Infrared Lasers in Chemistry Phillip John' Heriot-Watt University, Riccarton, Currie, Edinburgh EH14 4AS The advent of high power infrared lasers over the last decade has stimulated extensive research in many areas of aspects of the subject even in a broadly based article of this nature. Reported applications ( I ) using COz lasers include such diverse subjects as welding and cutting, eye surgery, communication technology, synthesis and purification of chemical com~oundsand laser isotone The latter . seoaration. . chemical applications rely on our understanding of the interaction of intense infrared radiation with molecules. Photochemistry is the study of the chemical changes induced in a molecule following the absorption of a quantum of light. A number of important physical processes simultaneously occur which do not involve an overall chemical transformation. The term photophysics may he applied to such processes which are intimately connected with the degradation of the energy originally supplied to the molecule in form of a quantum of light. Prior to lasers, radiation sources for photochemistry were predominantly limited to the visible and ultraviolet reaions of the electromagnetic spectrum. Ahof light of thesekavelengths creates sorption of a electronically excited states of atoms or molecules. Subsequent reaction of these states lead to a fascinating wealth of chemistry. The conventional flash photolysis technique (21, pioneered by Norrish and Porter, exposed molecules to a relatively intense flux of quanta in a short time duration. Even under such circumstances Einstein's Law of Photochemistry
(3) was rigorously obeyed in that the primary electronic transitions involved the absorption of a single photon. The . orooerties of laser radiation. namelv. . - , monochromacitv. ", intensity and coherence have made it a unique source of radiation for infrared photochemistry. Arguably, the major impact on chemistry has been in inducing photochemistry in the ground electronic state of polyatomic molecules. As such, the main theme of the present article is a description of the mechanism, and a review of the apulications, of chemical reactions induced by the multiple absorption of infrared manta. 1)iaiociarim ot molecules occur, atrcr direct rscitarim u t th(: vihr.lhnal stares. 111 [he ~ a s rdecumpcsirion , \.ia thi, ntufr has been limited to thermai heating; however, this is a very inefficient and indiscriminate means of molecular dissociation. In contrast, the photcdissociation of molecules after absorbing many infrared quanta in an intense laser beam may, under appropriate conditions, be highly selective. The consequences of the important differences between thermal and laser induced multiple photon dissociation will he emphasized in the preceding sections of this article. I t is imnossihle to cover in detail all asnects of infrared laser chemistr; in this article. Hopefully, however, it will stimulate the reader to delve more deeply into this fascinating subject. T o make this pursuit more enjoyable, I have included a hihlioera~hvof the chemistrv and ohvsics literature as well as re;iew akicles which cover indi;id;al topics in more depth. The C02 Laser-Workhorse
of Gas Lasers
Molecular Energy Levels Gas lasers are a class of lasers which are widelv. emdoved . . in research and indus~riilllithorntorit~i.They range fro~uthe puwerful('O1 laser 181 the ul~iquitouiHe-Ne h e r . .4n exposition on gas Isem is nor preaenred hrre. and the rradrr shu~tld inccr~tsult3 basic review ( 4 1 o r ttLxti;t l . 5 1 fur m~m~ilt.t.til(d formation. The C0z laser is an example of a molecular gas laser in which the lasing material is gaseous carbon dioxide. The linear symmetric COz molecule has three fundamental vibrational modes described by three quantum numbers (nlnzna). Each mode gives rise to a particular oscillatory motion which is 1. At anv instant the CO? zra~hicallvillustrated in Figure u molecule may he vibrating with an apparently complex oscillation comwrisine a combination of these fundamental modes. The rllativeenergies of the vibrational energy levels, "
Figure 1. The fundamental vibrational modes in GO2. (a)symmetric stretch: (b) doubly degenerate bend; (c) asymmetric stretch.
A
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Presented at the British Association for the Advancement of Science. Heriot-Wan University, Edinburgh in September 1979.
Volume 59
Number 2
February 1982
135
Figure 2. Vibrational energy levels (not to scale) of C02. Fast relaxation processes are indicated by full lines while the dashed line shows the laser transitions.
Characteristics of Pulsed CO. Radiation Tunability Monochromaticity Intensity Coherence Power
Beam Diameter Repetition Rate LOWCost Photons
selected wavelengths in 9.2-10.9 ~m range band width -0.05 cm-' untocussed pulse -5 J cm+ delivered in 200-800 ns
-
unfocus~ed-2 MW cm+ focussed beam produces >GW cm+ 1 cm diameter 0.2-2 Hz Tens ot cents per mole of photons based on consumed electrical energy at laser
pertinent to lasing action, of the C02 molecule is given in Figure 2. Each vibrational level exhibits closely spaced, discrete rotational levels indicated, in the latter figure, by the shading above the vibrational levels in the manifold of .t:#ten. The C 0 2 laser operatrs pumping moIecuI~s,usm:: ~ 0 1 1 vtntiundl? an elecrric,al t l i i ( harge, irtm t h ~ , g r ~ ~ iIk)01 . n d 1,) a h~xhervibratimsl siatt. ahich, rmentiallv, ppulnte r he (001I b l d t t ' by rndintivt and tr)ll:iimnl pr,,rt.>ws. .4 pupulatiun inwrsion h e t w m thv I ~ t t estate r and tht. I 11~1)and I I W I~atc.s. I 10.ljpm m(I within a laser o ~ v i t ?produces , 1;lst.r ~ I L P U81 -Y.ti um, n.slwcli~,elv.I\ num1x.r %,fnlloaed rorati
