Rotational Spectrum of Eugenol As Analyzed with Double Resonance and Grid-Based Autofit.

The molecule eugenol was extracted from cloves via steam-distillation, and its rotational spectrum from 3-18 GHz was acquired with a chirped-pulsed Fourier transform microwave spectrometer. This spectrum was analyzed via two separate methods in parallel, one employing several microwave-microwave double resonance measurements and the other using a newly written version of the Autofit program. Both methods yielded rotational constants in excellent agreement with predictions from ab initio calculations. The relative merits of the different analysis methods are discussed.

AC Stark Effect Observed in a Microwave-Millimeter/Submillimeter Wave Double-Resonance Experiment.

Microwave-millimeter/submillimeter wave double-resonance spectroscopy has been developed with the use of technology typically employed in chirped pulse Fourier transform microwave spectroscopy and fast-sweep direct absorption (sub)millimeter-wave spectroscopy. This technique offers the high sensitivity provided by millimeter/submillimeter fast-sweep techniques with the rapid data acquisition offered by chirped pulse Fourier transform microwave spectrometers. Rather than detecting the movement of population as is observed in a traditional double-resonance experiment, instead we detected the splitting of spectral lines arising from the AC Stark effect. This new technique will prove invaluable when assigning complicated rotational spectra of complex molecules. The experimental design is presented along with the results from the double-resonance spectra of methanol as a proof-of-concept for this technique.

Fast sweep direct absorption (sub)millimeter-wave spectroscopy.

Direct absorption spectroscopy has been the mainstay for spectral acquisition in the millimeter and submillimeter wavelength regimes because of the sensitivity offered by standard hot electron bolometer detectors. However, this approach is limited in its utility because of the slow spectral acquisition speeds. A few rapid acquisition techniques that offer reasonable levels of sensitivity have been developed, but these rely on specialized and costly equipment. We present here a new instrument design for a (sub)millimeter spectrometer that offers both rapid spectral acquisition and highly sensitive detection while using equipment from existing chirped-pulse Fourier transform spectrometers and direct absorption spectrometers. We report on spectrometer design and performance and compare the results to standard lock-in detection techniques.

Interplay of phenol and isopropyl isomerism in propofol from broadband chirped-pulse microwave spectroscopy.

The conformational equilibrium of the general anesthetic propofol (2,6-diisopropylphenol) has been studied in a supersonic expansion using broadband chirped-pulse microwave spectroscopy. Three conformers originated by the combined internal rotation of the hydroxyl and the two isopropyl groups have been detected in the jet-cooled rotational spectrum. The most stable conformer exhibits tunneling splittings associated with the internal rotation of the hydroxyl group, from which we determined the torsional potential and barrier heights (905-940 cm(-1)). The carbon backbone structure was derived from the spectral assignments of all 12 (13)C monosubtituted isotopologues in natural abundance and confirmed a plane-symmetric gauche orientation of the two isopropyl groups (Gg) for this conformer. In the other two detected conformers (EG and GE) one of the isopropyl groups is eclipsed with respect to the ring plane while the other is gauche, differing in a ∼180° rotation of the hydroxyl group. Supporting ab initio calculations provided information on the potential energy surface and molecular properties of the title compound.

Determination of precise relative energies of conformers of n-propanol by rotational spectroscopy.

The rotational spectrum of n-propanol (n-CH(3)CH(2)CH(2)OH) was studied with several techniques of contemporary broadband rotational spectroscopy at frequencies from 8 to 550 GHz. Rotational transitions in all five conformers of the molecule, Gt, Gg, Gg', Tt, and Tg, have been unambiguously assigned. Over 6700 lines of the Gt, Gg, and Gg' species, for quantum number values reaching K(a) = 33 and J = 67, were fitted in a joint analysis leading to the determination of DeltaE(Gg-Gt) = 47.82425(25) cm(-1) and DeltaE (Gg'-Gg) = 3.035047(11) cm(-1). Stark effect measurements in supersonic expansion were used to further confirm the assignment. The results are compared with those for the ethanol molecule and with ab initio calculations, allowing several inferences to be drawn concerning the differences in the large amplitude torsional potential of the hydroxyl group in the two molecules.

A broadband Fourier transform microwave spectrometer based on chirped pulse excitation.

Designs for a broadband chirped pulse Fourier transform microwave (CP-FTMW) spectrometer are presented. The spectrometer is capable of measuring the 7-18 GHz region of a rotational spectrum in a single data acquisition. One design uses a 4.2 Gsampless arbitrary waveform generator (AWG) to produce a 1 mus duration chirped pulse with a linear frequency sweep of 1.375 GHz. This pulse is sent through a microwave circuit to multiply the bandwidth of the pulse by a factor of 8 and upconvert it to the 7.5-18.5 GHz range. The chirped pulse is amplified by a traveling wave tube amplifier and broadcast inside the spectrometer by using a double ridge standard gain horn antenna. The broadband molecular free induction decay (FID) is received by a second horn antenna, downconverted, and digitized by a 40 Gsampless (12 GHz hardware bandwidth) digital oscilloscope. The second design uses a simplified pulse generation and FID detection scheme, employing current state-of-the-art high-speed digital electronics. In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 Gsampless AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 Gsampless digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7-18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle-Flygare-type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.

Vibrational dynamics of carboxylic acid dimers in gas and dilute solution.

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl(4)). Ultrafast excitation of the broad O-H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O-H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm(-1)) and the chemical identity of the dimer.

Determination of band curvatures by angle-resolved two-photon photoemission in thin films of C60 on Ag(111).

The thickness-dependent interfacial band structure was determined for thin films of C(60) on Ag(111) by angle-resolved two-photon photoemission spectroscopy. Dispersions of molecular-orbital derived bands (HOMO, LUMO+1, and LUMO+2) were acquired, and limits were placed on their possible effective masses. A group theoretic approach is also incorporated to further understand the properties of these states. The HOMO, LUMO+1, and LUMO+2 bands possess (best-fit) effective masses of -7 m(e), -7 m(e), and -12 m(e), respectively. These values are consistent with theoretical calculations, averaged over the closely spaced subbands for each state, and provide practical limits on the effective fundamental charge-transport properties of C(60) films.

Ultrafast electron dynamics at metal interfaces: intraband relaxation of image state electrons as friction.

Two-photon photoemission of image potential states above monolayers of p-xylene/Ag(111) shows that electrons with different momenta have very different rise and decay rates as a function of parallel momentum. The dynamics are due to energy and momentum loss (intraband relaxation), which we model as a stochastic process isomorphic to the overdamped motion of a harmonic oscillator. The method extracts a friction coefficient from the data which can be explained by electron-electron scattering in a formalism based on the Lindhard dielectric function. One-electron excitations (interband transistions) dominate the dissipation mechanism, with a smaller contribution from collective electronic excitations (plasmons).