Beam expansion

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.

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. 

The advent of supersonic molecular beam expansion techniques and high spectral-and time-resolved laser spectroscopic probing has transformed experimental... 

FifTure 4.14 Dependence of the beam expansion angle S on the reduced source pressure polpomvn for the converging nozzle, pmiiin Mm) - 29 mm punnn (0.365 /xm) 51 mm pomin (1-3 pm) = 330 inmHg. 

We limit the presentation of experimental data to those experiments utilizing supersonic molecular beam expansions in combination with UHV techniques. Supersonic beam experiments control the incident kinetic energy and angle of the molecule to a high degree, and thus are generally easier to interpret at the microscopic dynamical level than those experiments utilizing effusive beams. 

For example, let us contrast the dissociations of OCS and H202 in a molecular beam. Because of the strong rotational cooling present in molecular beam expansions, the parent angular momentum will be very small. Thus, in the case of OCS, the dissociation can be considered to occur in a single plane. The OCS bending motion, which provides the torque to give... 

Clusters are generally observed in molecular beam expansions. The low-temperature characteristic of this environment permits the condensation of molecules into dimers, trimers and, if the cooling is sufficient, veritable snowballs with thousands of monomer units. These broad distributions of clusters in a beam cause major problems in detection. While it is possible to control the expansion so that dimers are the dominant cluster (in the presence of a large excess of monomers), the trimers can only be studied in the presence of an excess of dimers, etc. Thus a major experimental problem in the study of neutral clusters is the detection of a particular cluster in the presence of many other sized clusters. 

In many respects the development of tunable infrared and ultraviolet laser sources when combined with molecular beam expansions, mrurked the start of the modern or contemporary period of cluster studies. First, it offered the opportunity to selectively excite specific rovibrational or rovibronic levels in a complex. Second, variations in the spectra (linewidth, intensity and frequency) gave insight into dynamical behavior and the presence of nearby perturbing states. 3 Finally, the availability of widely tunable sources has enabled the experimentalist to select quantum states that would provide the maximum information content on a cluster system, an impetus that continues to drive the development of new lasers and laser systems. As this is an extremely wide field of research, primary emphasis in this chapter will be placed on vibrational spectroscopic studies of neutral and ionic clusters. 

The small amount of radiation that leaks out of the cavity is measured with an appropriate detector. The decay lifetime is affected by the presence of an absorbing species within the cavity. The high finesse, resulting from extremely well fabricated high reflective coatings on the cavity mirrors, translates into an effective path length of tens of meters. If a molecular beam expansion is performed within the cavity, the direct absorption by molecular clusters can be observed. Considerable attention has been paid to the theory associated with the use of both pulsed and cw lasers. This method, while still in its infancy, promises to be another useftil tool in the characterization of hydrogen-bonded neutral molecular clusters. 

A common problem to all Time-of-Flight mass spectrometers is the so called "turn around time" that is due to different initial velocities of the neutrals in the ion source. Even in a RETOF these kinetic energies can destroy the achievable mass resolution of the instrument. The low translational cooling, resulting from the supersonic beam expansion, yields small kinetic energy distributions of the neutral molecules /17/. 

FIGURE 5 The orginal short-pulse tunable dye oscillator, (a) The layout by Hansch, with an intracavity telescope to increase the resolution of the grating, (b) One-dimensional beam expansion in a prism with high incidence angle on the input face and normal incidence on the exit face. A series of four prisms, arranged for no net dispersion, makes up the quad-prism expander that has replaced the Hansch telescope. [Adapted with permission from Klauminzer, G. K. (1977). IEEE J. Quantum Electronics QE-13, 103. 1977 IEEE.]... 

Since prism beam expanders and subsequently multiple-prism arrays have become rather important in many areas of optics, it is appropriate to consider them in some detail and to mention a few relevant historical aspects. The prism as a beam expander was first depicted by Newton, in his book Opticks, in 1704. He also considered prism arrays to control dispersion. Prism pairs applied to beam expansion were introduced by Brewster, in 1813. The prism was first used as a beam expander in a dye laser by Myers in 1971 and by Stokes et al in 1972. Beam expanders comprised of prism pairs and several prisms were introduced independently to tunable lasers by Kasuya et al and Klauminzer in the late 1970s. 

The attraction in REMPI experiments is the potential for mass selectivity in the recorded data, which can be extremely useful for example in the investigation of molecular reactions or van der Waals complexes formed in a supersonic beam expansion. This... 

In laser beam manipulation, two of the mostimportant functions are beam expansion (enlarging the beam diameter) and beam reduction (reducing the beam diameter). As shown in Figure 10.12, there are two different approaches to producing beam expansion or beam reduction, bofli utilizing a combination of two lenses. 

In applications, in which high-powered lasers are used, the second method of beam expansion is the one of choice. This is because the tiny beam spot size created at the common focal point in a Keplerian beam expander results in energy densities that may be high enough to cause the air to break down or ionize. 

An important consequence of the beam expansion is that, to a good approximation, the divergence angle of the expanded beam is related to the divergence angle of the incident unexpanded beam by... 

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