Page 150 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN003H-565 June 13, 2001 20:37

212 Coherent Control of Chemical Reactions

Typical experimental results are shown in Fig. 6 for the
reaction

∗
HI + ω 3 , 3ω 1 → H + I , (18a)
−
→ HI + e . (18b)
+
The excited iodine atom produced in reaction (18a) ab-
+
sorbs one or two photons to yield the I ion. The Xe pres-
sure in the third harmonic cell is adjusted so that the one-
and three-photon signals are approximately equal. Varia-
tion of the H 2 pressure in the phase-tuning cell produces
the sinusoidal variation of the ion signals shown in Fig. 6.
◦
Evident in this figure is a phase lag of 150 between the
+
two products, HI and I. Also shown is modulation of the
signal produced by photoionization of H 2 S, which pro-
+
vides a reference phase for the HI and I signals.
+
Coherent phase control has been used to populate
both bound and continuum eigenstates. Bound-to-bound
state control has been demonstrated for many molecules,
FIGURE 4 Theoretical calculation of the coherent phase control including HCl, CO, NH 3 ,CH 3 I, N(CH 3 ) 3 , N(C 2 H 5 ) 3 ,
of the photodissociation of IBr by one- and three-photon excitation. (CH 3 ) 2 N 2 H 2 , and c-C 8 H 8 . Bound-to-continuum control
[Reproduced with permission from Chan, C. K., Brumer, P., and
has been achieved for the photoionization of Hg, HI, DI,
Shapiro, M. (1991). J. Chem. Phys. 94, 2688. Copyright American
H 2 S, and D 2 S, and for the photodissociation of HI, DI,
Institute of Physics.]
and CH 3 I. In all of these studies, the use of one- vs
three-photon excitation ensures that the parity changes
for the two paths are the same. If the parities for the two
2

IBr + ω 3 , 3ω 1 → I + Br P 3/2 (17a)
paths are not equal, as for one- vs two-photon excitation,
2
the average over scattering angles in Eq. (15) causes the
→ I + Br P 1/2 (17b)
cross term to vanish. In this case the differential cross sec-
as a function of relative laser phase and intensity is given tion (i.e., the distribution of recoil angles) may still be
2
in Fig. 4. The contours show the fraction of Br( P 1/2 ) pro- controlled. One- vs two-photon control of angular distri-
duced from an initial rovibrational level with quantum butions has been demonstrated for the photoionization of
numbers v = 0, J = 42, averaged over initial M J , with a Rb and NO and for the photodissociation of HD . This
+
−1
photon energy of ω 1 = 6635.0cm . The abscissa is the method has also been used to control the direction of an
3
2
2
dimensionless ratio x /(1 + x ), where x =|ε 1 | /(¯ε|ε 3 |), electric current in a GaAs/AlGaAs quantum well and in
and ¯ε is defined as a single unit of electric field. an amorphous GaAs semiconductor.
An apparatus used for coherent control experiments is It is possible to design multipath control schemes in
depicted in Fig. 5. A molecular beam intersects the two which the laser phase cancels out of the interference
laser beams in a plane lying between the repeller and term. One possibility is a “diamond” path configuration,
extractor electrodes of a time-of-flight mass spectrom- ω 1 + ω 2 vs ω 2 + ω 1 , with a resonance near ω 1 contributing
eter. A uv laser beam of frequency ω 1 is focused into a phase to the first path and a resonance near ω 2 con-
a cell containing a rare gas such as xenon. Third har- tributing a phase to the second path. As before, the total
monic generation (THG) by the rare gas produces coher- probability is the square of the sum of the amplitudes for
ent vacuum ultraviolet (vuv) radiation of frequency ω 3 each path, but here the phases of the two laser beams
with a definite relative phase, ϕ,defined by Eq. (13). The appear in both paths and cancel in the cross term. In
two laser beams enter a phase-tuning cell containing a this case the control parameters are the laser frequencies,
transparent gas such as hydrogen. Because the difference which determine the detuning from the resonances. This
between the indices of refraction at ω 1 and ω 3 is pro- technique was used by Daniel Elliott and coworkers to
portional to the pressure of the phase-tuning gas, an in- control the differential cross sections for the ionization of
crease in the gas pressure produces a linear increase of ϕ. Ba and NO.
Ions produced in the reaction region are repelled into a Another example of phase-insensitive control utilizes
field-free flight tube and detected by a microchannel plate the “lambda” scheme depicted in Fig. 7. In this case a
(MCP). strong coupling (ω 2 ) field mixes an excited state with