Andrei Tokmakoff, MIT Department of Chemistry, 4/16/2009
10. NONLINEAR SPECTROSCOPY
Spectroscopy comes from the Latin “spectron” for
and the Greek “
. These roots are very telling, because in molecular spectroscopy you use light to interrogate
matter, but you actually never see the molecules, only their influence on the light.
spectroscopies give you different perspectives. This indirect contact with the microscopic targets
means that the interpretation of spectroscopy in some manner requires a model, whether it is
stated or not. Modeling and laboratory practice of spectroscopy are dependent on one another,
and therefore a spectroscopy is only as useful as its ability to distinguish different models.
observables that we have to extract microscopic information in traditional spectroscopy are
resonance frequencies, spectral amplitudes, and lineshapes. We can imagine studying these
spectral features as a function of control variables for the light field (amplitude, frequency,
polarization, phase, etc.) or for the sample (for instance a systematic variation of the physical
properties of the sample).
In complex systems, those in which there are many interacting degrees of freedom and in
which spectra become congested or featureless, the interpretation of traditional spectra is plagued
by a number of ambiguities.
This is particularly the case for spectroscopy of disordered
condensed phases, where spectroscopy is the primary tool for describing molecular structure,
interactions and relaxation, kinetics and dynamics, and tremendous challenges exist on
understanding the variation and dynamics of molecular structures.
This is the reason for using
nonlinear spectroscopy, in which multiple light-matter interactions can be used to correlate
different spectral features and dissect complex spectra. We can resonantly drive one
spectroscopic feature and see how another is influenced, or we can introduce time delays to see
how properties change with time.
Absorption or emission spectroscopies are referred to as linear spectroscopy, because
they involve a weak light-matter interaction with one primary incident radiation field, and are
typically presented through a single frequency axis. The ambiguities that arise when interpreting
linear spectroscopy can be illustrated through two examples: