Innovative Laser Techniques in Chemical Kinetics A Pedagogical Survey Laurie J. Kovalenko and Stephen R. Leone1 Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, and Department ol Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309
When vou think of a laser. what do vou envision? A beam of lights; highly collimated that it can reach the moon from here? A beam so powerful it can initiate nuclear reactions? Such pictures correctly depict some of the extraordinary properties of lasers. Additional properties include: monochromaticity (single frequency or color), polarization (single electric field direction), and coherence (single frequency, direction, and phase) ( I ) . In our everyday life we encounter lasers in many places. Lasers are used a t check-out counters to read bar codes on items being purchased, the resulting signal being transformed into your receipt. They read information coded on compact discs, acquiring frequencies and amplitudes that are transformed into sound. In the field of medicine the laser is used to reattach retinas. to clear blocked arteries, and to do root-canal work. In the field of communications. lieht sienals from lasers are sent alone fiber optics, replacing theelectrical signals sent along metal wires. In chemistry the laser is proving to be invaluable for investigating many chemical phenomena (2-4).In this article we focus our discussion on a few of the myriad ways in which lasers are used in the quest for chemical understanding, and thus for measuring, controlling, and creating chemical reactions. Our main thrust will be a consideration of the applications of lasers in the field of chemical kinetics. Examples of recent progress using lasers are presented in the context of atmosoheric and combustion chemistrv. We consider two types of laser applications in chemistry. The first is the use of short laser ~ u l s e to s oreoare . . areactant in a known state whose subsequent time evolution can then be monitored bv fluuresccnce detection. The second is the use of a continuous laser as a probe to monitor the time behaviors of specific species in a reaction. Investigation of these ideas wifi require-a consideration of a range of types of lasers, from those which produce ultra-short femtosecond (10-Is S) pulses of light t i those which provide continuous beams of light. We will develop a number of kinetic principles and see how chemists exploit many of the laser's special properties, such as short pulses, monochromaticity, power, and polarization, in carefully designed experiments to study kinetic phenomenaiThe first consideration will be a review of what a laser is, or in other words, how it works. How Lasers Work A laser is a source of radiation, or light, just as is a candle, a light bulb, or a shooting star. These lieht sources transform energy of one kind intoenergy of another kind, namely light. The candle transforms chemical energy into light hy the mechanism of combustion. The light bulb converts electrical energy into light by the process of resistive heating. The shooting star transforms mechanical energy into light by frictional heating. Likewise, lasers also convert energy of various kinds into light. But, whereas the light produced in the first three cases is incoherent (photons of random frequencies, directions, and phases), a laser provides highly coherent radiation (Fig. 1).
Fig~re1. Comparison of a coherenl sadrce lrfghll and an mcaherenl sa.rce (,eft)ol iignt Note that although the i r o pholons traveling lo lne iell have the Same frequency and direction, they are opposite in phase. By contrast we see that the properties of coherent light are a single frequency, direction, and phase.
T o see how a laser produces coherent radiation, consider the three types of ,interactions between light and matter (between a photon and a molecule or atom). Absorption is the process whereby a molecule captures a photon and is thus "excited" to a higher energy quantum state (Fig. 2a). Will a ohoton of anv enerw ... be absorbed? Recall Einstein's quantization concept ( 5 ) .which was first used to explain the ohotuelectric effect: the enerev of an ahsorl~cdvhotun. En. kquals the difference in energy between the tu;o quantu& states of the molecular transition: E, = (El - Eo). Also recall that the energy of a photon corresponds to a frequency, u, by Planck's equation (5):E, = hu, where h is Planck's constant. Therefore, the wavelength, or frequency, of an ahsorbed photon will be a precise value determined by the energy difference between the quantum states. The second process, spontaneous emission, is where an "excited" molecule emits aphoton and thus becomes "de-excited" to a lower energy quantum state (Fig. 2b). The emitted photon can leave in any direction with any phase, although there is again a constraint on its frequency. The third process of interaction between lieht and matter is called stimulated emission: a photon stimulates an excited molecule into emitting a photon identical to itself (Fie. . - 20. . . thus "de-exritina" the molecule to a lower energy quantum state. This proce& has several unique properties. I t results in two photons identical in frequency, direction, and phase: i.e., coherent light. Although in general all three processes will be occurring in a laser, the third process, stimulated emission, is amplified with respect to the other two (a kind of a chain reaction is produced, where one photon results in two, which then result in four, and so on). Thus the acronym Laser: Light Amplification by Stimulated Emission of Radiation. T o have a laser i t must he more probable that a photon will stimulate the
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'
Staff Member, Quantum Physics Division, National Bureau of Standards. Volume 65
Number 8
August 1988
681
N3.
