Lasers and molecular beams

By means of femtochemistry, investigation of elementary reactions on a timescale of femtoseconds (10-15s) is possible. The method employs a combination of pulsed-laser and molecular-beam technologies. Investigation of a unimolecular reaction by femtosecond spectroscopy involves two ultra-fast laser pulses being passed into a beam of reactant molecules. 

A. Me Ilroy, T.D. Hain, H.A. Michelsen, and T.A. Cool. A Laser and Molecular Beam Mass Spectrometer Study of Low-Pressure Dimethyl Ether Flames. Proc. Combust. Inst., 28 1647-1653, 2000. 

We thank Frank Tittel, Y. Liu and Q. Zhang for helpful discussions, encouragement and technical support. This research was supported by the Army Research Office and the Robert A. Welch Foundation, and used a laser and molecular beam apparatus supported by the NSF and the US Department of Energy. H.W.K. acknowledges travel support provided by SERC, UK. J.R.H. and S.C.O B. are Robert A. Welch Predoc-toral Fellows. 

The pioneering work of Wilson and co-workers [71-75] established cross-ed-laser and molecular-beam photofragmentation spectroscopy with time-of-flight (TOF) mass-spectrometric detection as a universal and detailed... 

Figure 5.2 Doppler profiles for IR absorption of 6% HF in a He beam obtained with a laser arrangement shown in Figure 5.1. The sharp narrow peak is obtained by crossing the laser beam at an angle of 90° with respect to the molecular beam, while the broad peak is the result of a 30° angle between the laser and molecular beams. The center line absorption for the P, line is 3920 cm . The shift in the two peaks is a measure of the beam velocity. A 40- jLm-diameter nozzle was used with a stagnation pressure of 6.5 atm. Adapted with permission from Bohac et al. (1992).

Due to the simple and open ion-trap structure, laser and molecular beams can be integrated more easily into the SCSI-MS technique (see Figures 10.2 and 10.3,[11]) than into a FT-ICR mass spectrometer with its large bulky super-conducting solenoid cooled cryogenically. Furthermore, because the SCSI-MS technique is compatible with micro-traps that are under development currently by the ion-trapping community (see for example Stick et al. [12]), this technique has the potential for possible future commercialization. 

How many photoelectrons are counted with a photomultiplier if the fluorescence emitted from the crossing volume Vc = 10 " cm of both the laser and molecular beams is imaged by a lens with P) = 4 cm at a distance L = 8 cm from Vc onto the photocathode with quantum efficiency 7ph = 0.2 ... 

The advent of modem technologies, like lasers and molecular beams, makes it possible to select reactants in a given quantum state and to probe products in a specific quantum state. Thus, we can measure the state-to-state reaction cross-section ctif. It measures the reactive area for a species in the initial i state forming a product that departs in a final / quantum state. Likewise, we can also define the state-to-state differential cross-section which measures the... 

Obviously, the reaction cross-section represents the effective area for which the binary collision produces a chemical reaction. This parameter is necessary for a molecular description of the chemical reaction. It can be measured by laser and molecular beam techniques applied to the study of chemical reactions it can also be calculated by using molecular reaction dynamical theories. The reaction cross-section depends, among other variables, on the collision energy, so that we may write this dependence as... 

For Doppler-free resolution of fluorescence lines (Chap. 9), the laser-induced fluorescence of molecules in a collimated molecular beam can be imaged through a FPI onto the entrance slit of the monochromator (Fig. 4.47). In this case, the crossing point of laser and molecular beams, indeed, represents nearly a point source. 

ABSTRACT. Laser and molecular beam techniques allow detailed study of many dynamical properties of single reactive collisions. The chemical scope of these methods is now very wide and includes internal state preparation of reactants, change of collision energies, state detection of products, and thus determination of state-to-state reaction rates. The great impact of laser spectroscopy on knowledge in the field of structure, molecular energy transfer and the mechanism of elementary chemical reactions is illustrated by two selected examples, i.e. studies in which laser-induced fluorescence (LIF) has been used to determine the specific impact parameter dependence of the Ca + HF -> CaF(X) + H reaction and the product state distributions for the reaction of metastable Ca with SF5. 

FIGURE 1. Crossed laser and molecular beam apparatus for LIF-studies. In addition, the experimental setup for polarization measurements is given [9]. 

Fig. 4.50). In this case, the erossing point of laser and molecular beam, indeed, represents nearly a point source. 

I have chosen to include only those lines whose production requires what may be termed standard techniques . Electrically excited and optically pumped lasers are included, except those involving multi-photon systems, tunable pump lasers, or stark-shifted [1.16] far-infrared lasing. These are all useful techniques but beyond the purposes of this book. Gas-dynamic lasers [1.17], chemical lasers and molecular beam masers [1.18] are also excluded. 

Grice, R. and A. H. Zewail (eds.) (1996). Laser and Molecular Beam Studies of Chemical Reaction Dynamics. Chem. Phys. Amsterdam, Elsevier. 

Lee, Y. T. and Y. R. Shen (1980). Studies with crossed lasers and molecular beams. Phys. Today 33, 52. 

Theory and calculations on the chemical reactions of polyatomic molecules are very active areas of research, " There are several reasons for this. The most modem experimental techniques using lasers and molecular beams are being applied to study the microscopic details of such chemical reactions including how different vibrational modes of polyatomic molecules influence reactivity," and measurements of the lifetimes of reaction complexes. State-selected experiments of this type require detailed quantum reactive scattering theory in their interpretation. Furthermore, there is a need for the accurate calculation of kinetic data such as rate constants of polyatomic reactions that are sometimes difficult to study in the laboratory but are important in areas such as atmospheric, combustion, and interstellar chemistry. 

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