arXiv subjects

Stephen L. Coy

Publications and source records attributed to Stephen L. Coy.

6 recordsLinked to original sources

arXiv2022

Long-range model of vibrational autoionization in core-nonpenetrating Rydberg states of NO

In high orbital angular momentum ($\ell \geq 3$) Rydberg states, the centrifugal barrier hinders close approach of the Rydberg electron to the ion-core. As a result, these core-nonpenetrating Rydberg states can be well described by a simplified model in which the Rydberg electron is only weakly perturbed by the long-range electric properties (i.e., multipole moments and polarizabilities) of the ion-core. We have used a long-range model to describe the vibrational autoionization dynamics of high-$\ell$ Rydberg states of nitric oxide (NO). In particular, our model explains the extensive angular momentum exchange between the ion-core and Rydberg electron that had been previously observed in vibrational autoionization of $f$ ($\ell=3$) Rydberg states. These results shed light on a long-standing mechanistic question around these previous observations, and support a direct, vibrational mechanism of autoionization over an indirect, predissociation-mediated mechanism. In addition, our model correctly predicts newly measured total decay rates of $g$ ($\ell=4$) Rydberg states because, for $\ell\geq4$, the non-radiative decay is dominated by autoionization rather than predissociation. We examine the predicted NO$^+$ ion rotational state distributions generated by vibrational autoionization of $g$ states and discuss applications of our model to achieve quantum state selection in the production of molecular ions.

physics.atom-ph↗

arXiv2020

Nonlinear rotational spectroscopy reveals many-body interactions in water molecules

Because of their central importance in chemistry and biology, water molecules have been the subject of decades of intense spectroscopic investigations. Rotational spectroscopy of water vapor has yielded detailed information about the structure and dynamics of isolated water molecules as well as water dimers and clusters. Nonlinear rotational spectroscopy in the terahertz regime has been developed recently to investigate the rotational dynamics of linear and symmetric-top molecules whose rotational energy levels are regularly spaced, but it has not previously been applied to water or other lower-symmetry molecules with irregularly spaced levels. We report the use of recently developed two-dimensional terahertz rotational spectroscopy to observe high-order rotational coherences and correlations between rotational transitions that could not be observed previously. The results include two-quantum (2Q) peaks at frequencies that are shifted slightly from the sums of distinct rotational transitions on two molecules, which directly reveal the presence of previously unseen metastable water complexes with lifetimes of 100 ps or longer. Several such peaks observed at distinct 2Q frequencies indicate that the complexes have multiple preferred bimolecular geometries. Our results demonstrate sensitivity of rotational correlations measured in 2D THz spectroscopy to molecular interactions and complexation in the gas phase.

physics.chem-ph↗

arXiv2016

Direct single-shot observation of millimeter wave superradiance in Rydberg-Rydberg transitions

We have directly detected millimeter wave (mm-wave) free space superradiant emission from Rydberg states ($n \sim 30$) of barium atoms in a single shot. We trigger the cooperative effects with a weak initial pulse and detect with single-shot sensitivity and 20 ps time resolution, which allows measurement and shot-by-shot analysis of the distribution of decay rates, time delays, and time-dependent frequency shifts. Cooperative line shifts and decay rates are observed that exceed values that would correspond to the Doppler width of 250 kHz by a factor of 20 and the spontaneous emission rate of 50 Hz by a factor of $10^5$. The initial superradiant output pulse is followed by evolution of the radiation-coupled many-body system toward complex long-lasting emission modes. A comparison to a mean-field theory is presented which reproduces the quantitative time-domain results, but fails to account for either the frequency-domain observations or the long-lived features.

physics.atom-ph↗

arXiv2016

A gapless micro-dielectric-barrier-discharge ion source for analytical applications

Use of dielectric barrier discharge (DBD) as an ion source for sensitive chemical analysis is uncommon because barrier discharges generate excess noise due to spatial and temporal instability. This design uses contacted, crossed glass-coated micro-wires to focus the field into a gradually vanishing gap, suppressing spatial and temporal variability, reducing pressure, temperature, and humidity effects, stabilizing discharge initiation and limiting chemical fragmentation. Positive-ion-mode proton transfer, chemical fragmentation from a micro-discharge, and NO+ adducts combine to allow broad chemical sensitivity. We analyze noise properties of the ion source and report chemical responsivity for a wide range of volatile organic compounds. Source noise spectral density is compared for three systems: the contacted coated wires source, a gapped dielectric barrier discharge source, and a 5 mCi Ni-63 radioactive source. The crossed-wires source shows noise properties approaching those of the white-noise Ni-63 source, while gapped discharge exhibits 1/f noise from area-discharge random path and intensity variations. For chemical sensitivity testing, dilute samples are delivered by vapor flow injection or by gas chromatography, and then detected by differential ion mobility spectrometry (DMS / FAIMS) and, in a few cases by mass spectrometry. The compounds tested in positive ion mode include ketones and alcohols, simple aromatics (benzene, toluene, xylenes, ethyl and propyl benzene), chlorinated and nitrated solvents and aliphatics (hexanes and n-octane), with sensitivities from ppt to ppb levels. The wires source for trace vapor detection is stable under a range of environmental conditions from very low (ppb) humidity levels upward, has wider chemical coverage than the radioactive nickel source with nearly equivalent noise properties, and is adjustable to higher intensity.

physics.ins-det↗

arXiv1994

Microwave detected, microwave-optical double resonance of NH3, NH2D, NHD2, and ND3. I. Sturcture and force field of the A state

Microwave detected, microwave-optical double resonance was used to record the A state electronic spectrum of NH3, NH2D, and NHD2 with both vibrational and rotational resolution. To investigate ND3 with the same resolution as we had with our hydrogen containing isotopomers, a strip-line cell was constructed allowing the simultaneous passage of radio-frequency and ultraviolet radiation. Rotational constants were obtained as a function of nu2 excitation and an A state equilibrium bond length was estimated at 1.055(8) angstroms. In addition, the harmonic force field for the A state has been experimentally determined. fhh, falpha,alpha-falpha,alpha', and frr were found to be 1.06(4)aJ/angstrom2, 0.25(2)aJ, and 4.9aJ/angstrom^2 respectively. This calculated harmonic force field predicts that the asymmetry observed in the NH3 2(4) band is due to a strong anharmonic interaction with the 4(3) level and the broad feature observed in the dispersed fluorescence spectrum previously assigned to the 1(1) band is more likely attributable to the 4(2) level.

chem-ph↗

arXiv1994

Microwave detected, microwave-optical double resonance of NH3, NH2D, NHD2, and ND3. II. Predissociation dynamics of the A state

Using microwave detected, microwave-optical double resonance, we have measured the homogeneous linewidths of individual rovibrational transitions in the Ã state of NH3, NH2D, NHD2, and ND3. We have used this excited state spectroscopic data to characterize the height of the dissociation barrier and the mechanisms by which the molecule uses its excess vibrational and rotational energies to help overcome this barrier. To interpret the observed vibronic widths, a one dimensional, local mode potential has been developed along a N-H(D) bond. These calculations suggest the barrier height is roughly 2100cm-1, approximately 1000cm-1 below the ab initio prediction. The observed vibronic dependence of levels containing two or more quanta in nu2 is explained by a Fermi resonance between nu2 and the N-H(D) stretch. This interaction also explains the observed trends due to isotopic substitution. The rotational enhancement of the predissociation rates in the NH3 2(1) level is dominated by Coriolis coupling while for the same level in ND3, centrifugal effects dominate.

chem-ph↗