David F. Plusquellic

The Conformational Structures and Dipole Moments of Ethyl Sulfide in the Gas Phase

The pure rotational spectrum of ethyl sulfide has been measured from 12 to 21 GHz in a 1 K jet-cooled expansion using a Fourier-transform microwave (FTMW) spectrometer. Prominent features in the spectrum are assigned to transitions from three conformational isomers. Additional assignments of the 13C and 34S isotopomer spectra of these conformers effectively account for all of the remaining transitions in the spectrum. Accurate “heavy-atom” substitution structures are obtained via a Kraitchman analysis of 14 rotational parameter sets, permitting definitive identification of the molecular structures of the three conformers. Two of the structures designated as the gauche–gauche (GG) and trans–trans (TT) conformers have symmetric forms with C2 and C2v symmetries, respectively, and the third trans–gauche (TG) configuration is asymmetric. The components of the electric dipole moment along the principal inertial axes have been determined from Stark measurements and are consistent with these structural assignment...

Interstellar isomers : The importance of bonding energy differences

We present strong detections of methyl cyanide (CH3CN), vinyl cyanide (CH2CHCN), ethyl cyanide (CH3CH2CN), and cyanodiacetylene (HC4CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH3NC) for its JK = 10-00 transition, which is the first interstellar report of this line. To determine the spatial distribution of CH3NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the JK = 4K-3K (K = 0-3) transitions, but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is >8500 cm-1 (>12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH3NC emission with a single antenna (Gaussian beam size ΩB = 1723 arcsec2) but not with an interferometer (ΩB = 192 arcsec2) strongly suggests that CH3NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH3CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH3NC appears to be deficient in the LMH hot core. Thus, large-scale, nonthermal processes in the surrounding medium may account for the conversion of CH3CN to CH3NC, while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH3NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH3CN can produce CH3NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are cospatial.

Chirped-pulse terahertz spectroscopy for broadband trace gas sensing

We report the first demonstration of a broadband trace gas sensor based on chirp-pulse terahertz spectroscopy. The advent of newly developed solid state sources and sensitive heterodyne detectors for the terahertz frequency range have made it possible to generate and detect precise arbitrary waveforms at THz frequencies with ultra-low phase noise. In order to maximize sensitivity, the sample gas is first polarized using sub-μs chirped THz pulses and the free inductive decays (FIDs) are then detected using a heterodyne receiver. This approach allows for a rapid broadband multi-component sensing with low parts in 10(9) (ppb) sensitivities and spectral frequency accuracy of <20 kHz in real-time. Such a system can be configured into a portable, easy to use, and relatively inexpensive sensing platform.

Multiheterodyne spectroscopy with optical frequency combs generated from a continuous-wave laser

Dual-drive Mach-Zehnder modulators were utilized to produce power-leveled optical frequency combs (OFCs) from a continuous-wave laser. The resulting OFCs contained up to 50 unique frequency components and spanned more than 200 GHz. Simple changes to the modulation frequency allowed for agile control of the comb spacing. These OFCs were then utilized for broadband, multiheterodyne measurements of CO2 using both a multipass cell and an optical cavity. This technique allows for robust measurements of trace gas species and alleviates much of the cost and complexity associated with the use of femtosecond OFCs produced with mode-locked pulsed lasers.

Experimental studies of peptide bonds: Identification of the C7eq conformation of the alanine dipeptide analog N-acetyl-alanine N′-methylamide from torsion-rotation interactions

Rotational spectra of the biomimetic molecule, alanine dipeptide and the double 15N(15N2) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal tunneling splittings from the torsional mode structure of two of its three methyl rotors. The torsional states assigned include one AA-state and two AE-states (i.e., AE and EA) for each isotopomer. The AA-states are well-fit to A-reduction asymmetricrotor Hamiltonians. The “infinite-barrier-limit” rotational constants of the 14N2 isotopomer are A=1710.97(8) MHz, B=991.89(9) MHz, and C=716.12(6) MHz. The AE-states are analyzed independently using “high-barrier” torsion-rotation Hamiltonians, yielding observedminus-calculated standard deviations of 100-fold for the 15N2 isotopomer) when analyzed in a ρ-axis frame where ρb=ρc=0. The best-fit torsion-rotation parameters provide accurate V3 barriers and C3 rotor axis angles for both methyl groups. The observed ...

Photodissociation dynamics in quantum state-selected clusters: A test of the one-atom cage effect in Ar-H2O

High resolution IR overtone pumping with an injection seeded optical parametric oscillator (OPO) is used in conjunction with excimer laser photolysis to investigate the state‐resolved dynamics of quantum state‐selected van der Waals clusters in a slit supersonic expansion. The narrow band IR light source (160 MHz, 5 mJ) preselects a specific upper state via the internal rotor band of Ar–H2O which correlates to the ‖03〉−←‖00〉+, 000←101 transition in H2O monomer. At fixed UV photolysis and probe wavelength, scanning the high resolution OPO yields the ‖03〉−←‖00〉+, Σ(000)←Σ(101) overtone action spectrum of Ar–H2O complexes. Conversely, at fixed IR pump wavelength, the state distribution of the OH photoproduct from photolysis of quantum state selected Ar–H2O clusters can be probed by laser induced fluorescence on the A 2Σ+←X 2Π (0,0) band. The OH distributions from H2O monomer vs Ar–H2O photolysis from the same internal rotor state are remarkably similar, though significant anomalies are observed for specific ...

Bond-selective photofragmentation of jet-cooled HOD at 193 nm: Vibrationally mediated photochemistry with zero-point excitation

Photofragment yields are reported for supersonically cooled H2O, D2O, and HOD via one photon, 193 nm photolysis in a slit jet expansion, with OH and OD fragments monitored by laser induced fluorescence methods. Detailed analysis of the dependence of OH vs OD photofragment signals on isotopic composition is used to extract relative photolysis cross sections and branching ratios for bond-selective cleavage in HOD, H2O, and D2O samples. Specified relative to the 193 nm cross sections for H2O→H+OH, the ratios are 0.392(20), 0.032(20), and 0.0157(19) for (i) HOD→H+OD, (ii) HOD→D+OH and D2O→D+OD channels, respectively. Specifically, these results indicate a propensity for H–OD vs D–OH bond cleavage in HOD of 12(8):1. This strong H/D isotopic selectivity reflects extreme non-Franck–Condon photolysis out of classically of forbidden regions of the ground-state wave function, i.e., bond-selective photochemistry mediated solely by zero-point vibrational excitation. However, when compared with theoretical predictions...

Photodissociation dynamics of jet-cooled H2O and D2O in the non-Franck–Condon regime: Relative absorption cross sections and product state distributions at 193 nm

Quantum state distributions for nascent OH and OD fragments generated by Franck–Condon “forbidden” 193 nm photodissociation of H2O and D2O are reported, with the two isotopomers initially prepared in their zero-point vibrational and lowest ortho/para nuclear spin allowed rotational states (i.e., JKaKc=101 and 000 in a 3:1 ratio for H2O and 1:2 ratio for D2O) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence (LIF) with cylindrical mirror collection optics optimized for the slit expansion geometry, which makes photodissociation studies feasible with cross sections as low as ≈10−26 cm2. The OH and OD fragments are formed exclusively in v=0, but with highly structured quantum state distributions in rotational, Λ-doublet, and fine structure levels (2Π3/2+, 2Π1/2+, and 2Π3/2−) that exhibit qualitatively different trends than observed in previous jet photolysis studies at 157 nm in the Franck–Condon “allowed” regime. The relative OH/OD fragment...

HIGH RESOLUTION VIBRATIONAL OVERTONE STUDIES OF HOD AND H2O WITH SINGLE MODE, INJECTION SEEDED RING OPTICAL PARAMETRIC OSCILLATORS

Design, performance, and applications of a pulsed, single mode optical parametric oscillator (OPO) for studies of high resolution spectroscopy and photodissociation dynamics are presented. Single mode operation is achieved by resonantly seeding a four-mirror OPO ring cavity with a tunable, continuous wave (cw) ring dye laser, providing continuous scanning capability at near Fourier transform limited resolution [Δν=160(20) MHz] with peak output energies ⩾10 mJ. The high spectral brightness of this OPO light source is sufficient to saturate Δv=3 stretching transitions in OH, NH, and CH vibrational manifolds, which makes feasible quantum state-selected multiple resonance spectroscopies at 0.005 cm−1 resolution. The capability of this single mode OPO is explicitly demonstrated via (i) vOH=3←0 overtone spectroscopy of HOD, (ii) near-IR optical saturation studies of H2O in the |03−〉 overtone vibrational manifold, (iii) high resolution LIF Dopplerimetry of OH radicals, and (iv) IR/UV multiple resonance spectrosc...

Segmented chirped-pulse Fourier transform submillimeter spectroscopy for broadband gas analysis.

Chirped-pulse Fourier transform spectroscopy has recently been extended to millimeter wave spectroscopy as a technique for the characterization of room-temperature gas samples. Here we present a variation of this technique that significantly reduces the technical requirements on high-speed digital electronics and the data throughput, with no reduction in the broadband spectral coverage and no increase in the time required to reach a given sensitivity level. This method takes advantage of the frequency agility of arbitrary waveform generators by utilizing a series of low-bandwidth chirped excitation pulses paired in time with a series of offset single frequency local oscillators, which are used to detect the molecular free induction decay signals in a heterodyne receiver. A demonstration of this technique is presented in which a 67 GHz bandwidth spectrum of methanol (spanning from 792 to 859 GHz) is acquired in 58 μs.