Calvin Mukarakate

Automated construction of potential energy surfaces

ABSTRACT Methods to construct molecular potential energy surfaces through automated generation of ab initio electronic structure data are reviewed. Given a chosen method for fitting ab initio data (electronic energies represented at particular geometries) into an analytic surface, the questions of how best to select the data point locations and how to interface an electronic structure software package with fitting codes in parallel on a high-performance computing cluster are addressed. It is shown that methods based on interpolating moving least squares fitting are useful as they lend themselves to an algorithm which iteratively refines the fitted surface towards arbitrary accuracy. Several variants of the method are illustrated through examples including spectroscopic potentials for van der Waals systems, systems with high permutation symmetry, reactive systems, and systems with multiple coupled electronic states. An outlook identifying areas for future development is given. GRAPHICAL ABSTRACT

Fluorescence excitation and emission spectroscopy of the ÃA″1←X̃A′1 system of CHBr

We report fluorescence excitation and emission spectra of CHBr in the 450–750nm region. A total of 30 cold bands involving the pure bending levels 20n with n=2–8 and combination bands 20n301(n=1–8), 20n302(n=1–6), 20n303(n=1–2), 10120n(n=5–7), 10120n301(n=4–6), and 10120n302(n=5) in the AA″1←XA′1 system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers (CHBr79,CHBr81). The derived AA″1 vibrational intervals are combined with results of [Yu et al. J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The AA″1 state C–H stretching frequency is determined here for the first time, and the observed ν3 dependence of the Br79–Br81 isotope splitting in the AA″1 state is in good agreement ...

Spectroscopy and dynamics of the predissociated, quasi-linear S2 state of chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S(2) state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S(1) state. This study, which represents the first observation of the S(2) state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S(2) state vibrational levels with angular momentum l = 1 were observed for CHCl, and 34 levels for CDCl. The range of l in this case was restricted by the pronounced Renner-Teller effect in the low-lying S(1) levels, which severely reduces the fluorescence lifetime for levels with K(a) > 0. Nonetheless, by exploiting different intermediate S(1) levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S(1) state. Plots of the predissociation linewidth (lifetime) vs. energy for various S(2) levels show an abrupt onset, which lies near the calculated threshold for elimination to form C((3)P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold.

Electronic spectroscopy of the ÃA″1↔X̃A′1 system of CDBr

We report fluorescence excitation and single vibronic level emission spectra of jet-cooled CDBr in the 450–750nm region. A total of 32 cold bands involving the pure bending levels 20n with n=3–10 and combination bands 20n301 (n=2–10), 20n302 (n=2–9), 10120n (n=7–10), and 10120n301 (n=6,8–9) in the AA″1←XA′1 system of this carbene were observed; most of these are reported and/or rotationally analyzed here for the first time. Rotational analysis yielded band origins and effective (B¯) rotational constants for both bromine isotopomers (CD79Br and CD81Br). The derived AA″1 vibrational intervals are combined with results of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The AA″1 state C–D stretching frequency (2350cm−1) is determined for the first time, in excellent agreement with theory, as are the Br79–Br81 isotope splittings in the excited state. Our emission spectra probe the vibrational structure of the XA′1 and aA″3 states up to ...