7136
J. Phys. Chem. 1991,95, 7136-7138
of retrograde heats of transport. A few cautionary remarks are required at this point. First, the single-ion heats of transport used to calculate the electrophoretic terms are not known precisely. Estimates of these quantities from different studies can disagree by about 1-2 kJ mol-'. The uncertainty in the limiting heat of transport of the NBu4+ion may be larger because of the difficulties associated with the nonlinear extrapolation of Q*(NBu4X) to infinite dilution. It should be emphasized, however, that the likely errors in the values of Q*"+ or would not alter the magnitude of the calculated electrophoretic terms significantly. Of more serious concern is the approximate nature of the calculated electrophoretic terms. Payton and his colleagues find that eq 5 gives qualitative but not quantitative agreement between the measured and predicted initial thermoelectric powers of hydrogen-electrode t h e r m o c e l l ~ . ~ ~ From Figure 1 it is clear that the predicted concentration dependence of Q* for aqueous NBu4CI is in poor agreement with experiment. In this case the electrophoretic effect may be so large that terms of higher order than cl/* are important. (By analogy, both first- and second-order electrophoretic terms16 are significant
e*"-
in the isothermal diffusion of NBuJ salts.17) With appropriate terms of order cl/* as well as order c or c In c, it would of course be possible to reproduce the observed minimum in the plot of Q* versus c. The question of higher order terms cannot be answered until a more rigorous, practical theory of thermal diffusion is developed for dilute electrolytes. Despite these limitations, the present results suggest that the electrophoreticterms for thermal diffusion can be as large or even larger than the direct ion-ion terms. It is hoped that the unusual properties of aqueous NBu4X salts will motivate additional theoretical work on the electrophoretic term. From the experimental side, it would be helpful to have thermal diffusion data for other electrolytes with large values of (r+- r-)(Q*"+to see if they too show unusual behavior.
e*"-)
Acknowledgment. This research was supported financially by the Natural Sciences and Engineering Research Council. (16) Robinson, R. A.; Stokes, R. H. Electrolyte Solutiotw, 2nd ed.; Academic Press: New York, 1959. (17) Renner, T. A.; Lyons, P. A. J . Phys. Chem. 1974, 78, 1667.
Rotational Constants of Vlbratlonally Excited Iodine from Purely Rotational Coherence Observed In Pump-Probe Experimentst D.M. Willberg, J. J. Breen,* M. Cutmann? and A. H. Zewail* Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125 (Received: July 12, 1991; In Final Form: August 8, 1991)
Pump-probe laser-indud fluorescence techniques are used to determine the rotational constants of vibrationally excited iodine, coaled by supersonic beam expansions. For comparison, it is shown that the results obtained from real time (purely rotational coherence) are in excellent agreement with literature values for the u' = 12-23 of the B state. Results for nascent I2 (0' = 22), from the predissociation of 12Ne(0' = 23), are also reported.
recurrence. Diatomics represent an ideal case for comparing the measured rotational constants of single vibronic levels obtained by this method and others.
Introduction The concept behind purely rotational coherence and its use for excited-state structural determination is described elsewhere.'