Chemistry Reference
In-Depth Information
7.2 EXPERIMENTAL AND COMPUTATIONAL METHODS
Because there are a variety of different experimental laser and detection systems that
may be employed for time-resolved resonance Raman (TR
3
) experiments, only two
common types of experimental techniques that are typically utilized to study
chemical reactions will be described in this chapter. The first experiment described
will be a typical nanosecond TR
3
spectroscopy apparatus that can acquire spectra of
intermediates from several nanoseconds to millisecond timescales by using elec-
tronic control of the pump and probe laser systems to set and vary the relative time-
delay between the pump and probe pulses. The second experiment described will be
a typical ultrafast TR
3
spectroscopy apparatus that may be employed to study
intermediates from the picoseconds to several nanosecond timescales by varying the
optical path length difference between the pump and probe laser pulses. Both types of
laser systems may be needed for some reaction systems to better investigate the
chemical reaction and intermediates of interest from the picoseconds to the micro-
second or millisecond timescales. For more details and information about the
nanosecond TR
3
and ultrafast TR
3
experiments described Sections 7.2.1 and 7.2.2,
the reader is referred to the original literature of Refs. 39-47 and to a recent topic
chapter on time-resolved resonance Raman spectroscopy given in Ref. 48.
7.2.1 Nanosecond Time-Resolved Resonance Raman (TR
3
)
Figure 7.1 presents a simple schematic diagram of the fundamental parts of a typical
two-pulse nanosecond TR
3
experiment where nanosecond lasers provide the light for
the pump and probe laser pulses for the TR
3
experiments. While a wide range of
nanosecond lasers (Nd:YAG, Nd:YLF, nitrogen, and others) may potentially be
employed for nanosecond TR
3
experiments, Nd:YAG laser systems are most
commonly utilized to generate the pump and probe laser pulses with 5-10 ns pulse
widths and 10-100 Hz pulse repetition rates and the 1064 nm fundamental wave-
length of the Nd:YAG lasers may be frequency doubled (SHG), tripled (THG), and
quadrupled (FHG) to produce 532, 355, and 266 nm laser pulses, respectively, and
these harmonics can pump optical parametric oscillators (OPOs) or dye lasers to
generate continuously tunable wavelengths of laser light in the visible region that can
in turn also be frequency doubled or mixed with the output of the Nd:YAG laser to
produce laser pulses in the near ultraviolet and ultraviolet wavelength regions.
Therefore, nanosecond laser pulses can be produced throughout most of the
ultraviolet and visible spectral regions for use as pump and probe laser pulses.
Raman shifters that are passive and less expensive can also be employed to produce
discrete wavelengths of laser pulses in the ultraviolet and visible spectral regions
from the harmonics of the Nd:YAG lasers.
Utilizing two laser systems allows the pump and probe laser pulses to be
independently changed for both their spectral wavelengths and their relative timing
in the TR
3
experiments. The relative timing of the pump and probe laser pulses are
determined by an electronic pulse generator that triggers both the flash lamps and
Q-switches of the two laser systems and a fast photodiode connected to a fast
