=Paper=
{{Paper
|id=Vol-452/paper-25
|storemode=property
|title=Degenerate and two-color resonant four-wave mixing of C2 in a molecular beam environment
|pdfUrl=https://ceur-ws.org/Vol-452/poster5.pdf
|volume=Vol-452
}}
==Degenerate and two-color resonant four-wave mixing of C2 in a molecular beam environment==
https://ceur-ws.org/Vol-452/poster5.pdf
Degenerate and two-color resonant four-wave mixing of C2- in a molecular beam
environment
1 2 2 2
M. Tulei , P. P. Radi , G. Knopp , T. Gerber*
1
Physics Institute, Space Research & Planetary Sciences,
University Bern, Bern, CH
2
Paul Scherrer Institut, Molecular Dynamics,
5232 Villigen/PSI, CH
Two-color resonant four-wave mixing spectroscopy can be used for the sensitive and selective characterization of
-
negative ions in a molecular beam environment. Results are shown for C 2 produced by an electric discharge in a
mixture of acetylene and Argon. The plasma expands immediately after the discharge into vacuum forming a super-
sonic beam containing C2-anions. High signal-to-noise ratios show that the preparation technique is suited for the
application of high-resolution optical double-resonance spectroscopy.
Introduction probing the signal beam at remote distances. The
Negatively charged molecular ions are of relev- resulting high sensitivity renders the techniques
ance in astronomy, in the upper atmosphere, in applicable to species that are present in very low
electrical discharges, and in combustion. Anions concentrations. This property has been successful-
are present in most combustion system and are ly exploited for DFWM measurements of trace
assumed to play an important role in reactions species in low pressure cells, flames and molecu-
forming pollutants like soot and aerosols from air- lar beams[10-14]. The nonlinear methods are often
craft-engines[1-4]. In a recent work, Warnatz and complementary to the more conventional linear
coworkers[5] included negative ions in the chemi- spectroscopic techniques. Because four-wave
cal reaction mechanism for the modeling of a fuel- mixing is based on absorption, the signal intensity
lean methane-oxygen flame. Detailed reaction is insensitive to the lifetime of the upper level[15].
mechanisms involving negatively charged species As a consequence, the large and important cate-
are required for assessing the kinetics prevalent in gory of molecules, exhibiting non-fluorescing or
the processes of technical applications. In spite of pre-dissociative states, are accessible. Recent-
their importance only few anion species are spec- ly[16] it has been shown that the high temporal
troscopically characterized, - reflecting the noto- resolution of four-wave mixing spectroscopy on the
rious difficulty to prepare anions in sufficient abun- order of a few ns is sufficient to discriminate be-
dance. Often, anions are generated in a plasma tween different species in a discharged molecular
where numerous, more abundant, neutral- and beam on the basis of their time shifted nascency.
cation-species coexist whose spectra may overlap. This is in stark contrast to to cavity ring-down spec-
Therefore, a high selectivity is required to disen- troscopy[17], which allows the assessment of time
tangle the spectral features of anions among all scales only in the µs-domain. An additional benefit
other light emissions. An additional challenge for is obtained using two distinct input frequencies for
spectroscopic investigations of anions is the typical TC-RFWM. A signal is obtained exclusively when
absence of stable electronically excited states that both frequencies interact with distinct molecular
can not exist due to the low binding energy of the transitions that share a common level. As for all
excess electron. Nevertheless, rotationally re- double-resonant techniques, the selectivity by in-
solved electronic spectroscopy of valence or dipole termediate level labeling is beneficial to the simpli-
bound states has been achieved for a number of fication of spectral congestion. Rotational charac-
anions. One of the prominently investigated anion terization of high-lying vibrational states on the
-
is C2 [6-9]. ground potential energy surface is feasible by the
We applied degenerate four-wave mixing stimulated emission pumping (SEP) variant of TC-
(DFWM) and two-color resonant four-wave mixing RFWM[18].
(TC-RFWM), two background-free and highly sen- Thus, owing to the background-free characteris-
-
sitive methods, to C2 . DFWM and TC-RFWM are tics of nonlinear four-wave mixing techniques, an
nonlinear spectroscopic tools exhibiting high sig- inherently optimal signal-to-noise ratio is obtained.
nal-to-noise ratios due to a fully resonant process. However, nonlinear methods suffer from their qua-
The coherent, laser-like signal beam ensures col- dratic dependence on species density and involved
lection of the entire signal rather than a small frac- cross-sections. Nevertheless, in recent works, we
tion as compared to an incoherent process like have shown that DFWM and TC-RFWM tech-
Raman scattering or laser-induced fluorescence. In niques are sufficiently sensitive to yield substantial
addition to the high collection efficiency, a coherent signal-to-noise ratios for transient species in a
signal beam allows the rejection of stray light by molecular beam that are generated in a pulsed
* Corresponding author: thomas.gerber@psi.ch
Towards Clean Diesel Engines, TCDE 2009
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