\
RELAXATION
POPULATION
..,,,*,,- #ION PHOTON
MOLECULE
H F VIBRATIONAL O U A N T U M STATES
VIBRATIONALLY EXCITED MOLECULE
a
-~
VIBRATIONALLY EXCITED MOLECULE
MOLECULE
HF VIBRATIONAL O U A N T U M STATES
PHOTON
.-
Fiatre 3. A mica1 four-level lasina Nq. reDresents the number of . " svstem. . molecules m level 1. k,o represents the rate Constant tor nonrad at ve relaxallan of a molecule in q u 8 n t m stale 1 to qvamum state 0. Thus tne rate for nonmdiat;ve relaxation 01 q a n t u m state 1 into quantum state 0 ts equal lo ~
-
-
PHOTON
VIBRATIONALLY EXClTEO M O L E C U L E
(3 2). An inverted population is thus established between levels 2 and 1.Spontaneous de-excitation of a few molecules in level 2 produces photons that then stimulate the emission from those molecules remaining in level 2. Rapid nonradiative relaxation from level 1to level 0 depletes the population of molecules in level 1 so that loss of laser nhotons throueh absorption (1 21 is minimized. Thecondition fortheestablishment ofa oooulation inversion between levels 2 and 1can be written&athematically. The maior for transitions between quantum states . orocesses . are: absorption, spontaneous emission, stimulated emission, and collisionallv induced nonradiative relaxation. T o s i m ~ l i fy the analysis we consider only the transitions between neighboring quantum states, and we consider a system where there are no quantum state degeneracies3 Let NI represent the number of molecules in level 1, klo the rate constant for nonradiative relaxation of a molecule in quantum state 1 to quantum state 0, AIDthe rate constant for soontaneous emission from auantum state 1 to ouantum state 0, also known as the in stein coefficient of spontaneous emission. and ,ollBI? .- .-the rate constant for absorotion from quantum state 1to quantum state 2, where PIP k the energy density of light a t the laser transition frequency, and BIZthe Einstein coefficient for absorption. (Note how the rate constant for absorption, a process requiring photons, depends on the amount of light present while the rate constant for s~ontaneousemission, a process not requiring phostate 1 tons, does-not). Stimulated e&is$on from to quantum state 0 is nedigible since the energy -~ density of light a t that nonlaser transition frequency, pol, is negligible. The rate of depopulating level 1 can then be written as: Nl(klo Alo plzB12). Similarly, the rate of populating level 1can be written as N z ( k z ~ +A21 + P I ~ B Z Iwhere ), now PI~BZI, the rate constant for stimulated emission from quantum
MOLECULE
TWO
HF VIBRATIONAL
IDENTICAL
Q U A N T U M STATES
PHOTONS
-
Figure 2. (a) Absorption of a photon by an HF molecule on the vlbrationai 1. The molecular vibration can be visualized as the transition Y = 0 oscillation of two balls connected by a spring. (b) Spontaneous emission of a photon by a vibrationally excited HF molecule an the transition v = 1 0. ( c ) Stimulated emlssion of a photon by a vibrationally excited HF molecule on the transition v = 1 0.
-
emission of another photon than that i t will be lost by absorption. An essentiai requirement to achieve this condition, called a population inversion, is that there be an excess of excited molecules, enough so that the rate of photon production bv stimulated emission is greater than the rate of photon loss by absorption. ~ o ~ u l a t i oinversions n are rare phenomena in naturally occurring systems.2 Under thermal equilibrium conditions, no matter how hot, the populations No and NI of quantum state energy levels Eo and El are described hy a Boltzmann distribution (5):
+
where k is the Boltzmann constant, T is the absolute temperature, and the levels are taken to be nondegenerate for simplicity. Thus in the thermal equilibrium conditions usually found in nature there are always fewer molecules in an excited quantum state than in a lower energy quantum state; i.e., no population inversions occur. How then is a no~ulationinversion oroduced? The answer is through clever kinetics. An example of a typical laser is eiven in Fieure 3. Initiallv some laree fraction of molecules :re excited:or "pumped';(electricafiy, chemically, optically . . .) to a high energy quantum state (0 3). They then rapidly relax to a lower energy, intermediate quantum state
..
