Download Review for the Final Exam

Review for the Final Exam
Chem 499/599
Many acronyms were used during the class such as CARS, CSRS, SRG, SRL, TIRE, COORS,
LIDAR, DIALS, LASER and PARS. What do each of these mean?
Be able to write out the expression for the polarization of the medium to third order in the
susceptibility. Which of these terms is responsible for COORS? Which is responsible for
CARS, SRG, SRL, Raman Shifting, and other nonlinear effects in isotropic media? Does the
term which depends on the second order susceptibility contribute to the nonlinear effects in
isotropic media? What about in anisotropic media. What is SHG? In terms of the idea that the
polarizability of the medium induces a dipole in the medium that oscillates as a result of
interaction with the primary electromagnetic wave, what is it about very high intensity light that
gives rise to nonlinear effects which are electromagnetic waves emitted at frequencies other than
that of the primary incoming EM radiation? What are some of the frequencies that can be
emitted? What about when two high intensity beams interact with the medium, each having a
different frequency, what are some of the frequencies that can be emitted? What is third
harmonic generation? Be able to calculate the second and third harmonic outputs for a given
primary wave.
In CARS there is an enhancement in the production of light emitted at a particular frequency
when the energy in cm- 1 of the incoming beams are given by 1-2 = vib. and the frequency of
the generated beam is 21 - 2 = 3. What is vib? This is shown by using Maxwells equations
to come up with an expression for the electromagnetic wave emitted due to the third order
nonlinear polarization. Remember from Maxwells relations that one can derive the GAIN EQN
and get an expression for the magnitude of the electric field emitted due to the 3rd order
susceptibility. Expressions for the intensity of the electric field, and the power of the electric
field emitted at 3 result. You should be able to write down an expression for the polarization in
terms of the third order susceptibility for CARS and SRG. How can altering the polarization of
the incoming electric fields allow just certain susceptibility terms to contribute to the CARS
beam generated?
You should be familiar with the expression that shows the frequency dependence of the 3rd order
susceptibility that is essentially a resonance denominator that approaches zero when 1-2 = vib.
Also the susceptibility expression shows the squared concentration dependence and the
dependence on the relative population of the states involved. You should be able to recognize
these in the susceptibility expression. Also we found that the CARS selection rules were the
same as those for COORS. What term shows this? You should be able to define all of the terms
in the equations for the power or the intensities of the CARS and SRG expressions. You must be
able to calculate the wavenumbers of the incoming lasers and generated beams for CARS, CSRS,
SRG. SRL, COORS given the pump or probe laser wavelength and the fundamental vibrational
frequency. Are there certain requirements for the angles of the incoming laser beams? What is
this called? What is Resonance CARS?
What are the differences between CARS and COORS in terms of the concentration dependence,
spatial and temporal coherence of light, the shape of the spectral lines, power dependence, etc?
What about CARS and SRG? Is CARS the technique of choice for detection of trace amounts of
gas in another solvent gas? Why or why not? Remember that the 3rd order susceptibility for
CARS is given by ( 3 ) N R +  + i" . On which susceptibility term does SRG and SRL
depend? You should be able to explain why a spectrum has a particular lineshape.
We talked about CARS being used as a diagnostic method. There are several reasons why CARS
is ideal for probing luminous systems. What are they? How can the vibrational and rotational
temperatures of the molecules be determined from a CARS spectrum? How does the nuclear
spin degeneracy affect the intensities of the peaks associated with rotation? How can pressure be
determined? What factors contribute to the linewidths? Be able to give the instrumentation
required for a CARS or SRG experiment.
You should be able to give the energy diagrams for the COORS, CARS, SRG, SRL techniques.
What is a Raman Shifter? How does it differ from an SRG experiment? Be able to show how
the different lines in the Raman shifter are generated and be able to calculate the wavelengths of
the lines. What is the general structure and purpose of a Raman Shifter? What threshold power
is needed before stimulate Raman occurs?
You should be able to identify each of the terms in the DIALS or LIDAR equations. What are
the differences between the Raman and Rayleigh LIDAR equations? Know and be able to
calculate the return wavelengths at the detector in the both cases. What molecule is primarily
used for Raman LIDAR? Which other one could be used? What information about the
atmosphere can LIDAR give you? What information about the atmosphere can DIALS provide?
You should have a basic idea of the instrumentation required to perform these experiments.
How do you determine the height resolution based on the lasers being used?
You should be able to identify the variables in the thermal lensing equations given in class.
What is the basic idea behind a thermal lens? You should know what the basic instrumental setup
looks like. What is the difference between a two laser beam experiment and a single beam
experiment? What do the signals look like? Is this experiment always performed with a pulsed
laser. Why does the probe laser need to be a cw laser? What properties does the pump laser
need to satisfy?
What is the enhancement factor? How does it affect the thermal lensing signal? How many
orders of magnitude larger than a typical UV-Vis signal can the signal for thermal lensing be?
Given a table of thermal constant you should be able to calculate relative enhancement factors.
If you change the solvent in a particular experiment and the signal is larger, does that always
mean that you will have a lower detection limit of the absorbing species in the new solvent?
Can thermal lensing be used to determine the temperature? If so what is the basis of the
temperature calculation? In the case of absorption and transition to a highly excited vibrational
combination band, what are the possibilities for contribution to the thermal lensing signal. Based
on studies presented in class, how can the number of collisions leading to the thermal lensing
signal be calculated.
The last part of the class was devoted to femtosecond lasers. How is a femtosecond laser pulse
generated? What is the difference between active and passive mode locking? What is a SESAM
and what is its purpose? What is the problem with direct amplification of the laser beam? How
can the beam be amplified? How is the bandwidth of the laser influenced by the pulsewidth of
the laser? Is it possible to get a very narrow bandwidth say 0.01 nm from a 1 fs pulse? What
about determining the pulsewidth? Can the pulsewidth of a femtosecond laser be determined
directly with a detector? How is the pulsewidth determined?
Be familiar with the timing and experimental particulars for time resolved resonance Raman.
Also how can the Raman signal be separated from fluorescence?
The final exam will be on Wed., May 2 at 12:30. Remember that the students in 599 will also
have an oral part to the exam based upon the written work that was handed in with their oral
report. It will follow the written exam.