Picosecond (and femtosecond) photofragment spectroscopy has recently provided time-resolved, state-to-state dynamics of molecular photofragmentation. The focus of these experiments was on unimolecular reactions, where two main issues are fundamental to the dynamics: the nature of the "half-collision" and the degree to which statistical theories account for the time evolution of product state distributions (PSDs)

Bernstein, Richard B.

Khundkar, Lutfur R.

Scherer, Norbert F.

Zewail, Ahmed H.

Caltech Authors - Main

Real‐time picosecond clocking of the collision complex in a bimolecular reaction:
The birth of OH from H+CO2
Norbert F. Scherer, Lutfur R. Khundkar, Richard B. Bernstein, and Ahmed H. Zewail 
 
Citation: The Journal of Chemical Physics 87, 1451 (1987); doi: 10.1063/1.453280 
View online: http://dx.doi.org/10.1063/1.453280 
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/87/2?ver=pdfcov 
Published by the AIP Publishing 
 
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Letters to the Editor 
a) StalfMember, Quantum Physics Division, National Bureau of Standards. 
IE. Carrasquillo M., T. S. Zwier, and D. H. Levy, J. Chem. Phys. 83, 4990 
(1985). 
2J. M. Hutson and B. J. Howard, Mol. Phys. 43, 493 (1981). 
3C. H. Joyner, T. A. Dixon, F. A. Baiocchi, and W. Klemperer, J. Chem. 
Phys. 74, 6550 (1981). 
4C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 86, 3151 (1987). 
~C. M. Lovejoy and D. J. Nesbitt, Rev. Sci. Instrum. 58, 807 (1987). 
6L. Andrews and O. L. Johnson, J. Chem. Phys. 76, 2875 (1982). 
'c. M. Lovejoy, M. D. Schuder, and D. J. Nesbitt, J. Chem. Phys. 86, 5337 
(1987). 
8A. M. Sapse and J. M. Howell, J. Chem. Phys. 78, 5738 (1983). 
9 A. D. Buckingham and P. W. Fowler, J. Chern. Phys. 79, 6426 (1983). 
lOp. A. Baiocchi, W. Reiher, and W. K1emperer, J. Chem. Phys. 79, 6428 
(1983). 
\lA. P. L. Rendell, O. B. Bacskay, and N. S. Hush, Chern. Phys. Lett. 117, 
400 (1985). 
12M. Spackman, J. Chern. Phys. 85, 6587 (1986). 
\3H. Jalink, D. H. Parker, and S. Stolte, J. Mol. Spectrosc. 121,236 (1987), 
and references therein. 
14T. J. Balle, E. J. Campbell, M. R. Keenan, and W. H. Flygare, J. Chem. 
Phys. 72, 922 (1980). 
ISO. Ouelachvili, Opt. Commun. 19, 150 (1976). 
Real-time picosecond clocking of the collision complex in a bimolecular 
reaction: The birth of OH from H + CO2 
Norbert F. Scherer,a) Lutfur R. Khundkar, Richard B. Bernstein,b) and 
Ahmed H. Zewailc) 
Arthur Amos Noyes Laboratory of Chemical Physics, d) California Institute of Technology, Pasadena, 
California 91125 
(Received 13 April 1987; accepted 12 May 1987) 
Picosecond (and femtosecond) photofragment spec-
troscopyl-S has recently provided time-resolved, state-to-
state dynamics of molecular photofragmentation.6-8 The fo-
cus of these experiments was on unimolecular reactions, 
where two main issues are fundamental to the dynamics: the 
nature of the "half-collision" and the degree to which statis-
tical theories account for the time evolution of product state 
distributions (PSDs). 
In contrast to unimolecular fragmentation, bimolecular 
reactions involve a full collision, and the dynamics9 of the 
collision complex is crucial to the fate ofproduct(s}. Tradi-
tionally, these reactions have been studied by the crossed 
molecular beam scattering technique. 10.1 I From the anisot-