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Journal of Chemical Education
+
Although population inversions are rare in naturally occurring systems, there is evidence that they do occur in space as masers (where the frequency of radiation that is amplifiedfalls in the microwave region, hence the replacement of the letter "I" in laser with the letter "mu). See for example Kegel, W. H. Appl. Phys. 1976, 9, 1. A system with no quantum state degeneracies has only one quantum state at each energy. For a discussion of degeneracy, in which there Is more than one quantum state corresponding to a given energy, see ref 5.
I (01
LASER
!
Y-Y
I
(bl APPARATUS
SIGNAL
/ D Y E ' C E L ~ ~I ~ ETALON I I OSCILLATOR
Figwe 4. (a) Experimental apparatus for modern laser-initiated reaction studies: The dye laser consists of an oscillator, an amplifier, and various optics. The oscillator is the actual iaser. It consists of the lasing medium (a dye solution), an optlcal resonator (consisting of a back minor M, and a front mirror M, called an output coupler). and an etalon (enables selection of a single laser frequency, w mode). The rear mirror, M,, is actually a grating (for tuning the frequency of the iaser beam). The dye molecules are optically excited with a Nd:YAG pump iaser. The amplifier is Just another dye cell whose dye molecuies are also excited by lhe Nd:YAG pump laser. When the oscillator laser beam traverses the amplifier, it stlmulstes emission from the excited dye molecules and so is ampllfled. (Note that the amplifier does not isse by itself because It has no mirrors.) The rest of the optics are for enhancing the quality of the beam. 8 = beam splitter (reflects a fraction of the beam in a direction perpendicular tothe incoming beam while ieningthe rest go through undeflected), L = lens. P = prism (reflects the beam by 90 '). (b) The laser beam p h o t o d i s ~ ~ ~ i a tthe e s CI-containing precursor molecule to initiate reaction. infrared fluorescence signals hom vibrationally exceed H C V ) product molecuies are detected wilh an IR detector, amplified, digitized, signal-averaged, and recorded. Small amounts of the laser beam are used to trigger the detection system and to monitor the iaser energy.
state 2 to quantum state 1, is a major process since the energy density of light a t the laser transition frequency is substantial. Thus the rate of change of population in level 1 can be expressed as
This differential equation is known as a rate equation. T o simplify analysis, we assume steady state conditions. This means the populations in each quantum state are nearly constant in time, so the time derivatives are zero. Setting the above equation equal to zero and solving for the ratio of populations in the two quantum states of the laser transition, we obtain
T o establish a oonulation inversion. Nq must be ereater than N1. Under ourconditions of no degeneracies, the absorption and stimulated emission rate constants between the two quantum states are equal (BIZ= Bz,). Thus the condition for the establishment of a population inversion is
which requires that the lower laser transition level depopulates faster than i t is oonulated. When a laser is opkrating, there is a delicate kinetic halance between the rates of excitation, spontaneous emission, stimulated emission, and nonradiative relaxation. Herein lies the difference between lasers and, for exam~le,candles. I n a candle numerous collisional deactivation mechanisms rapidly relax excited molecules into an equilibrium Boltz-
mann distribution and thus prevent population inversions. For a laser to operate it requires a clever adjustment of conditions kinetically to favor a population inuersion so that stimulated emission becomes the dominant process. By devising novel ways to achieve such adjustments, chemists have played important roles in the development of lasers. Once the stimulated emission process has begun, further amplifications are obtained by recycling the emitted photons back through the laser cavity by reflection. In a simple arrangement called an optical resonator, mirrors are placed at each end of the laser cavity (Fig. 4a). Only light that propagates along the mirror axis is further amplified. One of the mirrors, called an output coupler, is made partially transmitting, thus allowing some of the light beam to escape for a variety of uses as described below. Absolute Rate Constant Measurements
