Session P27: Focus Session: New Trends in Spectroscopy II

Coherent Broadband Microwave Spectroscopy

Recent advances in high-speed digital electronics have made it possible to develop a new type of Fourier transform microwave spectrometer that provides true broadband detection. These spectrometer use linear sweep chirped pulses for polarization of the molecular sample. The subsequent broadband free induction decay is digitized with a high-speed (50 Gs/s) 8-bit digitizer and the frequency domain spectrum is obtained by fast Fourier transformation. Spectrometer designs covering the 2-8 GHz, 6.5-18.5 GHz, and 25-40 GHz frequency bands will be presented. The spectrometer design is applicable to both pulsed jet molecular beam sources and to low-pressure gas samples at room-temperature. Measurement approaches to enhance the spectrometer sensitivity and reduce sample consumption will be presented. The technological advances in core spectrometer components expected over the next few years will also be described. Applications of this measurement technique to problems in molecular structure determination, chemical kinetics of isomerization reactions, and unbiased searches for molecules in the interstellar medium will be uses to illustrate the advantages of the spectrometer design.   

Design and Chemical Application of Chirped-Pulse Millimeter-Wave Spectroscopy

Chirped-Pulse Millimeter-Wave (CPmmW) Spectroscopy is the first truly broadband Fourier-transform technique for high-resolution spectroscopy in the millimeter-wave region. The design is based on the pioneering Fourier-Transform Chirped-Pulse Microwave (FT-CPMW) spectrometer developed at the University of Virginia, which operates at frequencies up to 20 GHz. We have built an instrument that covers the 70-102 GHz frequency region and can acquire up to 12 GHz bandwidth of spectrum in a single shot. Preliminary tests indicate a significant advantage in spectral acquisition time over existing millimeter-wave spectrometers, and further improvement to the sensitivity is expected as broadband millimeter-wave power amplifiers become affordable. The ability to acquire broadband Fourier-transform millimeter-wave spectra enables rapid measurement of survey spectra at sufficiently high resolution to measure diagnostically important electronic properties and also allows the accurate determination of relative line strengths. As an example of the usefulness of this tool to physical chemists, the preliminary results of several experiments will be discussed.   

Coherence-converted population transfer FTMW-IR double resonance spectroscopy of CH$_{3}$OD in the asymmetric CH-Stretch Region

State-selected infrared spectra of jet-cooled CH$_{3}$OD in the asymmetric CH stretch region (2890-3020 cm$^{-1}$) were obtained using the E-species microwave transitions: 1$_{0}\leftarrow $1$_{-1}$ at 18.957GHz; 2$_{0}\leftarrow $2$_{-1}$ at 18.991GHz; and 3$_{0}\leftarrow$3$_{-1}$ at 19.005GHz. The title technique enables the facile assignment of complex torsion-rotation spectra with multiple interacting vibrational bands. The two asymmetric CH-stretch fundamentals ($\nu _{2}$: 3000.2 cm$^{-1}$ and $\nu _{9 }$: 2955.7 cm$^{-1}$) are analyzed along with about 15 additional bands in the region of the binary combinations of the CH bends (2890- 2950 cm$^{-1}$). The number of observed vibrational bands indicates that the CH stretch bright states couple first to the binary CH bend combinations, and then to higher order combinations of the normal modes. The $J\prime $-dependence of the rotational structure is regular, but the $K\prime $-dependence is complicated by multiple strong $K\prime $-dependent interactions between vibrational states.   

Chirped pulses of microwave radiation as a light source in Fourier transform microwave spectroscopy

With recent advances in digital-to-analog and analog-to-digital speeds the possibility of performing high resolution ($<$ 100 kHz), broadband ($>$ 2 GHz per acquisition event) pure rotational spectroscopy has been realized. We have constructed several spectrometers of this type and will demonstrate their benefits. Target systems include mixed halogenated systems, functionalized hydrocarbons, and heavy metal-containing systems. Information obtained relates to molecular geometric structures, group electronegativities, and molecular electronic structures.   

Quantifying Excitonic Coupling in Disordered Semiconductor Quantum Wells

Monolayer fluctuations in the thickness of a quantum well lead to the formation of different types of excitons (bound electron-hole pairs) clearly resolvable in an optical spectrum.~ Coherent coupling between these spectrally resolved exciton resonances may modify the statistics of photon emission or affect energy transfer processes. We study a prototypical disordered GaAs quantum well sample using the newly developed electronic two-dimensional Fourier transform spectroscopy. Our experiments permit for the first time a quantitative measurement of the coupling strength between exciton resonances. Our theoretical modeling suggests that strong coherent coupling may only be observed when the Coulomb correlation length is greater than both the disorder correlation and confinement lengths. This rule should be applicable to other disordered systems, e.g. molecular aggregates, where coherent coupling critically affects charge transfer processes.   