ropy of the products' angular distribution (PAD) in the cen-
ter-of-mass coordinate system it is possible to infer whether 
the reaction proceeds by the "direct-mode" (subpicosecond 
interaction time) or via a "long-lived" (few rotational per-
iods) collision complex. 12.13 In this regard, quasiclassical 
trajectory calculations on semiempirical potential surfaces 14 
and statistical theories IS-24 of formation and decay of colli-
sion complexes have been instructive. More recently, spec-
troscopic methods have been utilized,2s-27 but the main body 
of experimental information28-30 has come from post-colli-
sion observables, i.e., PSDs (via chemiluminescence31 and 
LIF32), PADs,33 and products' polarization.34 
In this Communication, we report the first direct (real-
time) measurement of product formation from a bimolecu-
lar reaction: the birth of OH from the hot atom reaction 
H + OCO.35- 37 The reaction steps are: H + CO2 _ HOCO 
- HO + CO. In our pump-probe experiments, a picosecond 
laser pulse initiates the reaction via photodissociation of the 
van der Waals (vdW) molecule IH" 'OCO, prepared in a 
seeded supersonic beam expansion. The "clocking" of the 
reaction is accomplished by the use of a picosecond probe 
pulse (for OH LIF monitoring), delayed in time with re-
spect to the initiation pulse (Fig. I). UV photolysis of such a 
vdW precursor molecule ensures that the initial H velocity 
( - 20 km s - I) is preferentially directed along the hydrogen 
bond to the OCO, limiting the range of impact param-
eters38-40 and "orienting" the reactants.38-42 The essential 
element in our experiment is that the UV photolysis picose-
cond (pump) pulse acts as a precise trigger setting off the H 
atom "projectile," thus establishing the zero o/time for the 
bimolecular collision. 
The experimental apparatus is similar to that employed 
in Refs. 6-8. The seeded molecular beam was obtained by 
expanding mixtures of5% HI and 8% CO2 in He(P= 1800 
Torr}. The beam was characterized by electron impact (15-
30 eV) TOF mass spectrometry. The mass spectra displayed 
peaks for HI, CO2, HI' CO2, (C02 ) n and very little of 
HI· (C02 h Care was taken to trap 12 (at - 15 ·C) before 
nozzle expansion. The complex HI' CO2 concentration was 
typically 3%-5% relative to HI. 
The picosecond pump pulse (239 nm) was generated by 
mixing the second harmonic of the 616 nm light with the 
1.06 J.L of a Y AG laser. The probe was the second harmonic 
of the 616 nm pulse. The cross correlation between 616 and 
308 nm was obtained using a Lil03 crystal. The probe was 
tuned to the QI (I) transition of OH using a cell of HZOZ•7 
On-resonance enhancement for OH LIF was evident from 
probe wavelength tuning experiments. Typically, for the 
molecular beam X /D-40. The OH signal level was <0.2 
photons/pulse. Following Wittig et al., 38.39 care was taken to 
optimize the 1: 1 vdW complex in the expansion. Signal pro-
cessing and treatment of the data followed Refs. 6-8. 
The experimental results are displayed in Fig. 2. The rise 
time of the OH signal after deconvolution is found to be - 5-
15 ps. This first direct observation of the time evolution of 
the nascent product of a bimolecular reaction represents the 
transient decay time of the collision complex, formed with 