Suppose we want to prevent the process of ozone depletion in the earth's atmosphere (6)or increase the efficiency of combustion engines and decrease their pollutants (7). T o do this we need to understand these systems in detail. Many of these nrocesses are alreadv oartiallv understood. For examthe ozone depletion is ple, &ere is substantial e4dence in oart attributable to a comolex seouence of chemical reactions in the atmosphere involving the photochemistry of chlorofluorocarbons (CFC's). As a result. in the United States CFC's are banned for' use in aerosol'sprays. I t is also known that the octane rating of easoline affects comhustion efficiency and that the use of unTeaded gas can decrease the amount of heavy metal pollutants produced. 'Chus there is a strong interest to conveit to unleaded gasoline. Even though many general aspects of these problems are solved, there still remain pollution problems, ozone depletion problems, and a desire for more miles for our gallon. So scientists continue to study the finer details of chemical structure and reactivity in three complex systems. There are various strateeies used to solve these orohlems. Take combustion as an example. At one extreme are bulk studies: i.e., try different mixtures of hydrocarbons and air, ignite them, and measure how much energy is produced and how much pollution. At the other extreme are detailed studies of isolated reactions: react butane with oxygen atoms, and measure individual rate constants, determine what products are formed, and, since combustion dramatically raises the temperature of the reactants, study how temperature affects the reactions. T o understand the reactions that govern these complex systems we need to know their rates, and so we want to measure their rate constants. We start our survey with an example of how to measure a rate constant for an isolated reaction using a laser excitation method. The reaction is that of chlorine atoms with hydrogen sulfide (8,9):
teat
The progress of a reaction is measured in terms of the rate of chance of the concentration of a soecies. In this case advantage i s taken of the fact that some fraction of the HCI(v) produced is vibrationally excited (hence the indication v, which represents HCI in either vibrational quantum state 0 or 1). The intensity of photons spontaneously emitted, called fluorescence intensity, is proportional to the total concentration of HCI(v) produced. Thus the time dependence of the fluorescence corresponds to the time depeudence of the product HCl(v). In the modern kinetics laboratory, rather than using UV (ultraviolet) light from the sun or from a flashlamp, a pulsed UV laser is used to produce chlorine atoms by the photodissociation of S2CI2 (Fig. 4h): S,CL2
-
S,CI
+ C1
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Two advantages of using a laser for the photodissociation are: (1) the monochromaticity of the light ensures selective photon absorption by S& and not by HzS, which absorbs photons of different frequencies and (2) the short p u k e length produces chlorine atoms "instantaneously" on the time scale for subsequent reaction. A typical trace of the HCI(u = 1) fluorescence intensity as a function of time (on the microsecond scale) is shown in Figure 5a. Compare this with the duration of the flashlamp and laser pulse (Fig. 5b and c). The rate equation for this bimolecular (two collision partners) reaction is
0
150 TlME ( p s e c )
0
50
where [ ] indicates concentration. Under appropriate experimental conditions, called pseudo first order, where [HzS] is in such large excess that its value changes negligibly over the course of the reaction, the rate equation can be integrated4 to yield:
50
100
200
= ~ ( 1 -e-k~'.l[H'8t ) [HC~(")]
where C is aconstant. Thus the exponential rise time, ~,i.,, of the product HCI fluorescence signal can be expressed as:
If measurements of rise times (on the order of microseconds) are carried out for a ranee of HvS concentrations. a nlot of reciprocal rise time vs. [I&] a slope of h ~ c l1;. tgis case the room temperature rate constant was found to be (7.3 0.9) X lo-" cm3 molecule-' s-' (9). With shorter laser pulses, faster phenomena can be studied. I t is now possible to observe processes occurring over a few picoseconds using laser pulses only 400 fs long (1 fs = s). For example, in the photodissociation of iodine cyanide (10)
*
hr
ICN-CN
+I
a 400-fs laser pulse was used to excite the ICN molecule to a higher energy electronic quantum state. The CN fragment produced upon dissociation was monitored in time by laserinduced fluorescence (LIF). LIF is similar to the fluorescence detection technique described above, where instead of monitoring the fluorescence from chemically excited molecules, called chemiluminescence, fluorescence is monitored from product molecules excited with a laser. I t was found that the ICN molecule does not dissociate immediately upon absorbing a photon. Rather, the excited state lifetime was measured to be 600 100 fs. Measurement of excited state lifetimes aids in the undersranding of the dvnamirs of bond breakinr. Upon absorr~tion of a photon, dois the molecule have time to rotate defore breaking apart? Does the photon enerm distribute itself statistirally over the whole kalerule or &es it remain concentrated in one place, or vibrational model In the case of ICN. theshort lifetimeof theexcited ICN molecule indirates thatthe excited stateishighly unstable; i.e., the excitedstate potential energy surface is repulsive. The excited state molecule exists only for a small fraction of a rotational period before it dissociates. A similar study of the methyl iodide molecule (11) showed that the I-CH3 bond breaks even sooner after excitation, within the 400-fs duration of the laser pulse. A precise lifetime measurement will have to wait for the development of lasers with still shorter pulses.