-' Briefly, purely rotational coherence arises when a population of molecules is prepared in a coherent superposition of rotational states by a polarized picosecond/femtosecond laser pulse. This induced alignment of an ensemble of jet-cooled molecules decays, but rephases again at a time, T , determined by the rotational constant of the molecule. Because of its Doppler-free nature, the phenomenon has been used extensively and developed as a method for measuring excited-state rotational constants by the groups of Felker,&' McDonald,*-'O Topp" and at Caltech.l-3J2 The experiments have invoked the techniques of time-correlated single-photon c o ~ n t i n g , ~ pumpprobe - ~ * ~ ~ * ~ multiphoton ~ ionization,12 pump-probe fluorescence depleti~n,'*~*~-'~ stimulated Raman-induced fluorescence depletion! and pumpprobe laser-induced f l u o r e s c e n ~ e . ~ ~ The J ~ experimental and theoretical methodology has also been extended to the femtosecond regime, where it is used to study wave packet dynamics and real-time alignment in chemical r e a c t i o n ~ . ~ ~ J ~ For a diatomic molecule, the spacing between r = 0 and the (in-phase) recurrence is 1/28, while the spacing between r = 0 and the out-of-phase recurrence is 1/48, referred to as a half-
(1) Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987,86, 2460; J . Phys. Chem. 1986, 90, 724. (2) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987,86, 2483. (3) Baskin, J . S.; Zewail, A. H. J . Phys. Chem. 1989, 93, 5701. (4) Connell, L. L.; Corcoran, T. C.; Joireman, P.W.; Felker, P. M. J. Phys. Chem. 1990, 94, 1229. (5) Connell, L. L.; Joireman, P. W.; Felker, P. M. Chem. Phys. Lett. 1990, 166, 510. (6) Corcoran, T. C.; Connell, L. L.; Hartland, G. V.; Joireman, P. W.; Hertz, R. A.; Felker, P. M. Chem. Phys. Lett. 1990, 170, 139. (7) Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1991, 94,4668; J . Phys. Chem. 1991, 95,4935. (8) Cote, M. J.; Kauffman, J. F.; Smith, P. G.; McDonald, J. D. J . Chem. Phys. 1989, 90,2865. (9) Kauffman, J . F.; Cote, M. J.; Smith, P. G.; McDonald, J. D. J . Chem. Phys. 1989, 90,2874. (IO) Smith, P. G.; McDonald, J. D. J . Chem. Phys. 1990, 92, 3991. ( I I ) Kaziska, A. J.; Shchuka, M. 1.; Topp, M. R. Chem. Phys. Letr., in
'*Current Contribution no. 8474. address: Department of Chemistry, Columbia University, New
(14) Dantus, M.; Bowman, R. M.; Baskin, J. S.;Zewail, A. H. Chem. Phys. Lett. 1989, 159,406. Zewail, A. H. J . Chem. Soc., Faraday Trans. 2 1989,85. 1221. Dantus, M.; Bowman, R. M.; Gruebele, M.; Zewail, A. H. J . Chem. Phys. 1989, 91, 7437. (15) Gruebele. M.; Roberts, G.; Dantus, M.; Bowman, R. M.; Zewail, A. H.Chem. Phys. Len. 1990, 166, 459.
York, N Y . 8 Deutsche Forschungsgemeinschaft postdoctoral fellow from Germany.
0022-3654/91/2095-7136$02.50/0
Dress. r - ---
(12) Scherer, N . F.; Khundkar, L. R.; Rase, T. S.; Zewail, A. H. J. Phys. Chem. 1987, 91,6478. (13) Baskin, J. S.; Semmes, D.; Zewail, A. H. J . Chem. Phys. 1986, 85, 7488. ...
0 1991 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 95, No. 19, I991 7137 I
I
1
1
I
l
l
I
1
1
1
1
1
I
l
l
I
1
1
1
1
1
I
l
l
Theoretical
Probe Laser
Vlbronlc State
Experimental
Qround State
Figure 1. Pumpprobe (LIF) scheme. The pump laser prepares a coherent superposition of rotational states in a given excited vibronic level. The probe laser, at a given time delay, interrogates the coherence by utilizing the relative polarizations and the LIF of the state of interest. For 12, the transient is obtained by monitoring the fluorescence from the ion-pair state as a function of time delay between the pump and probe pulses.
In this Letter, we present picosecond experimental results of iodine, cooled in a supersonic expansion with an estimated rotational temperature of 5 K. The rotational states with J up to 25 are populated (assuming a Boltzmann distribution) and, therefore, centrifugal distortion effects are minimal. Rotational recurrences of iodine have been previously measured in femtosecond experiments, both in a bulb15 and in a molecular beam.” Since the bandwidth of the picosecond pulse in the reported experiments is 3 cm-’, a single vibrational level in the B electronic state can be excited, and the recurrence time 7 can be directly measured to give the B constant of that level. The experiments are reported here for v‘ = 12-23. For 12Ne,we observe the recurrences of fragment iodine. Excellent agreement with the literature values is found.
