AD-All? 077 CALIZFORNIA UNIV SAN DIEfO LA JOLLA DEPT OF CHEHMSTRY F/6 7/5PICOSECONO DYNAHICS 00' 1(2) PHOTOCZSSOCIATION. (U)JUN 89 P BADO, P H KBEN.N, J P KROSNA NOOOI 47"-C-031
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IMPICOSECOND DYNAMICS OF 1 2 PHOTODISSOCIATION IVO Technical Report
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P. Bado, P.11. Berens, J.P. Bergsma, S.B. Wilson,and K.R. Wilson ONR-NO0014-78 C-0325
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To be published in Picosecond Phenomena III, edited by K. Eisenthal,R. Hochstrasser, W. Kaiser and A. Laubereau (Springer-Verlag, Berlin,1982)
III KEY WORDS (Cantinueoan reverse side ilnocoeewy end #dent#&y by blOck nmber)
t~ CL.. molecular dynamics vibrational energyQ picosecond spectral measurement resonance R~aman spectraC 12 photodissociation
j20 ABSTRACT (Continue Onrvreede If necoeeiy, end Identify by block nimber)
LL. A combination of theoretical molecular dynamics and experimental picosecondspectral measurement is discussed and illustrated for I? 'photodissociation
which can potentially answer one of the fundamental unresolved questionsof chemistry, how, from a microscopic viewpoint, solutions reactions take
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TECHNICAL REPORT NO. 7
Picosecond Dynamics of I ? Photodissociation
by
P. Bado, P.H. Berens, J.P. Bergsma, S.B. Wilson and K. R. Wilson
Prepared for Publication
in
Picosecond Phenomena III
edited by
K. Eisenthal, R. Hochstrasser, W. Kaiser and A. Laubereau
University of California, San DiegoDepartment of Chemistry
La Jolla, CA 92093
Reproduction in whole or in part is permitted forany purpose of the United States Government Ac
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*Sec*
PICOSECOND DYNAMICS OF 12 .PHOTODISSOCIATION
P. Bado, P.H. Berens, ],P. Bergsma, S.B. Wilson and K.R. Wilson
Department of Chemistry, University of California, San Diego,La Jolla, CA 92093 USA
E.J. Heller
Theoretical Division, T-12, Los Alamos National Laboratory,Los Alamos, NM 87545 USA
1. IntroductionWhile liquid solution reactions are much more important in chemistry, gas phase reactions aremuch better understood. Given the central importance of solution reactions to inorganic, organic.industrial and biochemistry, it is rather surprising that, as yet, there is not a single such reactionwhose molecular dynamics are understood in detail. Theoretical and experimental evidence alreadymakes clear that much of the important molecular dynamic action in solution reactions occurs onthe picosecond and subpicosecond time scales. The dihalogen photodissociation and recombinationreactions, X2 + h&-- X + X- X 2 , involving the simplest possible molecular reactants and pro-ducts, diatomics, and in rare gas solution involving only two elements, seem excellent candidatesfor study.
2. TheoryThe first deterministic theoretical study of the molecular dynamics of reactions was by BUNKERand JACOBSON [I], who computed the classical trajectories for 12 in CCI4 solvent represented by 26spherical, structureless particles in a specular cube. MURREL, STACE and DAMMEL121modelled the photodissociation of 12 in dense inert gases, 12 plus 22 gas atoms in a spherical, soft-walled container. We have similarly modelled 12 plus 50 Xe atoms at liquid density in truncatedoctahedral periodic boundary conditions[3, 41, computing the photodissociation, solvent caging.atomic recombination and vibrational energy decay to the solvent from the new 12 molecule, asshown in Fig. 1. The conclusion of all three molecular dynamic studies is that geminate recombina-tion is usually a very fast process, over within a few picoseconds. An important caveat, and aweakness in these theoretical studies, is that the process whereby the I atoms dissociating on anexcited state potential surface refind the ground state surface on which they recombine is not wellunderstood, and is therefore handled in these calculations by arbitrary assumptions which may beincorrect. If so, the real time for geminate recombination may be longer than the few picosecondscalculated.
FIg. 1. Time evolution in liquid Xe solution of12 vibrational energy during reaction sequence
.- m of photodissociation (at time zero), solvent cag-ing of some of the recoiling I atoms, radical
.- recombination, and vibrational loss to solvent.Also shown is the equilibrium Boltzmann vibra-
,0-0 tional energy distribution before photodissocia-tion. The vibrational energy distribution al-ready bifurcates into two branches within the0-5 ps period, the higher one corresponding to
10-15 those I atom pairs which have escaped the cage0-5 and whose minimum energy is the 12 dissocia-
u,, n_ tion energy, and the lower one corresponding0 to recombined 12 progressively losing vibration-
V.*TlONA EN(RY (o;,) al energy to the solvent.