*
'To do the integration, make the substitution [CI] = [ClIo [HCI(v)],which just means that each CI atom lost to reaction produces one HCI(vJmolecule. (Initially. [HCI(v)] = 0, so [CI] = [CIJo. Finally, all the CI produced upon photolysis has reacted to form HCI(v) so [HCI(v)]= [ClIo.) 884
Journal of Chemical Education
150 2 0 0 TIME ( p s e c )
>-
100
LASER
0
100 150 TIME ( p s e c ) 50
200
Figure 5. A comparison of (a) the time for reaction of chlwine atoms with H2S to (b) the time for production of chlorine atoms by a flash lamp and (cl the time forproduction of chlorine atoms by a pulsed laser.
Laser Initlaled Chaln Reactions Combustion and many atmospheric processes involve chain reaction mechanisms. A basic example illustrating the various stages of a chain reaction is the laser-initiated reaction of chlorine with hydrogen bromide (12,13). In addition to the advantagesof monochromaticity, which enables selective dissociation of C12 while leaving HBr intact, and short pulses, which initiate the chain process "instantaneously" on the time scale for subsequent reaction, there is a third advantage in using a laser in the study of chain processes, namely its high power. An increase in laser power results in
an increase in radical density, which determines the rate of cIiain termination. In the first stage of a chain reaction free radicals are produced, in this case by absorption of an ultraviolet photon: h.
Initiation:
CI,
2Cl
+
In the second stage free radicals react to produce other free radicals: hi
Propagation:
C1+ HBr Br + CI,
HCl(v) + Br
+
k,
BrC1+ CI
+
Finally in the chain termination stage, free radicals recombine to produce nonreactive products (where M is any third collision partner):
-
k.dM1
Termination:
Br + Cl
BrCl
The evolution of the chain is followed as i t occurs by monitoring the intensity of fluorescence from vibrationally excited HCI(v) molecules as a function of time. With the use of a short-pulse, low-power laser, the three stages of the chain svstem can be time-resolved. Short pulses ensure an "instantaneous" initiation stage, while the low power results in slow termination staee reactions. Fieure 6 shows the fluorescence intensity of product HCI(v) molcules from such a case as a function of time. The initial fast rise corresponds to the fast reaction of all the chlorine atoms produced in the initiation stage. The fast decay corresponds to the collisionally induced nonradiative relaxation of the vibrationally excited HCI(v). In the absence of a chain reaction this fluorescence signal would decay tozero. However, the persistenceof a small HCI(v) fluorescence signal a t long times indicates that HCl(v) is continuously being produced. This is a result of the second chain propagation reaction, which slowly continues to produce reactive chlorine atoms. Thus a small,
TIME
lrnsec)
Figure 6. Concentration of vibrationally excited HCIIv) from the CI + HBr reaction as a function of time. The hitiation stage is instantaneouson this time bv reaction of ~cale. The fast rise anddecav carresoond to the HCIIvb . . ~roduced . Ci f o r d in the initiation stage. me steady slate s gna . I,, corresponastorne rlC4vI prOd~Ce0by react on ol C lormea m the cham propagation stage. Tne long Itme decay of lhor sognsl 0s due lo loss of CI an0 81 rad'cair n chain terminstion sfage reactions
steady state concentration (I,) of vibrationally excited HCl(v) molecules persists to long times. The eventual decay of this signal corresponds to the slow third order (three collision partners) chain termination stage reactions. The rate constants for the propagation reactions, kl and kz, were found to be (1.02 f 0.15) X 10-" and (1.1 0.4) X 10-l5 cm3 molecule-' s-1., resoectivelv. .(13). . The four orders of maenitude difference in rate constants can be understood in terms of enerev reauirements. Whereas the C1 atom reaction is 65 k~/mol"ixotiermic, the Br atom reaction is endothermic by 24 kJ/mol. We note that in one of these studies (12), the chain termination steps have been investigated by varying the laser power. ~ o r m a i l ycombustion systems are more complicated than this. As combustion proceeds, the reactants heat up, and the reaction rates increase. If the system can thermdly equilibrate faster than the reactions occur, i t can be characterized bv reaction rate constants a t narticular temoeratures. But combustion systems are usually not in thermal equilibrium. We therefore must further investieate the meanine of reac" tion rate constants.
