Microwave spectrum of the ethanol-methanol dimer

The hydrogen bond donor/acceptor competition in mixed alcohol clusters remains a fundamental question in physical chemistry. Previous theoretical work on the prototype ethanol-methanol dimer has been inconclusive in predicting the energetically preferred structure. Here, we report the microwave spectrum of the ethanol-methanol dimer between 8-18 GHz, using a chirped pulse Fourier transform microwave spectrometer. With the aid of ab initio calculations, 36 transitions have been fit and assigned to a t-ethanol-acceptor, methanol-donor structure in an argon-backed expansion. In a helium-backed expansion, a second excited conformer has been observed, and tentatively assigned to a g-ethanol-acceptor, methanol-donor structure. No ethanol-donor, methanol-acceptor structures have been found, suggesting such structures are energetically disfavored

Nonlinear terahertz coherent excitation of vibrational modes of liquids

We report the first coherent excitation of intramolecular vibrational modes via the nonlinear interaction of a TeraHertz (THz) light field with molecular liquids. A terahertz-terahertz-Raman pulse sequence prepares the coherences with a broadband, high-energy, (sub)picosecond terahertz pulse, that are then measured in a terahertz Kerr effect spectrometer via phase-sensitive, heterodyne detection with an optical pulse. The spectrometer reported here has broader terahertz frequency coverage, and an increased sensitivity relative to previously reported terahertz Kerr effect experiments. Vibrational coherences are observed in liquid diiodomethane at 3.66 THz (122 cm^(−1)), and in carbon tetrachloride at 6.50 THz (217 cm^(−1)), in exact agreement with literature values of those intramolecular modes. This work opens the door to 2D spectroscopies, nonlinear in terahertz field, that can study the dynamics of condensed-phase molecular systems, as well as coherent control at terahertz frequencies

ULTRAFAST TERAHERTZ KERR EFFECT SPECTROSCOPY OF LIQUIDS AND BINARY MIXTURES

The ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a nonlinear spectroscopic technique capable of measuring the dielectric relaxation of liquids. The true power of this technique lies in its ability to provide complementary information to measurements taken using heterodyne-detected optical Kerr effect (OKE) spectroscopy. The optical pulses in OKE measurements interact with the sample via the molecular polarizability, a rank-two tensor, in contrast with THz pulses that interact with the molecules via the dipole moment, a rank-one tensor. Given the different light-matter interactions in the two techniques, TKE measurements help complete the physical picture of intermolecular interactions at short timescales. We report here our implementation of heterodyne-detected TKE spectroscopy, along with measurements of pure liquids, and binary mixtures. Some of the liquids presented here were previously believed to be TKE inactive, thus showing that we have achieved a greater sensitivity than the previous implementation in the literature. In addition, we will discuss a variety of binary mixtures and show how the TKE data can be compared with OKE data to deepen our physical understanding of intermolecular interactions in liquids

DOPPLER-LIMITED SPECTROSCOPY WITH A DECADE-SPANNING TERAHERTZ FREQUENCY COMB

We report the generation and detection of a decade-spanning TeraHertz (THz) frequency comb (0.15-2.4 THz) using two Ti:Sapphire femtosecond laser oscillators and ASynchronous OPtical Sampling THz Time-Domain Spectroscopy (ASOPS-THz-TDS). The measured linewidth of the comb at 1.5 THz is 3 kHz over a 60 second acquisition. With time-domain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1-2 THz with an average Doppler-limited fractional uncertainty of $5.9times10^{-8}$. Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies and will enable future studies of jet-cooled hydrogen-bonded clusters

ULTRAFAST TERAHERTZ KERR EFFECT SPECTROSCOPY OF AROMATIC LIQUIDS

Ultrafast Terahertz Kerr Effect (TKE) spectroscopy is a relatively new nonlinear THz technique that is sensitive to the orientational dynamics of anisotropic, condensed-phase samples. The sample is excited by a single high field strength $sim$1 picosecond THz pulse, and the resulting transient birefringence is measured by a $sim$40 femtosecond 800 nm probe pulse. We have measured the TKE response of several aromatic liquids at room temperature, including benzene, benzene-d6, hexafluorobenzene, pyridine, and toluene. The measured decay constants range from $sim$1-10 ps, and, along with previous optical Kerr effect results in the literaturefootnote{Loughnane et al. JPCB 110.11 (2006): 5708-5720.}, give insights into intermolecular interactions in these liquids