"2-
In addition, theoretical calculations by NESB17T and HYNES[5, 6] for 12 in rare gases and inCO 4 and by our group[3, 4] in liquid Xe indicate that the decay to the solvent of the vibrationalenergy in the newly reformed 12 molecule will require the order of a hundred to hundreds ofpicoseconds, as shown in Fig. 1, considerably slower than the time required for geminate recombi-nation. Note that these calculations are for solvent atoms or molecules which are very weaklybound to one another, and that the vibrational relaxation might be quite different, for example, in astrongly hydrogen-bonded liquid.
From our calculated molecular dynamics, plus the potential curves and transition dipolemoments, we can compute transient electronic absorption spectra[71, as shown in Fig. 2, whichincludes the A-X, B-X, and the B" lu('rl) - X transitions. A related nonmolecular dynamicsspectral calculation has been carried out by NESBITT and HYNES[6]. A small quantum correctionby temperature scaling, which would be exact for the coordinate distribution in the harmonic limit,is made to the equilibrium spectra which agree well with the known 12 experimental spectralpointsi8). Note that spectra measured at different wavelengths follow different time histories,which can in principle be used to follow the time evolution of the vibrational energy distribution ofthe relaxing 12 r olecules.
WAVELENGTH (nm)20O 1OOO 600 400 300
1404-2600
96--2400
-2200
R 72 Fig. 2. Transient electronic absorption spectra7M 0-5 - - 1800 computed from molecular dynamics for 12 (in
PXe solution) reaction sequence of photodissoci-I600 ~ ation, solvent caging, radical recombination,
56 and vibrational decay. Time zero is the photo-2D A dissociation. The top curve is the computed
48 -1200 equilibrium spectrum before photodissociation40 -"/ _-on which are superimposed the experimental
4]0 .1 3 10 points from Tellinghuisen [81.95-100
~32 800
24 6
16- 4M03
0 8 12 1602004 8 32
' ENERGY ft0cmfl }
3. Expfiment3.e ealiest picosecd experimental mutts were by the EISENTHAL JouLP9, 10], who measured
the trasient electronic absorption spectra after excitation at 530 rim. Decay times of -70 PS for 12
in hexdecane and -- 140 ps in CC4 were observed. Subsuently theme studies wee extended bythe EISENTHAL group to 12 in aromatic olvents[I I] which are believed to form orplexes with I
-3-
atoms, and by LANGHOFFI121 who observed 12 photodissociation in several weakly associatedliquids, finding decay times in the -100-150 ps range. More recently, KELLEY andRENTZEPIS[13] have observed 12 photodissociation in fluid and liquid Xe with a decay time of-40 ps, as well as in CCI4, and similar experiments have been carried out by the PETERSgroup[14. All of the above studies used second harmonic Nd pump light at -530 nrm. which, atleast in the gas phase, results in excitation largely to the bound B 0( 3f11) state which presumablypredissociates, but may absorb another photon in the meantime[]5, 161. A delay of -20 psbetween excitation and the maximum in the absorption curve has been attributed to absorptionfrom the B state 19, 13, 171. Thus, the linking of these experimental transient spectra to the molecu-lar dynamics of the 12 photodissociation and recombination reaction is made difficult by the prob-able presence of the additional processes of B state predissociation and absorption. We haverecently reported[31 transient absorption for 12 excited using an Ar*-dye (DCM) synchronouslypumped source at 710 nm into the dissociative A state, thus avoiding the problem of predissocia-tion. In order to achieve the sensitivity needed to detect the very weak[8] 12 A state absorption, weuse a multiple modulation system[18] based on the discovery by HERITAGE[19] and LEVINE andBETHEA[20, 211 that the noise in Ar synchronously pumped dye lasers falls off by several ordersof magnitude in going from the audio to the radio frequency region. We modulate the pump andprobe beams at two different radio frequencies and detect at the difference frequency, using inex-pensive and readily available radio amateur equipment[I8]. In addition, we audio modulate thepump beam and synchronously detect at that frequency. Decay times are shown in Fig. 3. We sug-gest the hypothesis that vibrational decay might be expected to be faster for the more stronglyhydrogen-bonded solvents which are expected to have a greater spectral mode density in the rangeof 12 vibrational frequencies. All the above experimental measurements suffer in interpretationfrom the rather weak connection between molecular dynamics and electronic absorption, which isfurther complicated by the possible presence of 12- solvent , I - solvent, and 12 -12 complexes.
25. - 0 -o~f QW)_ C ;0o01 Fig. 3. Experimental transient electronic
absorption spectra for 12 in ethylene glycol_ (0.2 molar, 1.3xlO-2 mole fraction), in
__ ethyl alcohol (0.33 molar, 1.9x10-2 mole9.- 0 -__0+- fraction), and in CC14 (0.096 molar,
cc. 9.3x10- 3 mole fraction). All the spectraC o 05- are for perpendicular orientation of linearly:s 2,. polarized pump and probe beams.
"0 =W 0 +50 +0 -150 +200 250DELAY (PICOSECONDS)
4. Discussion and Conclusion
* In summary, two hypotheses have been advanced: i) that geminate recombination for 12 is relativelyslow and thus accounts for the observed range of transient absorption decay times and ii) that gem-inate recombination is relatively fast and that vibrational decay times to reach vibrational levels withhigher absorption instead account for the observed transient absorption decay times. At the presenttime, the available theoretical and experimental tools have not been sufficiently powerful to cleanlydisprove either or both of these hypotheses.