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Inside the Rate Constant: Energy Dependence In ordinary chemical systems such as the atmosphere or comhustinn engines, reagent molecules collide with various orientations and with all sorts of energies. Whether a collision results in reaction depends on the specific orientation, the speed a t which the molecules collide, and the internal enemies of the collision partners. For examde. . . usuallv... the greater the relative speed at which two molecules collide, the greater the ~rohabilitvforreaction. But in a thermal eauilib;ium distribution of molecules, very high energy colfisions are rare. Thus the rate constant for reaction in a chemical system is a weighted average of reaction probabilities a t all collision energies. The weighting factor is the Boltzmann distribution at that particular temperature. In addition, since a reaction may occur more readily when the reactants collide with a certain orientation, one also needs to average the reaction probabilities of individual collisions over all possible collision orientations. The final result is a reaction rate constant, which is used to describe a thermal equilibrium chemical system. For such a system, the dependence of the rate constant k, on temperature is empirically described by the Arrheuius expression (5). k = Ae-E.fiT rx
where E, is the activation energy, k is the Boltzmann constant, T is the absolute temperature, and A is the preexponential factor. In contrast, what if molecules produced in chemical reactions, such as in combustion, have insufficient time to equilibrate to a well-defined temperature before reacting further? Will energy concentrated in one type of motion, say vibration, affect the rate differently than the same amount of energy concentrated in a different form of motion, say translation? Lasers enable us to study reactions in greater detail, breaking down both the energy and the orientation (or alignment) dependence into their componentsso that we can start to answer some of these questions. We consider two examples, the first a study of energy dependence, the second of alignment5 dependence. An excellent example of an energy dependence study involves the reaction of calcium atoms with hydrogen fluoride (14) The meanings of orientation and alignment can be characterized for the simple case of a nonrotating molecule such as HF in the following way. Alignment specifies whether the H-F axis is perpendicular or parallel to the collision partner. Orientation furtherspecifies which of the two different ends of the molecule. H or F, is pointed toward the collision partner. Volume 65 Number 6 August 1988
685
El
I* ( 2 ~ 1 , 2 )
I (2~3,2)
Eo
I ATOM S P I N ORBIT ELECTRONIC Q U A N T U M STATES
reaction probability. In our final example we will see how putting energy in the wrong place can actually hinder a reaction. But first we consider an example of how alignment of reagents in a collision can affect reaction probability. Inside the Rate Constant: Reagent Alignment Dependence To gain an understanding of how reagent alignment affects reactivity (16),a fourth property of lasers is used: the emission of ~ o l a r i z e dlizht. Polarized laser light is used to excite the p ibrhital of cacium selectively to o11&1the orbital oeruendicular or oarallel m the direction of a beam of chlo;in; molecules (2i):
(a
Ca + CI,-CaCI(An)
+ CI
or
I t was found that the reaction to produce the electronically excited A n state is more probable for the perpendicular alignment of the Ca orbital than for the parallel alignment (k I /kt = 1.35). This can be understood bv a correlation of theatomic orbitals of the reactant atom, Es~cium,with the molecular orhitalsof the product molecule,calciumchloride. I t is proposed that the p-orbital of the calcium atom transforms into a particular CaCl molecular orbital. The perpendicular alienment thus results in a ll molecular orbital while the para112 alignment results in a 2' molecular orbital. Thus electronic state. a reaction involvto form the (All). product . ing perpendicular alignment presewes the orbital symmerry while a reaction involving parallel alignment break3 it.
Figure 7. (a) l d i n e atom spin-abit eiechonic quantum states. Phalcdissacialion of methyl iodide can produce iodine atoms in either electronic quantum state. The probe laser frequency, v , is tuned to the energy of the transition. (E, E,): u = (E, Eo)/h. (b) Change in probe laser beam intensity as a function of time Followingthe CHd photoiyris pulse. The initial amplificationof the probe laser beam is due to the 1' population inversion produced upon photoiysis. Subsequent decay of the signal corresponds to the collisional quenching of I' to ground state Iatoms, resulting in net absorptionof the probe beam.
