Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH AΣ+2 (v=4) radicals

Predissociation dynamics of the hydroxyl radical (OH) based on a five-state spectroscopic model

Multi-reference configuration interaction potential energy curves (PECs) and spin-orbit couplings for the X 2Π, A 2Σ+, 1 2Σ-, 1 4Σ-, and 1 4Π states of OH are computed and refined against empirical energy levels and transitions to produce a spectroscopic model. Predissociation lifetimes are determined by discretizing continuum states in the variational method nuclear motion calculation by restricting the calculation to a finite range of internuclear separations. Varying this range gives a series of avoided crossings between quasi-bound states associated with the A 2Σ+ and continuum states, from which predissociation lifetimes are extracted. 424 quasi-bound A 2Σ+ state rovibronic energy levels are analyzed, and 374 predissociation lifetimes are produced, offering good coverage of the predissociation region. Agreement with measured lifetimes is satisfactory, and a majority of computed results were within experimental uncertainty. A previously unreported A 2Σ+ state predissociation channel that goes via X 2Π is identified in the calculations. A Python package, binSLT, produced to calculate predissociation lifetimes, associated line broadening parameters, and lifetime uncertainties is made available. The PECs and other curves from this work will be used to produce a rovibronic ExoMol line list and temperature-dependent photodissociation cross sections for the hydroxyl radical.

Laser-Plasma Spectroscopy of Hydroxyl with Applications

This article discusses laser-induced laboratory-air plasma measurements and analysis of hydroxyl (OH) ultraviolet spectra. The computations of the OH spectra utilize line strength data that were developed previously and that are now communicated for the first time. The line strengths have been utilized extensively in interpretation of recorded molecular emission spectra and have been well-tested in laser-induced fluorescence applications for the purpose of temperature inferences from recorded data. Moreover, new experiments with Q-switched laser pulses illustrate occurrence of molecular recombination spectra for time delays of the order of several dozen of microseconds after plasma initiation. The OH signals occur due to the natural humidity in laboratory air. Centrifugal stretching of the Franck-Condon factors and r-centroids are included in the process of determining the line strengths that are communicated as a Supplementary File. Laser spectroscopy applications of detailed OH computations include laser-induced plasma and combustion analyses, to name but two applications. This work also includes literature references that address various diagnosis applications.

Empirical Line Lists in the ExoMol Database

The ExoMol database aims to provide comprehensive molecular line lists for exoplanetary and other hot atmospheres. The data are expanded by inclusion of empirically derived line lists taken from the literature for a series of diatomic molecules, namely CH, NH, OH, AlCl, AlF, OH + , CaF, MgF, KF, NaF, LiCl, LiF, MgH, TiH, CrH, FeH, C 2 , CP, CN, CaH, and triplet N 2 . Generally, these line lists are constructed from measured spectra using a combination of effective rotational Hamiltonian models for the line positions and ab initio (transition) dipole moments to provide intensities. This work results in the inclusion of 22 new molecules (36 new isotopologues) in the ExoMol database.

Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes

An ultraviolet-infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S₁ origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S₁ origin fluorescence, resulting in spectra with high signal-to-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (Ac-Phe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch-bend Fermi resonance.

A THEORETICAL ANALYSIS OF THE DISSOCIATION OF OH RADICAL: FINE-STRUCTURE DISTRIBUTIONS OF THE O(3PJ) PRODUCT

A theoretical treatment is presented for the nonadiabatic photodissociation of hydroxyl radical. Five electronic states, X 2Π, A 2Σ+, 14Σ-, 12Σ- and 14Π, are included in this simulation. Based on the accurate ab initio calculations of the potential energy curves, transition dipole moments and nonadiabatic couplings between the relevant states, the dissociation dynamics are investigated using the time-dependent quantum wave packet approach. The direct dissociation, following the excitation from the specific vibrational levels of the ground state X 2Π to the repulsive state 12Σ- is examined, where the total and partial cross sections and the branching fractions of spin–orbit fine-structures of O (3 P J) are calculated over a wide range of the incident photon frequencies. For the predissociation via the bound excited state A 2Σ+, the influence of the nonadiabatic interactions in different internuclear regions and the role of the repulsive states in the dissociation starting at different vibrational levels are also analyzed through the spin–orbit branching fractions of products.