Improvements in both theoretical and experimental tools should lead to more stringent tests.On the theoretical side, molecular dynamics and spectral calculations can certainly be extended to avariety of different solvents, and to different pump photon energies and thus different I atom recoilenergies. In addition, different assumptions as to the mechanism for relaxation to the ground statepotential curve can be tried out. Transient electronic absorption measurements suffer from theintrinsic limitation of a weak connection to molecular dynamics. In contrast, transient Raman spec-tra can directly reveal, for example, rotational and vibrational periods in the evolving reactants andproducts. We have computed such transient spectra 13,4 for this reaction sequence. The
.4.
equivalent resonance Raman spectra can also be calculated and they also will reveal much of theunderlying molecular dynamics, possibly including upper state recurrence times[22]. Thus Ramanspectra could provide a definitive test of the above hypotheses.
It is surprising, but true, that for no chemical reaction in solution, not even for one as sim-ple and as well studied as 12 photodissociation and recombination, are the detailed atomic motionsby which it occurs yet known. Even such basic aspects are not certain as the order of magnitude ofthe time required for caging and geminate recombination, and whether a direct deterministic or astochastic diffusional approach to geminate recombination is most appropriate.
This situation may soon change, as there is now a four order of magnitude time range, -100fs to I ns, over which molecular dynamic calculations and short light pulse experiments can over-lap. Transient infrared, Raman and electronic absorption spectra all reflect the underlying molecu-lar dynamics of chemical reactions and can provide an interface at which theory and experimentmay meet. By comparing transient spectra computed from molecular dynamics with the equivalentmeasured spectra, one can hope to discover the microscopic dynamics by which many chemicalprocesses occur.
Thanks for the support which has made this work possible to NSF Chemistry, ONR Chemis-try, NASA-Ames, NIH Division of Research Resources, and Fonds National Suisse for fellowshipsupport to P. Bado.
ReferencesI. D. L. Bunker and B. S. Jacobson, J. Amer. Chem. Soc. 94, 1843 (1972).2. J. N. Murrel, A. J. Stace, and R. Dammel, J. C. S. Faraday Transactions 1174, 1532 (1978).3. P. Bado, P. H. Berens, and K. R. Wilson, in Picosecond Lasers and Applications, edited by L. S.
Goldberg (Proc. Soc. Photo-Optic. Engin., Bellingham, WA, 1982) Vol. 322, p. 230. (Seeref. 16 below.)
4. P. H. Berens, J. P. Bergsma, and K. R. Wilson, J. Chem. Phy., to be submitted (1982).5. D. J. Nesbitt and J. T. Hynes, Chem. Phys. Lett. 82, 252 (1981).6. D. J. Nesbitt and J. T. Hynes, J. Chem. Phys., in press.7. P. H. Berens, J. P. Bergsma, K. R. Wilson, and E. J. Heller, to be submitted.8. J. Tellinghuisen, J. Chem. Phys. 76, 4736 (1982).9. T. J. Chaung, G. W. Hoffman, and K. B. Eisenthal, Chem. Phys. Lett. 25, 201 (1974).10. K. B. Eisenthal, in Ultrashort Light Pulses; Picosecond Techniques and Applications, edited by S.
L Shapiro (Springer-Verlag, Berlin, 1977) p. 275.11. C. A. Langhoff, K. Gnidig, and K. B. Eisenthal, Chem. Phys. 46, 117 (1980).12. C. A. Langhoff, B. Moore, and W. Nugent, in Picosecond Phenomena /, edited by R.
Hochstrasser, W. Kaiser, and C. V. Shank (Springer-Verlag, Berlin, 1980) p. 249.13. D. F. Kelley and P. M. Rentzepis, Chem. Phys. Let. 85, 85 (1982).14. K. Peters, Harvard University, (private communication).
f IS. G. E. Busch, R. T. Mahoney, R. 1. Morse, and K. R. Wilson, J. Chem. Phys. 51, 837 (1969).16. R. K. Sander and K. R. Wilson, J. Chem. Phys. 63, 4242 (1974).
7 17. W. S. Struve, Chem. Phys. Len. 51, 603 (1977).18. P. Bhdo, S. B. Wilson, and K. R. Wilson, Rev. Sd. Instrum., in press, plus addendum noting
that response of receiver is quadratic, as pointed out by D.B. McDonald and G.R. Fleming,which explains difference in decay times from ref. 3.
19. J. P. Heritage, in Picosecond Phenomena II, edited by R. Hochstrasser, W. Kaiser, and C. V.Shank (Springer-Verlag. Berlin, 1980) p. 343.
20, B. F. Levine and C. G. Bethea, Appl. Phys. Len. 36, 245 (1980).21. B. F. Levine and C. C. Bethea, IEEEJ. Quantum Eectron. QE-16, 85 (1980).22. E. J. Heller and K. R. Wilson, to be submitted.
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