HYDROGEN-BONDING AND HYDROPHOBIC INTERACTIONS IN THE ETHANOL-WATER DIMER

The conformational energy landscape of the ethanol-water dimer is determined by the relative hydrogen-bond donor and acceptor strengths of the two molecules, as well as weaker hydrophobic interactions between the water and the ethyl group. Using a combination of textit{ab initio} calculations and chirped-pulse Fourier transform microwave spectroscopy, we have recorded the first rotationally-resolved, jet-cooled spectrum of the ethanol-water dimer between 8-18.5 GHz and identified two water-donor ethanol-acceptor conformers. The lowest energy conformer is chiral, has ethanol in the gauche configuration, and is consistent with previous raman and infrared results.footnote{Nedi_, Marija, et al. PCCP 13.31 (2011): 14050-14063.} The second conformer corresponds to the trans-ethanol configuration, and exhibits a significant splitting

Hydrogen bonding in the ethanol–water dimer

We report the first rotational spectrum of the ground state of the isolated ethanol–water dimer using chirped-pulse Fourier transform microwave spectroscopy between 8–18 GHz. With the aid of isotopic substitutions, and ab initio calculations, we identify the measured conformer as a water-donor/ethanol-acceptor structure. Ethanol is found to be in the gauche conformation, while the monomer distances and orientations likely reflect a cooperation between the strong (O–H⋯O) and weak (C–H⋯O) hydrogen bonds that stabilizes the measured conformer. No other conformers were assigned in an argon expansion, confirming that this is the ground-state structure. This result is consistent with previous vibrationally-resolved Raman and infrared work, but sheds additional light on the structure, due to the specificity of rotational spectroscopy

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH3_3CHCH2_2O)

Life on Earth relies on chiral molecules, that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH$_3$CHCH$_2$O), in absorption toward the Galactic Center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found

Hydrogen bond competition in the ethanol–methanol dimer

Previous theoretical work on the ethanol–methanol dimer has been inconclusive in predicting the preferred hydrogen bond donor/acceptor configuration. Here, we report the microwave spectrum of the dimer using a chirped pulse Fourier transform microwave spectrometer from 8–18 GHz. In an argon-backed expansion, 50 transitions have been assigned to a trans-ethanol-acceptor/methanol-donor structure that is likely stabilized by a secondary weak C–H⋯O hydrogen bond. A higher energy conformer was observed in a helium-backed expansion and tentatively assigned to a gauche-ethanol-acceptor/methanol-donor structure. No ethanol-donor/methanol-acceptor dimers have been found, suggesting such interactions are energetically disfavored. A preliminary analysis of the A–E splitting due to the internal rotation of the methanol methyl group in the ground state species is also presented. We find evidence of the Ubbelohde effect in the measured A–E splittings of three deuterated isotopologues and the normal species of this conformer

Sub-Doppler Cooling and Compressed Trapping of YO Molecules at μ\muK Temperatures

Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices. Yttrium monoxide (YO) has unique intramolecular interactions. The Fermi-contact interaction dominates over the spin-rotation coupling, resulting in two manifolds of closely spaced states, with one of them possessing a negligible Land\'e g-factor. This unique energy level structure favors dual-frequency DC magneto-optical trapping (MOT) and gray molasses cooling (GMC). We report exceptionally robust cooling of YO at 4 $\mu$K over a wide range of laser intensity, detunings (one and two-photon), and magnetic field. The magnetic insensitivity enables the spatial compression of the molecular cloud by alternating GMC and MOT under the continuous operation of the quadrupole magnetic field. A combination of these techniques produces a laser-cooled molecular sample with the highest phase space density in free space.Comment: 9 pages, 6 figure