-
-
A laser was used to excite the hydrogen fluoride molecule to a well-defined vibrational ouantum state (v = 1). I t was found that at room temperature, calcium reacts with vibrationallv excited HF ( u = 1) four orders of maenitude faster than i t reacts with ground state HF(u = 0). ?his startling difference can he explained bv a simple picture of bond breakage. A general ihservatioh of such endothermic reactions is that they are highly sensitive to excitation of the vibrational mode along the reaction coordinate. Vibrational excitation of the H F bond in this case puts energy right where it is needed most. Suppose we had instead ~ r i e dtu speed up the reaction by heating. Heatit~gputs energy into all decrees of freedom of the molecule: translation. vihration. rotati&, and electronic excitation, even though sdme are not as useful as others for bond breakaee. Bv" channeline " enerev -. to where it is most effective, chemists are currently attemptinu to develoo laser-controlled chemistry for use in technofor example in the dectronics industry logical (15). It is not always the case that adding energy increases the
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Lasers as Probes In this final section we give an example of using a laser to monitor the progress of a reaction rather than initiate it. The technique of laser gain vs. absorption spectroscopy measures the change in intensity of a continuous probe laser beam in time. Under normal conditions. nhen liiht passes thrnugh a sample, it is reduced in intensity due toahsorption.The final intensity,I,,can bedescribed bvrhr Beer-Lamhert Law (51:
where Ii is the initial intensity of light, 1 the path length, a the absorption coefficient, and C = Co - C,, the difference in concentration between the lower and upper energy levels of the transition. In a thermal equilibirum system more molecules are in lower energy states than in higher energy states, so Co - C1 is positive, and the process of absorption domi: nates (It < Ii).But under certain conditions more molecules can be in hieher enerev -.states than in lower enerev -.states so Co - CI can he negative, a population inversion is established, and the process of stimulated emission dominates. The probe laser beam is then amplified (If > Ii),a phenomenon called gain. For example, a laser is used to photodissociate methyl iodide (18): h.,
CHJ-
CH, + I
A second, cw (continuous wave) probe laser is tuned to the energy of the transition between the excited and ground states of the iodine atom (Fig. la). Figure 7b shows the change in the intensity of the probe beam as a function of . orohe beam is initime followine the ~hotolvsis~ u l s e The tially amplifgd on' passihg through the'sample, evidence that ~hotodissociatonof CH-I oroduces an iodine atom ooo. ulatibn inversion. The pophation of excited state iodine atoms decreases with time due to collisional relaxation: the population inversion is then lost, and the process of ahsmp.
.
tion dominates. A comparison of the relative intensities for the probe laser gain and absorption forms the basis of a highly successful technique to measure photochemical quantum yields (la), which can be useful to study the photochemistry of some atmospheric constituents. This technique has also been used (19) to monitor the reaction of bromine atoms, Br, with iodine monohromide. IBr, whichrevealsan intriguing specificity in this reaction. A small fraction ot' IRr molecules are photodissociated. Prompt gain in a probe laser tuned to the frequency of the Br Br* transition is seen, due to a Br* population inversion produced directly from the photodissociation (Fig. 8a). Following this prompt gain a further increase in gain is inverobserved, due to an enhancement in the . oooulation . sion by pieferential loss of ground state atoms through reactiun with IHr. T o measure the rate constant fur reaction of ground state Br with IBr, a fast Br* quencher, COz, was added to the reactant mixture. As seen in Figure 8b, the Br' can he rapidly deactivated to ground state Br in a time short compared with subsequent Br reaction with IBr. From the decay of the absorption signal in Figure8b a rate constant of 4.6 X lo-'' cm' molecde-I s-I was measured for the react ion of ground state atoms (19). Although the experiment precludes measurement of the rate constant for reaction of Br* with IBr (it measures the sum of the ouenchine and reaction rate constants), this rate constant was determined to be a t least 40 times slower. Thus it is seen that the lower enerev ground electronic state is actually more reactive with 1% than the higher energy electronic state. I t is not always the case that reactivity increases with energy. The decrease in reactivitv with electronic enerw in this case can he understood by an adiabatic correlation description and thermodvnamic considerations. An adiabatic reaction is a reaction that proceeds along a single electronic potential energy surface of the svstem. Thus for this svstem an adiabatic reaction would result in the same product electronic states as reactant electronic states: Br reacts to aive I. and Br* reacts to give I*. But the latter is endothermicby 32 kJImol and thus is not expected to occur as readily as the former. The reaction of Br* to give ground state I, which is thermodynamically favorable, requires a nonadiabatic transition between electronic states. In this particular system, however, such nonadiabatic effects are believed to be weak.
