Laser vaporization source

Method. The laser vaporization source eliminates the material constraints inherent in conventional oven sources. This is accomplished by localizing the heating to a very small area at the surface of the sample and by entraining the vapor produced in a rapid flow of high pressure gas. 

Figure 1 is a schematic of the laser vaporization source. This diagram depicts a pulsed valve on the left which supplies high pressure helium flow directly towards the right. Several workers have also chosen to use continuous helium f ows(2,6,9). In general these sources are modifications of conventional supersonic beam sources. 

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

The field of gas-phase transition metal cluster chemistry has expanded rapidly due to the development of the laser vaporization source and the fast flow chemical reactor. The work from the three major laboratories have been reviewed. Many additional laboratories are developing cluster chemistry programs and will soon certainly make significant contributions. 

The Ti+ ions, produced by using a laser vaporization source (cooled by collisions with He) have been reacted with NH3 to produce the dehydrogenated product ions, [TiNH]+ (100). Many early transition metal ions and Os+ produce the [MNH]+ ion (9,106). The ion [TiNH]+ was reactive toward NH3 and increasing the concentration of NH3 in the drift tube (100) allowed up to four NH3 molecules to add to the [TiNH]+ ion, thus producing five-coordinate Ti in the gas phase. 

Figure 9.61 ToF mass spectrum of metal-carbon cluster ions (TiC2+ and ZrnCm+ cluster ions) using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, = 532 nmj and ionization by a XeCI excimer laser (308 ). ( . M. Davis, S. J. Peppernick and A. W Castleman, J. Chem. Phys., 124, 164304(2006). Reproduced by permission of American Institute of Physics.)...

The ionization potentials (IPs) of ammonia clusters containing alkali metal atoms, such as Li [10], Na [8] and Cs [9], have been reported by Hertel s and Fuke s groups. These clusters have been prepared by pickup sources coupled with a heated oven (Na and Cs) or a laser-vaporization source (Li). The IP(n) values decrease almost linearly with (n-f 1) , which is approximately proportional to the inverse of the cluster radius. Although the IPs of free atoms are different (5.392, 5.139 and 3.894 eV for Li, Na and Cs, respectively), those of the clusters (n > 5) are almost the same irrespective to the metal atoms. The intercept at (n + 1) 0... 

Another remarkable endohedral complex which, has recently been created is U C2s.[Gu92] This affords a counterexample to the observation that endohedral complexes are typically based on fullerenes larger than Ceo- Indeed, C28 appears to be the smallest fullerene produced in supersonic cluster beams from a laser-vaporization source. Photofragmentation experiments ve an additional sense of how strongly the internal U stabilizes C28- Larger, empty fullerenes can be blasted down to C32, but... 

Figure 1 is a schematic of a typical laser vaporization source used in our laboratory. A pulsed valve introduces high-pressure helium over the target, synchronized with the firing of a pulsed laser used to produce the metal vapor. For experiments such as the measurement of absolute reaction rate constants, it is advantageous to use a continuous-flow supersonic beam. ... 

Figure 1. Schematic of laser vaporization source/fast-flow reactor configuration currently employed in our laboratory.

In a seeded supersonic expansion source [35-37[, the material is heated in an oven and a mixture of gaseous material and a seed gas expands through a nozzle into the vacuum. This source produces a highly intense beam of small pure and mixed metal clusters, but it is limited to metals with a low boiling point (Li, Na, K, etc.). Laser vaporization sources [13,38[ are more widely used as they produce pure and mixed clusters of most elements and when operated at low frequency (1-10 Hz) they have been successfully used... 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

Fig. 1.29. Schematic sketch of the collision cell method for the study of metal cluster reactivity. The supersonic laser vaporization source is depicted on the right hand side. The clusters subsequently pass two collision cells in which reactions can take place. Finally, laser ionization mass spectrometry serves to detect the neutral reaction products [3]...

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

P. Pradel, P. Monchicourt, J.J. Laucagne, M. Perdrix, and G. Watel, Carbon cluster ion formation in a direct laser vaporization source, Chem. Phys. Lett. 1989, 158, 412-416. 

FIG. 5. Schematic overview of the set-up with the laser vaporization source and the nozzle for supersonic expansion used in the production of clusters at Goteborg. The figure shows the laser vaporization source with the target material, the laser beam for evaporation, the small volume where a plasma of atoms and ions exist and the region where the clusters are formed in the expansion through the nozzle. 

The breakthrough experiment was carried out by Whitham et al. [39,40] in France. They used a Smalley-type laser vaporization source (Fig. 4) to provide a molecular beam of Ca atoms entrained in He or Ar gas. The second harmonic (532 nm) from a pulsed Nd YAG laser was focused (Fig. 4) on a rotating calcium rod. About 500 jus prior to this, a pulsed valve (left side of Fig. 4) is opened and the plume of vaporized metal is entrained in Ar or He gas. The carrier gas is seeded with a few percent of the oxidant such as H20. The plume of excited- and ground-state metal atoms are carried down a short channel and react with the oxidant. At the end of the channel, the product molecules such as CaOH expand into the vacuum chamber and cool. After a short expansion, the pressure has dropped so low that the molecules are effectively in a collisionless, ultracold (<10K) environment. 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

BeOH and MgOH (along with A-X transition of BaOH) are in need of rotational analysis. Very little work is available for other more highly excited states, except in the case of CaOH. Jarman and Bemath [85] located the C2A state of CaOH and very recently Pereira and Levy [67] found the D2E+, 2Z+, and F states at 28,153, 29,879, and 30,215cm-1, respectively. In these experiments, pulsed lasers were used with a pulsed laser vaporization source. The F state is the first bent covalent state to be identified for CaOH and it correlates with the C2n state of CaF (Fig. 7). The first unambiguous measurement of the OH and OD stretching frequency (Table 2) was also possible because of the favorable Franck Condron factors in the F (bent) - X (linear) transition. 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

The aromaticity concept has been introduced to the chemistry literature through benzene and other cyclic molecules and their extension to inorganic molecules like borazine and silicazine. The seminal work of Boldyrev and coworkers has paved the way to have a recent upsurge of interest in the field of aromaticity and antiaromaticity of metal clusters (Fig. 13-17). Negative ion photoelectron spectroscopy using a laser vaporization source augmented by ab initio calculations have authenticated the aromatic nature... 

In this subsection we describe, in more detail, the Ba FCH3 + liz/ —> product intra-cluster reaction, which has been extensively studied in frequency- and time-domain experiments. The weakly bound complex Ba - -FCH3 is produced in a laser vaporization source, followed by supersonic expansion. A gas pulse from a mixture of He with CH3F is generated, and the output of an Nd YAG laser is focused onto the surface of a rotating barium disk to produce a vapour of Ba that is injected into the gas pulse. This mixture expands supersonically into the vacuum chamber. 

Piuzzi P, Dimicoli 1, Mons M, Tardivel B, Zhao Q (2000) A simple laser vaporization source for thermally fragile molecules coupled to a supersonic expansion application to the spectroscopy of tryptophan. Chem Phys Lett 320 282... 

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