J. Chern. Phys. 87 (2),15 July 1987 0021-9606/87/141451-03$02.10 @ 1987 American Institute of Physics 1451  Reuse of AIP Publishing content is ubject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to  IP:  131.215.220.162 On: Mon, 29
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1452 Letters to the Editor 
Photolysis of 
HI 
Hot H atom attack 
on OCO Formation of Collision Complex HOCO 
Product formation 
HO+CO 
t 
fA 
picosecond 
pump 
o 
c 
t 
i 
u 
0 
If) 
?:-
'0; 
c 
" E 
... 
::; 
J: 
0 
0 
... 
:.: .... 
-50 
Time 
E9¥ of H+OCO Collision energy 
p 
Time (picoseconds) 
HOCO 
(cis/trans) 
50 100 
FIG. 2. Experimental results. Typical "transient" showing the OH LlF sig-
nal vs delay time 'T. The experimental data points have been fitted (solid 
curve) assuming an exponential risetime (15 ps) and convoluting with the 
measured cross-correlation function (shown in the insert). The weighted 
residuals are also plotted (above) vs 'T. From a number of experiments. the 
rise time range is 5-15 ps. More recent results (Ref. 47) deconvoluted with 
the proper system response function for pump and probe pulses. and ob-
tained with improved SIN. give'T, = 5 ± 2 ps. 
(t =rl 
i.. 
picosecond 
probe 
FIG. 1. Scheme ofthe experiment and p0-
tential energy surface (PES) of the bimo-
lecular reaction H + CO2, The picose-
cond photolysis pulse initiates the 
hot-atom reaction forming the HOCO col-
lision complex. This dissociates to yield 
the OH product which is detected by the 
probe picosecond laser pulse delayed by a 
time 'T with respect to the pump. The insert 
is a simplified picture of the PES taken 
from theoretical and experimental work 
(see the text). 
the given available energy. For the photolysis wavelength 
used, the initial relative translational energy, Etr (~Eav) of 
the H + OCO reagents is calculated to be 200 kJ mol-I as-
suming collinear incidence. The slower H atoms from the 1* 
branch, in contrast, yields Etr = 112 kJ mol- I, only slightly 
above threshold. 35 
For the subject reaction there is considerable knowledge 
of the potential surface based on theory43 and experi-
ment. 35-40.44,45 The minimum energy path has an entrance 
barrier corresponding to a threshold energy of - 105 
kJ mol-I, 35,38 followed by a potential well supporting a com-
plex, HOCO (itself studied by matrix isolation spectrosco-
py),46 followed by a small exit barrier44 to OH + CO. The 
initial H atom velocity is - 200 Alps and the available ener-
gy of collision with CO2 is above the transition state bar-
rier.39 Thus, the initial step is assumed to be complete within 
our pulse, but the rapid H-atom attack slows down depend-
ing on the vibrational motion in HOCO, which leads to a 
slower rate for the formation of HO + CO. The measured 
OH rise time can now be used to model the PSD and possible 
deviations from statistical behavior. Experiments at differ-
ent photolysis wavelengths will yield the energy dependence 
of the formation and decay of the complex. 
Planned experiments also include direct detection of the 
appearance of the HOCO and extension of the technique to 
the femtosecond time domain.5,48 This will make it possible 
to clock not only complex-mode bimolecular reactions, but 
also subpicosecond, direct-mode hot atom bimolecular reac-
tions. 