Br, Br*+ I B r + B r 2 + I
LOSS O F Br BY Br+ I B r REACTION
T
-
INITIAL
0
5
0
TIME ( p s )
(a
-.
Br*
l-
a 0 V)
m
a
I).
0
With lasers as initiators and orobes of chemical orocesses. many fundamental and subtle questions can be'answered about chemical reactivity. We have seen that simply adding energy to a system does not necessarily enhance reactivity. The form that energy takes, whether electronic, vibrational. translational, or rotational, is important. The monochroma: ticity of lasers enables such selective placement of energy. We have seen how the alignment of colliding partners can affect reaction probability. The polarization of laser light enables selection of specific reaction alignments. The short pulses and high powers of lasers enable study of ultra-fast reactions and the time resolution of chain reactions.
~
~
Freeman: San Francis~o,1978. 6. Chsm Enz. News 19R7.1Feh 21) d 7. &me$. ~ . kP .~ Y ES~ U C .1985, 292. ~~~
m,
8 Brsifhwaite. M.:Ilone.S.R. J. Chem Phys. 1978.69.839. 9. No8hiff.D. J.;Leono,S. R . J Chom. Phys. 1980, 72,1722. Sehere,N.F.;Knee.J, L.:Smith,D. D.:Zewsil, A. H. J.Phys.Chem. 1985.89,5141,
10.
11. Knee.J.L.:Khundksr,L.R.:Zewail,A.H.J.Chem.Phys. 1985.63.1996.
12. Nerhilt, D.J.;Loone,S.R.J. Cham.Phys. 1981.75.4949.
10
2(
I5
Figure 8. (a) Change in probe laser beam intensity as a function 01 time foliowing IBr photolysis. In this case the laser frequency is tuned to the Br + Br' transition frequency. The prompt rise indicates amplification of the beam due to a Br' population inversion produced by the photolysis. The subsequent slower rise corresporms to a further increase in the probe beam amplitude due to an enhancement of the population inversion. This enhancement is due notto an increase inthe Br' population but to a decrease in the Br population through preferential reaction of ground state Br with IBr. (b) Change in probe laser beam intensity as a function of time following IBr photolysis where addition of Con causes rapid quenching of Br* to Br (the Coat r e p r e sent5 vibratlonally excited COz). The Subsequent decay of the absorption C o n e ~ p o n dtothe ~ loss of Br throughthe reaction: Br IBr Br, I, enabling measurement of its rate constant.
Literature Clted ~
5
TIME ( p s )
Concluslon
1. O'Shea.O.C.;Callen. W. R.;Rhodes.W.T.;Introduction ruLosersond Their Applicnlions: Addiaon-Wedey: Reading, MA, 1977. 2. Find8en.E.W.:Ondriaa. M. R. J. Chcm. Edur lgR6.6:t.179 ~ ~ , ~ ~ 9. Lmnc,S.R.~ ~ 19&,227,889. h n ~ ~ 4. Lmne,S.R.J. Chem.Educ. 1976.53, 13. S. See 8 phpieal chemistry text, such as Atkins. P. W. Physicoi Chemistry; W . H.
+ C O p B r t C02
+
,
-
+
~
19. 14.
Dolmn. D. A.: Leone, S. R. J. Ph?s. Chem.. i n n r e . Ksmy,Z.:Zare.R. N. J. Chpm P h y s 1978.6R. 3360.
IS. 0sgood.R. M., Jr. Ann. Re". Phys. Chsm. 1963,34,77. 18. Wondward,R. 8: Hofmsn, R.: The C n n r e r v o t i m i~/OrbildSymmetr~.; Chemie: Weinheim, 1970. 17. Rettner.C.T.:Zare.R.N. J. Chem Ph?l. 1982. 77, 2416. 18. Herr. W . P.: Koh1ar.S. J.; Haugen. H. K.:Le0ne.S. R. J. Chem. P h p . 1986.8,. 2143. 19. Haown,H. K.: Wcilr,E.: Lenne, S. R. Chem. Phys.Lau. 1985.119.75,
Volume 65
Number 8
August 1988
667
