Experimental probes of reactive collisions

Detailed experimental information about the intimate processes that occur during reactive encounters comes from molecular beams, especially crossed molecular beams (Fig. 24.12). The detector for the products of the collision of two beams can be moved to different angles, so the angular distribution of the products can be determined. Because the molecules in the incoming beams can be prepared with different energies (for example, with different translational energies by using rotating sectors and supersonic nozzles, with different vibrational energies by using selective excitation with lasers, as shown in Section 24.9b, and with different orientations (by using electric fields), it is possible to study the dependence of the success of collisions on these variables and to study how they affect the properties of the outcoming product molecules.

Fig. 24.12 In a crossed-beam experiment, state-selected molecules are generated in two separate sources, and are directed perpendicular to one another. The detector responds to molecules (which may be product molecules if chemical reaction occurs) scattered into a chosen direction.

One method for examining the energy distribution in the products is infrared chemiluminescence, in which vibrationally excited molecules emit infrared radiation as they return to their ground states. By studying the intensities of the infrared emis sion spectrum, the populations of the vibrational states may be determined (Fig. 24.13). Another method makes use of laser-induced fluorescence. In this technique, a laser is used to excite a product molecule from a specific vibration-rotation level; the intensity of the fluorescence from the upper state is monitored and interpreted in terms of the population of the initial vibration-rotation state. When the molecules being studied do not fluoresce efficiently, coherent anti-Stokes Raman spectroscopy (CARS, Section 13.16) can be used to monitor the progress of reaction. Multiphoton ionization (MPI) techniques are also good alternatives for the study of weakly fluorescing molecules. In MPI, the absorption of several photons by a molecule result in ionization if the total photon energy is greater than the ionization energy of the molecule. One or more pulsed lasers are used to generate the molecular ions, which are commonly detected by time-of-flight mass spectrometry (TOF-MS, Section 19.2). The angular distribution of products can also be determined by reaction product imaging. In this technique, product ions are accelerated by an electric field towards a phosphorescent screen and the light emitted from specific spots where the ions struck the screen is imaged by a charge-coupled device (CCD, Further information 13.1). An important variant of MPI is resonant multiphoton ionization (REMPI), in which one or more photons promote a molecule to an electronically excited state and then additional photons are used to generate ions from the excited state. The power of REMPI lies in the fact that the experimenter can choose which reactant or product to study by tuning the laser frequency to the electronic absorption band of a specific molecule.

Fig. 24.13 Infrared chemiluminescence from CO produced in the reaction O + CS → CO+S arises from the non-equilibrium populations of the vibrational states of CO and the radiative relaxation to equilibrium.

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مواضيع ذات صلة

Photosensitization

Rate laws of complex photochemical reactions

The overall quantum yield of a photochemical reaction

Electron transfer reactions

Resonance energy transfer

The primary quantum yield

Timescales of photophysical processes

The catalytic efficiency of enzymes

The Michaelis–Menten mechanism of enzyme catalysis

Enzymes

Application of the techniques

Monitoring the progress of a reaction