Experimental Section The picosecond laser system and the molecular beam apparatus used in these experiments are described in detail elsewhere.’* Briefly, the laser system consists of a frequency-doubled modelocked, Q-switched Nd:YAG laser that pumps two etalon-tuned cavity-dumped dye lasers. The system repetition rate is 800 Hz. The pump laser (rhodamine 590 in methanol) operates in the region 550-575 nm. The visible output of the probe laser (DCM in dimethyl sulfoxide) is frequency doubled, and operates at 335-345 nm. An optical delay line is used to provide variable temporal separation between the pump and probe pulses before they intersect the molecular beam. The cross-correlation for these two pulses is 40 ps, and the visible pulse has an energy of 5 pJ, while the UV pulse has an energy of 0.25 pJ. The relative polarization of the probe laser with respect to the pump laser is set using a X/2 waveplate on the pump laser output. The iodine is cooled in a supersonic expansion of helium through an approximately 50-pm glass pinhole at 43 O C . The iodine is introduced by passing the helium (22 psig) over iodine crystals (Aldrich 99.99995, no further purification) at room temperature. The laser pvlses intersect the jet 75 nozzle diameters downstream from the nozzle. Laser-induced fluorescence (LIF) is collected, collimated, and focused onto a photomultiplier tube (RCA, 1P28). The fluorescence is spatially filtered with an iris to minimize scattered UV light signal, and passed through a high-pass UV filter (UGl 1) to eliminate scattered visible light from the pump (16) Zewail, A. H.Faraday Discuss. 1991, 91. (17) Dantus, M.;Janssen, M. H. M.;Zewail, A. H. Chem. Phys. Lett. 1991, 181, 281. (18) Breen, J. J.; Willberg, D. M.;Gutmann, M.;Zewail, A. H. J . Chem. Phys. 1990, 93,9180. Willberg, D.M.;Gutmann, M.;Breen, J. J.; Zewail, A. H.J . Chem. Phys., submitted for publication.
Time (picoseconds)
Figure 2. Purely rotational coherence of the single vibronic level u’ = 12 under beam conditions. Also shown is the theoretical simulation of the expected transient (see text). Both experiments and theory show the rotational recurrences (in phase and out of phase) at times determined precisely by the rotational constant.
$ m
-
0.027
-
L
a
0.026-
0
10
15
20
25
Vibrational Level, v’
Figure 3. Graph showing the values of the rotational constants, as a function of u’, obtained from this work. Also shown, for comparison, are the literature values for B as reported in ref 20. The error bars can be reduced further if an actual simulation of all recurrences is made, as discussed in ref 3; the error is indicated for each u’measurement (for one data point the error bar is not plotted).
pulse and fluorescence from the B electronic state.
Results and Discussion The scheme of the experiments is illustrated in Figure 1. The pump pulse prepares the rotational coherence in a given v’state, and the probe pulse monitors this coherence through a detected fluorescence from the ion-pair state.19 We have measured the B(v? of different vibronic states of iodine over the range u’ = 12-23. Figure 2 shows the results for v’ = 12, where the polarization of the probe pulse is parallel to that of the pump pulse. Also shown is a simulation based on the theoretical development given in ref 1. The recurrences give the rotational constant directly. The simulation is made for a rotational temperature of 5 K (Boltzmann distribution) and a 40-ps fwhm Gaussian cross-correlation. The fast initial dephasing of the rotational coherence, reported in refs 15 and 17, is decreased considerably here due to the convolution with the relatively (temporarily) broad laser pulses. The recurrence time, 7 , gives the rotational constant of the specific vibronic state by the simple relationship: 7 = 1/2B. Thus as the rotational constant decreases with increasing v’, the value of 7 becomes larger. To determine 7 , each recurrence peak is fit to a Gaussian function, and the peak-to-peak differences for a given transient are measured (for more accurate determination (19) Brand, J. C. D.; Hoy, A. R.Appl. Spectrosc. Rev. 1987,23,285 and references therein.
7138
J. Phys. Chem. 1991, 95, 7138-7142
A/----"-
1. (v'=22)
Besides the ease with which the rotational constants for individual o'states of I2 can be extracted by this method, the rotational recurrences observed have proven to be useful in identifying the vibrational levels produced in vibrational predissociation experiments. In a set of experiments described in detail in ref 18, the state-to-state rates of vibrational predissociation have been measured for the 12X van der Waals clusters (where X = Ne and Ar):
+
I~X(B,U'J4 I ~ ( B , u ' ~ ) X
-
W e (v'-23) I
I
I
I
I
I
1
I
I
1. (v'=22)
+ Ne
f
,
,
l
,
I
l
I
,
I
" h e (picoseconds)
Fipre 4. Shown are two transients: (top) transient obtained for the D' vibronic level of I,; (middle) transient obtained for the vibrational predissociation of the IINe cluster excited to the v ' = 23 level. In the latter case, the build-up of nascent Iz(u' = 22) is monitored ( T = 53 ps). A cross-correlation of the pump and probe laser pulses is also shown.
of B one can use the entire ~imulationl-~). The differences between full recurrenm and half-recurrences are both measured. The error values reported are 1 standard deviation of the value of T measured for each particular vibrational level. Figure 3 gives a graph plotting both the experimentally determined B constants of this work and those of the literature values20 as a function of u'. (20)
Gerstcnkorn, S.;Luc, P. J . Phys. (Paris)1985, 46, 867.