a) Recipient of a Procter and Gamble Award. American Chemical Society. 
1986. 
b) Sherman Fairchild Distinguished Scholar. Caltech (1986); permanent 
address: Department of Chemistry, University of California, Los An-
geles. CA 90024. 
c) John Simon Guggenheim Foundation Fellow. 
d) Contribution No. 7572; Research supported by the National Science 
Foundation under Grant No. DMR-8521191. 
'J. L. Knee. L. R, Khundkar. and A. H. Zewail. J. Chem. Phys. 82. 4715 
J. Chern. Phys., Vol. 87, No.2. 15 July 1987 
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Letters to the Editor 
(1985). 
2J. K. Knee, L. R. Khundkar, and A. H. Zewail, J. Chern. Phys. 83, 1996 
(1985). 
3J. L. Knee, L. R. Khundkar, and A. H. Zewail, J. Phys. Chern. 89, 4659 
(1985). 
4N. F. Scherer, F. E. Doany, A. H. Zewail, and J. W. Perry, J. Chern. Phys. 
84, 1932 (1986). 
'N. F. Scherer, J. L. Knee, D. D. Smith, and A. H. Zewail, J. Phys. Chern. 
89,5141 (1985). 
6L. R. Khundkar, J. L. Knee, and A. H. Zewail, J. Chern. Phys. 87, 77 
(1987). 
7N. F. Scherer and A. H. Zewail, J. Chern. Phys. 87, 97 (1987). 
8J. L. Knee, L. R. Khundkar, and A. H. Zewail, J. Chern. Phys. 87, 115 
(1987). . 
9R. D. Levine and R. B. Bernstein, Molecular Reaction Dynamics and 
Chemical Reactivity (Oxford University, N.Y., 1987). 
IOE. H. Taylor and S. Datz, J. Chern. Phys. 23, 1711 (1955). 
l1D. R. Herschbach, Discuss. Faraday Soc. 33,149 (1962); 55, 233 (1973). 
12W. B. Miller, S. A. Safron, and D. R. Herschbach, Discuss. Faraday Soc. 
44, 108 (1967). 
13S. Stolte, A. E. Proctor, and R. B. Bernstein, J. Chern. Phys. 65, 4990 
(1976). 
14p. Brurner and M. Karplus, Faraday Discuss. Chern. Soc. 55, 80 (1973). 
I'C. E. K1ots, J. Phys. Chern. 75,1526 (1971). 
16g. A. Safron, N. D. Weinstein, D. R. Herschbach, and J. C. Tully, Chern. 
Phys. Lett. 12, 564 (1972). 
17M. Quack and J. Troe, Ber. Bunsenges. Phys. Chern. 78, 240 (1974); 79, 
170 (1975). 
IBR. A. Marcus, J. Chern. Phys. 62, 1372 (1975). 
19G. Worry and R. A. Marcus, J. Chern. Phys. 67, 1636 (1977). 
20J. C. Light, in Atom-Molecule Collision Theory, edited by R. B. Bernstein 
(Plenurn, N.Y., 1979), p. 647. 
21R. D. Levine and J. L. Kinsey, in Atom-Molecule Collision Theory, edited 
by R. B. Bernstein (Plenum, N.Y., 1979), p. 693. 
221. Nadler, M. Noble, H. Reisler, and C. Wittig, J. Chern. Phys. 82, 2608 
(1985). 
23C. Wittig, I. Nadler, H. Reisler, M. Noble, J. Catanzarite, and G. Radhak-
rishnan, J. Chern. Phys. 83, 5581 (1985). 
NOTES 
24C. X. Qian, M. Noble, I. Nadler, H. Reisler, and C. Wittig, J. Chern. Phys. 
83,5573 (1985). 
2Sp. Hering, P. R. Brooks, R. Curt, R. Judson, andR. S. Lowe, Phys. Rev. 
Lett. 44, 687 (1980). 
26H._J. Foth, J. C. Polanyi, and H. H. Telle, J. Phys. Chern. 86,5027 (1982). 
27T. C. Maguire, P. R. Brooks, R. F. Curl J. H. Spence, and S. J. Ulvick, J. 
Chern. Phys. 85, 844 (1986). 
28R. N. Zare and R. B. Bernstein, Phys. Today 33 (No. 11),43 (1980). 
29y. T. Lee and Y. R. Shen, Phys. Today 33 (No. 11),52 (1980). 
3~. B. Bernstein and A. H. Zewall, J. Phys. Chern. 90, 3467 (1986). 
31A. M. Ding, L. J. Kirsch, D. S. Perry, J. C. Polanyi, and J. L. Schreiber, 
Faraday Discuss. Chern. Soc. 55, 252 (1973). 
32H. W. Cruse, P. J. Dagdigian, and R. N. Zare, Faraday Discuss. Chern. 
Soc. 55, 277 (1973). 
33J. M. Farrar and Y. T. Lee, Annu. Rev. Phys. Chern. 25, 357 (1974); D. 
