THE ELECTROMAGNETIC SPECTRUM IN CHEMISTRY MANSEL DAVIES University ~ d ~ e ofg Wales, e Aberystwyth, Wales
ONEof the major topics of chemical physics is the interaction of matter and radiation. While it appears that the essential nature of the electromagnetic waves does not change over the whole of the observable spectrum, their interaction with matter changes very markedly from one region t o another. This is a consequence of the range of energy levels which play a determinative role in the structural organization of material systems. Given the Bohr relation, E = hv, it is not difficult to understand why the most significant effect of slowly varying fields is the linear displacement of charged particles or the angular displacement of dipoles (i. e., small changes in the (classically) continuously variable linear or angular momenta of particles) while the highest frequencies are able to shake apart even the constituents of atomic nuclei. The overriding characteristic of the electromagnetic spectrum is the velocity of propagation of the waves, c = 3 X 10'O cm. per sec. For most chemical aspects it suffices to treat the waves as electrical oscillations of sinusoidal form, taking place transversely to the direction of propagation. The associated magnetic oscillation lies in the plane perpendicular to that of the electric field. With their constant velocity, the waves are sufficiently described either by their length (A) in cm., representing the distance from crest to crest in the wave displacement, or alternately, by their frequency (v) in see.-', i. e., the number of waves passing per second. These are of course related by: vh = c. I n various regions it is convenient to use multiples or submultiples of the centimeter in designating?; e. g., km. = lo5cm.; M (micron) = 10W4 em.; A. (Angstrom) = cm. Another unit closely related to the preceding is the wave number (w), defined as w = 1/X (i. e., the number
Comic ond 7-rays. Quanta orlbofe or demm. pore nudei. R.diochemirtr*.
Ultr~riolet. Eledronic spectrn. Abrorpfion. with oct;r.tia, ioni..,ion, and decomposition of rno1&dn.
of waves per cm.), whose dimensions are thus cm.-', and w = u/c. It may be recalled that the total energy needed to give one quantum t o each molecule in one gram mol, Nhv = N h w , is the einstein. This energy measure is clearly proportional to the frequency or wave number and is, in the chemical units of calories per mol, conveniently related to the wave number by the constant factor Nhc. The einstein for w = 1 em.-', or A = 1em., is 2.858 cal. per mol. Some of the salient points of interest to the chemist in various regions of the electromagnetic spectrum are indicated in the figure.' It need scarcely be emphasized that the regions are arbitrarily defined, even the visible range varying from one individual to another and more markedly from one species to another. I t is equally clear that the various regions arise from the changing reactions of matter to the radiation. The size of the particles which can be moved by the waves decreases from microscopic or colloidal, through molecular, to electronic and nuclear as the wave length decreases from very large (infinite for direct currents) to very small values. An appreciable fraction of a systematic course in physical chemistly can be traversed by following the continuous line of the spectrum, and it must suffice here to comment briefly on some items. Of the chemical effects of a direct current ( v = 0, X = m), those of eIectrolysis and electrode polarization by ionic discharge are especially characteristic. The generation of direct currents by chemical change, studied in the e. m. f. measurements of cells, is a typical instance of emission. i. e . . the reverse Drocess of ahsomA chart of this kind can well take its place in the lecture room alongside the periodic t.ahle.
M k r o r o r e ond infrared region. Molecular r o b tion,. bond ribroliar. Abm % p irt, ,pec,r. for Itrudurc ."d
anolyrir.