The 12X cluster is prepared in an excited vibrational level of the B electronic state by the pump laser; energy then transfers from this mode to the van der Waals stretching mode, causing the bond to rupture. The nascent I2 is excited by the probe laser to an ion-pair state, from which UV fluorescence is detected. If the predissociation process is fast compared with the rotational period of the molecule (Le., T < T,), rotational coherence is retained by the nascent 12(B,u;). %IS is shown in Figure 4 for o: = 23. In this case, the risetime for nascent I2 is 53 f 5 ps, and the criterion of T" C 7, ensures a preserved coherence, as shown by Baskin et alaB The rotational constant extracted from the rotational recurrences present in this transient is B = 0.024 83 f O.OO0 3 cm-I, which corresponds well to the literature value of B = 0.02485 cm-' for u' = 22. Therefore, the observed rotational recurrences confirm that the vibrational predissociation process in this system proceeds via the loss of one quantum of energy in the I2 stretch mode. The results reported for 12Neare part of an ongoing investigation of the dynamics of small clusters involving 12, and more details are given in ref 18. In the future, we hope to extend these studies to the measurements of rotational recurrences of the larger clusters, 12X,.
Acknowledgment. This work was supported by a grant from the National Science Foundation.
Slmulatlomr of Cbo Collisions wlth a Hydrogen-Terminated Dlamond {I 11) Surface R. C. Mowrey,* D. W. Brenner, B. 1. Dunlap, J. W. Mintmire, and C. T. White Theoretical Chemistry Section, Code 61 19, Naval Research Laboratory, Washington, D.C. 20375-5000 (Receiued: July 19, 1991)
The stability of C, during collisionswith a hydrogen-terminateddiamond { 1111 surface was studied by using molecular dynamics simulations. At a collision energy of 150 eV only nonreactive collisions occur. At higher energies nonreactive scattering and two types of reactive collisions occur: ( I ) exchange of one or more atoms between the molecule and the surface; (2) chemisorption of the C, molecule. No dissociation of the rebounding moleculeswas observed on the time scale of the simulations. However, the scattered molecules contain a large amount of internal energy, which suggests that dissociation may occur at longer times.
The recent development of techniques for synthesizing and purifying macrawpic quantities of the highly symmetric molecule Csoand the related fullerenes'-' has intensified experimental and ( I ) Kdgtclchmer, W.; Fostiropoulos, K.; Huffman, D. R. Chcm. Phys. Lerr. 1990, 170, 167. (2) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (3) Taylor, R.; Hare, J. P.; Abdul-Dada, A. K.; Kroto, H. W. J . Chcm. Soc., Chcm. Commun. 1990,20, 1423. (4) Johnson, R. D.; Meijer, 0.;Bethune, D. S.;J . Am. Chrm. Soc. 1990, I 1 2, 8983. (5) Ajie, H.; Alvarez, M. M.;Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kritschmer, W.; Rubin, Y.;Schriver, K. E.;Sensharma, D.; Whetten. R. L. J. Phys. Chcm. 1990, 94, 8630. (6) Haufler, R. E.; Conceicao, J.; Chibente, L. P. F.;Chai, Y.;Byme, N. E.;Flanagan,,S.; Haley, M.M.; OBrien, S.C.; Pan, C.; Xiao, Z.; Billups, W. E.; Ciufolini, M.A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990, 94, 8634.
theoretical efforts aimed a t determining the properties of these molecules. One property being studied is the stability of the fullerenes with respect to surface-induced dis~ociation.~*~ Detection of the charged gas-phase products of collisions of Cm*, CY0+, and CS4+with silicon and graphite surfaces8 at energies up to 200 eV and of Cso+and CW2+with a stainless steel surface9 at energies up to 60 and 120 eV, respectively, revealed no fragmentation. This high stability with respect to surface-induced dissociation was unexpected because experiments studying photoinduced dissociation of Cm+ detected sequential loss of Cz (7) Hawkins, J. M.; Lewis, T. A.; Loren, S.D.; Meyer, A.; Heath, J. R.; Saykally, R. J. J . Org. Chcm. 1990, 55, 6250. (8) Beck, R. D.; St. John, P.;Alvarez, M.M.; Diederich, F.; Whetten. R. L. J . Phys. Chcm., to be published. (9) McElvany, S. W.; Ross, M. M.;Callahan, J. H. Marcr Res. Soc., Symp. Proc. 1991, 206, 697.
This article not subject to US.Copyright. Published 1991 by the American Chemical Society
