Laser beam expansion, detection

Laser beam expansion was also employed in LPA laser detection of HO (36-39). Hubler et al. (38) calculated an asymptotic laser-generated HO concentration in their quasi-continuous-wave expanded laser beam by assuming a chemical decay lifetime of 1 s for the excess HO. Chemical recycling of this HO was assumed to be slow with respect to the residence time of air in the laser beam. 

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.

The deformation appears as a surface bump that can be detected during the read operation because the intensity of light reflected back to the detector will be different from that reflected from a flat area. Erasure is effected by exposure to a second laser beam at 780 nm. Here, light is absorbed by the dye in the retention layer but not the expansion layer the retention layer is softened, and the elastic energy stored in the expansion layer restores planarity to the laminated structure. 

If the first laser beam is interrupted at the time to for a time interval At that is short compared to the transit time T = z2 — Z )/v (this can be realized by a Pock-els cell or a fast mechanical chopper), a pulse of molecules in level /) can pass the pump region without being depleted. Because of their velocity distribution, the different molecules reach zi at different times t = to- -T. The time-resolved detection of the fluorescence intensity /fi(0 induced by the second, noninterrupted laser beam yields the distribution n T) = n(Az/v), which can be converted by a Fourier transformation into the velocity distribution n(v). Figure 4.13 shows as an example the velocity distribution of Na atoms and Na2 molecules in a sodium beam in the intermediate range between effusive and supersonic conditions. If the molecules Na2 had been formed in the reservoir before the expansion, one would expect the relation Up(Na) = V2up(Na2) because the mass m(Na2) = 2m(Na). The result of Fig. 4.13 proves that the Na2 molecules have a larger most probable velocity Up. This implies that most of the dimers are formed during the adiabatic expansion [410]. 

Bulk analytical methods, including laser beam deflection and X-ray or neutron diffraction techniques, have been used to measure lattice strains and stresses. These methods, however, are limited to measuring lattice volume expansion due to li-ion intercalation on the molecular scale. They cannot detect extended defects, cracks, or microfractures, and thus cannot be directly used for mechanics analysis of particles or porous particle networks. Even with this limitation, diffraction or deflection techniques can stiU provide valuable information on lattice structural... 

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

Cluster synthesis by supersonic expansion normally yields broad distribution of cluster sizes. The measurement of any size-dependent property depends on the availability of a mass-specific detection scheme [20]. Relatively nondestructive and selective methods for assaying neutral cluster size distributions have been provided by combinations of mass spectrometry with (a) laser techniques (resonant two-photon ionization (R2PI) [21-26], depletion spectroscopy [27]) and (b) helium cross-beam scattering techniques [28]. 

Fig. 2.17. Side elevation of the vacuum setup of the molecular beam machine. Chi vacuum chamber to generate the molecular beam by adiabatic expansion. The oven shown in Fig. 2.19 is inside. Ch2 vacuum chamber where the molecules interact with the laser pulses and are detected by a quadrupole mass spectrometer (QMS) or a Langmuir-Taylor detector (Fig. 2.22). Chi is pumped by an oil diffusion pump (DP) with a cold trap (CT), Ch2 by a turbomolecular pump (TP). Prevacuum is provided by a Roots (RVP) and a rotary valve vacuum pump (RVVP)...

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