M. Neurnark, A. M. Wodtke, G. N. Robinson, C. C. Hayden, and Y. T. 
Lee, J. Chern. Phys. 82, 3045 (1985). 
34R. Altkorn and R. N. Zare, Annu. Rev. Phys. Chern. 35, 265 (1984). 
3'G. A. Oldershaw and D. A. Porter, Nature 223, 490 (1969). 
36C. R. Quick, Jr. and J. J. Tiee, Chern. Phys. Lett. 100, 223 (1983). 
37K. K1einermanns and J. Wolfrum, Chern. Phys. Lett. 104, 157 (1984). 
38S. Buelow, G. Radhakrishnan, J. Catanzarite, and C. Wittig, J. Chern. 
Phys. 83, 444 (1985). 
39G. Radhakrishnan, S. Buelow, and C. Wittig, J. Chern. Phys. 84, 727 
(1986). 
.as. Buelow, M. Noble, G. Radhakrishnan, H. Reisler, C. Wittig, and G. 
Hancock, J. Phys. Chern. 90, 1015 (1986). 
41C. Jouvet and B. Soep, J. Chern. Phys. SO, 2229 (1984). 
42C. Jouvet and B. Soep, Chern. Phys. Lett. 96, 426 (1983). 
43L. B. Harding (unpublished), quoted in Ref. 39, G. Schatz and L. Hard-
ing (to be published). 
441. W. M. Smith and R. Zellner, J. Chern. Soc. Faraday Trans. 2 69, 1617 
(1973). 
4'1. W. M. Srnith, Chern. Phys. Lett. 49,112 (1977). 
46D. E. Milligan and M. E. Jacox, J. Chern. Phys. 54, 927 (1971). 
47N. F. Scherer, L. R. Khundkar, R. B. Bernstein, and A. H. Zewail (to be 
published). 
48M. Dantus, M. Rosker, and A. H. Zewail (subrnitted for publication). 
Ab initio calculation of the equilibrium geometry of NHt and NH3 
and the symmetric stretching potential of NHt 
Peter Botschwina 
Fachbereich Chemie der Universitiit Kaisers/autem, D-6750 Kaisers/autem, West Germany 
(Received 24 February 1987; accepted I April 1987) 
Although the NH/ ion has been the subject of intensive 
investigation by high-resolution infrared spectroscopy over 
the last four years, 1-5 it has so far not been possible to derive 
an accurate value for the equilibrium bond length solely 
from experimental data. Estimates of r. on the basis of ex-
perimental data and some assumptions have been made by 
Crofton and Oka/ who reported a value of 1.026(5) A and 
Nakanaga and Amano,4 who arrived at 1.021 ± 0.003 A. 
Probably the most reliable ab initio values have been pub-
lished by Yamaguchi and Schaefer.6 Their largest basis set, 
consisting of (l0s,6p,2d) for nitrogen and (6s, 1p) for hydro-
gen and contracted to [6,4,2/4,1], yielded r. = 1.0185 A at 
the CI-SD level and 1.0217 A when Davidson's correction 
was employed to approximately account for the effects of 
higher substitutions. 
In this note, the results of more extended ab initio calcu-
lations at the CEP A 7 level of approximation (coupled elec-
tron pair approximation) are reported for the equilibrium 
geometries of NH/ and NH3• Two different basis sets were 
einployed. The smaller one, which is already larger than the 
extended basis set of Yamaguchi and Schaefer6 and which 
consists of 73 contracted Gaussian-type orbitals, 
( lIs, 7p,2d /6s.2p) in contraction [8,5,2/4,2], was previous-
ly employed in CEPA-I calculations for HN2+ 8 and pro-
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Aug 2016 19:19:18


Real-time picosecond clocking of the collision complex in a bimolecular reaction: The birth of OH from H+CO_2

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Real-time picosecond clocking of the collision complex in a bimolecular reaction: The birth of OH from H+CO_2

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