Stephen C. Ross

SiC2: A molecular pinwheel

We present the results of a combined experimental and theoretical study of the large‐amplitude motion in SiC2 in which the C2 fragment undergoes hindered internal rotation. Stimulated emission pumping (SEP) is used to obtain rovibrational term energies for levels with up to 14 quanta of excitation in the large‐amplitude vibration. We analyze the SEP data, as well as other available experimental data, using a semirigid bender model that allows for complete internal rotation within a triatomic molecule. From the least‐squares fitting of this model to the data, we determine the potential energy along the minimum energy path of the large‐amplitude vibration, the harmonic energies of the small‐amplitude vibrations, and the variations of these energies and of the molecular geometry with the large‐amplitude coordinate. The fitting is aided by results obtained from ab initio calculations we perform on the triangular and linear configurations of the molecule. The current data set is consistent with a large‐amplitude potential energy function in which the energy difference between the triangular and linear configurations is 1883 cm−1. The statistical error on this energy difference is 22 cm−1, but we estimate the physical uncertainty to be about 200 cm−1. This result is in excellent agreement with the energy difference of 1819 cm−1 we obtain in our best ab initio calculations. The semirigid bender fitting and our best ab initio calculations are also both consistent with a potential energy function having no local minimum at linearity.

Chemical reaction jet spectroscopy, molecular structure, and the bending potential of the à 1A″ state of monofluorosilylene (HSiF)

The jet‐cooled laser induced fluorescence excitation spectrum of the A 1A″–X 1A′ band system of HSiF has been observed with the chemical reaction jet technique. Vibrational analysis of the spectrum gave upper state fundamental vibrational frequencies of ν1=1547 cm−1, ν2=558 cm−1, and ν3=857 cm−1. Seven bands in the spectrum were recorded at high resolution and rotationally analyzed, providing excited state molecular constants. The upper state vibrational and rotational bending levels were fitted to a semirigid bender model to obtain the equilibrium geometry and the potential energy barrier to linearity. Due to correlations in the parameters, it was necessary to fix the bond angle at the ab initio value of 114.5°. The resulting fitted model yielded re(Si–F)=1.602 A, re(Si–H)=1.548 A with a potential energy barrier to linearity of 9130 cm−1.

Decay dynamics of the long-range H¯Σg+1 state of D2 and H2: Experiment and theory

We present accurate experimental measurements of the lifetimes of rovibrational levels of the long-range H¯Σg+1 state for both D2 and H2, obtained directly from the observation of the time-dependent decay of the fluorescence from these excited levels. These results improve upon and extend those of Reinhold et al. [J. Chem. Phys. 112, 10754 (2000)]. Several decay pathways are open to these levels including fluorescence, predissociation, and autoionization. We present theoretical results for each of these processes, each calculated using the simplest but still appropriate level of theory. In particular, the theoretical calculations provide a quantitative explanation of the dramatic vibrational dependence of the observed lifetimes, the isotope dependence of the lifetimes for levels well localized within the H¯ potential well and therefore not subject to significant tunneling, and an insight into the role of enhanced tunneling in autoionization. In these calculations each of the rovibrational levels of the H¯...

The v4 large-amplitude bending potential of HNCO as determined by a semirigid bender analysis of the ground state rotational spectra

The ground state microwave, millimeter wave, and far-infrared spectroscopic data (J < 10) of HNCO and its isotopomers were analyzed using the semirigid bender (SRB) Hamiltonian. The geometry of the molecule and the potential energy function of the large-amplitude HNC-bending motion were determined by a non-linear least-squares fitting procedure. The barrier to the linear configuration was found to be 1899 cm−1, which is much lower than the previously reported value. The parameters determined for the molecular structure are in excellent agreement with the rs values.

An MQDT Primer

The purpose of this article is to give a simple beginners introduction to the basic concepts of Multichannel Quantum Defect Theory (MQDT). This will be done by proceeding through a series of examples. Each example will add something to our understanding and will also illustrate an area in which the theory can be used. In particular MQDT is used to study Rydberg levels of atoms and molecules and the ionization continuum that lies above each Rydberg series. We will also see that MQDT can account for the interactions between the Rydberg series and continua that occur in an atom or a molecule. A short bibliography at the end includes references to several review articles to which interested readers are directed if they wish to deepen their knowledge.