JOURNAL OF CWZMICAL EDUCATION
tion of radiation. Most frequently, chemical potential is degenerated into heat. This, a t room temperatures, is characterized by wave lengths of the order of 10 r (w = lo3 cm.-'), but it may appear initially, e. g., in chemiluminescent systems, a t the far higher frequencies of the visible or ultraviolet regions. Similarly, nuclear reactions produce gamma or cosmic rays. The molecular response to alternating fields of fre10 km.) is quencies below about 3 X lo4 set.-I (A very similar to that for static fields, because the times of oscillation of the field, sec. or more, are much greater than the intervals needed by the ions or molecules to adjust themselves to the changing fields. As detection and amplification of these frequencies in electrical bridges is often far more convenient than the use of direct currents, they are of great service in conductance and dielectric measurements. The latter provide the means of determining permanent molecular dipole moments. As the frequency increases up to 3 X 101° set.-' (A = 1 cm.) the equilibrium distributions of ions ( i . e., the Debye-Hiickel ionic atmospheres) and the rotation of polar molecules fail to keep up with the rapid oscillations. Accordingly, typical dispersion effects appear, the conductance or dielectric constant changing with the frequency. The lack of synchronism between oscillating field and dipole rotation results in electrical energy's being dissipated in the medium-. e., dielectric absorption (or dielectric loss) occurs. This is a maximum for approximately that frequency whose time of oscillation coincides with the time taken by the dipoles to t,um around when the field is instantaneously reversed. When the wave lengths become of the same dimeusions as the conductance and dielectric cells and other items used in studying the behavior of materials, the conventional bridge methods have to be replaced by wave guide techniques. This change comes in a t wave lengths of about 1 meter or v = 3 X 10"ec.-I Wave lengths below 1 em. encounter typical quantized responses. First, the natural rotations of molecules in the gaseous state are observed, as contrasted with the electrically imposed dipole orientations in liquids at much longer wave lengths, giving rise to very sharp absorption lines from whose positions the moments of inertia of the molecules may be evaluated. The frequencies of these lines can be determined with exceptional accuracy (1 part in 103,2and from them bond lengths in many molecules have now been evaluated to within 0.001 11. The rotational spectra of the lighter molecules are found in what is conventionally called the far infrared, a t wave lengths of the order of lo-%cm., or o = 100 cm.-' Here they overlap with the vibrational frequencies of heavy molecules, vibrations which are most generally studied as absorptions characteristic of each individual molecule a t somewhat higher wave numbers by using prism (for example, NaC1, CsBr) spectrometers. 2 The eonstancv and recision in these fretlueneies is the basis '
>
of the so-called "atomici' (properly, moleoul&) olaok.
The visible region is best given as 3800 A! t o 7800 A. Its range defines no unique effects. The quanta on absorption now produce electronic excitation of the molecules, the energies being such as correspond to great activation of the molecules chemically without being so large as to involve their radical disintegration. It is no mere chance that the principal absorption of chlorophyll is centered a t about 4500 A,, which coincides with the maximum in the solar emission. The resulting photochemical activation is the starting paint of almost the whole of our planet's life processes. With a notable exception in the case of the rare earth atoms, only the outer electrons are involved in visible absorptions or emissions. On going t o shorter wave lengths in the ultraviolet, the larger quanta can excite the deeper electrons and strip them from the atoms. In the very far ultraviolet the spectrum of a copper atom which has lost 19 electrons has been observed in intense electric discharges. Of far greater chemical interest are those spectra in the ultraviolet analogous t o the Balmer series for the H atom, in which an electron of a polyatomic molecule is observed a t a succession of levels preceding ionization. The study of these molecular Rydberg-series spectra provides a source of p r e cise information concerning the states of bonding and non-bonding electrons, and has contributed basic features to current quantum theories of valence. The ultraviolet absorptions are the principal cause of the refractive-index dispersion in the visible region. Very intense absorptions are found, for instance, when the delocalized electrons of extended conjugated systems are involved. With further decrease in wave length, the dimensions of molecules and atoms are reached in the X-ray region. Here any regularity in the distribution of atomic centers in the gaseous, liquid, or solid states gives rise to diffraction effects in the radiation scattered by the material. Most simply, in the solid state where the regularities in atomic and molecular patterns are greatest, the principal effects can be treated in terms of a reflection of the radiation from the families of atomic or molecular planes, the condition for which is given by Bragg's relation. X-ray structure analysis has contributed more t o the advance of chemistry in this century than has any other single technique. Even more generally, it is the photographic process which has made possible the systematic study of the whole range from 3000 11. t o shorter wave lengths. The scientific progress of the last 50 years would have taken place almost incomparably more slowly without the photographic plate. Beyond the X-ray, the gamma ray and cosmic ray region extends, as far as we know, without limit. Here the quanta correspond t o the differences between the energy levels possible in the nuclei. The process of the ejection of electrons (ionization) from atoms by ultraviolet radiation is analogous to the nuclear disruption which gamma rays can effect. As it is only some 20 years since the discovery of the neutron and the invention of the cvclotron. the basic analvsis of nuclear en-
-
ergy levels is still in progress. The guiding principles which should result from this study will have immediate interest for the chemist in so far as they account for the relative stabilities and ahundances of the chemical elements. To an even greater extent than in other parts,
there must be in this extreme region of the electromagnetic spectrum interactions between matter and radiation which are as yet entirely unsuspected. The advancing study of these interactions is one of the main high-roads in physical science.
