612 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics
of the other halogens. The reactant molecules pass into a mass spectrometer that can
be moved to different angles so that the angular distribution of the products can be
studied. The TOF selector allows for determination of the velocity distribution of the
products, giving information on the distribution of energy between translational and
internal degrees of freedom. Other types of detectors are used. For example, a surface
ionization detector can detect alkali metal atoms.^39
It is also possible to carry out a reaction by bringing a beam of molecules into a
stationary gaseous sample. The reaction cross section can be measured by the atten-
uation of the beam. The reaction cross section generally depends on the states of the
reactants and products and on the collision energy. An ordinary chemical reaction is
a sum of such reactions, since various states of the reactant and product molecules
are represented in a system of many molecules. The “ideal” molecular beam kinetics
experiment would give the reaction cross section for different values of the collision
energy and for different states of the reactants, the angular distribution of products,
the velocity distribution of the product molecules, and the distribution of electronic,
vibrational, and rotational states of the products. No single experiment has given all
of these pieces of information, but each of them has been obtained in at least one kind
of experiment. Some techniques used in molecular beam experiments are:^40- Chemiluminescence. In this method, radiation emitted by excited products is spec-
troscopically analyzed as it is emitted. The intensities of radiation due to various
transitions can be used to determine the population distribution for product states.
Modern techniques also allow time-resolved spectra to be observed (intensity as a
function of time as well as of wavelength). Measurements in the picosecond region
are becoming common and femtosecond measurements are being carried out. - Chemical lasers. Some reactions produce product molecules with an inverted pop-
ulation distribution. That is, the population of some state of higher energy is larger
than that of some state of lower energy. In this case, achemical laseris possible,
in which incident radiation can cause stimulated emission, and in which radiation
of the same wavelength is emitted. For example, the flash photolysis of trifluo-
roiodomethane in the presence of hydrogen and a buffer gas can produce excited
HF molecules with a population inversion:
(1) CF 3 I
uv flash
−→ F+CF 2 I (13.6-3a)(2) F+H 2 −→H+HF∗ (13.6-3b)(3) HF∗
stimulated emission
−→ HF+hν (13.6-3c)Incident radiation of the proper frequency can cause emission of radiation from the
excited HF molecules. Figure 13.23 shows the laser emission as a function of time
for a number of transitions in this system.- Laser pump and laser probe. In this technique one laser is trained on a beam of
reactant molecules, essentially using photons as one of the reagents. A second laser
is trained on the beam in the product region, raising product molecules to excited
states, from which they fluoresce. Spectroscopic analysis of the fluorescent radiation
gives information about the distribution of products and their states.
(^39) G. G. Hammes,Principles of Chemical Kinetics, Academic Press, New York, 1978, p. 113ff.
(^40) G. G. Hammes,op. cit., p. 210ff (note 39).