on two samples of 5 and 7 mm thickness present(M in Figure 11.12 show that the SH si al does not depend any more on the relative phase difference between seeding beams. It can be inferred from Equation 11.6... 

FIG. 11.15 Intensity of the SH generated signal at saturation in function of the reluive intensities of the writing beams. Samples are spin-coated films of the DR I-MMA 35/65 copolymer with various thicknesses. Each experimental point is measured for an optimized value of the relative phase difference AO between the seeding beams. 

When the seeding conditions are optimized, the generated SH signal is clearly visible on a white screen. In the case of a 0.3 pm thick film of the DRl-MMA 35/65, it is found that after saturation, the intensity ratio between the 2co generated generated beam 12 and the 2co seeding beam /2a... 

FIG. 11.19 Erasure of the photoinduced polar orientation with a monochromatic light at 2co frequency. The sample was a 0.1. m thick film of the DRI-MMA 35/65 copolymer. After seeding, the sample was continuously irradiated with a 2e> beam (fluence used was a few MW/cm, which corresponds to the 2cn seeding beam intensity). The curve in the insert presents the evolution of the photoinduced birefringence in the same conditions. 

In a recent experiment [50] the sticking of ethylene molecules on a (001) Ag surface, at 80 K and saturated with O2 molecules, has been studied as a function of the degree of alignment of ethylene as produced in a velocity selected supersonic seeded beam. It has been found that the sticking coefficient strongly depends on such an alignment when... 

Note the dependence of the rate on the intensity of the stimulating harmonic or seeding beam at 2(0. On converting the sum over k4 to an integral, utilizing our knowledge of the delta function and converting to a harmonic intensity, we have... 

The seeding of molecular harmonics is in some sense a throwback to experiments where second harmonics were first observed in condensed-matter isotropic systems. For example, in glass fibers it was observed that a harmonic was produced after long exposures to fundamental frequency laser light [99,100]. It was later found that, by introduction of a low intensity seeding beam at the harmonic frequency, the onset of the harmonic in the glass was essentially instantaneous [101]. It was at this time that the proposal of a SWM mechanism... 

---