The microwave spectrum and semirigid bender analysis of isocyanatoethyne, HCCNCO

Abstract Isocyanatoethyne, HCCNCO, has been prepared and its microwave spectrum recorded. The molecule behaves as a slightly asymmetric prolate rotor, the spectrum indicating deviations from linearity in the equilibrium structure. Vibrationally excited states with up to three quanta of the CNC bending mode were observed, allowing analysis of the spectrum to be carried out using a semirigid bender model. The results of this analysis give the following structural parameters: r(CC) = 1.2237 (20) A , r(CN) = 1.3025 (54) A , r(NC) = 1.2139 (60) A , r(CO) = 1.1741 (48) A , ∠(CNC) = 140.67 (48)° and, ∠(NCO) = 170.02 (93)°, with all other bond angles assumed to be 180°. The barrier to linearity was calculated to be 537.2(5.4) cm−1.

Correlation diagrams between the free-rotor and the hindered rotor states of a diatomic molecule: energy levels and transition moments obtained by numerical integration of the schrödinger equation

We present a numerical technique for the direct calculation of the rotational eigenvalues and transition moments of a rigid diatomic 1Σ rotor in an external axially-symmetric potential. Because this technique does not require an expansion in spherical harmonics, the wavefunctions and energy eigenvalues are obtained without having to perform integrals over the potential energy function, or having to calculate (and truncate) a Hamiltonian matrix. We use our technique to show the effect of the external field on the molecular spectra and present correlation diagrams of energy levels and transition moments between high barrier and free-rotor limits. The case of double minima potential energy functions, including those with inequivalent minima, is presented. The general features of the rotational states and transition probabilities of rotors in such potential energy functions is explored to serve as a guide for future experimental work. The presentation of the technique and of the intricacies of the energy levels and transition moments of the double minima potential energy functions is intentionally pedagogical.

Progress in the rotational analysis of the ground and low-lying vibrationally excited states of malonaldehyde

Despite being an important prototype molecule for intramolecular proton tunnelling, the far-IR spectrum of the internally hydrogen-bonded species malonaldehyde (C3O2H4) is not yet well understood. In the talk I gave at the ISMS meeting in 2015 I discussed the high-resolution spectra we obtained at the Canadian Light Source synchrotron in Saskatoon, Saskatchewan. These spectra include a number of fundamental vibrational bands in the 100-2000 cm−1 region. In our efforts to analyze these bands we have noticed that our ground state combination differences show a large drift (up to an order of magnitude larger than our experimental error) away from those calculated using constants established by Baba et al.,a particularly in regions of high J (above 30) and low Ka (below 5). An examination of the previous microwave and far-IR studiesbc reveals that this region of J-Ka space was not represented in the lines that Baba et al. used to generate the values for their fitting parameters. By including our own measurements in the fitting, we were able to improve the characterization of the ground state so that it is now consistent with all of the existing data. This characterization now covers a much larger range of J-Ka space and has enabled us to make significant progress in analyzing our far-IR synchrotron spectra. These include an excited vibrational state at 241 cm−1 as well as several states split by the tunnelling effect at higher wavenumber.

FITTING THE HIGH-RESOLUTION SPECTROSCOPIC DATA FOR NCNCS

ZBIGNIEW KISIEL, ON2, Institute of Physics, Polish Academy of Sciences, Warszawa, Poland; BRENDA P. WINNEWISSER, MANFRED WINNEWISSER, FRANK C. DE LUCIA, Department of Physics, The Ohio State University, Columbus, OH, USA; DENNIS TOKARYK, STEPHEN CARY ROSS, Department of Physics, University of New Brunswick, Fredericton, NB, Canada; BRANT E BILLINGHURST, EFD, Canadian Light Source Inc., Saskatoon, Saskatchewan, Canada.