Development and application of four-wave mixing and cavity ring-down technique for spectroscopic studies

Optical diagnostic methods provide powerful tools for characterizing the molecular composition and detailed physical properties of molecular systems. The high intensity, coherent, and spectrally pure radiation provided by lasers, together with recent advances in laser spectroscopy, has revolutionized optical diagnostics. The unique properties of the laser techniques allow quantum state specific measurements to be performed with high temporal and spatial resolution. Applications of laser diagnostics are numerous and diverse, starting from fundamental spectroscopic measurements of isolated molecules in supersonic expansions, in combustion at high temperatures, to detecting complex macromolecules in biological systems. There is a variety of spectroscopic techniques available, each with its particular strengths and weaknesses that determine the area of their application. Important considerations for choosing a technique include sensitivity, applicability to a variety of chemical species, the capability for quantitative and qualitative measurements, and its experimental complexity. During the past several years a relatively mature nonlinear technique, degenerate four-wave mixing (DFWM), has received renewed attention because of its potential role as an optical diagnostic. In the DFWM experiment three incident beams are used which have identical (degenerate) frequency ω but are distinguishable due to having their directional and/or polarization characteristics. Nonlinear interactions with the target molecules give rise to a fourth output beam, with energy conservation demanding that the frequency of this coherently “scattered” radiation be equal to ω. The direction of propagation and polarization of the emerging signal wave also obey various conservation criteria. The availability of four-wave mixing as a probe of molecular structure comes from frequency-dependent variations in the efficiency of the signal production. In particular, these changes in optical response reflect fundamental properties of the molecules of interest (e.g. transition energies, decay rates), with exceptionally strong DFWM interactions expected to accompany resonant excitation of allowed molecular transitions. Thus by monitoring the intensity of the output radiation as a function of the incident frequency ω an essentially background-free, “absorption-like” spectrum can be measured for the species of interest. Because the four-wave mixing process does not rely upon “secondary process” (like fluorescence or ionization) to identify the location of a transition, such techniques can be applied to any system that shows resonant attenuation or amplification of light.
Interest in investigation of hydrocarbon families like the cumulene carbenes (H2C(=C)n) arises from their relevance in combustion and photochemical processes as well as their detection in the interstellar medium. Several members with n = 2-8 have been detected in the laboratory by microwave spectroscopy and n = 2, 3, 5 in dark molecular clouds. In the literature cumulene carbenes have also been suggested as possible diffuse interstellar bands (DIBs) carriers [92]. Consequently the electronic spectra of these species in the gas phase are required. The first electronic spectrum of propadienylidene H2C=C=C: was recorded in an argon matrix [89] and later on the vibrationally resolved spectrum was observed in solid neon. Three electronic transitions were observed: a strong C~ – X~band system in the 39051-47156 cm-1 range, weaker B~– X~ transition in the 16161-24802 cm-1 region, and the hardly detectable forbidden A~– X~ one at 13885-16389 cm-1. Based on these observations one can search for these absorptions in the gas phase.
In this work the electronic spectrum of propadienylidene has been measured using cavity ring down spectroscopy (CRDS). The stronger ˜ B 1B1 – ˜ X A11 transition has not been observed presumably because the short lifetime of the B~ state leads to broad absorptions which are difficult to detect. Two forbidden vibrational bands in the ˜ A12 - ˜ X A11 system are however seen. The transition at 15813 cm gains intensity through a-type Coriolis coupling in the excited state, and the one at 16233 cm borrows intensity by vibronic coupling with the nearby states of appropriate symmetry. The rotational analysis carried out gives the molecular constants for the excited state, which can be used to calculate a geometrical structure. -1-1
Interest in studying sulfur-bearing conjugated hydrocarbons arises not only due to their astrophysical relevance, but also their applications in molecular electronic devices. Sulfur terminated the carbon chains add strong polarization to the π-system, resulting in a strong dipole moment and consequently a large electronic oscillation strengths. Because of the sufficient abundance of sulfur in the interstellar medium one can expect the sulfur containing species to be found there. Sulfur-bearing molecules such as C3S and C5S have been already detected in the space by radioastronomy.
Up to now the optical spectra of the first five members of the HC2nS family have been measured. Thus, HC2S, HC4S, HC6S have been studied by means of LIF, where the spectrum of the HC6S has been also recorded by two-color two-photon ionization. HC8S and HC10S were studied by CRDS.
Recently, the spectrum of the 223/23/2AXΠ←Π%% system of HC4S, produced in a supersonic discharge, has been studied by LIF and dispersed fluorescence. In the latter, the authors report an extensive study of several vibronic transitions, while in the former a rotationally resolved spectrum
was observed with a 0.02 cm-1 laser linewidth, allowing spectroscopic constants to be determined. However in their experiment the 221/21/2AXΠ←Π%% spin-orbit component was not observed due to low temperatures in the jet.
The first part of this work is devoted to spectroscopic studies of two radicals, C3H5 and C3H2, using cavity ring-down spectroscopy, a wildly used spectroscopic technique known for its high sensitivity and experimental simplicity. However, problems were encountered when measuring optical spectra: namely, the CRD spectrum of C3H5 was misassigned due to another absorber overlapping in the same spectral region. In the case of C3H2, the strong B~ 1B1 -X~ A11 transition could not be detected due to the short lifetime of the B~ state. Therefore a nonlinear optical probe (degenerate four wave mixing) was developed and combined with a slit nozzle source. The first experiments investigated C2, which was previously studied using FWM technique only on high temperature (3000 K) oxy-acetylene flames. Direct comparison shows that while DFWM spectroscopy provides slightly less sensitivity (typically two or three orders of magnitude less then first order processes, such as CRDS), SN ratios are roughly 10 times greater. Moreover, spectra obtained by FWM are background free and very well structured. DFWM can also provide species selectivity, because the generated signal depends quadratically on concentration. After C2 detection, C3 was studied to improve the DFWM experimental setup and explore saturation effects.
Detection of HC4S demonstrated that this technique is also applicable toward lager molecules, provided that one has synthesized sufficient concentrations. Thus, replacing a circular orifice with a slit nozzle increased the sample interaction length. This led to significant enhancement in the sensitivity. Reducing the cross angle between the three incident beams from 2º to under 1.5º lengthen the interaction path from 10-15 mm to 15-20 mm, also resulting in greater sensitivity.
Indeed, much of the interest in DFWM stems from its unique capabilities in situations where more conventional linear methods to fail. Thereby, one should be able to detect the strong B~ 1B1 - X~ A11 transition of C3H2, which has never been measured in the gas phase and is of high interest due to its astrophysical relevance.