Nuclear magnetic resonance spectroscopy is a powerful tool for the investigation of material properties,
which has found applications in many areas of physics, chemistry, biology
and medicine. Its physical foundations, the excitation of transitions between
nuclear spin states, was explored
in the years after the second world war, and is well characterised by now. Today's interest in the field is based largely on the immense potential for applications:
spins can serve as probes for their environment because they are weakly
coupled to other degrees of freedom. In most magnetic resonance experiments,
these couplings are used to monitor the environment of the nuclei, like spatial
structure or molecular dynamics.
While the direct excitation of nuclear spin
transitions requires radio frequency
irradiation, it is often possible to use light for polarizing the spin
system or for observing its dynamics. This possibility arises from the coupling
of spins with the electronic degrees of freedom: optical photons excite transitions
between states that differ both in electronic excitation energy as well
as in their angular momentum states. In general, the light can serve three different
purposes: it can polarize the spin system, thereby creating the 'raw material'
for spectroscopic experiments;
it can measure the spin polarization, serving as a detector; and it can influence
the dynamics of the system. Some motivations for using light in magnetic
resonance experiments include
In many cases, the possible
sensitivity gains are the primary reason for using optical methods. Compared
to NMR, sensitivity gains of more than 10 orders of magnitude are possible.
In fact, is was recently demonstrated, that it is possible to measure magnetic
resonance spectra of individual molecules [Koehler et. al. 1993; Wrachtrup et. al. 1993],
while a typical NMR sample contains of the order of 1017 spins.
- Selectivity: Lasers can
be used to selectively observe signals from certain regions in space, like surfaces,
at certain times which may be defined by laser pulses with a resolution
of 10-14 sec, or from a particular chemical environment defined, e.g. by the chromophore
of a molecule or the quantum confined electrons in a semiconductor. Crystal
imperfections can change the
optical properties of an atomic or molecular site in such a way that it those
sites can be distinguished against a background that is orders of magnitude larger.
Conventional magnetic resonance requires the presence
of a population differences between spin states to excite transitions between
them. This population difference is usually established by thermal relaxation through coupling with the lattice, i.e. the spatial degrees of freedom of the system. At low temperatures, this
coupling process may be too slow for magnetic resonance experiments. In the case
of optical excitation, the population differences are established directly by
a polarizing laser beam. The time required for polarization of the system depends
then only on the laser intensity and can be instantaneous on the time scale
of the magnetic resonance experiment, independent of temperature.
- Experiments in electronically excited states: If information about an electronically
excited state is desired
that is not populated in thermal equilibrium, it may by necessary to use light
to populate this state. It is then certainly advantageous to populate the different
spin states unequally to obtain at the same time the polarization differences
that are needed to excite and observe spin transitions.
1.2. Laser Spectroscopy
Apart from magnetic resonance, the interaction of matter with laser light
is the second most important ingredient
for the experiments that will be discussed below. In laser spectroscopy,
light induces transitions between different electronic states. The differences
between laser spectroscopy and magnetic resonance are related to the different
frequency scales: optical frequencies are more than 6 orders of magnitude higher
than rf frequencies. An important consequence is that laser spectroscopy
is much more sensitive as a technique: optical experiments on individual atoms
[Hurst et. al. 1979; Dehmelt 1990],
and molecules [Nguyen et. al. 1987; Moerner et. al. 1989] have become routine
in the last few years. While the first experiments of this type were performed
in ultrahigh vacuum, they can now also be performed in condensed matter, like
solutions [Nguyen et al. 1987] or solid matrices [Moerner et al. 1989].
characteristic property of laser spectroscopy is the high time resolution
which is of course also intimately linked to the high frequencies of optical
radiation. Technical limits to
the time resolution are now of the order of a few femtoseconds, corresponding
to only a few optical cycles. These characteristics are of course used extensively
in various applications. In chemistry, the high time resolution has been used
for 'real time' monitoring of chemical reactions, e.g. by measuring the intermolecular
potential as a function of time during the reaction. Also in solid state
physics, laser spectroscopy is used extensively to monitor fast processes,
like excitation and recombination
of electrons and holes in semiconductors.
In the field of high-resolution
spectroscopy, it is also possible to achieve very high frequency resolution:
some systems can now reach resolutions of better than 1 Hz, corresponding to a
relative frequency stability of some 1014. This high frequency resolution is especially
attractive for metrology, where many groups try to build new, more precise
frequency standards that are based on optical transitions. The properties of
laser spectroscopy, high frequency
resolution and high temporal resolution can potentially also be used in optically
enhanced magnetic resonance experiments - obviously an attractive perspective
for many applications.
The article is structured as follows: the next
section discusses the most important physical phenomena that permit the use of
laser radiation in magnetic resonance experiments. Examples of applications to
specific systems are discussed in the third section.
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