Expansion of Two-Dimensional Spectroscopy into the Microwave: Implementation and Applications

Over the past several decades the developments of pulsed, two-dimensional, spectroscopic techniques have covered a broad range of the electromagnetic spectrum, essentially spanning the radio frequency to the UV. However, a notable gap has been present in the microwave/millimeter wave region of the spectrum owing, in large part, to the lack of suitable pulsed broadband excitation and detection equipment in this part of the EM spectrum. In the past five years, fast digital electronics (40 Gs/s; 12 GHz bandwidth) have allowed the development for ultra-broadband spectroscopic techniques to be developed in the 8-18 GHz region of the EM spectrum. We will discuss our current efforts to develop an ultra-broadband two-dimensional spectroscopy, direct analogs of 2D-NMR techniques, in the 8-18 GHz region. Unique spectroscopic applications of the technique, such as the spectroscopic determination of chirality, will be discussed.   

Ultrafast vibrational decoherence probed in the infrared near-field of molecular bonds

The strong intra- and intermolecular coupling in molecular systems is responsible for the dominance of the non-radiative over the radiative pathway in the ultrafast decoherence of vibrational excitations. The traditional coherent vibrational spectroscopy techniques therefore rely on nonlinear wavemixing to probe the underlying dynamics. Here we demonstrate the enhanced radiative far-field coupling from the optical near-field of vibrational excitations in block-copolymer nanostructures chosen as model system, by scattering scanning near-field optical microscopy (s-SNOM). The modified tip-scattered free-induction decay of the coherently excited vibrational mode following a broadband femtosecond infrared excitation is temporally resolved by interferometric homodyne detection. The results provide insight into spatial coherence and inhomogeneity with nanometer spatial resolution.   

Fluorescence Dynamics Utilizing Fluorescence OPA FROG

It has been demonstrated that fluorescence can be amplified by use of optical parametric amplification (OPA). By then using the FROG (Frequency Resolved Optical Gating) technique, fluorescence dynamics can be measured on an ultrashort timescale. This technique has several attractive features compared to current state-of-the-art fluorescence upconversion because it has the potential to amplify weak fluorescence, detection occurs at the wavelength of the fluorescence signal in the visible or near IR spectral region and the phase-matching condition is $\vec{k}_{pump}$ = $\vec{k}_{signal}$ + $\vec{k}_{idler}$. We will demonstrate time gating, effective amplification of fluorescence, and the dependence on concentration, and its effectiveness for resolving the stokes shift for common fluorescent dyes.   

Shape-dependent exciton spin polarization studied by time-resolved magneto-optical spectroscopy

Shape-dependent exciton spin polarization of semiconducting nanoparticles will be presented. Time- and polarization-resolved magneto-photoluminescence spectroscopy is carried out at low temperature in magnetic fields up to 17.5 Tesla to investigate the extent of spin polarization in CdSe quantum dots and nanorods. One-dimensional CdSe nanorods exhibit a large degree of circular polarization when even small magnetic fields are applied. The large spin polarization achieved in 1-D nanostructures is not observed in 0-D quantum dots. The experimentally measured polarization is attributed to strong mixing of ``dark'' and ``bright'' exciton fine-structure states in 1-D nanostructures, which leads to the formation of spin-polarized excitons. The polarized emission is also confirmed by wavelength-resolved intensity-integrated and time-correlated single-photon counting measurements. The findings may have significant impacts on devices based on the nanocrystals platform, including; solar-to-electric energy conversion, spintronics and chemical lasers.   

Rovibrational Phase-Space Surfaces for High Resolution Analysis of Polyad Bands

After nearly a century of scientific effort the spectra of spherical top molecules, such as methane, are notoriously problematic to evaluate both experimentally and theoretically. At the same time, astronomical applications require ever higher resolution data of exactly these molecules. Several theoretical and computational tools exist to predict these spectra quantitatively, but Rovibrational Phase-Space analyses, such as the Rotational Energy Surface (RES) serve as qualitative tools. Some such analysis exists in the literature, but advances in computing hardware and computational tools has made it much easier and more practical. Previous efforts have evaluated the rotational level clustering in vibrational singlet and doublets. Here we show a more complicated RES analysis, evaluating the $\nu_3 / 2 \nu_4$ polyad band of $CF_4$