Ultrashort Laser Pulse Phenomena, 2nd Edition

550 Examples of Ultrafast Processes in Matter

displacement of the atoms and substantially less energy than required for the

actual bond breaking that occurs with melting.

With the availability of ultrafast X-ray sources, X-ray diffraction became a

powerful new spectroscopic tool for time-resolved spectroscopy [41]. Because

X-ray diffraction is sensitive to the crystal symmetry laser-induced phase changes

can be probed directly by this technique [42].

11.5. PRIMARY STEPS IN PHOTO–BIOLOGICAL

REACTIONS

In the progression of increasingly complex systems, we have come to the role

of fs tools in analyzing the most complex biological systems. Two important

biological problems connected to fs spectroscopy are photosynthesis and vision.

In both cases, light energy is converted to biochemical energy, either for the

purpose of energy storage–transfer or for the purpose of detection. The primary

processes in the complex chain of reactions following light absorption, in vision,

or photosynthesis, takes place on a fs time scale. The quantum yield of these

ultrafast transformations is remarkably high—typically between 50 and 100%.

11.5.1. Femtosecond Isomerization of Rhodopsin

The primary process of vision takes place in rhodopsin, a pigment embedded

in the membranes of specialized photoreceptor cells, the rod and cone cells of

the retina. The role of the pigment is light absorption followed by a molecular

conformational change, which leads eventually to a change in membrane poten-

tial. This change in electrical potential across the photoreceptor cells is eventually

transmitted to the nervous system [43]. We are interested here in the primary pro-

cess of vision, which is the isomerization of the pigment following absorption of

a photon.

The pigment is a complex molecule called rhodopsin consisting of an “opsin”

protein bound to the 11-cis form of retinal chromophore. The absorption band of

rhodopsin peaks at 500 nm, which corresponds to the peak sensitivity of vision.

This main absorption corresponds to a transition from the S

0

ground state to a

S

1

excited state in the potential energy surface representation of Figure 11.12(a).

The potential energy is plotted as a function of an angular torsional coordinate of

the molecule. Absorption of a photon at 500 nm is followed by isomerization

to the red-absorbing trans-isomer bathorhodopsin. The classical representation

of the transformation is a “twist” of the chain [Fig. 11.12(b)]. The potential

surfaces as a function of the corresponding coordinate angle have a minimum

corresponding to the cis- and trans-configurations.