Fluorescence-detected Raman-optical double-resonance spectroscopy

A few forms of coherent Raman spectroscopy1
·2 have demonstrated the capability of
creating excess populations in specific molecular
rovibrational states. Although this
produces an undesirable saturation effect in
some contexts,1 it is intrinsic to the signal
strength of others2 and in.akes several
Raman-optical double-resonance schemes
feasible. One such scheme,3which combines
stimulated Raman spectroscopy with ionization
detection, has been employed in
double-resonance studies of NO. Subsequently,
4 we have employed visible laserinduced
fluorescence (LIF) to detect coherent
Raman excitation of a-polyatomic molecule,
D2CO.
The excitation scheme for our Raman-LIF
double-resonance technique is shown in Fig.
WFF6-1, the details being specific to D2CO.
A pulsed Nd:YAG/dye laser system provides
fixed-wavelength pump and tunable Stokes
radiation, the difference frequency of which
excites the Raman-active v2 (CO stretch)
mode of vibration and generates an excess
rovibrational population, whi~h is probed by
a second pulsed dye laser tuned to an appropriate
vibronic hot band. The optical
bandwidth ('""'0.5 cm-1), sample pressure
(""-'0;5 Torr), and minimum pump-probe
timing delay (""-'20 nsec) provide rotationally
selective excitation of D2CO within a detection
interval 10 times shorter than that between
gas-kinetic molecular collisions. An
increase of timing delay or pressure enables
sta.te-specific collision-induced processes to
be investigated.
Observations of Raman-induced intensity
changes in LIF excitation spectra of D2CO
indiCate4 a 1000-fold enhancement of sensitivity
over conventional coherent Raman
spectroscopy, comparable with that obtained3
by ionization detection in NO.
Conversely, by fixing the probe wavelength
arid scanning the Raman difference frequency,
it is possible to project out specific
features from the overall Raman spectrum.
The experiments on D2CO have resolved individualO-,
P-, R-, and S-branch Raman
lines, avoiding the spectral congestion of a Q
branch and enabling rotational selectivity to
be achieved with relatively broadband pulsed
lasers.4
We have performed similar Raman-optical
double-resonance experiments in the centrosymmetric
molecule glyoxal, (CHO)z.
The results appear to be complicated by
electronic resonance effects, which possibly
implicate the low-abundance cis-isomer of
glyoxal; further experiments are in progress
to test this interpretation.
Our principal objective has been to develop
a flexible means of exciting specific molecuhir
rovibrational states and of following the
subsequent transfer of population to other
states, the pump-probe timing delay being
varied to obtain kinetic information. Molecular
processes of interest include rotationai
relaxation, vibrational energy transfer, fast
isomerization, and intramolecular energy
dissipation. Our approach may be viewed as
an alternative to nonlinear-optical difference-
frequency generation of tunable infrared
radiation, in that coherent Raman
excitation amounts to difference-frequency
mixing in the molecule of interest, rather
than in an external nonlinear medium.
(12 min.)
1 M. D. Duncan, P. Oesterlin, F. Konig, and R. L.
Eyer, Chern. Phys. Lett. 80, 253 (19131)~ S. A. J .
Druet and J.P. Taran, Prog. Quant. Electron. 7, 1
(1981).
2 J. J. Barrett and D. F. Heller, J. Opt. Soc. Am. 71,
1299 (1981). .
3 P. Esherick and A. Owyoung, Chern. Phys. Lett.
101 (1983).
4 D. A. King, R. Haines, N. R. Isenor, and B. J. Orr,
Opt. Lett. s; 629 (1983).