Page 167 - 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

Coherent Control of Chemical Reactions 229

FIGURE 26 Nomogram of the dimensionless alignment parameter ω as a function of laser intensity, field strength,
and polarizability anisotropy. [Reproduced with permission from Freidrich, B., and Herschbach, D. R. (1995). J. Phys.
Chem. 99, 15686. Copyright American Chemical Society.]

ω (⊥) = α (⊥) ε 2 4B, (70) λ = 1.064 µm), a circularly polarized dissociating pulse
0
12
2
(100 fs, 3 × 10 W/cm , 688 nm), and a circularly polar-
2
13
where B is the rotational constant. Noting that I = ized ionizing pulse (100 fs, 7 × 10 W/cm , 800 nm). The
2
2
0.00265ε , where I is the laser intensity in W/cm , and fragment ions were accelerated toward a microchannel
0
ε 0 is in V/cm, we obtain plate, and an image produced by electrons ejected onto a
phosphor screen was captured by a charge-coupled device
˚ 3
−1
2
ω (⊥) = 5.28 × 10 −12 α (⊥) [A ]I[W/cm ]/B[cm ].
(CCD) camera. Typical images demonstrating alignment
(71) of C 6 H 5 I are shown in Fig. 27. The image on the left
The condition for strong alignment is ω = ω − shows the anisotropic recoil of the iodine atom produced
−1
˚ 3
∼
ω ⊥ = 10, which, for α = 10 A and B = 1cm , corres- by a linearly polarized dissociation laser in the absence of
11
2
ponds to I = 2 × 10 W/cm . A nomogram for ω, plot- an aligning pulse. The middle image shows isotropic re-
ted in Fig. 26, shows that many molecules may be aligned coil produced by a circularly polarized dissociation laser
with fields readily achieved in the laboratory. in the absence of an aligning pulse. Finally, the image
To align a molecule, it is necessary that the intensity on the right shows the highly anisotropic recoil produced
of the aligning laser pulse lie below the threshold for by a circularly polarized dissociation laser in the pres-
multiphoton ionization. From Eq. (70) it is evident that ence of an aligning pulse. Other molecules that have been
the required intensity varies inversely with polarizabil- so aligned include I 2 , ICl, CS 2 , and CH 3 I. If instead the
ity. Because molecules with low polarizabilities generally aligning pulse is elliptically polarized, it is possible to
have higher ionization potentials, the conditions for laser align all three axes of a molecule, as was demonstrated
alignment are fairly robust. An experiment demonstrat- for 3,4-dibromothiophene (C 4 H 2 Br 2 S).
2
ing alignment utilized three laser pulses: a linearly po- The average value of cos θ is a measure of the ex-
2
2
12
larized aligning pulse (τ = 3.5 ns, I = 1.4 × 10 W/cm , tent of alignment. Values of
cos θ > 0.9 obtained with